Abstract
Benzenoids (C6–C1 aromatic compounds) play important roles in plant defense and are often produced upon herbivory. Black cottonwood (Populus trichocarpa) produces a variety of volatile and nonvolatile benzenoids involved in various defense responses. However, their biosynthesis in poplar is mainly unresolved. We showed feeding of the poplar leaf beetle (Chrysomela populi) on P. trichocarpa leaves led to increased emission of the benzenoid volatiles benzaldehyde, benzylalcohol, and benzyl benzoate. The accumulation of salicinoids, a group of nonvolatile phenolic defense glycosides composed in part of benzenoid units, was hardly affected by beetle herbivory. In planta labeling experiments revealed that volatile and nonvolatile poplar benzenoids are produced from cinnamic acid (C6–C3). The biosynthesis of C6–C1 aromatic compounds from cinnamic acid has been described in petunia (Petunia hybrida) flowers where the pathway includes a peroxisomal-localized chain shortening sequence, involving cinnamate-CoA ligase (CNL), cinnamoyl-CoA hydratase/dehydrogenase (CHD), and 3-ketoacyl-CoA thiolase (KAT). Sequence and phylogenetic analysis enabled the identification of small CNL, CHD, and KAT gene families in P. trichocarpa. Heterologous expression of the candidate genes in Escherichia coli and characterization of purified proteins in vitro revealed enzymatic activities similar to those described in petunia flowers. RNA interference-mediated knockdown of the CNL subfamily in gray poplar (Populus x canescens) resulted in decreased emission of C6–C1 aromatic volatiles upon herbivory, while constitutively accumulating salicinoids were not affected. This indicates the peroxisomal β-oxidative pathway participates in the formation of volatile benzenoids. The chain shortening steps for salicinoids, however, likely employ an alternative pathway.
Highlights
As sessile organisms, plants have to cope with a variety of biotic stressors, such as herbivores and pathogens
Herbivory of C. populi beetles induces the production of benzenoid volatiles, but nonvolatile benzenoid compounds are barely affected Herbivory often results in the induced formation of specialized metabolites
To test the influence of a specialist herbivore on poplar-induced defense compounds, we investigated the formation of volatile and nonvolatile specialized metabolites in leaves of P. trichocarpa upon herbivory of the poplar leaf beetle C. populi
Summary
Plants have to cope with a variety of biotic stressors, such as herbivores and pathogens. Plants produce a huge variety of specialized metabolites, which can either act as direct defense compounds against the herbivores or pathogens, as indirect defense, for instance through the attraction of parasitoids or predators, or as defense signals (Unsicker et al., 2009; Mithofer and Boland, 2012; Furstenberg-Hagg et al, 2013; Maag et al, 2015). Estimates assume that around 200,000 to 1 million structurally different compounds are produced as specialized metabolites by plants (Pichersky and Lewinsohn, 2011). These metabolites can be classified based on their biosynthetic origin or common chemical core structures, and the three major compound classes are terpenes, nitrogen-containing compounds, and aromatic.
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