Floral Aldehyde Dehydrogenases: Biochemistry and Involvement in Scent Production

Plants use floral and fruit volatiles as chemical cues to interact with their environment by attracting pollinators and seed dispersers, thus ensuring plant reproductive success. These volatiles also have a significant economic value as they contribute directly to the quality, and indirectly to the yield, of crops. Arising from diverse biochemical pathways, floral and fruit volatiles can be divided into four major classes according to their metabolic origin: benzenoids/phenylpropanoids, terpenoids, fatty acid derivatives and amino acid derivatives. Benzenoids are of particular interest in the floral scent of snapdragon and petunia flowers where methylbenzoate is a dominant volatile arising from benzoic acid (BA). Benzoic acid (BA) is also an important building block in a wide spectrum of compounds from primary metabolites to secondary products. Benzoic acid biosynthesis from L-phenylalanine requires shortening the propyl side chain by two carbons, which can occur via a β-oxidative pathway or a non-β-oxidative pathway with benzaldehyde (BD) as a key intermediate. The non-β-oxidative route requires benzaldehyde dehydrogenase (BALDH) to convert BD to BA. Using a functional genomics approach, we identified an Antirrhinum majus (snapdragon) BALDH, which exhibits 40% identity to bacterial BALDH. Transcript profiling, biochemical characterization of the purified recombinant protein, molecular homology modeling, in vivo stable isotope labeling, and transient expression in petunia flowers reveal that BALDH is capable of oxidizing BD to BA in vivo. GFP localization and immunogold labeling studies show that this biochemical step occurs in mitochondria, raising a question about the role of subcellular compartmentalization in BA biosynthesis. To determine if a similar enzyme is responsible for BA formation in petunia flowers, we isolated and analyzed three ALDH candidates for expression in limb tissue. One candidate, whose recombinant protein had activity with BD, was down-regulated by RNAi. However, the resulting emission profile in these plants was inconsistent with its function as a BALDH