The lack of floral synthesis and emission of isoeugenol inPetunia axillarissubsp.parodiiis due to a mutation in theisoeugenol synthasegene

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Floral scent has been extensively investigated in plants of the South American genusPetunia. Flowers ofPetunia integrifoliaemit mostly benzaldehyde, while flowers ofPetunia axillarissubsp.axillarisemit a mixture of volatile benzenoid and phenylpropanoid compounds that include isoeugenol and eugenol. Flowers of the artificial hybridPetunia hybrida, a cross betweenP. integrifoliaandP. axillaris, emit a similar spectrum of volatiles asP. axillarissubsp.axillaris. However, the flowers ofP. axillarissubsp.parodiiemit neither isoeugenol nor eugenol but contain high levels of dihydroconiferyl acetate in the petals, the main scent-synthesizing and scent-emitting organs. We recently showed that both isoeugenol and eugenol inP. hybridaare biosynthesized from coniferyl acetate in reactions catalyzed by isoeugenol synthase (PhIGS1) and eugenol synthase (PhEGS1), respectively, via a quinone methide-like intermediate. Here we show thatP. axillarissubsp.parodiihas a functionalEGSgene that is expressed in flowers, but itsIGSgene contains a frame-shift mutation that renders it inactive. Despite the presence of active EGS enzyme inP. axillarissubsp.parodii, in the absence of IGS activity the coniferyl acetate substrate is converted by an as yet unknown enzyme to dihydroconiferyl acetate. By contrast, suppressing the expression ofPhIGS1inP. hybridaby RNA interference also leads to a decrease in isoeugenol biosynthesis, but instead of the accumulation of dihydroconiferyl acetate, the flowers synthesize higher levels of eugenol.

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