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Angiosperms developed floral nectaries that reward pollinating insects (1). Although nectar function and composition have been characterized, the mechanism of nectar secretion has remained unclear (2). Here we identify SWEET9 [...]

Plants emit green leaf volatiles (GLVs) in response to herbivore damage, thereby attracting predators of the herbivores as part of an indirect defense. The GLV component of this indirect defense was thought to be a general wound signal backing herbivore-specific information. We found that Manduco sexto--infested Nicotiana attenuato attract the generalist hemipteran predator Geocoris spp. as the result of an herbivore-induced decrease in the (Z)/(E) ratio of released GLVs, and that these changes in the volatile bouquet triple the foraging efficiency of predators in nature. These (E)-isomers are produced from plant-derived (Z)-isomers but are converted by a heat-labile constituent of herbivore oral secretions. Hence, attacking herbivores initiate the release of an indirect defense a full day before the attacked plants manufacture their own defensive compounds. 10.1126/science.1191634

Byline: Jianqiang Wu; Ian T. Baldwin, Department of Molecular Ecology, Max Planck Institute for Chemical Ecology, Hans-Knoll-Stra[sz]e 8, 07745 Jena, Germany; email: jwu@ice.mpg.de, baldwin@ice.mpg.de Keywords: plant-herbivore interaction, elicitor, defense, signaling, systemic response, natural variation Abstract Plants have evolved sophisticated systems to cope with herbivore challenges. When plants perceive herbivore-derived physical and chemical cues, such as elicitors in insects' oral secretions and compounds in oviposition fluids, plants dramatically reshape their transcriptomes, proteomes, and metabolomes. All these herbivory-induced changes are mediated by elaborate signaling networks, which include receptors/ sensors, Ca.sup.2+ influxes, kinase cascades, reactive oxygen species, and phytohormone signaling pathways. Furthermore, herbivory induces defense responses not only in the wounded regions but also in undamaged regions in the attacked leaves and in distal intact (systemic) leaves. Here, we review recent progress in understanding plant perception of herbivory and oviposition, and the herbivory-induced early signaling events and their biological functions. We consider the intraspecific phenotypic diversity of plant responses to herbivory and discuss the underlying genetic variation. We also discuss new tools and technical challenges in studying plant-herbivore interactions.

Phytohormones mediate the perception of insect-specific signals and the elicitation of defenses during insect attack. Large-scale changes in a plant's transcriptome ensue, but how these changes are regulated remains unknown. Silencing of RNA-directed RNA polymerase 1 (RdR1) makes Nicotiana attenuata highly susceptible to insect herbivores, suggesting that defense elicitation is under the direct control of smalI-RNAs (smRNAs). Using 454-sequencing, we characterized N. attenuata's smRNA transcriptome before and after insect-specific elicitation in wild-type (WT) and RdR1-silenced (irRdR1) plants. We predicted the targets of N. attenuata smRNAs in the genes related to phytohormone signaling (jasmonic acid, JA-Ile, and ethylene) known to mediate resistance responses, and we measured the elicited dynamics of phytohormone biosynthetic transcripts and phytohormone levels in time-course experiments with field- and glasshouse-grown plants. RdR1 silencing severely altered the induced transcript accumulation of 8 of the 10 genes, reduced JA, and enhanced ethylene levels after elicitation. Adding JA completely restored the insect resistance of irRdR1 plants. irRdR1 plants had photosynthetic rates, growth, and reproductive output indistinguishable from that of WT plants, suggesting unaltered primary metabolism. We conclude that the susceptibility of irRdR1 plants to herbivores is due to altered phytohormone signaling and that smRNAs play a central role in coordinating the large-scale transcriptional changes that occur after herbivore attack. Given the diversity of smRNAs that are elicited after insect attack and the recent demonstration of the ability of ingested smRNAs to silence transcript accumulation in lepidopteran larvae midguts, the smRNA responses of plants may also function as direct defenses. 454-sequencing | herbivore resistance | JA signaling | phytohormone regulation

Herbivore attack elicits costly defenses that are known to decrease plant fitness by using resources that are normally slated for growth and reproduction. Additionally, plants have evolved mechanisms for tolerating attack, which are not understood on a molecular level. Using [sup.11]C-photosynthate labeling as well as sugar and enzyme measurements, we found rapid changes in sink-source relations in the annual Nicotiana attenuata after simulated herbivore attacks, which increased the allocation of sugars to roots. This herbivore-induced response is regulated by the [beta]-subunit of an SnRK1 (SNF1-related kinase) protein kinase, GAL83, transcripts of which are rapidly down-regulated in source leaves after herbivore attack and, when silenced, increase assimilate transport to roots. This C diversion response is activated by herbivore-specific elicitors and is independent of jasmonate signaling, which regulates most of the plant's defense responses. Herbivore attack during early stages of development increases root reserves, which, in turn, delays senescence and prolongs flowering. That attacked GAL83-silenced plants use their enhanced root reserves to prolong reproduction demonstrates that SnRK1 alters resource allocation so that plants better tolerate herbivory. This tolerance mechanism complements the likely defensive value of diverting resources to a less vulnerable location within the plant. carbon-11 | defense | plant-herbivore interactions | tolerance

Far-red radiation (FR) reflected by green tissues is a key signal that plants use to detect the proximity of future competitors. Perception of increased levels of FR elicits a suite of responses collectively known as the shade-avoidance syndrome, which includes increased stem elongation, production of erect leaves, and reduced lateral branching. These responses improve the access to light for plants that occur in crowded populations. Responses to the proximity of competitors are known to affect the susceptibility to disease and predation in several organisms, including social animals. However, the impacts of warning signals of competition on the expression of defenses have not been explicitly investigated in plants. In the experiments reported here, we show that reflected FR induced a dramatic down-regulation of chemical defenses in wild tobacco (Nicotiana longiflora). FR altered the expression of several defense-related genes, inhibited the accumulation of herbivore-induced phenolic compounds, and augmented the performance of the specialist herbivore Manduca sexta. Complementary studies with tomato suggested that the effects of FR on defenses are mediated by the photoreceptor phytochrome B. The central implication of these results is that shade-intolerant species such as wild tobacco and tomato activate functional changes that affect their ability to cope with herbivore attack in response to phytochrome signals of future competition, even in the absence of real competition for resources. These findings suggest that competition overshadowed herbivory during the evolution of this group of species and add a new axis to the definition of the shade-avoidance syndrome. chlorogenic acid | far-red | Nicotiana | phytochrome | shade avoidance

The plant traits that are important for resistance to herbivore attack in nature are complex and operate on many spatial scales. They involve direct defenses (toxins, digestibility reducers, etc.) (1), [...], We transformed the native tobacco, Nicotiana attenuata, to silence its lipoxygenase, hydroperoxide lyase, and allene oxide synthase genes in order to inhibit oxylipin signaling, known to mediate the plant's direct and indirect defenses. When planted into native habitats, lipoxygenase-deficient plants were more vulnerable to N. attenuata's adapted herbivores but also attracted novel herbivore species, which fed and reproduced successfully. In addition to highlighting the value of genetically silencing plants to study ecological interactions in nature, these results show that lipoxygenase-dependent signaling determines host selection for opportunistic herbivores and that induced defenses influence herbivore community composition.

Plant trypsin proteinase inhibitors (TPIs) are potent herbivore- and jasmonate (JA)-induced defenses, but support for the commonly invoked explanation for their inducible expression, namely their associated fitness costs, has been elusive. To determine whether the expression of TPIs incurs fitness costs, we expressed 175 bp of the seven-domain pi from Nicotiana attenuata in an antisense orientation in a TPI-producing genotype (WT) of N. attenuata to reduce TPI expression. Moreover, we expressed the full-length seven-domain pi in a sense orientation under control of a constitutive promoter to restore TPI activity in a natural genotype unable to produce TPIs because of a mutation in its endogenous pi gene. Lifetime reproductive output was determined from high and low TPI-producing plants of the same genetic background with and without JA elicitation and grown in the same pot to simulate natural competitive and nutrient regimes. Transformants with either low or no TPI activity grew faster and taller, flowered earlier, and produced more seed capsules (25-53%) than did neighboring TPI-producing genotypes, and JA elicitation increased TPI production and decreased seed capsule production further. Growth under high light levels only marginally reduced these fitness costs. Results were similar regardless of whether TPI activity was suppressed or restored by transformation: the larger the difference in TPI activity between neighbors, the larger the difference in seed capsule production ([R.sup.2] = 0.57). TPI production is costly for a plant's components of fitness when grown under realistic competitive regimes and is consistent with the hypothesis that inducibility evolved as a cost-saving mechanism.

Allopolyploid speciation occurs instantly when the genomes of different species combine to produce self-fertile offspring and has played a central role in the evolution of higher plants, but its consequences for adaptive responses are unknown. We compare herbivore-recognition and -resistance responses of the diploid species and putative ancestral parent Nicotiana attenuata with those of the two derived allopolyploid species Nicotiana clevelandii and Nicotiana bigelovii. Manduca sexta larvae attack all three species, and in N. attenuata attack is recognized when larval oral secretions are introduced to wounds during feeding, resulting in a jasmonate burst, a systemic amplification of trypsin inhibitor accumulation, and a release of volatile organic compounds, which function as a coordinated defense response that slows caterpillar growth and increases the probability of their being attacked. Most aspects of this recognition response are retained with modifications in one allotetraploid (N. bigelovii) but lost in the other (N. clevalandii). Differences between diploid and tetraploid species were apparent in delays (maximum 1 and 0.5 h, respectively) in the jasmonate burst, the elicitation of trypsin inhibitors and release of volatile organic compounds, and the constitutive levels of nicotine, trypsin inhibitors, diterpene glycosides, rutin, and caffeoylputrescine in the leaves. Resistance to M. sexta larvae attack was most strongly associated with diterpene glycosides, which were higher in the diploid than in the two allotetraploid species. Because M. sexta elicitors differentially regulate a large proportion of the N. attenuata transcriptome, we propose that these species are suited for the study of the evolution of adaptive responses requiring trans-activation mechanisms.

Genotypes of the wild tobacco Nicotiana attenuata from different geographic regions in North America vary considerably in the level of constitutive and inducible trypsin protease inhibitors (TrypPIs), a potent direct defense, as well as in the production of herbivore-induced volatiles that function as indirect defense. Genotypes collected from Arizona were found to lack the ability to produce TrypPIs at a transcriptional level, had decreased volatile production, but exhibited nicotine and growth responses that were not distinguishable from genotypes collected in Utah. In field trials with naturally occurring herbivores and in lab experiments with Manduca sexta larvae, Arizona genotypes were damaged more and sustained greater herbivore growth than the Utah genotypes. When Arizona and Utah genotypes were grown in competition, Arizona genotypes produced significantly more seed capsules than the Utah neighbor did. Moreover, jasmonate elicitation, which dramatically increased TrypPI production in only the Utah genotypes, reduced lifetime fitness measures of the Utah genotypes more than of the Arizona genotypes, demonstrating that TrypPI production is correlated with a fitness cost. The loss of both a direct and an indirect defense suggests a functional linkage between these types of defense. Key words: chemical polymorphism, costs; genetic variation; herbivory; induced defenses; intraspecific plant competition; Nicotiana attenuata; phenotypic plasticity; phenotypic variation; protease inhibitors; volatile emission.

To examine the hypothesis that intra-plant oviposition preferences of Manduca quinquemaculata on Nicotiana attenuata optimize predation risk and nutritional needs of developing larvae and eggs, we measured oviposition behavior of adults and larval mortality, movement, performance, and body temperatures at different leaf positions in a natural population. Nearly 70% of the eggs were oviposited on young central stem leaves of elongated plants. Intra-plant movement was very rare in the first and common in the second to fourth larval instars. The oviposition preference for, and larval movement toward, younger leaves was correlated with a 40% lower predation risk and a 6.3-fold greater mass gain, suggesting higher nutritive value despite 2.1-fold higher nicotine concentrations and 4.6-fold higher polyphenol oxidase activities. The predatory bug, Geocoris pallens, which consumed eggs and larvae with instar- and leaf position-specific preferences, was responsible for the vast majority of M. quinquemaculata mortality and may shape the moth's oviposition preference. Key words: food quality; Geocoris pallens; hornworm; Lepidoptera; Manduca quinquemaculata; Nicotiana attenuata; oviposition; predation; Sphingidae; top-down control.

The authors review the publications on plants' response to herbivore attack. The topics of interest include molecular control over the response diversity elicited by herbivore attack.

Herbivore attack is widely known to reduce food quality and to increase chemical defenses and other traits responsible for herbivore resistance. Inducible defenses are commonly thought to allow plants to forgo the costs of defense when not needed; however, neither their defensive function (increasing a plant's fitness) nor their cost-savings function have been demonstrated in nature. The root-produced toxin nicotine increases after herbivore attack in the native, postfire annual Nicotiana attenuata and is internally activated by the wound hormone, jasmonic acid. I treated the roots of plants with the methyl ester of this hormone (MeJA) to elicit a response in one member of each of 745 matched pairs of plants growing in native populations with different probabilities of attack from herbivores, and measured the lifetime production of viable seed. In populations with intermediate rates of attack, induced plants were attacked less often by herbivores and survived to produce more seed than did their uninduced counterparts. Previous induction did not significantly increase the fitness of plants suffering high rates of attack. However, if plants had not been attacked, induced plants produced less seed than did their uninduced counterparts. Jasmonate-induced responses function as defenses but are costly, and inducibility allows this species to forgo these costs when the defenses are unnecessary.

Nicotine, an inducible defense in Nicotiana sylvestris, is produced in the roots and transported throughout the plant after leaf damage. Root nicotine production is activated by an endogenously produced signal, jasmonic acid (JA), which is synthesized in response to leaf damage and either directly through transport or indirectly via another signal, increases JA pools in the roots. The addition of JA or its methyl ester (MJ) to the roots of hydroponically grown plants stimulates nicotine production in the same way that leaf damage does. In this paper we use MJ to stimulate the induced defense without damaging plants in order to examine the hypothesis that these plants have an immunological 'memory' of prior inductions. Such a 'memory' can be measured as a change in either the speed or the amount of induced nicotine production or accumulation that results from prior inductions. 'Memory' is an important component of the induced defenses of animals and is predicted to evolve in systems where an initial attack is a reliable predictor of future attacks. We induced 200 plants 1, 2, or 3 times during an 18-d period of rosette growth, allowed 6 d between inductions for the relaxation of the response between inductions and quantified changes in biomass, whole-plant nicotine pools and de novo rates of nicotine production from 15N-labeled nitrate acquired at the time of induction. Induced changes in nicotine pools and the rates of nicotine production varied across the three induction periods: the highest rates and induced pools occurred during the second induction period, but the highest concentrations were found during the first induction period. The rates of de novo nicotine production from 15N-labeled nitrate were dramatically increased by MJ stimulations during all three induction periods. We found no evidence for alterations in the rates of nicotine production in plants not currently exposed to MJ; 6 d after an induction, plants had relaxed their rates of nicotine production to levels that were not significantly different from those found in plants without a prior induction history. Moreover, the maximum pools of 15N-labeled nicotine were equivalent among all induced plants regardless of prior induction history. In summary, one or two prior inductions did not markedly affect nicotine production from recently acquired nitrate as measured in either induced or uninduced plants. However, the ecologically more relevant measure of the speed of induction is the change in whole-plant nicotine pools, and plants with one and two prior inductions increased their whole-plant nicotine pool significantly faster during the third induction period than did plants not previously induced. Plants with two prior inductions attained significant increases in their nicotine pools 2 d earlier than did plants with one or no prior inductions. Moreover, the effect was additive: plants with two prior inductions were faster than plants with one. The effects of prior inductions on the speed of the induced response did not translate into increases in the magnitude of the response at the end of the 18-d experiment. The lack of change in the magnitude of the induced response may reflect the methods used in our experiment more than the plant's capabilities. The whole-plant induced response is limited by the amount of signal transported to the roots, and since we used the same amount of MJ in all stimulations, our experiment would not detect 'memory' in the production of the signal in the damaged leaf or its transport to the roots. These results demonstrate that plants, like animals, alter their induced defense in response to their prior experiences. Key words: immunological 'memory'; induced defenses; jasmonic acid; methyl jasmonate; 15N; Nicotiana sylvestris; nicotine., INTRODUCTION Inducible defenses allow an organism to change its allocation of resources to defense in response to cues from the environment. The advantage of an inducible allocation rather than a [...]

Nicotine is thought to be an excellent example of a 'mobile' (sensu Coley et al. 1985) plant defense metabolite: it is synthesized in the roots, transported to leaves, and reported to be metabolically labile, with a half-life of

Abstract. As plants become nitrogen-limited in their growth, carbon/nutrient (C/N) theory predicts a decreasing allocation of nitrogen to nitrogen-intensive defense compounds, while optimal defense (OD) theory predicts an increasing allocation to defense; C/N theory predicts no alteration in allocation to defense after damage if the C:N ratio of the plant does not change, while OD theory does. We compare C/N and OD theory predictions for the inducible nicotine production of a native tobacco, Nicotiana sylvestris. We define allocation as the slope of the ln--ln allometric relationship between whole-plant nicotine and nicotine-free biomass in undamaged and damaged plants, harvested every other day in eight consecutive harvests. Relative growth, nitrogen, and nitrogen in nicotine accumulation rates (RGRs, RNARs, and RNicARs) were calculated to estimate the instantaneous investment of nitrogen in nicotine and in the growth of plant tissues. Nicotine accumulation continues during nitrogen stress, resulting in a 3.29-fold increase in the nicotine pools of undamaged plants, and is accelerated in response to leaf damage, resulting in a 4.06-fold increase in the nicotine pools of damaged plants. Nitrogen pool size did not change after day 2, and both undamaged and damaged plants allocated an increasing proportion of their nitrogen pool to nicotine throughout the experiment. Damage increased the allocation of nitrogen to nicotine; the slope of the ln nicotine vs. ln nicotine-free biomass regression for damaged plants was significantly greater (16.9%; P < .041) than the slope for undamaged plants as measured during the period of damage-induced nicotine accumulation (days 2--10 after damage). Allocation of nitrogen to nicotine accumulation is clearly not a passive function of the nitrogen in excess of growth requirements, and damage increases this allocation. Trade-offs between biomass and nicotine production over the course of the experiment between damaged and undamaged plants were not apparent. However, the short-term estimates indicate otherwise. On day 2, there was a significant decrease in RGR, with a correlated increase in RNicAR in response to damage, suggesting that there was a short-term trade-off between allocation to growth and nicotine accumulation or other induced processes. By the end of the experiment, each damaged plant had accumulated 0.595 mg N in nicotine above that accumulated by undamaged plants, an additional investment representing 1.94% of the total nitrogen pool. However, 2 d after damage, damaged plants allocated 0.131 mg more N to nicotine than undamaged plants, representing a short-term investment of 17.7% of the nitrogen potentially made available by the decrease in RGR (0.740 mg). Plants invest substantial portions of their nitrogen in an induced defense, but our estimates do not include the nitrogen costs of production, storage, transport, detoxification, or the indirect costs of this induced defense.

Abstract. Defense and regrowth after herbivore attack are not mutually exclusive alternatives for most plants, yet few studies have examined the coordination of the processes responsible for these two plant functions. To this end, we studied the coordination of alkaloidal and photosynthetic responses to simulated herbivory in the context of changes in leaf nitrogen in plants grown under a range of nitrate supply rates in two experiments. In the first experiment, damage-induced changes in leaf nicotine, total nitrogen, nitrate, and photosynthetic rate (PR) were monitored in same-aged undamaged leaves of young Nicotiana sylvestris plants grown in pots. In the second, the alkaloidal response to damage was uncoupled from damage by growing plants in pots for > 150 d, causing them not to respond to leaf damage with increased nicotine concentrations ('uninducible'). We propose that the changes in PR and nicotine content induced by damage reflect the allocation of resources to regrowth and defense, respectively, and examine the predictions of the optimal defense (OD) theory regarding these responses. We have previously established that neither constitutive nor induced nicotine production is a passive consequence of a nitrogen imbalance in excess of growth requirements as is predicted by the carbon/nutrient (C/N) theory. If PR reflects the fitness value of the leaf and damage reflects a high probability of future damage, we interpret the OD theory to predict that PR and nicotine content should be correlated, and that damage should increase the amount of nicotine allocated for a given PR. Nicotine, nitrogen, and PR increased in a coordinated fashion in response to leaf damage in the inducible plants. In both experiments, PR and nitrogen were highly correlated, but damage did not affect the PR--nitrogen relationship. In the first experiment with inducible plants, nicotine and nitrogen were also highly correlated. However, damage significantly increased the slope of the nicotine--nitrogen relationship 1.6-fold. Similarly, nicotine and PR were significantly correlated and the slope of the nicotine--PR relationship increased significantly (1.9-fold) in response to damage. These results are consistent with the predictions of the OD theory. Despite this coordination, alkaloidal and photosynthetic responses can be uncoupled. Regardless of nitrate supply rate, damaged 'uninducible' plants exhibited no significant increase in nicotine content, but significantly increased their PR in response to damage with a correlated increase in leaf nitrogen content. Nicotine and PR were not significantly correlated in undamaged plants, but were significantly correlated in damaged plants. Unless other defenses are activated in uninducible plants, these results may reflect a priority of growth over defense in uninducible plants with slow growth rates or reduced rooting volume.

The pit-dwelling ant lion, Myrmeleon carolimus, kills formic acid-spraying ants, Camponotus floridanus, without making them spray, and extracts the contents of the ants' bodies without rupturing the acid sac. Normally when threatened, Camponotos reacts by simultaneously biting and spraying, but when the ant lion drags the ant to the floor of the pit, the ant fails to grip the ant lion and it is killed before spraying.

Author(s): Ian T. Baldwin (corresponding author) [1] Leaf Defence Edward E. Farmer Oxford Univ. Press: 2014. Plant Sensing and Communication Richard Karban Univ. Chicago Press: 2015. Plant Behaviour and Intelligence [...]

Author(s): Ian T. Baldwin (corresponding author) [1] Fortress Plant: How to Survive When Everything Wants to Eat You Dale Walters Oxford University Press: 2017. [illus. 1] The view of plants [...]

Plants defend themselves against herbivores with chemical and physical defenses that directly influence herbivore performance and indirectly through traits that attract the natural enemies of herbivores (1-3). One such indirect [...], Herbivore attack is known to increase the emission of volatiles, which attract predators to herbivore-damaged plants in the laboratory and agricultural systems. We quantified volatile emissions from Nicotiana attenuata plants growing in natural populations during attack by three species of leaf-feeding herbivores and mimicked the release of five commonly emitted volatiles individually. Three compounds (cis-3-hexen-1-ol, linalool, and cis-[Alpha]-bergamotene) increased egg predation rates by a generalist predator; linalool and the complete blend decreased lepidopteran oviposition rates. As a consequence, a plant could reduce the number of herbivores by more than 90% by releasing volatiles. These results confirm that indirect defenses can operate in nature.

THOMAS E. OHNMEISS [1] IAN T. BALDWIN [1,2,3] Abstract. Optimal Defense (OD) theory predicts that the within-plant allocation of secondary metabolites that function as defenses will be positively correlated with [...]

For Love of Insects is a story of passionate scientific inquiry into the chemical ecology of insects by a researcher, Thomas Eisner, whose life's work defines the discipline. Like any [...]

The litter of many plant species is known to inhibit germination, and this phenomenon is commonly interpreted as allelopathic inhibition of one species by another. However, an alternative interpretation is that seeds may be using environmental signals to inform the timing of their germination and thereby use dormancy as a mechanism of habitat choice. Nicotiana attenuata Torn ex Wats. (Solanaceae) is typically found for less than three years after fire in the sagebrush, blackbrush, and pinyon-juniper forests of the Great Basin desert of North America. A dormant seed bank is established during this ephemeral post-fire period, and pyrolysis products of [Alpha]-cellulose (containing only C, H, and O) in wood smoke are known to initiate germination in dormant seeds of this species. We demonstrated in a glasshouse experiment that germination into burned soils (as compared to unburned) results in a 12-fold increase in lifetime seed production, which reflects a minimum estimate of the fitness benefit of making accurate germination decisions. With seed bioassays, we examined the distribution of this smoke signal in the [A.sub.1] soil layer at several burned areas in southwestern Utah, United States to determine whether the presence of this smoke-derived germination cue predicts the spatial and temporal occurrence of N. attenuata populations after fires. Although we found no evidence for the germination signal in areas that had not been burned for 30 yr, the occurrence of the germination signal did not perfectly coincide with the distribution of populations. We found evidence for its transport by wind and water into adjacent unburned areas (from 40 m to 1 km away from a burned site) and its persistence over time (for ≥ 7 yr), making this signal an unreliable indicator of the plant's habitat. To resolve this discrepancy, we examined the effect of unburned [A.sub.0] soil horizon on smoke-induced germination. The litter-containing [A.sub.0] soil horizon (and aqueous extracts thereof), collected from underneath seven dominant species from later stages of post-fire succession, completely inhibited germination of both dormant and nondormant seeds, even in the presence of a smoke cue in excess of that required to elicit germination. The inhibitory effect was limited to the early stages of germination (48 h after exposure to smoke), and we confirmed these results with natural seed banks. We demonstrated that the [A.sub.0] soil horizons and their aqueous extracts are not toxic to N. attenuata seeds or growing plants, and they have no effect on lifetime seed production. Moreover, they do not inhibit the germination of the nondormant, conspecific native tobacco, N. trigonophylla, which grows in the same area but is not associated with fire. Hence, these negative factors do not function in allelopathically mediated competitive interactions between N. attenuata and later successional species. We propose that the occurrence of N. attenuata populations after fires can be explained by the combined stimulatory effect of smoke-derived signals on the dormant seed bank and the inhibitory effect of signals from unburned litter, and that both signals are required for N. attenuata to identify its germination niche. Key words: [A.sub.0] and [A.sub.1] soil horizons; allelopathy; fire; germination signals; litter-inhibited germination; Nicotiana attenuata; positive and negative control; seed banks; seed dormancy; smoke-induced germination., INTRODUCTION Dormancy functions to distribute organisms in time and, much like its spatial counterpart, dispersal, can be directed or random, allowing plants to synchronize growth with suitable environmental conditions (Bouwmeester [...]