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Summary: Plants face attackers aboveground and belowground. Insect root herbivores can lead to severe crop losses, yet the underlying transcriptomic responses have rarely been studied.We studied the dynamics of the transcriptomic response of Brussels sprouts (Brassica oleracea var. gemmifera) primary roots to feeding damage by cabbage root fly larvae (Delia radicum), alone or in combination with aboveground herbivory by cabbage aphids (Brevicoryne brassicae) or diamondback moth caterpillars (Plutella xylostella). This was supplemented with analyses of phytohormones and the main classes of secondary metabolites; aromatic, indole and aliphatic glucosinolates.Root herbivory leads to major transcriptomic rearrangement that is modulated by aboveground feeding caterpillars, but not aphids, through priming soon after root feeding starts. The root herbivore downregulates aliphatic glucosinolates. Knocking out aliphatic glucosinolate biosynthesis with CRISPR‐Cas9 results in enhanced performance of the specialist root herbivore, indicating that the herbivore downregulates an effective defence.This study advances our understanding of how plants cope with root herbivory and highlights several novel aspects of insect–plant interactions for future research. Further, our findings may help breeders develop a sustainable solution to a devastating root pest. [ABSTRACT FROM AUTHOR]

Plants developing into the flowering stage undergo major physiological changes. Because flowers are reproductive tissues and resource sinks, strategies to defend them may differ from those for leaves. Thus, this study investigates the defences of flowering plants by assessing processes that sustain resistance (constitutive and induced) and tolerance to attack. We exposed the annual plant Brassica nigra to three distinct floral attackers (caterpillar, aphid and bacterial pathogen) and measured whole‐plant responses at 4, 8 and 12 days after the attack. We simultaneously analysed profiles of primary and secondary metabolites in leaves and inflorescences and measured dry biomass of roots, leaves and inflorescences as proxies of resource allocation and regrowth. Regardless of treatments, inflorescences contained 1.2 to 4 times higher concentrations of primary metabolites than leaves, and up to 7 times higher concentrations of glucosinolates, which highlights the plant's high investment of resources into inflorescences. No induction of glucosinolates was detected in inflorescences, but the attack transiently affected the total concentration of soluble sugars in both leaves and inflorescences. We conclude that B. nigra evolved high constitutive rather than inducible resistance to protect their flowers; plants additionally compensated for damage by attackers via the regrowth of reproductive parts. This strategy may be typical of annual plants. Results show that flowering Brassica nigra protect their inflorescences with constitutive resistance rather than with inducible resistance, and compensate for floral dammage with early changes in primary metabolism and regrowth. Metabolic profiles reflected high resource investment into reproduction. [ABSTRACT FROM AUTHOR]

Summary: The phenotypic plasticity of flowering plants in response to herbivore damage to vegetative tissues can affect plant interactions with flower‐feeding organisms. Such induced systemic responses are probably regulated by defence‐related phytohormones that signal flowers to alter secondary chemistry that affects resistance to florivores. Current knowledge on the effects of damage to vegetative tissues on plant interactions with florivores and the underlying mechanisms is limited.We compared the preference and performance of two florivores on flowering Brassica nigra plants damaged by one of three herbivores feeding from roots or leaves. To investigate the underlying mechanisms, we quantified expression patterns of marker genes for defence‐related phytohormonal pathways, and concentrations of phytohormones and glucosinolates in buds and flowers.Florivores displayed contrasting preferences for plants damaged by herbivores feeding on roots and leaves. Chewing florivores performed better on plants damaged by folivores, but worse on plants damaged by the root herbivore. Chewing root and foliar herbivory led to specific induced changes in the phytohormone profile of buds and flowers. This resulted in increased glucosinolate concentrations for leaf‐damaged plants, and decreased glucosinolate concentrations for root‐damaged plants.The outcome of herbivore–herbivore interactions spanning from vegetative tissues to floral tissues is unique for the inducing root/leaf herbivore and receiving florivore combination. [ABSTRACT FROM AUTHOR]

There is increasing awareness that interactions between plants and insects can be mediated by microbial symbionts. Nonetheless, evidence showing that symbionts associated with organisms beyond the second trophic level affect plant‐insect interactions are restricted to a few cases belonging to parasitoid‐associated bracoviruses. Insect parasitoids harbour a wide array of symbionts which, like bracoviruses, can be injected into their herbivorous hosts to manipulate their physiology and behaviour. Yet, the function of these symbionts in plant‐based trophic webs remains largely overlooked. Here, we provide the first evidence of a parasitoid‐associated symbiont belonging to the group of ichnoviruses which affects the strength of plant‐insect interactions. A comparative proteomic analysis shows that, upon parasitoid injection of calyx fluid containing ichnovirus particles, the composition of salivary glands of caterpillars changes both qualitatively (presence of two viral‐encoded proteins) and quantitatively (abundance of several caterpillar‐resident enzymes, including elicitors such as glucose oxidase). In turn, plant phenotypic changes triggered by the altered composition of caterpillar oral secretions affect the performance of herbivores. Ichnovirus manipulation of plant responses to herbivory leads to benefits for their parasitoid partners in terms of reduced developmental time within the parasitized caterpillar. Interestingly, plant‐mediated ichnovirus‐induced effects also enhance the performances of unparasitized herbivores which in natural conditions may feed alongside parasitized ones. We discuss these findings in the context of ecological costs imposed to the plant by the viral symbiont of the parasitoid. Our results provide intriguing novel findings about the role played by carnivore‐associated symbionts on plant‐insect‐parasitoid systems and underline the importance of placing mutualistic associations in an ecological perspective. [ABSTRACT FROM AUTHOR]

Plants have evolved resistance traits that negatively affect attackers, and tolerance traits that sustain plant growth despite herbivore damage. These mechanisms often co‐occur in a mixed‐defence strategy, balancing resistance and tolerance. These plant defences can be enhanced upon interaction with soil micro‐organisms.Here we investigated the effects of volatiles emitted by soil‐borne fungi on plant defence to insect herbivory, and on plant phenology.We exposed roots of Brassica rapa plants to volatiles emitted by four soil‐borne fungi. As a proxy of plant resistance, we assessed the performance of Pieris brassicae, a caterpillar feeding on leaves and inflorescences, and of Delia radicum, an insect root herbivore. As a proxy of plant tolerance, we compared growth of volatile‐exposed plants challenged with or without insects. Additionally, we assessed the effects on plant phenology by recording bolting time and by counting the number of buds and flowers.Plant exposure to fungal volatiles differentially affected plant resistance to above‐ and below‐ground herbivory. Performance of P. brassicae caterpillars differed between the fungal volatile‐exposed plants but was variable between experimental batches. In contrast, the effects of fungal volatiles on D. radicum performance were predominantly negative, indicating an increased plant resistance. Despite root consumption by D. radicum, root dry weight remained unchanged in infested plants compared with uninfested ones, irrespectively of the volatile exposure, suggesting compensation for the tissue loss, sometimes at the cost of undamaged above‐ground tissues. When B. rapa plants were attacked by P. brassicae caterpillars, only exposure to volatiles of some fungi led to compensation for the loss of above‐ground tissues consumed by the caterpillars, which differed between leaves and inflorescences. Furthermore, bolting was accelerated in response to volatiles of some fungi, resulting in more buds and flowers, which suggests a potential enhancement of plant fitness.Our data show that fungal volatiles can modulate the mixed‐defence strategies of B. rapa plants, balancing plant resistance and tolerance to above‐ and below‐ground herbivory. These effects may be variable and were fungus specific. Ultimately, plant fitness may be enhanced upon root exposure to fungal volatiles. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

Plant‐mediated interactions are an important force in insect ecology. Through such interactions, herbivores feeding on leaves can affect root feeders. However, the mechanisms regulating the effects of above‐ground herbivory on below‐ground herbivores are poorly understood. Here, we investigated the performance of cabbage root fly larvae (Delia radicum) on cabbage plants (Brassica oleracea) previously exposed to above ground herbivores belonging to two feeding guilds: leaf chewing diamondback moth caterpillars (Plutella xylostella) or phloem‐feeding cabbage aphids (Brevicoryne brassicae). Our study focusses on root‐herbivore performance and defence signalling in primary roots by quantifying phytohormones and gene expression. We show that leaf herbivory by caterpillars, but not by aphids, strongly attenuates root herbivore performance. Above‐ground herbivory causes changes in primary roots in terms of gene transcripts and metabolites involved in plant defence. Feeding by below‐ground herbivores strongly induces the jasmonate pathway in primary roots. Caterpillars feeding on leaves cause a slight induction of the primary root jasmonate pathway and interact with plant defence signalling in response to root herbivores. In conclusion, feeding by a leaf chewer and a phloem feeder differentially affects root‐herbivore performance, root‐herbivore‐induced phytohormonal signalling, and secondary metabolites. Caterpillars (Plutella xylostella) and aphids (Brevicoryne brassicae) feeding on leaves of Brassica oleracea differentially affect gene expression, phytohormone concentrations and defence against cabbage root fly larvae (Delia radicum) in the primary root. [ABSTRACT FROM AUTHOR]

In perennial plants, interactions with other community members during the vegetative growth phase may influence community assembly during subsequent reproductive years and may influence plant fitness. It is well‐known that plant responses to herbivory affect community assembly within a growing season, but whether plant–herbivore interactions result in legacy effects on community assembly across seasons has received little attention. Moreover, whether plant–herbivore interactions during the vegetative growing season are important in predicting plant fitness directly or indirectly through legacy effects is poorly understood.Here, we tested whether plant–arthropod interactions in the vegetative growing season of perennial wild cabbage plants, Brassica oleracea, result in legacy effects in arthropod community assembly in the subsequent reproductive season and whether legacy effects have plant fitness consequences. We monitored the arthropod community on plants that had been induced with either aphids, caterpillars or no herbivores in a full‐factorial design across 2 years. We quantified the plant traits 'height', 'number of leaves' and 'number of flowers' to understand mechanisms that may mediate legacy effects. We measured seed production in the second year to evaluate plant fitness consequences of legacy effects.Although we did not find community responses to the herbivory treatments, our data show that community composition in the first year leaves a legacy on community composition in a second year: predator community composition co‐varied across years. Structural equation modelling analyses indicated that herbivore communities in the vegetative year correlated with plant performance traits that may have caused a legacy effect on especially predator community assembly in the subsequent reproductive year. Interestingly, the legacy of the herbivore community in the vegetative year predicted plant fitness better than the herbivore community that directly interacted with plants in the reproductive year.Synthesis. Thus, legacy effects of plant–herbivore interactions affect community assembly on perennial plants across growth seasons and these processes may affect plant reproductive success. We argue that plant–herbivore interactions in the vegetative phase as well as in the cross‐seasonal legacy effects caused by plant responses to arthropod herbivory may be important in perennial plant trait evolution such as ontogenetic variation in growth and defence strategies. [ABSTRACT FROM AUTHOR]

Plants balance the allocation of resources between growth and defence to optimize fitness in a competitive environment. Perception of neighbour‐detection cues, such as a low ratio of red to far‐red (R:FR) radiation, activates a suite of shade‐avoidance responses that include stem elongation and upward leaf movement, whilst simultaneously downregulating defence. This downregulation is hypothesized to benefit the plant either by mediating the growth‐defence balance in favour of growth in high plant densities or, alternatively, by mediating defence of individual leaves such that those most photosynthetically productive are best protected. To test these hypotheses, we used a 3D functional–structural plant model of Brassica nigra that mechanistically simulates the interactions between plant architecture, herbivory, and the light environment. Our results show that plant‐level defence expression is a strong determinant of plant fitness and that leaf‐level defence mediation by R:FR can provide a fitness benefit in high densities. However, optimal plant‐level defence expression does not decrease monotonically with plant density, indicating that R:FR mediation of defence alone is not enough to optimize defence between densities. Therefore, assessing the ecological significance of R:FR‐mediated defence is paramount to better understand the evolution of this physiological linkage and its implications for crop breeding. Plants use ratios of red to far‐red (R:FR) light to predict future competition and to subsequently activate shade‐avoidance responses and downregulate defences. R:FR‐mediated defence is suggested to be advantageous to decrease plant defence with increasing plant competition and to adjust defence to the photosynthetic value of a leaf as signalled by R:FR. Using a 3D plant model, this study shows that adjusting leaf‐level defence can result in a fitness benefit to the plant, whereas R:FR ratios alone cannot account for the optimal relation between the level of plant defence and the degree of competition. [ABSTRACT FROM AUTHOR]

Summary: Susceptibility to the root‐knot nematode Meloidogyne incognita in plants is thought to be a complex trait based on multiple genes involved in cell differentiation, growth and defence. Previous genetic analyses of susceptibility to M. incognita have mainly focused on segregating dominant resistance genes in crops. It is not known if plants harbour significant genetic variation in susceptibility to M. incognita independent of dominant resistance. To study the genetic architecture of susceptibility to M. incognita, we analysed nematode reproduction on a highly diverse set of 340 natural inbred lines of Arabidopsis thaliana with genome‐wide association mapping. We observed a surprisingly large variation in nematode reproduction among these lines. Genome‐wide association mapping revealed four quantitative trait loci (QTLs) located on chromosomes 1 and 5 of A. thaliana significantly associated with reproductive success of M. incognita, none of which harbours typical resistance gene homologues. Mutant analysis of three genes located in two QTLs showed that the transcription factor BRASSINAZOLE RESISTANT1 and an F‐box family protein may function as (co‐)regulators of susceptibility to M. incognita in Arabidopsis. Our data suggest that breeding for loss‐of‐susceptibility, based on allelic variants critically involved in nematode feeding, could be used to make crops more resilient to root‐knot nematodes. [ABSTRACT FROM AUTHOR]

Plants have developed diverse defence mechanisms to ward off herbivorous pests. However, agriculture still faces estimated crop yield losses ranging from 25% to 40% annually. These losses arise not only because of direct feeding damage, but also because many pests serve as vectors of plant viruses. Herbivorous thrips (Thysanoptera) are important pests of vegetable and ornamental crops worldwide, and encompass virtually all general problems of pests: they are highly polyphagous, hard to control because of their complex lifestyle, and they are vectors of destructive viruses. Currently, control management of thrips mainly relies on the use of chemical pesticides. However, thrips rapidly develop resistance to these pesticides. With the rising demand for more sustainable, safer, and healthier food production systems, we urgently need to pinpoint the gaps in knowledge of plant defences against thrips to enable the future development of novel control methods. In this review, we summarize the current, rather scarce, knowledge of thrips-induced plant responses and the role of phytohormonal signalling and chemical defences in these responses. We describe concrete opportunities for breeding resistance against pests such as thrips as a prototype approach for next-generation resistance breeding. [ABSTRACT FROM AUTHOR]

One of the responses of plants to insect attack is the production of volatile organic compounds that mediate indirect defence of plants by attracting natural enemies of the attacking herbivores. Herbivore-induced plant volatiles (HIPVs) include terpenoids that play key roles in the attraction of natural enemies. Crosstalk between phytohormonal signalling pathways is well known to affect the regulation of plant defences, including the emission of HIPVs. Thus, simultaneous feeding on the same plant by caterpillars and aphids, can affect the attraction of parasitoids by the plant compared to single insect attack. The role of aphid density in the regulation of HIPV emission by plants under dual attack has not been studied previously. Here, we investigated the attraction of Diadegma semiclausum, a parasitoid of the Diamondback moth Plutella xylostella, to volatiles emitted by Arabidopsis thaliana plants, simultaneously attacked by host caterpillars, and by the non-host aphid Brevicoryne brassicae. Our study shows that the effect of aphid infestation on parasitoid attraction is influenced by the density of the aphids. Biosynthesis and emission of ( E, E)-α-farnesene could be linked to the observed preference of D. semiclausum parasitoids for the HIPV blend emitted by plants dually infested by caterpillars and aphids at a high density compared to dually infested plants with a low aphid density. Parasitoids such as D. semiclausum are important enemies of herbivorous insects and a better understanding of how plants express indirect defence mechanisms in response to multiple insect attack will provide important knowledge on plant-herbivore-parasitoid interactions under multiple stress conditions. [ABSTRACT FROM AUTHOR]

Plants can prepare for future herbivore attack through a process called priming. Primed plants respond more strongly and/or faster to insect attack succeeding the priming event than nonprimed plants, while the energetic costs of priming are relatively low., To better understand the evolution of priming, we developed a simulation model, partly parameterized for Brassica nigra plants, to explore how the fitness benefits of priming change when plants are grown in different biotic environments., Model simulations showed that herbivore dynamics (arrival probability, arrival time, and feeding rate) affect the optimal duration, the optimal investment and the fitness benefits of priming. Competition for light increases the indirect costs of priming, but may also result in a larger payoff when the nonprimed plant experiences substantial leaf losses., This modeling approach identified some important knowledge gaps: herbivore arrival rates on individual plants are rarely reported but they shape the optimal duration of priming, and it would pay off if the likelihood, severity and timing of the attack could be discerned from the priming cue, but it is unknown if plants can do so. In addition, the model generated some testable predictions, for example that the sensitivity to the priming cue decreases with plant age. [ABSTRACT FROM AUTHOR]

1. When herbivores of distinct feeding guilds, such as phloem feeders and leaf chewers, interact, the outcome of these interactions often shows facilitation. However, whether this facilitation turns into competition at stronger herbivory pressure remains unknown. 2. Using an integrative approach that links ecological processes (behavioural choices of insects) with physiological plant mechanisms (nutrient and phytohormone levels) for the wild crucifer Brassica nigra ( L.) Koch., this study evaluates preferences of leaf chewers for plants previously infested with several densities of the specialist aphid Brevicoryne brassicae L. ( Hemiptera, Aphididae). As leaf chewers, four species of caterpillars ( Lepidoptera) were selected that differ in their degree of specialisation in crucifers. 3. These results show that, whereas at low and medium aphid densities caterpillars displayed a preference for aphid-infested plants or no preference, at high aphid infestation density, all four species of caterpillar preferred uninfested plants, with a significant difference for Pieris rapae and Mamestra brassicae. 4. In contrast to our expectation, the consistent preference for uninfested plants at a high aphid density could not be associated with a decrease in plant nutrition. However, while jasmonate concentrations [i.e. 12-oxo-phytodienoic acid and jasmonic acid ( JA)] at medium aphid-density infestation decreased compared with low levels of infestation, at high infestation level, the jasmonates JA as well as JA conjugated with the amino acid isoleucine were present at higher levels compared with low-infestation treatments. 5. This work provides evidence that positive interactions observed in herbivore communities can be transient, leading to negative interactions mediated by changes in plant defences rather than in plant nutrition. [ABSTRACT FROM AUTHOR]

Attraction of parasitoids to plant volatiles induced by multiple herbivory depends on the specific combinations of attacking herbivore species, especially when their feeding modes activate different defense signalling pathways as has been reported for phloem feeding aphids and tissue feeding caterpillars. We studied the effects of pre-infestation with non-host aphids ( Brevicoryne brassicae) for two different time periods on the ability of two parasitoid species to discriminate between volatiles emitted by plants infested by host caterpillars alone and those emitted by plants infested with host caterpillars plus aphids. Using plants originating from three chemically distinct wild cabbage ( Brassica oleracea) populations, Diadegma semiclausum switched preference for dually infested plants to preference for plants infested with Plutella xylostella hosts alone when the duration of pre-aphid infestation doubled from 7 to 14 days. Microplitis mediator, a parasitoid of Mamestra brassicae caterpillars, preferred dually-infested plants irrespective of aphid-infestation duration. Separation of the volatile blends emitted by plants infested with hosts plus aphids or with hosts only was poor, based on multivariate statistics. However, emission rates of individual compounds were often reduced in plants infested with aphids plus hosts compared to those emitted by plants infested with hosts alone. This effect depended on host caterpillar species and plant population and was little affected by aphid infestation duration. Thus, the interactive effect of aphids and hosts on plant volatile production and parasitoid attraction can be dynamic and parasitoid specific. The characteristics of the multi-component volatile blends that determine parasitoid attraction are too complex to be deduced from simple correlative statistical analyses. [ABSTRACT FROM AUTHOR]

The effects of multiple insect attacks on herbivore-induced plant volatiles and carnivorous arthropods are increasingly studied. Phytopathogens also represent an important threat to plants, and plant defence strategies against pathogens and insects are strongly interconnected, yet the potential impact of pathogens on insect-induced volatiles has been largely overlooked, and degree of pathogenicity is rarely considered., We investigated how pathogen challenge, with virulent and avirulent strains of Xanthomonas campestris either alone or with simultaneous Pieris brassicae caterpillar herbivory, affected the volatile emissions of Brassica nigra plants. The impact of these volatiles on the foraging behaviour of Cotesia glomerata parasitoids wasps was then assessed., Pathogens themselves induced volatiles that were highly attractive to parasitoids, and enhanced the attractiveness of host-infested plant volatiles. Chemical analyses revealed that virulent and avirulent strains differentially induced plant volatiles, with primarily sesquiterpene, homoterpene and green leaf volatile compounds contributing to the differences. Strong similarities were found in the blends induced by the virulent strain and caterpillar herbivory., Challenge by either virulent or avirulent pathogens has a significant impact on plant chemistry and its interactions with other community members, demonstrating the importance of integrating pathogen- and insect-based research to broaden our knowledge of plant defences under conditions of increasing complexity., A is available for this article. [ABSTRACT FROM AUTHOR]

Arthropod herbivory induces the emission of plant volatiles that can be used by natural enemies of the herbivores to find their prey. Recently it has been shown that insectivorous birds also use these volatiles to locate their prey. Results of a previous study showed that birds with experience in foraging for caterpillars in trees were able to discriminate between caterpillar-infested and uninfested trees, even in the absence of caterpillars or their damage on leaves. Here, we tested whether the attraction to caterpillar-infested trees is exhibited in birds naïve with respect to finding caterpillars on trees. Results show that naïve great tits (Parus major) were not attracted to infested trees, when they could not see the larvae or their feeding damage. Naïve birds cannot discriminate between caterpillar-infested and uninfested trees. Therefore, the attraction to caterpillar-infested trees does not seem to be innate in great tits, and may be acquired through learning. [ABSTRACT FROM AUTHOR]

Plants evolved strategies to attract pollinators that are essential for reproduction. However, plant defence against herbivores may trade off with pollinator attraction., Here, we investigated the role of inducible plant secondary metabolites in such a trade-off. Our objective was to reveal the mechanisms underlying the effects of induced plant responses to pollination and herbivory. We assessed how responses of plants to pollination and insect herbivory affect the behaviour of flower visitors. Subsequently, we investigated how the production of volatile and non-volatile compounds changes after pollination and herbivory., Both herbivores and pollinators induced important phenotypic changes in flowers. Brassica nigra plants respond to pollination and herbivory with changes in the profile of volatiles and non-volatiles of their flowers. Our results show that butterflies use different cues when searching for an oviposition site or a nectar source. Pollination status influenced the behaviour of butterflies, but not that of syrphid flies., We discuss the results in the context of the trade-off between defence and reproduction in plants and suggest that systemic responses to herbivores can interfere with local responses to pollination. Therefore, these responses must be addressed in an integrated way because, in nature, plants are simultaneously exposed to herbivores and pollinators. [ABSTRACT FROM AUTHOR]

Beneficial root-associated microbes modify the physiological status of their host plants and affect direct and indirect plant defense against insect herbivores. While the effects of these microbes on direct plant defense against insect herbivores are well described, knowledge of the effect of the microbes on indirect plant defense against insect herbivores is still limited. In this study, we evaluate the role of the rhizobacterium Pseudomonas fluorescens WCS417r in indirect plant defense against the generalist leaf-chewing insect Mamestra brassicae through a combination of behavioral, chemical, and gene-transcriptional approaches. We show that rhizobacterial colonization of Arabidopsis thaliana roots results in an increased attraction of the parasitoid Microplitis mediator to caterpillar-infested plants. Volatile analysis revealed that rhizobacterial colonization suppressed the emission of the terpene ( E)-α-bergamotene and the aromatics methyl salicylate and lilial in response to caterpillar feeding. Rhizobacterial colonization decreased the caterpillar-induced transcription of the terpene synthase genes TPS03 and TPS04. Rhizobacteria enhanced both the growth and the indirect defense of plants under caterpillar attack. This study shows that rhizobacteria have a high potential to enhance the biocontrol of leaf-chewing herbivores based on enhanced attraction of parasitoids. [ABSTRACT FROM AUTHOR]

In nature, plants are exposed to attacks by multiple herbivore species at the same time. To cope with these attacks, plants regulate defenses with the production of hormones such as salicylic acid (SA) and jasmonic acid (JA). Because herbivore densities are dynamic in time, this may affect plant-mediated interactions between different herbivores attacking at the same time. In Arabidopsis thaliana, feeding by Brevicoryne brassicae aphids interferes with induced defenses against Plutella xylostella caterpillars. This is density dependent: at a low aphid density, the growth rate of P. xylostella was increased, whereas caterpillars feeding on plants colonized by aphids at a high density have a reduced growth rate. Growth of P. xylostella larvae was unaffected on sid2-1 or on dde2-2 mutant plants when feeding simultaneously with a low or high aphid density. This shows that aphid interference with caterpillar-induced defenses requires both SA and JA signal transduction pathways. Transcriptional analysis revealed that simultaneous feeding by caterpillars and aphids at a low density induced the expression of the SA transcription factor gene WRKY70 whereas expression of WRKY70 was lower in plants induced with both caterpillars and a high aphid density. Interestingly, the expression of the JA transcription factor gene MYC2 was significantly higher in plants simultaneously attacked by aphids at a high density and caterpillars. These results indicate that a lower expression level of WRKY70 leads to significantly higher MYC2 expression through SA–JA cross-talk. Thus, plant-mediated interactions between aphids and caterpillars are density dependent and involve phytohormonal cross-talk and differential activation of transcription factors. [ABSTRACT FROM AUTHOR]

Attack by multiple herbivores often leads to modification of induced plant defenses compared to single herbivory, yet little is known about the effects on induced indirect plant defense. Here, we investigated the effect of sequential induction of plant defense by Mamestra brassicae caterpillar oral secretion and an infestation by Tetranychus urticae spider mites on the expression of indirect plant defense in Lima bean plants. The effect on indirect defense was assessed using behavior assays with the specialist predatory mite Phytoseiulus persimilis in an olfactometer, headspace analysis of 11 major herbivore-induced plant volatiles (HIPVs) including ( E)-β-ocimene, and transcript levels of the corresponding gene Phaseolus lunatus ( E)- β- ocimene synthase ( PlOS). Predatory mites were found to distinguish between plants induced by spider mites and caterpillar oral secretion but not between plants with single spider mite infestation and plants induced by caterpillar oral secretion prior to spider mite infestation. Indeed, the volatile blends emitted by plants induced by spider mites only and the sequential induction treatment of caterpillar oral secretion followed by spider mite infestation, were similar. Our results suggest that plant indirect defense is not affected by previous treatment with oral secretion of M. brassicae caterpillars. [ABSTRACT FROM AUTHOR]

Induced plant defenses against either pathogens or herbivore attackers are regulated by phytohormones. These phytohormones are increasingly recognized as important mediators of interactions between organisms associated with plants. In this review, we discuss the role of plant defense hormones in sequential tri-partite interactions among plants, pathogenic microbes, and herbivorous insects, based on the most recent literature. We discuss the importance of pathogen trophic strategy in the interaction with herbivores that exhibit different feeding modes. Plant resistance mechanisms also affect plant quality in future interactions with attackers. We discuss exemplary evidence for the hypotheses that (i) biotrophic pathogens can facilitate chewing herbivores, unless plants exhibit effector-triggered immunity, but (ii) facilitate or inhibit phloem feeders. (iii) Necrotrophic pathogens, on the other hand, can inhibit both phloem feeders and chewers. We also propose herbivore feeding mode as predictor of effects on pathogens of different trophic strategies, providing evidence for the hypotheses that (iv) phloem feeders inhibit pathogen attack by increasing SA induction, whereas (v) chewing herbivores tend not to affect necrotrophic pathogens, while they may either inhibit or facilitate biotrophic pathogens. Putting these hypotheses to the test will increase our understanding of phytohormonal regulation of plant defense to sequential attack by plant pathogens and insect herbivores. This will provide valuable insight into plant-mediated ecological interactions among members of the plant-associated community. [ABSTRACT FROM AUTHOR]

Plants have a complex immune system that defends them against attackers (e.g. herbivores and microbial pathogens) but that also regulates the interactions with mutualistic organisms (e.g. mycorrhizal fungi and plant growth-promoting rhizobacteria). Plants have to respond to multiple environmental challenges, so they need to integrate both signals associated with biotic and abiotic stresses in the most appropriate response to survive., Beneficial microbes such as rhizobacteria and mycorrhizal fungi can help plants to 'deal' with pathogens and herbivorous insects as well as to tolerate abiotic stress. Therefore, beneficial microbes may play an important role in a changing environment, where abiotic and biotic stresses on plants are expected to increase. The effects of beneficial microbes on herbivores are highly context-dependent, but little is known on what is driving such dependency. Recent evidence shows that abiotic stresses such as changes in soil nutrients, drought and salt stress, as well as ozone can modify the outcome of plant-microbe-insect interactions., Here, we review how abiotic stress can affect plant-microbe, plant-insect and plant-microbe-insect interactions, and the role of the network of plant signal-transduction pathways in regulating such interactions., Most of the studies on the effects of abiotic stress on plant-microbe-insect interactions show that the effects of microbes on herbivores (positive or negative) are strengthened under stressful conditions. We propose that, at least in part, this is due to the crosstalk of the different plant signalling pathways triggered by each stress individually. By understanding the cross-regulation mechanisms we may be able to predict the possible outcomes of plant-microbe-insect interactions under particular abiotic stress conditions. We also propose that microbes can help plants to deal with insects mainly under conditions that compromise efficient activation of plant defences., In the context of global change, it is crucial to understand how abiotic stresses will affect species interactions, especially those interactions that are beneficial for plants. The final aim of this review is to stimulate studies unravelling when these 'beneficial' microbes really benefit a plant. [ABSTRACT FROM AUTHOR]

Plant hormones play important roles in regulating plant growth and defence by mediating developmental processes and signalling networks involved in plant responses to a wide range of parasitic and mutualistic biotic interactions., Plants are known to rapidly respond to pathogen and herbivore attack by reconfiguring their metabolism to reduce pathogen/herbivore food acquisition. This involves the production of defensive plant secondary compounds, but also an alteration of the plant primary metabolism to fuel the energetic requirements of the direct defence., Cytokinins are plant hormones that play a key role in plant morphology, plant defence, leaf senescence and source-sink relationships. They are involved in numerous plant-biotic interactions., These phytohormones may have been the target of arthropods and pathogens over the course of the evolutionary arms race between plants and their biotic partners to hijack the plant metabolism, control its physiology and/or morphology and successfully invade the plant. In the case of arthropods, cytokinin-induced phenotypes can be mediated by their bacterial symbionts, giving rise to intricate plant-microbe-insect interactions., Cytokinin-mediated effects strongly impact not only plant growth and defence but also the whole community of insect and pathogen species sharing the same plant by facilitating or preventing plant invasion. This suggests that cytokinins (CKs) are key regulators of the plant growth-defence trade-off and highlights the complexity of the finely balanced responses that plants use while facing both invaders and mutualists. [ABSTRACT FROM AUTHOR]

Upon herbivore attack, plants activate an indirect defense, that is, the release of a complex mixture of volatiles that attract natural enemies of the herbivore. When plants are simultaneously exposed to two herbivore species belonging to different feeding guilds, one herbivore may interfere with the indirect plant defense induced by the other herbivore. However, little is understood about the mechanisms underlying such interference., Here, we address the effect of herbivory by the phloem-feeding whitefly Bemisia tabaci on the induced indirect defense of Arabidopsis thaliana plants to Plutella xylostella caterpillars, that is, the attraction of the parasitoid wasp Diadegma semiclausum., Assays with various Arabidopsis mutants reveal that B. tabaci infestation interferes with indirect plant defense induced by P. xylostella, and that intact jasmonic acid and ethylene signaling are required for such interference caused by B. tabaci. Chemical analysis of plant volatiles showed that the composition of the blend emitted in response to the caterpillars was significantly altered by co-infestation with whiteflies. Moreover, whitefly infestation also had a considerable effect on the transcriptomic response of the plant to the caterpillars., Understanding the mechanisms underlying a plant's responses to multiple attackers will be important for the development of crop protection strategies in a multi-attacker context. [ABSTRACT FROM AUTHOR]

In terrestrial food webs, the study of multitrophic interactions traditionally has focused on organisms that share a common domain, mainly above ground. In the last two decades, it has become clear that to further understand multitrophic interactions, the barrier between the belowground and aboveground domains has to be crossed. Belowground organisms that are intimately associated with the roots of terrestrial plants can influence the levels of primary and secondary chemistry and biomass of aboveground plant parts. These changes, in turn, influence the growth, development, and survival of aboveground insect herbivores. The discovery that soil organisms, which are usually out of sight and out of mind, can affect plant-herbivore interactions aboveground raised the question if and how higher trophic level organisms, such as carnivores, could be influenced. At present, the study of above-belowground interactions is evolving from interactions between organisms directly associated with the plant roots and shoots (e.g., root feeders - plant - foliar herbivores) to interactions involving members of higher trophic levels (e.g., parasitoids), as well as non-herbivorous organisms (e.g., decomposers, symbiotic plant mutualists, and pollinators). This multitrophic approach linking above- and belowground food webs aims at addressing interactions between plants, herbivores, and carnivores in a more realistic community setting. The ultimate goal is to understand the ecology and evolution of species in communities and, ultimately how community interactions contribute to the functioning of terrestrial ecosystems. Here, we summarize studies on the effects of root feeders on aboveground insect herbivores and parasitoids and discuss if there are common trends. We discuss the mechanisms that have been reported to mediate these effects, from changes in concentrations of plant nutritional quality and secondary chemistry to defense signaling. Finally, we discuss how the traditional framework of fixed paired combinations of root- and shoot-related organisms feeding on a common plant can be transformed into a more dynamic and realistic framework that incorporates community variation in species, densities, space and time, in order to gain further insight in this exciting and rapidly developing field. [ABSTRACT FROM AUTHOR]

Plants can respond to feeding or egg deposition by herbivorous arthropods by changing the volatile blend that they emit. These herbivore-induced plant volatiles (HIPVs) can attract carnivorous natural enemies of the herbivores, such as parasitoids and predators, a phenomenon that is called indirect plant defense. The volatile blends of infested plants can be very complex, sometimes consisting of hundreds of compounds. Most HIPVs can be classified as terpenoids (e.g., (E)-β-ocimene, (E,E)-α-farnesene, (E)-4,8-dimethyl-1,3,7-nonatriene), green leaf volatiles (e.g., hexanal, (Z)-3-hexen-1-ol, (Z)-3-hexenyl acetate), phenylpropanoids (e.g., methyl salicylate, indole), and sulphur- or nitrogen-containing compounds (e.g., isothiocyanates or nitriles, respectively). One highly intriguing question has been which volatiles out of the complex blend are the most important ones for the carnivorous natural enemies to locate "suitable host plants. Here, we review the methods and techniques that have been used to elucidate the carnivore-attracting compounds. Electrophysiological methods such as electroantennography have been used with parasitoids to elucidate which compounds can be perceived by the antennae. Different types of elicitors and inhibitors have widely been applied to manipulate plant volatile blends. Furthermore, transgenic plants that were genetically modified in specific steps in one of the signal transduction pathways or biosynthetic routes have been used to find steps in HIPV emission crucial for indirect plant defense. Furthermore, we provide an overview on biotic and abiotic factors that influence the emission of HIPVs and how this can affect the interactions between members of different trophic levels. Consequently, we review the progress that has been made in this exciting research field during the past 30 years since the first studies on HIPVs emerged and we highlight important issues to be addressed in the future. Les plantes peuvent réagir à l’alimentation ou la ponte d’œufs par les arthropodes herbivores en modifiant le mélange de composés volatils qu’elles émettent. Ces composés volatils induits par les herbivores chez les plantes (HIPV) peuvent attirer des carnivores ennemis naturels des herbivores, tels que des parasitoïdes et des prédateurs, un phénomène connu sous le nom de défense indirecte des plantes. Le mélange de composés volatils des plantes infestées peut être très complexe, comprenant quelquefois des centaines de produits. La plupart des HIPV peuvent se classifier en terpénoïdes (par ex., (E)-β-ocimène, (E,E)-α-farnésène, (E)-4,8-diméthyl-1,3,7-nonatriène), en composés volatils des feuilles vertes (par ex., hexanal, (Z)-3-hexén-1-ol, acétate de (Z)-3-hexényle), en phénylpropanoïdes (par ex., salicylate de méthyle, indole) et en composés contenant du soufre ou de l’azote (par ex., les isothyocyanates ou les nitriles, respectivement). Une question particulièrement intrigante est de savoir lesquels des composés volatils du mélange complexe sont les plus importants pour permettre aux ennemis naturels carnivores de trouver les plantes hôtes convenables. Nous passons en revue les méthodes et les techniques utilisées pour mettre en évidence les composés qui attirent les carnivores. Les méthodes électrophysiologiques, comme l’électroantennographie, servent chez les parasitoïdes à déceler les composés perceptibles par les antennes. Divers types d’éliciteurs et d’inhibiteurs sont couramment utilisés pour manipuler les mélanges de composés volatils de plantes. De plus, des plantes transgéniques modifiées génétiquement à des étapes spécifiques de l’une des voies de transduction des signaux ou des voies de biosynthèse permettent de découvrir les étapes dans l’émission des HIPV essentielles pour la défense indirecte des plantes. Nous présentons aussi une revue synthétique des facteurs biotiques et abiotiques qui influencent les émissions de HIPV; nous examinons comment cela peut affecter les interactions entre les membres de différents niveaux trophiques. En définitive, nous passons en revue les progrès accomplis dans cet excitant champ de recherche au cours des 30 dernières années depuis les premières études sur les HIPV et nous soulignons les questions importantes à étudier dans l’avenir. [ABSTRACT FROM AUTHOR]

The indirect defense mechanisms of plants comprise the production of herbivore-induced plant volatiles that can attract natural enemies of plant attackers. One of the often emitted compounds after herbivory is methyl salicylate (MeSA). Here, we studied the importance of this caterpillar-induced compound in the attraction of the parasitoid wasp Diadegma semiclausum by using a mutant Arabidopsis line. Pieris rapae infested AtBSMT1-KO mutant Arabidopsis plants, compromised in the biosynthesis of MeSA, were more attractive to parasitoids than infested wild-type plants. This suggests that the presence of MeSA has negative effects on parasitoid host-finding behavior when exposed to wild-type production of herbivore-induced Arabidopsis volatiles. Furthermore, in line with this, we recorded a positive correlation between MeSA dose and repellence of D. semiclausum when supplementing the headspace of caterpillar-infested AtBSMT1-KO plants with synthetic MeSA. [ABSTRACT FROM AUTHOR]

Although exogenous treatment of plants with jasmonic acid (JA) may result in induced responses similar to plant defences induced by herbivory, few studies have compared the details of insect herbivory and JA-mimicked responses. We compared volatiles of two crucifer species, Cardamine impatiens and Lepidium virginicum, in response to Plutella xylostella larval feeding and exogenous application of JA, over the entire period of time when induced changes were detectable. Significant differences in the composition and timing of volatiles occurred between herbivory and JA treatments in both plants. The quantity of nitrile and isothiocyanate released in response to herbivory was significantly larger than that upon JA treatment. In each of the two plant species, most volatile components were emitted immediately upon larval feeding and their quantity dropped rapidly once feeding ceased. In contrast, the emission of volatiles in response to JA treatment lasted for a longer period of time, and the maximum emission rate was recorded 2 and 3 days after JA treatment in L. virginicum and C. impatiens respectively. These findings are discussed in the context of signal-transduction pathways and mechanisms involved in induced emissions of plant volatiles, as well as induced defences mediated by plant volatiles. [ABSTRACT FROM AUTHOR]

Herbivore-induced plant defences influence the behaviour of insects associated with the plant. For biting–chewing herbivores the octadecanoid signal-transduction pathway has been suggested to play a key role in induced plant defence. To test this hypothesis in our plant—herbivore—parasitoid tritrophic system, we used phenidone, an inhibitor of the enzyme lipoxygenase (LOX), that catalyses the initial step in the octadecanoid pathway. Phenidone treatment of Brussels sprouts plants reduced the accumulation of internal signalling compounds in the octadecanoid pathway downstream of the step catalysed by LOX, i.e. 12-oxo-phytodienoic acid (OPDA) and jasmonic acid. The attraction of Cotesia glomerata parasitoids to host-infested plants was significantly reduced by phenidone treatment. The three herbivores investigated, i.e. the specialists Plutella xylostella, Pieris brassicae and Pieris rapae, showed different oviposition preferences for intact and infested plants, and for two species their preference for either intact or infested plants was shown to be LOX dependent. Our results show that phenidone inhibits the LOX-dependent defence response of the plant and that this inhibition can influence the behaviour of members of the associated insect community. [ABSTRACT FROM AUTHOR]

In recent years, increasing attention has been paid in exploring the role of direct plant defence, through the production of allelochemicals, on the performance of parasitoid wasps and their hosts. However, few studies have determined if parasitoids can detect differences in plant quality and thus preferentially attack hosts on which their progeny develop most successfully. In this study we examined the development and preference of two endoparasitoids, Diadegma semiclausum and Cotesia glomerata, developing in larvae of their respective hosts, Plutella xylostella and Pieris brassicae. In turn, these were reared on different wild populations of black mustard Brassica nigra originating in the Netherlands and Sicily (Italy), as well as single cultivated strains of B. nigra and brown mustard, B. juncea. Chemical analyses of foliar glucosinolates and volatile emissions by P. xylostella-damaged plants revealed large differences between B. nigra and B. juncea plants, with smaller differences among the B. nigra populations. The four mustard populations differentially affected development time and body mass of the herbivores and parasitoids. Contrasts among the means revealed significant differences mainly between B. nigra and B. juncea. Both parasitoids, however, preferred to alight on plants in which their progeny developed most successfully. In behavioural bioassays, D. semiclausum did not discriminate among the B. nigra populations and preferred to alight on B. juncea, which was the best plant population for parasitoid development. By contrast, C. glomerata females exhibited the lowest preference for Italian B. nigra populations, on which adult parasitoid size was the smallest. These results reveal that parasitoids can detect even small differences in plant quality presumably through their volatile blends and that plant preference and offspring performance in the two species are ‘optimally synchronized’. [ABSTRACT FROM AUTHOR]

Plants can recruit parasitic wasps in response to egg deposition by herbivorous insects-a sophisticated indirect plant defense mechanism. Oviposition by the Large Cabbage White butterfly Pieris brassicae on Brussels sprout plants induces phytochemical changes that arrest the egg parasitoid Trichogramma brassicae. Here, we report the identification of an elicitor of such an oviposition- induced plant response. Eliciting activity was present in accessory gland secretions released by mated female butterflies during egg deposition. In contrast, gland secretions from virgin female butterflies were inactive. In the male ejaculate, P. brassicae females receive the anti-aphrodisiac benzyl cyanide (BC) that reduces the females' attractiveness for subsequent mating. We detected this pheromone in the accessory gland secretion released by mated female butterflies. When applied onto leaves, BC alone induced phytochemical changes that arrested females of the egg parasitoid. Microarray analyses revealed a similarity in induced plant re- sponses that may explain the arrest of T. brassicae to egg-laden and BC-treated plants. Thus, a male-derived compound endangers the offspring of the butterfly by inducing plant defense. Recently, BC was shown to play a role in foraging behavior of T. brassicae, by acting as a cue to facilitate phoretic transport by mated female butterflies to oviposition sites. Our results suggest that the anti-aphrodisiac pheromone incurs fitness costs for the butterfly by both mediating phoretic behavior and inducing plant defense. [ABSTRACT FROM AUTHOR]

Induction of plant defences by early season herbivores can mediate interspecific herbivore competition. We have investigated plant-mediated competition between three herbivorous insects through studies at different levels of biological integration. We have addressed (i) gene expression; (ii) insect behaviour and performance under laboratory conditions; and (iii) population dynamics under field conditions. We studied the expression of genes encoding a trypsin inhibitor and genes that are involved in glucosinolate biosynthesis in response to early season herbivory by Pieris rapae caterpillars in Brassica oleracea plants. Furthermore, we studied the interaction of these transcriptional responses with responses to secondary herbivory by the two specialist herbivores, P. rapae and Plutella xylostella, and the generalist Mamestra brassicae. P. rapae-induced responses strongly interacted with plant responses to secondary herbivory. Sequential feeding by specialist herbivores resulted in enhanced or similar expression levels of defence-related genes compared to primary herbivory by specialists. Secondary herbivory by the generalist M. brassicae resulted in lower gene expression levels than in response to primary herbivory by this generalist. Larval performance of both specialist and generalist herbivores was negatively affected by P. rapae-induced plant responses. However, in the field the specialist P. xylostella was more abundant on P. rapae-induced plants and preferred these plants over undamaged plants in oviposition experiments. In contrast, the generalist M. brassicae was more abundant on control plants and preferred undamaged plants for oviposition. P. rapae did not discriminate between plants damaged by conspecifics or undamaged plants. Our study shows that early season herbivory differentially affects transcriptional responses involved in plant defence to secondary herbivores and their population development dependent upon their degree of host plant specialization. [ABSTRACT FROM AUTHOR]

Most studies on plant defenses against insect herbivores investigate direct and indirect plant defenses independently. However, these defenses are not necessarily mutually exclusive. Plant metabolites can be transmitted through the food chain and can also affect the herbivore's natural enemies. A conflict may arise when a natural enemy is attracted to a plant that is suboptimal in terms of its own fitness. In addition, plant defenses are often studied in cultivated plant species in which artificial selection may have resulted in reduced resistance against insect herbivores. In this study, we investigated both direct and indirect plant defenses in two closely related wild brassicaceous plant species, Brassica nigra L. and Sinapis arvensis L. The herbivore Pieris brassicae L. (Lepidoptera: Pieridae), which is specialized on brassicaceous plant species, developed faster and attained higher pupal mass when reared on B. nigra than on S. arvensis. In contrast, Cotesia glomerata L. (Hymenoptera: Braconidae), which is a gregarious endoparasitoid of P. brassicae caterpillars, developed equally well on P. brassicae irrespective of the food plant on which its host had been reared. The feeding strategy of the parasitoid larvae, that is, selectively feeding on hemolymph and fat body, is likely to allow for a much wider host-size range without affecting the size or development time of the emerging parasitoids. In flight chamber experiments, C. glomerata, which had an oviposition experience in a host that fed on Brussels sprout, exhibited significant preference for host-damaged B. nigra over host-damaged S. arvensis plants. Headspace analysis revealed quantitative and qualitative differences in volatile emissions between the two plant species. This parasitoid species may use a range of cues associated with the host and the host's food plant in order to recognize the different plant species on which the host can feed. These results show that there is no conflict between direct and indirect plant defenses for this plant–host–parasitoid complex. [ABSTRACT FROM AUTHOR]

Plants are members of complex communities and interact both with antagonists and beneficial organisms. An important question in plant defense-signaling research is how plants integrate signals induced by pathogens, beneficial microbes and insects into the most appropriate adaptive response. Molecular and genomic tools are now being used to uncover the complexity of the induced defense signaling networks that have evolved during the arms races between plants and their attackers. Molecular biologists and ecologists are joining forces to place molecular mechanisms of plant defense into an ecological perspective. Here, we review our current understanding of the molecular mechanisms of induced plant defense and their potential ecological relevance in nature. [Copyright &y& Elsevier]

Background: Transcriptional profiling after herbivore attack reveals, at the molecular level, how plants respond to this type of biotic stress. Comparing herbivore-induced transcriptional responses of plants with different phenotypes provides insight into plant defense mechanisms. Here, we compare the global gene expression patterns induced by Pieris rapae caterpillar attack in two white cabbage (Brassica oleracea var. capitata) cultivars. The two cultivars are shown to differ in their level of direct defense against caterpillar feeding. Because Brassica full genome microarrays are not yet available, 70-mer oligonucleotide microarrays based on the Arabidopsis thaliana genome were used for this non-model plant. Results: The transcriptional responses of the two cultivars differed in timing as characterized by changes in their expression pattern after 24, 48 and 72 hours of caterpillar feeding. In addition, they also differed qualitatively. Surprisingly, of all genes induced at any time point, only one third was induced in both cultivars. Analyses of transcriptional responses after jasmonate treatment revealed that the difference in timing did not hold for the response to this phytohormone. Additionally, comparisons between Pieris rapae- and jasmonate-induced transcriptional responses showed that Pieris rapae induced more jasmonate-independent than jasmonate-dependent genes. Conclusion: The present study clearly shows that global transcriptional responses in two cultivars of the same plant species in response to insect feeding can differ dramatically. Several of these differences involve genes that are known to have an impact on Pieris rapae performance and probably underlie different mechanisms of direct defense, present in the cultivars. [ABSTRACT FROM AUTHOR]

Many plants have an indirect defense against herbivores by emitting volatiles that attract carnivorous enemies of the herbivores. In cucumber (Cucumis sativus) the production of carnivore attractants can be induced by herbivory or jasmonic acid spraying. From the leaves of cucumber plants with and without spider mite infestation, two subtractive cDNA libraries were made that were enriched in cDNA fragments up- or down-regulated by spider mite infestation. A total of 713 randomly selected clones from these libraries were used to make a cDNA microarray. Subsequently, cucumber plants were sprayed with jasmonic acid, mechanically damaged, infested with spider mites, or left untreated (control). Leaf samples were taken at a range of different time points, and induced volatile compounds and mRNA (from the same leaves) were collected, cDNAs prepared from the mRNA were hybridized to the clones on the microarray. The resulting gene expression profiles were analyzed in combination with volatile production data in order to gain insight in the possible involvement of the studied genes in the synthesis of those volatiles. The clones on the microarray and the induced cucumber volatiles could be grouped into a number of clusters in which specific biosynthetic genes clustered with the product of that pathway. For example, lipoxygenase cDNA clones clustered with the volatile (Z)-3-hexenyl acetate and the volatile sesquiterpene (E,E)-α-farnesene clustered with an upregulated sesquiterpene synthase fragment. This fragment was used to screen a cDNA library which resulted in the cloning of the cucumber (E,E)-α-farnesene and (E)-β-caryophyllene synthases. The use of combined global gene expression analysis and metabolite analysis for the discovery of genes Involved in specific biosynthetic processes is discussed. [ABSTRACT FROM AUTHOR]

Abstract: Inducible defences allow plants to be phenotypically plastic. Inducible indirect defence of plants by attracting carnivorous enemies of herbivorous arthropods can vary with plant species and genotype, with herbivore species or instar and potentially with other environmental conditions. So far, inducible indirect defence has mostly been studied for simple linear food chains. However, ultimately, ecologists should address inducible indirect defence in a food web context, where more than one organism (different herbivores and pathogens) may attack a plant and where a plant that emits herbivore-induced volatiles is surrounded by other plants that emit odours that can mix with the herbivore-induced volatiles from the attacked plant. Evolutionary ecologists are interested in the costs and benefits of interactions between plants and their attackers. These may be investigated by comparing different plant genotypes. The best comparison is between plant individuals that differ in only a single or restricted number of known traits. Such genotypes are difficult to obtain by conventional methods. However, rapid progress in the study of mechanisms of plant-attacker interactions and in the field of molecular genetics and genomics provides new tools that can be exploited by ecologists. For instance, genomic knowledge on Arabidopsis thaliana and the availability of characterized mutants and transgenes that are altered in one or a restricted number of genes can be exploited to address functional aspects of inducible indirect defence. In this paper we review progress in the knowledge of mechanisms of inducible indirect defence of plants and its importance for investigating the functional aspects of plant responses to herbivorous arthropods. Finally we identify some of the ecological questions that can be addressed by exploiting mechanistic aspects of inducible indirect defence. Induzierbare Abwehr ermöglicht Pflanzen phänotypische Plastizität. Bei der induzierbaren indirekten Abwehr von Pflanzen gegen Herbivore werden carnivore Feinde der herbivoren Arthropoden angelockt. Diese indirekte Abwehrreaktion kann je nach Art und Genotyp der Pflanze variieren, je nach Art und Stadium der Herbivoren und potentiell auch mit anderen Umweltfaktoren. Bisher wurde induzierbare indirekte Abwehr zumeist in einfachen, linearen Nahrungsketten untersucht. Letztlich sollten Ökologen induzierbare indirekte Abwehr aber im Kontext von Nahrungsnetzen untersuchen und berücksichtigen, dass mehr als ein Organismus (verschiedene Herbivore und Pathogene) die Pflanze attackieren können. Weiterhin ist eine Pflanze, die durch Herbivorenbefall induzierte Düfte freisetzt, auch noch umgeben von anderen Pflanzen, die ebenfalls flüchtige Verbindungen abgeben, die sich mit den Düften der induzierten Pflanze mischen können. Evolutionäre Ökologen sind an den Kosten und Nutzen der Interaktionen zwischen Pflanzen und ihrer Angreifern interessiert. Kosten-Nutzen-Analysen können anhand eines Vergleichs verschiedener Genotypen durchgeführt werden. Am besten werden Pflanzenindividuen verglichen, die sich nur in einem oder einer sehr begrenzten Zahl von Merkmalen unterscheiden. Solche Genotypen sind aber mit konventionellen Methoden schwer erhältlich. Aber die schnellen Fortschritte sowohl bei den Studien der Mechanismen von Pflanzen-Angreifer-Interaktionen als auch in der Molekulargenetik und Genomforschung eröffnen neue Möglichkeiten, die auch von Ökologen genutzt werden können. Um funktionale Aspekte der induzierbaren indirekten Verteidigung zu analysieren, kann man z.B. die Kenntnisse zum Genom von Arabidopsis thaliana nutzen sowie die Verfügbarkeit bestimmter Mutanten und transgener Individuen von A. thaliana, die nur in einem Gen oder einer bestimmten Anzahl von Genen verändert sind. Diese Publikation gibt einen Überblick der Kenntnisse über die Mechanismen der induzierbaren indirekten Verteidigung von Pflanzen und ihre Bedeutung für Analysen funktionaler Aspekte der pflanzlichen Reaktionen auf herbivore Arthropoden. Abschließend verweisen wir auf ökologische Fragen, die man untersuchen kann, wenn man mechanistische Aspekte der induzierbaren indirekten Antwort nutzt. [Copyright &y& Elsevier]

Abstract: Phenotypic plasticity enables invididuals to change their phenotype in response to their environment. These phenotypic changes can affect many interactions between the members of a community. Plants are able to respond towards herbivore attack by defensive mechanisms directly affecting the herbivore and by so-called indirect defences that negatively affect the herbivore by maintenance or attraction of carnivores. The phenotypic changes of plants caused by induced defences may vary with the type of attackers. Different attackers can evoke different plant responses due to specific elicitors or wounding. These different plant responses may be mediated by different choreographies of gene expression. Symbiotic and pathogenic microorganisms associated with the plant or the herbivore may play an important role in the induction process. Thus, a plethora of biotic factors affect the physiological, chemical, and molecular characteristics of plants in response to attack. The adaptiveness of phenotypic plasticity in terms of induced responses depends on the balance of their physiological and ecological costs and benefits. An integrated approach is necessary considering ecological, molecular and chemical aspects to gain deeper insight into induced defence and its application in environmentally benign crop protection. Phänotypische Plastizität befähigt Individuen, ihren Phänotyp in Reaktion auf Umweltreize zu ändern. Die phänotypischen Veränderungen können zahlreiche Interaktionen zwischen den Mitgliedern einer Biozönose beeinflussen. Angriffe von Herbivoren können in Pflanzen direkte und indirekte Verteidigungsmechanismen induzieren. Während die direkte Verteidigung sich unmittelbar gegen die Herbivoren richtet, wirkt die indirekte Abwehr über die Beherbergung oder Anlockung von Carnivoren auf die Herbivoren ein. Die induzierten phänotypischen Veränderungen der Pflanze sind je nach Typ des Angreifers sehr unterschiedlich. Unterschiedliche Herbivore können aufgrund verschiedener Fraßverhalten und aufgrund spezifischer Elicitoren jeweils ganz unterschiedliche pflanzliche Reaktionen hervorrufen. Die Vielfalt der pflanzlichen Antworten auf Herbivorenfraß wird bestimmt durch unterschiedliche Muster in der Expression der an der Abwehr beteiligten Gene. Symbiontische und pathogene Mikroorganismen, die mit der Pflanze oder den Herbivoren assoziiert sind, können eine wichtige Rolle im Induktionsprozess spielen. Demnach beeinflusst eine Fülle biotischer Faktoren die physiologischen, chemischen und molekularen Charakteristika der fraßinduzierten pflanzlichen Verteidigung. Der adaptive Wert der phänotypischen Plastizität, die sich in der Vielfalt der induzierten Verteidigungsmechanismen spiegelt, hängt vom Gleichgewicht der physiologischen und ökologischen Kosten und Nutzen der Abwehrreaktionen ab. Zum tieferen Verständnis der induzierten pflanzlichen Abwehr gegen Herbivore sowie zur Nutzung der induzierten Abwehrreaktion im umweltverträglichen Pflanzenschutz ist eine integrierte Herangehensweise notwendig, die sowohl ökologische wie auch molekulare und chemische Aspekte berücksichtigt. [Copyright &y& Elsevier]

Summary So far, genetic engineering of plants in the context of insect pest control has involved insertion of genes that code for toxins, and may be characterized as the incorporation of biopesticides into classical plant breeding. In the context of pesticide usage in pest control, natural enemies of herbivores have received increasing attention, because carnivorous arthropods are an important component of insect pest control. However, in plant breeding programmes, natural enemies of herbivores have largely been ignored, although there are many examples that show that plant breeding affects the effectiveness of biological control. Negative influences of modified plant characteristics on carnivorous arthropods may induce population growth of new, even more harmful pest species that had no pest status prior to the pesticide treatment. Sustainable pest management will only be possible when negative effects on non-target, beneficial arthropods are minimized. In this review, we summarize the effects of insect-resistant crops and insect-resistant transgenic crops, especially Bt crops, from a food web perspective. As food web components, we distinguish target herbivores, non-target herbivores, pollinators, parasitoids and predators. Below-ground organisms such as Collembola, nematodes and earthworms should also be included in risk assessment studies, but have received little attention. The toxins produced in Bt plants retain their toxicity when bound to the soil, so accumulation of these toxins is likely to occur. Earthworms ingest the bound toxins but are not affected by them. However, earthworms may function as intermediaries through which the toxins are passed on to other trophic levels. In studies where effects of insect-resistant (Bt) plants on natural enemies were considered, positive, negative and no effects have been found. So far, most studies have concentrated on natural enemies of target herbivores. However, Bt toxins are structurally rearranged when they... [ABSTRACT FROM AUTHOR]

Introduction: The oxylipin methyl jasmonate (MeJA) is a plant hormone active in response signalling and defence against herbivores. Although MeJA is applied experimentally to mimic herbivory and induce plant defences, its downstream effects on the plant metabolome are largely uncharacterized, especially in the context of primary growth and tissue-specificity of the response. Objectives: We investigated the effects of MeJA-simulated and real caterpillar herbivory on the foliar metabolome of the wild plant Brassica nigra and monitored the herbivore-induced responses in relation to leaf ontogeny. Methods: As single or multiple herbivory treatments, MeJA- and mock-sprayed plants were consecutively exposed to caterpillars or left untreated. Gas chromatography (GC) and liquid chromatography (LC) time-of-flight mass-spectrometry (TOF-MS) were combined to analyse foliar compounds, including central primary and specialized defensive plant metabolites. Results: Plant responses were stronger in young leaves, which simultaneously induced higher chlorophyll levels. Both MeJA and caterpillar herbivory induced similar, but not identical, accumulation of tricarboxylic acids (TCAs), glucosinolates (GSLs) and phenylpropanoids (PPs), but only caterpillar feeding led to depletion of amino acids. MeJA followed by caterpillars caused higher induction of defence compounds, including a three-fold increase in the major defence compound allyl-GSL (sinigrin). When feeding on MeJA-treated plants, caterpillars gained less weight indicative of the reduced host-plant quality and enhanced resistance. Conclusions: The metabolomics approach showed that plant responses induced by herbivory extend beyond the regulation of defence metabolism and are tightly modulated throughout leaf development. This leads to a new understanding of the plant metabolic potential that can be exploited for future plant protection strategies. [ABSTRACT FROM AUTHOR]

Low-dose crosstalk between jasmonic acid and salicylic acid signalling pathways results in a dynamic plant phenotype in terms of plant volatile emission and behavioural responses of an herbivorous spider mite.The jasmonic acid (JA) and salicylic acid (SA) signalling pathways, which mediate induced plant defence responses, can express negative crosstalk. Limited knowledge is available on the effects of this crosstalk on host-plant selection behaviour of herbivores. We report on temporal and dosage effects of such crosstalk on host preference and oviposition-site selection behaviour of the herbivorous spider mite Tetranychus urticae towards Lima bean (Phaseolus lunatus) plants, including underlying mechanisms. Behavioural observations reveal a dynamic temporal response of mites to single or combined applications of JA and SA to the plant, including attraction and repellence, and an antagonistic interaction between SA- and JA-mediated plant responses. Dose-response experiments show that concentrations of 0.001mM and higher of one phytohormone can neutralize the repellent effect of a 1mM application of the other phytohormone on herbivore behaviour. Moreover, antagonism between the two signal-transduction pathways affects phytohormone-induced volatile emission. Our multidisciplinary study reveals the dynamic plant phenotype that is modulated by subtle changes in relative phytohormonal titres and consequences for the dynamic host-plant selection by an herbivore. The longer-term effects on plant–herbivore interactions deserve further investigation. [ABSTRACT FROM AUTHOR]

Abstract: Plants are faced with a trade-off between on the one hand growth, development and reproduction and on the other hand defence against environmental stresses. Yet, research on insect–plant interactions has addressed plant–pollinator interactions and plant–attacker interactions separately. Plants have evolved a high diversity of constitutive and induced responses to attack, including the systemic emission of herbivore-induced plant volatiles (HIPVs). The effect of HIPVs on the behaviour of carnivorous insects has received ample attention for leaf-feeding (folivorous) species and their parasitoids and predators. Here, we review whether and to what extent HIPVs affect the interaction of plants in the flowering stage with mutualistic and antagonistic insects. Whereas the role of flower volatiles in the interactions between plants and insect pollinators has received increased attention over the last decade, studies addressing both HIPVs and pollinator behaviour are rare, despite the fact that in a number of plant species herbivory is known to affect flower traits, including size, nectar secretion and composition. In addition, folivory and florivory can also result in significant changes in flower volatile emission and in most systems investigated, pollinator visitation decreased, although exceptions have been found. Negative effects of HIPVs on pollinator visitation rates likely exert negative selection pressure on HIPV emission. The systemic nature of herbivore-induced plant responses and the behavioural responses of antagonistic and mutualistic insects, requires the study of volatile emission of entire plants in the flowering stage. We conclude that approaches to integrate the study of plant defences and pollination are essential to advance plant biology, in particular in the context of the trade-off between defence and growth/reproduction. [Copyright &y& Elsevier]

Antagonistic interactions between insect herbivores and plants impose selection on plants to defend themselves against these attackers. Although selection on plant defense traits has typically been studied for pairwise plant–attacker interactions, other community members of plant-based food webs are unavoidably affected by these traits as well. A plant trait might, for example, affect parasitoids and predators feeding on the herbivore. Consequently, defensive plant traits structure the diversity and composition of the complex community associated with the plant, and communities as a whole also feed back to selection on plant traits. Here, we review recent developments in our understanding of how plant defense traits structure insect communities and discuss how molecular mechanisms might drive community-wide effects. [Copyright &y& Elsevier]

Association mapping is rapidly becoming an important method to explore the genetic architecture of complex traits in plants and offers unique opportunities for studying resistance to insect herbivores. Recent studies indicate that there is a trade-off between resistance against generalist and specialist insects. Most studies, however, use a targeted approach that will easily miss important components of insect resistance. Genome-wide association mapping provides a comprehensive approach to explore the whole array of plant defense mechanisms in the context of the generalist–specialist paradigm. As association mapping involves the screening of large numbers of plant lines, specific and accurate high-throughput phenotyping (HTP) methods are needed. Here, we discuss the prospects of association mapping for insect resistance and HTP requirements. [Copyright &y& Elsevier]

● Supplemental far-red light increases performance of four herbivores on tomato. ● Effects of far-red light are stronger for phloem feeders than for tissue feeders. ● Supplemental red light opposes shade avoidance morphology in tomato. ● Red light reduces the performance of a tissue-feeding herbivore. Changing the spectral composition of light has numerous advantages for the production and quality of horticultural crops. However, changing the light spectrum to stimulate growth can have negative consequences for plant responses to biotic stress. Especially changes in the ratio between red and far-red light (R:FR) have an important effect on plant-herbivore interactions. A low R:FR is indicative of shading and competition for light and induces strong growth responses which are known as shade avoidance responses, including stem elongation and leaf hyponasty. Exposure to low R:FR also inhibits plant defensive responses against biotic agents and increases the performance of pests and pathogens. In this study we tested whether an increase in R:FR, through the supplementation of red light LEDs can be used to reduce the performance of arthropod herbivores. Tomato plants (Solanum lycopersocum) were exposed to three different R:FR ratios; 0.5 (shading), 1.2 (sunlight) and 5.2 (artificial) and infested with either caterpillars (Manduca sexta), spider mites (Tetranychus urticae), aphids (Myzus persicae) or whiteflies (Trialeurodes vaporariorum). Plants exposed to a low R:FR showed clear stem elongation and leaf hyponasty consistent with the shade-avoidance responses, while plants exposed to high R:FR showed reduced stem length and reduced hyponasty. The performance of all four herbivore species was significantly increased after exposure to low R:FR, although the strength of this effect varied between phloem feeding and tissue feeding herbivores. Increasing the R:FR only reduced the performance of the caterpillars and did not significantly affect the other herbivore species. These results indicate that herbivore species with different feeding strategies respond differently to changes in light quality. These findings have implications for the use of LEDs in greenhouse horticulture. [ABSTRACT FROM AUTHOR]