Phytochemical Diversity and Interactions with Herbivores

Dear Colleagues,

For over a century, biologists have asked why plants produce such large diversities of chemical compounds and hypothesized about their role in plant–insect interactions. Recent advances in metabolomics have enabled ecologists to measure large numbers of compounds simultaneously and efficiently, resulting in major quantitative developments (a measure of phytochemical diversity). Outstanding advances have been made in elucidating phytochemical diversity as a complex phenotype that can predict defensibility within plants as it relates to interactions with herbivores. Despite these advances, more progress is needed to achieve a nuanced view of phytochemical diversity and its interactions with herbivores, community dynamics, and ecosystem processes. Some specific gaps in this literature include: understanding the different components of phytochemical diversity (i.e., richness, evenness, and dissimilarity); the macro- and micro-evolutionary scale of phytochemical diversity; linking molecular structure to function; the identification of gene function (as it relates to phytochemical diversity); utilizing or manipulating phytochemical diversity to protect crop plants from herbivores or manage invasive species; and quantifying the metabolomic changes along environmental clines.

For this Special Issue, articles (original research papers, perspectives, hypotheses, opinions, reviews, modelling approaches, and methods) may focus all aspects of phytochemical diversity, but manuscripts describing theoretical or experimental studies are particularly welcome.

Dr. Andrea E. Glassmire
Prof. Dr. Michael J. Stout
Guest Editors

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• phytochemical diversity
• herbivore
• tritrophic
• chemical ecology
• plant secondary metabolites
• plant defense

Title: Herbivore and soil microbe effects on VOC produc7on in a wetland plant experiencing widespread dieback
Authors: Andrea Glassmire, Ana Salgado, Alex Ga2e, Michael Stout, Aaron Devries, Vinson Doyle, and James T. Cronin
Affiliation: Department of Biological Sciences, Louisiana State University, Baton Rouge, LA, USA
Abstract: Plant volaAles can be a signal to insect herbivores and predators that plants are toxic, anA- nutriAve, infested with herbivores, and/or have limited accessibility, affecAng plant fitness by repelling herbivores or aJracAng beneficial natural enemies. There is a gap in the literature, however, on the consequences of simultaneous below- and above-ground stressors on the producAon of plant VOCs and whether concurrent stressors can disrupt the ability of plant VOCs to effecAvely communicate resistance. We tested whether herbivory by a scale insect and the presence of beneficial versus harmful soil microbes influence the type and diversity of plant VOCs being emiJed. We used a dominant plant of wetland communiAes, Phragmites australis, that is experiencing widespread dieback in coastal marshes of Louisiana. Some possible causal agents in the dieback of P. australis are non-naAve scale insects, Nipponaclerda biwakoensis, and soil pathogens. Furthermore, lineages of P. australis vary in their resistance to scale infestaAon and dieback, with the resistant non-naAve European haplotype more resistant than the suscepAble naturalized Delta haplotype and the naAve Gulf haplotype. We used a full factorial design in which we manipulated soil microbes by establishing individual plants in soils from healthy and dieback Phragmites stands in the presence or absence of the scale insect from three lineages of Phragmites. We collected and idenAfied 59 volaAle compounds using GC-MS, with 24 of those compounds being classified as terpenes. Phytochemical diversity for all lineages was driven by metabolite evenness rather than metabolite richness. For within lineage differences across treatments, we found that healthy soils and presence of scales increased overall terpene evenness in Delta and European plants. In contrast, dieback soils and scale presence inhibited overall terpene evenness in Delta but had no effect on European plants. Gulf plants, however, had slightly reduced terpene evenness across all treatment combinaAons. For among lineage differences across treatments, we found that European plants had roughly 2x higher terpene evenness than Delta plants established in healthy soils, and 3x higher terpene evenness than Delta plants established in dieback soils. Overall, our results suggest that soil type and presence of scales significantly influenced producAon of terpene diversity in the suscepAble Delta lineage and disrupted the ability of the Delta haplotype to combat herbivory. Thus, the importance of geneAc variaAon in VOC producAon plays a key role in plant resistance, not only to natural enemies but also to climate-induced stressors like large-scale dieback.

Title: Plasticity in phytochemical diversity: complex determinants of biosynthesis
Authors: Lee Dyer
Affiliation: Biology Department, University of Nevada Reno, 1664 N. Virginia St., Reno, NV 89557, USA
Abstract: Plasticity is one of the most studied attributes of specialized metabolites, but the causes and consequences of plasticity of phytochemical diversity are mostly unknown. As with individual metabolites, there may be a tendency to gravitate towards appealing just-so adaptive stories, like talking trees and concerted community defenses against invaders. In contrast, the complex responses to mineral resources, light availability, CO2, and other determinants of chemical phenotypes are likely to be downplayed. This opinion piece explores the factors that contribute to plasticity in phytochemical diversity with a focus on two issues that are often left out of the adaptationist view of plant chemistry. First there are clear physiological constraints that limit the range of plasticity and limit some mixtures of metabolites to constitutive levels. Second, the relevance of effect sizes associated with such plastic responses needs to be established - many “statistically significant” levels of plasticity in phytochemical diversity are likely to be associated with relatively small effect sizes, which diminishes the potential selective pressures on herbivores and other organisms impacted by phytochemistry. These two issues juxtapose instances where compelling model systems show that plasticity plays a substantial role in the dynamics of phytochemical diversity with those studies where plasticity in phytochemical mixtures have been inappropriately oversold.