Specific Regulation of Pyrethrin Biosynthesis in Chrysanthemum cinerariaefolium by a Blend of Volatiles Emitted from Artificially Damaged Conspecific Plants

Plants emit specific blends of volatile organic compounds (VOCs) in response to mechanical wounding. Such induced VOCs have been shown to mediate in plant and interplant communication, yet little is known about the time- and dose-response relationships in VOC-mediated communications. Here, we employed young seedlings of Chrysanthemum cinerariaefolium to examine the effects of volatiles emitted by artificially damaged seedlings on the biosynthesis of the natural insecticides pyrethrins in intact conspecific plants. Wounded leaves emitted (Z)-3-hexenal, (E)-2-hexenal, (Z)-3-hexen-1-ol, (Z)-3-hexen-1-yl acetate and (E)-β-farnesene as dominant wound-induced VOCs. Exposing intact seedlings to a mixture of these VOCs at concentrations mimicking those emitted from wounded seedlings, as well as placing the intact seedlings next to the wounded seedlings, resulted in enhanced pyrethrin contents in the intact seedlings. Thus we quantified mRNA transcripts of 1-deoxy-D-xylulose 5-phosphate synthase (DXS), chrysanthemyl diphosphate synthase (CPPase), 13-lipoxygenase (13-LOX) and allene oxide synthase (AOS) genes in intact seedlings exposed to the VOC mixture to show that DXS and 13-LOX gene expression reached a maximum at 3 h, whereas CPPase and AOS reached it at 6 h. Interestingly, both increasing and decreasing the VOC mixture concentrations from those observed on injury reduced the expression of DXS, CPPase and AOS genes to the control level. Also, separating the VOC mixture into individual components eliminated the ability to enhance the expression of all the biosynthetic genes examined. This is the first study showing that the wound-induced VOCs function as a blend to control the biosynthesis of second metabolites at specific concentrations.