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Plants can produce and release allelochemicals to interfere with the establishment and growth of conspecific and interspecific plants. Such allelopathy is an important mediator among plant species in natural and managed ecosystems. This review focuses on allelopathy and allelochemicals in grasslands and forests. Allelopathy drives plant invasion, exacerbates grassland degradation and contributes to natural forest regeneration. Furthermore, autotoxicity (intraspecific allelopathy) frequently occurs in pastures and tree plantations. Various specialized metabolites, including phenolics, terpenoids and nitrogen-containing compounds from herbaceous and woody species are responsible for allelopathy in grasslands and forests. Terpenoids with a diversity of metabolites are qualitative allelochemicals occurring in annual grasslands, while phenolics with a few specialized metabolites are quantitative allelochemicals occurring in perennial forests. Importantly, allelochemicals mediate below-ground ecological interactions and plant–soil feedback, subsequently affecting the biodiversity, productivity and sustainability of grasslands and forests. Interestingly, allelopathic plants can discriminate the identity of neighbors via signaling chemicals, adjusting the production of allelochemicals. Therefore, allelochemicals and signaling chemicals synergistically interact to regulate interspecific and intraspecific interactions in grasslands and forests. Allelopathy and allelochemicals in grasslands and forests have provided fascinating insights into plant–plant interactions and their consequences for biodiversity, productivity and sustainability, contributing to our understanding of terrestrial ecosystems and global changes. [ABSTRACT FROM AUTHOR]

Summary: Plants actively respond to their neighbors by altering root placement patterns. Neighbor‐modulated root responses involve root detection and interactions mediated by root‐secreted functional metabolites. However, chemically mediated root placement patterns and their underlying mechanisms remain elusive.We used an allelopathic wheat model system challenged with 60 target species to identify root placement responses in window rhizobox experiments. We then tested root responses and their biochemical mechanisms in incubation experiments involving the addition of activated carbon and functional metabolites with amyloplast staining and auxin localization in roots.Wheat and each target species demonstrated intrusive, avoidant or unresponsive root placement, resulting in a total of nine combined patterns. Root placement patterns were mediated by wheat allelochemicals and (–)‐loliolide signaling of neighbor species. In particular, (–)‐loliolide triggered wheat allelochemical production that altered root growth and placement, degraded starch grains in the root cap and induced uneven distribution of auxin in target species roots.Root placement patterns in wheat–neighbor interactions were perception dependent and species dependent. Signaling (−)‐loliolide induced the production and release of wheat allelochemicals that modulated root placement patterns. Therefore, root placement patterns are generated by both signaling chemicals and allelochemicals in allelopathic plant–plant interactions. [ABSTRACT FROM AUTHOR]

Neighbor detection and allelochemical response are important mediators in plant–plant interactions. Although there is increasing knowledge about plant allelochemicals released in response to the presence of competitors and involved in neighbor-derived signaling, less is known about which signaling chemicals are responsible for the neighbor-induced allelochemical response. Here, we experimentally demonstrate that (–)-loliolide, a carotenoid metabolite, acts as a signaling chemical in barnyardgrass–rice allelopathic interactions. The production of the rice allelochemicals momilactone B and tricin was increased in the presence of five biotypes of barnyardgrass. (–)-Loliolide was found in all the biotypes of barnyardgrass and their root exudates and rhizosphere soils. There were significant positive relationships between rice allelochemicals and (–)-loliolide concentrations across the biotypes of barnyardgrass. Furthermore, (–)-loliolide elicited the production of momilactone B and tricin. Comparative transcriptomic analysis revealed regulatory activity of (–)-loliolide on the diterpenoid and flavonoid biosynthesis pathway. The expression of key genes involved in the biosynthesis of momilactone B (CPS4 , KSL4 , and MAS) and tricin (CYP75B3 and CYP75B4) was up-regulated by (–)-loliolide. These findings suggest that (–)-loliolide acts as a signaling chemical and participates in barnyardgrass-rice allelopathic interactions. Allelopathic rice plants can detect competing barnyardgrass through the presence of this signaling chemical and respond by increasing levels of their allelochemicals to achieve an advantage for their own growth. [ABSTRACT FROM AUTHOR]

Productivity decline of monospecific forest plantations has remained a serious problem. Despite increasing knowledge of the problem involved in the build-up of soil-borne pathogens and allelopathy, relatively little is known about tree-derived allelochemicals and their impacts on the soil microbial community and root growth. Therefore, the objective of this study was to examine a novel allelochemical cyclic dipeptide in relation to the soil microbial community and phytotoxicity to tree roots in 25-year-old monospecific Chinese fir ( Cunninghamia lanceolata ) forest plantations. We sampled soils and fine roots in situ and quantified soil cyclic dipeptide, microbial and root characteristics along with their correlation analyses. When compared with soil from a plantation established following removal of natural forest vegetation, soil from a replanted plantation contained a greater amount of cyclic dipeptide. Furthermore, increased soil potentially pathogenic fungi and reduced root biomass, root surface area and root length density were observed in the replanted plantations. There were negative relationships among cyclic dipeptide concentration, microbial community composition and root biomass in given plantations. Phospholipid fatty acid profiling showed that the signature lipid biomarkers of soil bacteria and fungi, and soil microbial community structure were affected under cyclic dipeptide application. Soil degradation dynamics indicated that cyclic dipeptide declined rapidly. The results demonstrated that allelochemical cyclic dipeptide not only had direct phytotoxicity to tree roots but also indirectly altered soil microbial community compositions, suggesting that productivity decline of continuous Chinese fir monocultures may be a negative feedback interaction between allelochemical-mediated soil microbial community and root phytotoxicity. [ABSTRACT FROM AUTHOR]

The interference of allelopathic weeds with crop plants might be mediated by volatile allelochemicals. In this study, the essential oil constituents of two weeds, wild oat ( Avena fatua) and crabgrass ( Digitaria sanguinalis), were investigated in relation to their effects on the growth and allelochemical production of wheat ( Triticum aestivum). Subsequently, by means of gas chromatography and gas chromatography-mass spectrometry, 52 compounds were identified from the crabgrass essential oil, particularly a signaling compound called methyl jasmonate, while 28 constituents were detected in the wild oat essential oil. Both essential oils were rich in terpenoids and inhibited the growth of wheat in air, filter paper and soil media but their inhibition varied with the growth medium and the weed species. In both the air and the filter paper media, there were not significant differences in the dry weight of wheat between the wild oat and the crabgrass essential oils. However, there was a greater reduction in the dry weight of the wheat root and plant with the wild oat essential oil than with the crabgrass essential oil in the soil medium. Furthermore, the production of the allelochemical, 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one, in wheat was induced by the essential oils. The results suggest that allelopathic interference with wheat by wild oat and crabgrass affects not only the biomass allocation, but also the allelochemical production, of wheat. [ABSTRACT FROM AUTHOR]

Abstract: Despite an increase in the understanding of the soybean isoflavones involved in root-colonizing symbioses, relatively little is known about their levels in the rhizosphere and their interactions with the soil microbial community. Based on a 13-year experiment of continuous soybean monocultures, in the present study we quantified isoflavones in the soybean rhizosphere and analyzed the soil microbial community structure by examining its phospholipid fatty acid (PLFA) profile. Two isoflavones, daidzein (7, 4′-dihydroxyisoflavone) and genistein (5,7,4′- trihydroxyisoflavone), were detected in the rhizosphere soil of soybean plants, with the concentrations in the field varying with duration of mono-cropping. Genistein concentrations ranged from 0.4 to 1.2 μg g−1 dry soil over different years, while daidzein concentrations rarely exceeded 0.6 μg g−1 dry soil. PLFA profiling showed that the signature lipid biomarkers of bacteria and fungi varied throughout the years of the study, particularly in mono-cropping year 2, and mono-cropping years 6–8. Principal component analysis clearly identified differences in the composition of PLFA during different years under mono-cropping. There was a positive correlation between the daidzein concentrations and soil fungi, whereas the genistein concentration showed a correlation with the total PLFA, fungi, bacteria, Gram (+) bacteria and aerobic bacteria in the soil microbial community. Both isoflavones were easily degraded in soil, resulting in short half-lives. Concentrations as small as 1 μg g−1 dry soil were sufficient to elicit changes in microbial community structure. A discriminant analysis of PLFA patterns showed that changes in microbial community structures were induced by both the addition of daidzein or genistein and incubation time. We conclude that daidzein and genistein released into the soybean rhizosphere may act as allelochemicals in the interactions between root and soil microbial community in a long-term mono-cropped soybean field. [Copyright &y& Elsevier]

Ambrosia trifida is worldwide spreading in ecological terms as pioneer species and is one of the most economically destructive weeds occurring on arable lands in North China. It has long been suspected of using allelopathy to interfere with the growth and establishment of crop plants, but the mechanisms involved in allelopathy is largely unknown. In this study, a series of experiments were conducted to assess the phytotoxicity and to identify the allelochemicals of A. trifida against wheat (Triticum aestivum). The results showed that wheat growth could be significantly inhibited in A. trifida infested or residue amended soils in Northeast China. Two carotane-type sesquiterpenes, 1α-angeloyloxy-carotol and 1α-(2-methylbutyroyloxy)-carotol, were subsequently isolated and identified from the toxic soils. Both compounds had high inhibitory activity on wheat growth. Their inhibition thresholds were 11.5μg/g soil for 1α-angeloyloxycarotol) and 16.3μg/g soil for1α-(2-methylbutyroyloxy)-carotol. Furthermore, these two compounds were detected from A. trifida infested and residue amended soils. Their amounts ranged from 13.7 to 43.2μg/g soil. As a result, A. trifida could release sufficient quantities of 1α-angeloyloxycarotol and 1α-(2-methylbutyloyloxy)-carotol into the soil to act as allelochemicals inhibiting the growth of wheat. The study suggested that 1α-angeloyloxy-carotol and 1α-(2-methylbutyroyloxy)-carotol could be key allelochemicals in A. trifida infested wheat fields and that A. trifida interferes with wheat growth via allelopathy. [Copyright &y& Elsevier]

Plants abound with active ingredients. Among these natural constituents, allelochemicals and signaling chemicals that are released into the environments play important roles in regulating the interactions between plants and other organisms. Allelochemicals participate in the defense of plants against microbial attack, herbivore predation, and/or competition with other plants, most notably in allelopathy, which affects the establishment of competing plants. Allelochemicals could be leads for new pesticide discovery efforts. Signaling chemicals are involved in plant neighbor detection or pest identification, and they induce the production and release of plant defensive metabolites. Through the signaling chemicals, plants can either detect or identify competitors, herbivores, or pathogens, and respond by increasing defensive metabolites levels, providing an advantage for their own growth. The plant-organism interactions that are mediated by allelochemicals and signaling chemicals take place both aboveground and belowground. In the case of aboveground interactions, mediated air-borne chemicals are well established. Belowground interactions, particularly in the context of soil-borne chemicals driving signaling interactions, are largely unknown, due to the complexity of plant-soil interactions. The lack of effective and reliable methods of identification and clarification their mode of actions is one of the greatest challenges with soil-borne allelochemicals and signaling chemicals. Recent developments in methodological strategies aim at the quality, quantity, and spatiotemporal dynamics of soil-borne chemicals. This review outlines recent research regarding plant-derived allelochemicals and signaling chemicals, as well as their roles in agricultural pest management. The effort represents a mechanistically exhaustive view of plant-organism interactions that are mediated by allelochemicals and signaling chemicals and provides more realistic insights into potential implications and applications in sustainable agriculture. [ABSTRACT FROM AUTHOR]

Despite increasing knowledge of allelochemicals as leads for new herbicides, relatively little is known about the mode of action of allelochemical-based herbicides on herbicide-resistant weeds. In this study, herbicidal activities of a series of allelochemical tricin-derived compounds were evaluated. Subsequently, a benzothiazine derivative 3-(2-chloro-4-methanesulfonyl)-benzoyl-hydroxy-2-methyl-2H-1,2-benzothiazine-1,1-dioxide with 4-hydroxyphenyl-pyruvate dioxygenase (HPPD) inhibiting activity was identified as a target compound on photosynthetic performance of penoxsulam-resistant versus -susceptible barnyardgrass ( Echinochloa crus-galli ). Regardless of barnyardgrass biotype, the benzothiazine derivative greatly affected chlorophyll fluorescence parameters (Fv/Fm, ETR 1 min and NPQ 1 min ), reduced the chloroplast fluorescence levels and expression of HPPD gene. In particular, the benzothiazine derivative interfered with photosynthetic performance of resistant barnyardgrass more effectively than the allelochemical tricin itself. These results showed that the benzothiazine derivative effectively inhibited the growth of resistant barnyardgrass and its mode of action on photosynthesis system was similar to HPPD-inhibiting sulcotrione, making it an ideal lead compound for further development of allelochemical-based herbicide discovery. [ABSTRACT FROM AUTHOR]

Abstract: Despite increasing knowledge of jasmonic acid (JA) and salicylic acid (SA) as signaling compounds involved in the defense of rice against attacking microbes and insect predators, relatively little is known about their levels in the growth media and their interactions with other plant competitors. In present study we quantified JA and SA in a rice-barnyardgrass coexistence system followed by correlation analysis to access rice allelochemicals. Both rice and barnyardgrass biosynthesized JA and SA, but their contents varied greatly with species, tissues and coexistence. There was a positive correlation in contents between rice allelochemicals and JA in roots or SA in shoots. Endogenous JA was exuded from barnyardgrass roots eliciting the production of rice allelochemicals. SA was not detected in growth media as an exogenous signaling compound in a rice-barnyardgrass coexistence system, but SA content in rice shoots was an indicator for distinguishing the allelopathic rice traits from the non-allelopathic ones. [Copyright &y& Elsevier]