Abstract
Jasmonic acid (JA) plays a central role in induced plant defence e.g. by regulating the biosynthesis of herbivore-induced plant volatiles that mediate the attraction of natural enemies of herbivores. Moreover, exogenous application of JA can be used to elicit plant defence responses similar to those induced by biting-chewing herbivores and mites that pierce cells and consume their contents. In the present study, we used Lima bean (Phaseolus lunatus) plants to explore how application of a low dose of JA followed by minor herbivory by spider mites (Tetranychus urticae) affects transcript levels of P. lunatus (E)-β-ocimene synthase (PlOS), emission of (E)-β-ocimene and nine other plant volatiles commonly associated with herbivory. Furthermore, we investigated the plant's phytohormonal response. Application of a low dose of JA increased PlOS transcript levels in a synergistic manner when followed by minor herbivory for both simultaneous and sequential infestation. Emission of (E)-β-ocimene was also increased, and only JA, but not SA, levels were affected by treatments. Projection to latent structures-discriminant analysis (PLS-DA) of other volatiles showed overlap between treatments. Thus, a low-dose JA application results in a synergistic effect on gene transcription and an increased emission of a volatile compound involved in indirect defence after herbivore infestation.
Highlights
Plants possess a whole arsenal of mechanisms to resist attacks by pathogens and herbivorous arthropods
Plants treated with 0.1 mM Jasmonic acid (JA) or four T. urticae alone showed higher (P
The combination treatment resulted in a PlOS transcript level that is twice the level that would be obtained if the effects of JA and four T. urticae were additive, revealing a synergistic effect of the two treatments on PlOS transcript levels
Summary
Plants possess a whole arsenal of mechanisms to resist attacks by pathogens and herbivorous arthropods. A general component of the response to chewing herbivores and foliar wounding is elicitation of the jasmonic acid (JA) signalling pathway in which the phytohormone JA plays a central role (McConn et al, 1997; Kessler and Baldwin, 2002). Piercing-sucking insects and biotrophic pathogens commonly induce the salicylic acid (SA) signalling pathway, which antagonizes the JA pathway (Kempema et al, 2007; Thaler et al, 2012). Both pathways regulate large-scale changes in defence-related parts of the plant transcriptome, proteome, and metabolome, which underlie plant direct and indirect resistance mechanisms (Kessler and Baldwin, 2002; Pieterse and Dicke, 2007).
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