Using Y-tube olfactometers, the researchers revealed a preference order of PDMs (TSM-damaged leaves were most preferred, followed by flowers and then intact leaves) that is driven by the attractive odours released from flowers and TSM-damaged leaves. Analysis of VOCs showed that flowers and damaged leaves emitted both specific and common VOCs, some of which are highly attractive to PDMs. The shared attractive VOCs were emitted in greater quantities from TSM-attacked leaves than from flowers. Application of ten of these VOCs to intact leaves disrupted PDM congregation in flowers and caused their downward movement, supporting the theory that differential VOC emission functions as a signal that guides predator movement.
It is further shown that the differential emission of VOCs stems from differential phytohormonal signalling between the plant parts. Specifically, the emission patterns of some attractive VOCs tracked the levels of salicylic acid (SA) and jasmonic acid (JA) in flowers and leaves, and VOC emission was induced by supply of methyl salicylate (MeSA) to flowers or MeSA and methyl jasmonate (MeJA) to leaves. The enhanced JA signalling, but not SA signalling, has a greater effect in leaves, as it can specifically induce the emission of attracting VOCs from leaves, while the VOC emission from flowers is elicited by enhanced signalling of SA but not JA. The role of the phytohormones in driving tissue-specific VOC emission received further support from the inhibited synthesis of SA or JA using biosynthetic inhibitors and RNA silencing.
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