Abstract
Dihydroflavonol-4-reductase (DFR) is a key enzyme in the catalysis of the stereospecific reduction of dihydroflavonols to leucoanthocyanidins in anthocyanin biosynthesis. In the purple sweet potato (Ipomoea batatas Lam.) cv. Ayamurasaki, expression of the IbDFR gene was strongly associated with anthocyanin accumulation in leaves, stems and roots. Overexpression of the IbDFR in Arabidopsis tt3 mutants fully complemented the pigmentation phenotype of the seed coat, cotyledon and hypocotyl. Downregulation of IbDFR expression in transgenic sweet potato (DFRi) using an RNAi approach dramatically reduced anthocyanin accumulation in young leaves, stems and storage roots. In contrast, the increase of flavonols quercetin-3-O-hexose-hexoside and quercetin-3-O-glucoside in the leaves and roots of DFRi plants is significant. Therefore, the metabolic pathway channeled greater flavonol influx in the DFRi plants when their anthocyanin and proanthocyanidin accumulation were decreased. These plants also displayed reduced antioxidant capacity compared to the wild type. After 24 h of cold treatment and 2 h recovery, the wild-type plants were almost fully restored to the initial phenotype compared to the slower recovery of DFRi plants, in which the levels of electrolyte leakage and hydrogen peroxide accumulation were dramatically increased. These results provide direct evidence of anthocyanins function in the protection against oxidative stress in the sweet potato. The molecular characterization of the IbDFR gene in the sweet potato not only confirms its important roles in flavonoid metabolism but also supports the protective function of anthocyanins of enhanced scavenging of reactive oxygen radicals in plants under stressful conditions.
Highlights
Anthocyanins, a class of flavonoids that is responsible for the colors in fruits and most flowers of higher plants, are major watersoluble pigments [1,2]
The anthocyanin levels in the cotyledons could be restored as close as 105% (DFR1) and 97.9% (DFR2) in the WT plants (Figure 3C). Taken together these results demonstrate that the purple sweet potato dihydroflavonol 4-reductase (DFR) gene is fully functional for anthocyanin biosynthesis in Arabidopsis
DFR is a key enzyme in the catalysis of the stereospecific reduction of dihydroflavonols to leucoanthocyanidins that uses NADPH as a cofactor
Summary
Anthocyanins, a class of flavonoids that is responsible for the colors in fruits and most flowers of higher plants, are major watersoluble pigments [1,2]. The substrate dihydroflavonol of DFR can be catalyzed by FLS to produce flavonols, and the leucoanthocyanidins that result from DFR can subsequently be converted to proanthocyanidin by leucoanthocyanidin reductase (LAR) [15,16,17] These are the two key branches in flavonoid pathways. Expression of the maize A1 gene encoding a DFR under the control of the CaMV 35S promoter in the tt mutants could restore the pigmentation within the cotyledon and seed coat under low-nitrogen conditions [24] Due to their crucial role in the flavonoid pathway, various DFR genes have been isolated from other species such as grape (Vitis vinifera), apple (Malus domestica), pear (Pyrus communis), sweet orange (Citrus sinensis) and petunia (Petunia hybrida) [25,26,27,28]. The protective function of anthocyanins in sweet potato was evidenced by enhanced scavenging of reactive oxygen species (ROS) at low temperature
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