Plants have developed a multitude of defense mechanisms against herbivores and pathogens. One class of defense mechanism against herbivores is referred to as 'induced indirect defense'. An example of ...induced indirect defense is to emit specific blends of volatiles in response to herbivore damage (herbivore-induced plant volatiles : HIPVs), which attract carnivorous natural enemies of the herbivores. It has been reported that jasmonate, salicylate and ethylene are signalling molecules which are involved in the production of HIPVs in lima bean. Lima bean leaves infested with two spotted spider mite (Tetranychus urticae) induce an expression of gene for S-adenosyl methionine decarboxylase (SAMDC) that catalyzes the rate-limiting step in the biosynthesis of spermidine (SPD) and spermine (SPM). Polyamines (PAs) including SPD and SPM are basic small molecules that are ubiquitous in all plant cells. While evidence has accumulated that PAs are involved in resistance to microbial attack and environmental stress in plants, the effects of PAs in response to herbivory are still unknown. In this study, we investigated whether exogenous polyamines affect the biosynthesis of induced volatiles in lima bean leaves. Lima bean leaves exogenously treated with cadaverine, putrecine, SPD or SPM, did not induce volatiles. However, SPM together with a low concentration of jasmonic acid (JA), induced the production of (Z)-3-hexenyl acetate and five terpenoids, which were not induced by the low concentration of JA alone. Furthermore, expression of genes for lipoxygenase (LOX) and farnesyl diphosphate synthetase (FPS) that are involved in volatile biosynthesis was enhanced by exogenous JA and SPM treatments.
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