Integrating biochemical and genomic approaches to elucidate C6-C2 volatile production: improvement of floral scent and fruit aroma

Abstract

The specific objectives of approved proposal include to: 1. Elucidate the C6-C2 biochemical pathways leading to the biosynthesis of phenylacetaldehyde, phenylethyl alcohol and phenylethyl acetate in floral tissues of ornamentally important plants, pefunia and roses. 2. Isolate and characterrze genes responsible for the production of these C6-C2 compounds and those involved in the regulation of the pathway using genomic and transcriptomic tools. 3. Determine whether altering the expression of key genes of this pathway can result in changing the aroma characteristics of flowers. Aldehydes are intermediates in a variety of biochemical pathways including those involved in the metabolism of carbohydrates, vitamins, steroids, amino acids, benzylisoquinoline alkaloids, hormones, and lipids. In plants they are also synthesized in response to environmental stresses such as salinity, cold, and heat shock or as flavors and aromas in fruits and flowers. Phenylacetaldehyde along with 2-phenylethanol and its acetate ester, are important scent compounds in numerous flowers, including petunias and roses. However, little is known about the biosynthesis of these volatile compounds in plants. We have shown that the formation PHA and 2-phenylethanol from Phe does not occur via trans-cinnamic acid and instead competes with the key enzyme of phenypropanoid metabolism Pheammonia-lyase (PAL) for Phe utilization. Using functional genomic approach and comparative gene expression profiling, we have isolated and characterized a novel enzyme from petunia and rose flowers that catalyzes the formation of the Ca-Czcompound phenylacetaldehyde (PHA) from L-phenylalanine (Phe) by the removal of both the carboxyl and amino groups. This enzyme, designated as phenylacetaldehyde synthases (PAAS), is a bifunctional enzyme that catalyzes the unprecedented efficient coupling of phenylalanine decarboxylation to oxidation, generating phenylacetaldehyde, CO2, ammonia, and hydrogen peroxide in stoichiometric amounts. Down-regulation of PAAS expression via RNA interference-based (RNAi) technology in petunia resulted in no PHA emission when compared with controls. These plants also produced no 2-phenylethanol, supporting our conclusion that PHA is a precursor of 2-phenylethanol. To understand the regulation of scent formation in plants we have also generated transgenic petunia and tobacco plants expressing the rose alcohol acetyltransferase (RhAAT) gene under the control of a CaMV-35S promoter. Although the preferred substrate of RhAAT in vitro is geraniol, in transgenic petunia flowers, it used phenylethyl alcohol and benzyl alcohol to produce the corresponding acetate esters, not generated by control flowers. These results strongly point to the dependence of volatile production on substrate availability. Analysis of the diurnal regulation of scent production in rose flowers revealed that although the daily emission of most scent compounds is synchronized, various independently evolved mechanisms control the production, accumulation and release of different volatiles. This research resulted in a fundamental discovery of biochemical pathway, enzymes and genes involved in biosynthesis of C6-C2s compounds, and provided the knowledge for future engineering plants for improved scent quality.