Abstract
The distribution of flavan-3-ols in various parts of
Camellia sinensis seedlings was investigated along with the expression of genes encoding enzymes associated with the biosynthesis of flavan-3-ols and related phenolic compounds. The main biosynthetic pathways leading to (−)-epigallocatechin-3-
O-gallate and (−)-epicatechin-3-
O-gallate biosynthetic pathways in tea leaves are proposed.
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-
14C]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.