These large-scale alterations, which are mostly known to induce plant anti-herbivore defense responses, function either directly via mechanisms such as production of amino acid-catabolizing enzymes, anti-digestive proteins or toxic or repelling chemicals (Weber et al., 1999 Chen et al., 2005), or indirectly through production and release of volatile organic compounds (VOC) that attract natural enemies of the herbivores or coordinate other plant defenses (Pare and Tumlinson, 1997, 1999 Kessler and Baldwin, 2001 Engelberth et al., 2004 Kessler et al., 2004 van Poecke and Dicke, 2004 Chehab et al., 2008). These elicitors trigger herbivore-specific defense responses in their host plants through large-scale transcriptional, translational and post-translational alteration (Halitschke et al., 2001 Howe and Jander, 2008 Hilker and Meiners, 2010). For example, insect feeding inevitably inflicts mechanical wounding on plants, with simultaneous release of oral secretions (OS), which are potential carriers of herbivore-specific elicitors into damaged tissues. Some of the most potent plant defenses against herbivores are inducible, herbivore-specific and indirect (Baldwin et al., 2006 Heil et al., 2012). Research over the past 40 years has clearly demonstrated a wide range of plant defenses against herbivore attack, but less attention has been given to herbivore adaptations used to overcome these defenses (Karban and Agrawal, 2002 Allmann and Baldwin, 2010). Throughout their life cycles, plants are attacked by a wide range of organisms, including insect herbivores, with diverse feeding behaviors. Feeding studies suggested a potential evolutionary advantage of suppressing GLV production, as caterpillars preferably consumed leaf tissue from plants that had not been primed by these volatile cues. Use of lines engineered to express HPL constitutively, in conjunction with quantitative RT–PCR-based expression analyses, established a combination of transcriptional and post-transcriptional reprogramming of the HPL pathway genes as the mechanistic basis of insect-mediated suppression of the corresponding metabolites. Simultaneously, however, these chewing herbivores stimulated JA production, demonstrating targeted insect suppression of the HPL branch of the oxylipin pathway. We established that emission of GLVs is stimulated by wounding incurred mechanically or by aphids, but release of these volatiles is constitutively impaired by both generalist and specialist chewing insects. Specifically, we used three Arabidopsis genotypes that were damaged by mechanical wounding or by insects of various feeding guilds (piercing aphids, generalist chewing caterpillars and specialist chewing caterpillars). Here we provide evidence that chewing insects differentially alter the oxylipin profiles produced by the two main and competing branches of the plant defensive response pathway, the allene oxide synthase (AOS) and hydroperoxide lyase (HPL) branches, which are responsible for wound-inducible production of jasmonates (JAs), and green leafy volatiles (GLVs) respectively.

Insect herbivores have developed a myriad of strategies to manipulate the defense responses of their host plants.

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