Comprehensive Analysis of Metabolic Fluxes from Leucoanthocyanins to Anthocyanins and Proanthocyanidins (PAs).

Anthocyanins and PAs are the two most common flavonoids, which are widely present among diverse species. Great progress has been made in their synthesis and regulation. In this study, we analyzed the metabolic fluxes from their synthetic precursor leucoanthocyanins, which were obtained by overexpression of dihydroflavonol 4-reductase (DFR) in vitro and in vivo. The unstable product leucocyanidin generated in the CsDFRa enzymatic reaction was easily converted into C-type carbocations under weak acidic conditions, which could be further involved in the synthesis of C-type PAs in vitro. Additionally, the metabolites in tobacco overexpressing CsDFRa and Arabidopsis thaliana DFR and anthocyanidin synthase (ANS) mutants were investigated. In CsDFRa transgenic tobacco, the content of anthocyanins in the petals was greatly increased, but no catechin or PA was detected. In A. thaliana, EC-type carbocation was mainly accumulated in the wild type (WT), and the C-type carbocation was only detected in the ans mutant. In tea plant, the accumulation of C-type PAs is strong positively correlated with the expression of CsDFRa. In summary, leucocyanidin is not only involved in the synthesis of downstream anthocyanin and epicatechin but also can be converted into C-type carbocation to participate in the synthesis of C-type PAs. Hence, from leucocyanidin, three metabolic fluxes were formed toward catechin, cyanidin, and C-type carbocation. These results enriched the metabolic fluxes of leucoanthocyanins and further elaborated the roles of DFR in the process of C-type PA formation.