title: Tomato aromatic amino acid decarboxylases participate in synthesis of the flavor volatiles 2-phenylethanol and 2-phenylacetaldehyde.
abstract: An important phenylalanine-derived volatile compound produced by plants is 2-phenylethanol. It is a major contributor to flavor in many foods, including fresh fruits, such as tomato, and an insect-attracting scent in roses and many other flowers. Despite the centrality of 2-phenylethanol to flavor and fragrance, the plant genes responsible for its synthesis have not been identified. Here, we describe a biosynthetic pathway for 2-phenylethanol and other phenylalanine-derived volatiles in tomato fruits and a small family of decarboxylases (LeAADC1A, LeAADC1B, and LeAADC2) that can mediate that pathway's first step. These enzymes each catalyze conversion of phenylalanine to phenethylamine and tyrosine to tyramine. Although tyrosine is the preferred substrate in vitro, phenylalanine levels in tomato fruits far exceed those of tyrosine, indicating that phenylalanine is a physiological substrate. Consistent with this view, overexpression of either LeAADC1A or LeAADC2 in transgenic tomato plants resulted in fruits with up to 10-fold increased emissions of the products of the pathway, including 2-phenylacetaldehyde, 2-phenylethanol, and 1-nitro-2-phenylethane. Further, antisense reduction of LeAADC2 significantly reduced emissions of these volatiles. Besides establishing a biosynthetic route, these results show that it is possible to change phenylalanine-based flavor and aroma volatiles in plants by manipulating expression of a single gene.
authors.affiliation: Department of Horticultural Sciences, University of Florida, P.O. Box 110690, Gainesville, FL 32611-0690, USA.
authors.forename: Denise
authors.initials: D
authors.lastname: Tieman
authors.forename: Mark
authors.initials: M
authors.lastname: Taylor
authors.forename: Nicolas
authors.initials: N
authors.lastname: Schauer
authors.forename: Alisdair R
authors.initials: AR
authors.lastname: Fernie
authors.forename: Andrew D
authors.initials: AD
authors.lastname: Hanson
authors.forename: Harry J
authors.initials: HJ
authors.lastname: Klee
mesh_terms: D000079:Acetaldehyde / Q000031:analogs & derivatives* / Q000737:chemistry; D001142:Aromatic-L-Amino-Acid Decarboxylases / Q000502:physiology*; D008401:Gas Chromatography-Mass Spectrometry; D018551:Solanum lycopersicum / Q000201:enzymology*; D008956:Models, Chemical; D010649:Phenylalanine / Q000737:chemistry; D010626:Phenylethyl Alcohol / Q000737:chemistry*; D030821:Plants, Genetically Modified; D012313:RNA / Q000378:metabolism; D013649:Taste
chemical_list: D010649:Phenylalanine; D012313:RNA; D001142:Aromatic-L-Amino-Acid Decarboxylases; D000079:Acetaldehyde; D010626:Phenylethyl Alcohol; C013192:phenylacetaldehyde