The pigment-scent connection: Do mutations in regulatory vs. structural anthocyanin genes differentially alter floral scent production in Ipomoea purpurea?

Many recent studies attempting to assess the biochemical connections between anthocyanin biosynthesis and floral scent production have yielded limited insights due in part to a focus on either field phenotypes of unknown genetic background or artificial genetic manipulation. In this study, we seek to more precisely explore the mechanistic connections between floral scent and color in Ipomoea purpurea by comparing inbred lines of wild-type purple flowered plants to lines of two naturally occurring color mutants: albino individuals created by a chalcone synthase (A locus) loss-of-function mutation and rayed individuals that result from a non-functional transcription factor (W locus). We found that I. purpurea floral scent is dominated by the two sesquiterpene hydrocarbons, (E)-β-caryophyllene and germacrene D, with small amounts of several other sesquiterpenoid compounds. These 15 carbon volatiles are derived from the mevalonic acid biosynthetic pathway, which has no structural precursor relationship with anthocyanin pigments. Thus, there is no direct pleiotropic relationship and, accordingly, we found no differences in overall scent production between purple-flowered and albino individuals. In contrast, rayed plants showed greater emission of several compounds when compared to their wild-type counterparts, suggesting that the specific mutant regulatory region in this phenotype could have an indirect effect on volatile production either through changes to overall metabolic flux or alteration of sesquiterpene synthase gene expression or enzyme activity. Future research should explore these possible roles for transcription factors across multiple biochemical pathways. There were no differences in floral scent composition or emission rate between the offspring of parents from the same line, suggesting that scent phenotype was conserved within each inbred line. However, there were differences in floral scent between inbred lines, suggesting that a number of genetic elements must contribute to overall scent production in this species.