Minisymposium 3: Secondary metabolism

Abs # 13001: Identification of genes involved in tomato volatile synthesis.

Presenter:	Denise, Tieman M, dmtieman@mail.ifas.ufl.edu
Authors	Denise, Tieman M (A)   Schmelz, Eric A (C)   Loucas, Holly M (B)   Clark, David G (B)   Hanson, Andrew D (A)   Klee, Harry J (A)
Affiliations:	(A): Universit y of Florida, Horticultural Sciences Department, Gainesville, FL (B): University of Florida, Environmental Horticulture Department, Gainesville, FL (C): USDA Center for Medical, Agricultural, and Veterinary Entomology, Gainesville, FL

The volatile compounds phenylacetaldehyde and 2-phenylethanol are important for the aroma and flavor of ripe tomato fruit. We have identified an L. pennellii introgression line (IL 8-2-1) with greatly increased levels of these compounds in ripe fruit. IL8-2-1 fruit have a distinct floral (rose-like) aroma consistent with the floral aromas of phenylacetaldehyde and 2-phenylethanol. We have used these lines as tools to elucidate the biochemical pathway for the synthesis of phenylacetaldehyde and 2-phenylethanol from phenylalanine. We have identified an aromatic amino acid decarboxylase that converts phenylalanine to phenethylamine, a potential intermediate in the pathway to phenylacetaldehyde and phenylethanol. Tomato fruit pericarp disks fed ¹⁴C-labeled phenylalanine produced ¹⁴C-phenethylamine, and this activity was higher in IL8-2-1 fruit. Since the conversion of phenylalanine to phenethylamine would likely be catalyzed by an aromatic amino acid decarboxylase, we conducted a database search for tomato homologs of known aromatic amino acid decarboxylases from other species. Two of the proteins encoded by the candidate genes had phenylalanine decarboxylase activity when expressed in E. coli. One of these genes mapped to the IL 8-2-1 L. pennellii introgression, further supporting the role of this gene in the increased levels of phenylacetaldehyde and 2-phenylethanol in the fruits from this line. The last step in the pathway, the conversion of phenylacetaldehyde to phenylethanol is catalyzed by an alcohol dehydrogenase. Microarrays containing approximately 3,000 tomato cDNAs indicated that an alcohol dehydrogenase gene was more highly expressed in IL8-2-1 fruit than in control M82 fruit. The alcohol dehydrogenase had the highest activity on phenylacetaldehyde as a substrate when expressed in E. coli. The tomato 2-phenylethanol dehydrogenase cDNA was introduced into petunia (cv. Mitchell Diploid) under control of a constitutively expressed promoter. Wild-type petunia flowers emit relatively high levels of phenylacetaldehyde and lower levels of 2-phenylethanol. However, transgenic petunia flowers have higher levels of 2-phenylethanol and lower levels of phenylacetaldehyde than wild-type flowers. Overall, these data indicate that the introduction of the transgene results in the conversion of phenylacetaldehyde to 2-phenylethanol in petunia flowers.