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.
植物制造的一种重要的苯丙氨酸衍生挥发性化合物是2-苯乙醇。它是许多食物的味道的主要贡献者,包括新鲜水果,如番茄,以及玫瑰和许多其他花卉的吸引昆虫的气味。尽管2-苯乙醇对于风味和香味至关重要,但其合成的植物基因尚未被确定。在这里,我们描述了番茄果实中2-苯乙醇和其他苯丙氨酸衍生挥发物的生物合成途径,以及可以介导该途径第一步的一小组脱羧酶(LeAADC1A,LeAADC1B和LeAADC2)。这些酶分别催化苯丙氨酸转化为苯乙胺和酪氨酸转化为酪胺。尽管酪氨酸是体外首选底物,但番茄果实中苯丙氨酸的水平远高于酪氨酸,表明苯丙氨酸是一种生理底物。与此观点一致的是,转基因番茄植物中LeAADC1A或LeAADC2的过表达导致途径产物的排放增加了多达10倍,包括2-苯乙醛、2-苯乙醇和1-硝基-2-苯乙烷。此外,LeAADC2的反义减少显着降低了这些挥发物的排放。除了建立一种生物合成途径外,这些结果还表明,通过操纵单个基因的表达,可以改变植物中基于苯丙氨酸的风味和香气挥发物。