Differential Production of meta Hydroxylated Phenylpropanoids in Sweet Basil Peltate Glandular Trichomes and Leaves Is Controlled by the Activities of Specific Acyltransferases and Hydroxylases

Abstract

Abstract Sweet basil (Ocimum basilicum) peltate glandular trichomes produce a variety of small molecular weight phenylpropanoids, such as eugenol, caffeic acid, and rosmarinic acid, that result from meta hydroxylation reactions. Some basil lines do not synthesize eugenol but instead synthesize chavicol, a phenylpropanoid that does not contain a metahydroxyl group. Two distinct acyltransferases,p-coumaroyl-coenzyme A:shikimic acidp-coumaroyl transferase andp-coumaroyl-coenzyme A:4-hydroxyphenyllactic acidp-coumaroyl transferase, responsible for the production of p-coumaroyl shikimate and ofp-coumaroyl 4-hydroxyphenyllactate, respectively, were partially purified and shown to be specific for their substrates.p-Coumaroyl-coenzyme A:shikimic acidp-coumaroyl transferase is expressed in basil peltate glands that are actively producing eugenol and is not active in glands of noneugenol-producing basil plants, suggesting that the levels of this activity determine the levels of synthesis of some meta-hydroxylated phenylpropanoids in these glands such as eugenol. Two basil cDNAs encoding isozymes of cytochrome P450 CYP98A13, whichmeta hydroxylates p-coumaroyl shikimate, were isolated and found to be highly similar (90% identity) to the Arabidopsis homolog, CYP98A3. Like the Arabidopsis enzyme, the basil enzymes were found to be very specific for p-coumaroyl shikimate. Finally, additional hydroxylase activities were identified in basil peltate glands that convert p-coumaroyl 4-hydroxyphenyllactic acid to its caffeoyl derivative andp-coumaric acid to caffeic acid.