Distribution and biosynthesis of flavan-3-ols in Camellia sinensis seedlings and expression of genes encoding biosynthetic enzymes

Abstract

The distribution of phenolic compounds in young and developing leaves, stems, main and lateral roots and cotyledons of 8-week-old tea (Camellia sinensis) seedlings was investigated using HPLC-MS(2). Fourteen compounds, flavan-3-ols, chlorogenic acids, and kaempferol-O-glycosides, were identified on the basis of their retention time, absorbance spectrum, and MS fragmentation pattern. The major phenolics were (-)-epigallocatechin-3-O-gallate and (-)-epicatechin-3-O-gallate, located principally in the green parts of the seedlings. Considerable amounts of radioactivity from [ring-(14)C]phenylalanine were incorporated in (-)-epicatechin, (-)-epigallocatechin, (-)-epicatechin-3-O-gallate and (-)-epigallocatechin-3-O-gallate, by tissues of young and developing leaves and stems. Expression of genes encoding enzymes involved in flavan-3-ol biosynthesis, CHS, CHI, F3H, F3'5'H, DFR, ANS, ANR and LAR was investigated. Transcripts of all genes, except LAR, were more abundant in leaves and stems than in roots and cotyledons. No significant difference was found in the amount of transcript of LAR. These findings indicate that in tea seedlings flavan-3-ols are produced by a naringenin-chalcone-->naringenin-->dihydrokaempferol pathway. Dihydrokaempferol is a branch point in the synthesis of (-)-epigallocatechin-3-O-gallate and other flavan-3-ols which can be formed by routes beginning with either a flavonoid 3'-hydroxylase mediated conversion of the flavonol to dihydroquercetin or a flavonoid 3',5'-hydroxylase-catalysed conversion to dihydromyricetin with subsequent steps involving sequential reactions catalysed by dihydroflavanol 4-reductase, anthocyanidin synthase, anthocyanidin reductase and flavan-3-ol gallate synthase.