Abstract
During fruit ripening, strawberries show distinct changes in the flavonoid classes that accumulate, switching from the formation of flavan 3-ols and flavonols in unripe fruits to the accumulation of anthocyanins in the ripe fruits. In the common garden strawberry (Fragaria×ananassa) this is accompanied by a distinct switch in the pattern of hydroxylation demonstrated by the almost exclusive accumulation of pelargonidin based pigments. In Fragaria vesca the proportion of anthocyanins showing one (pelargonidin) and two (cyanidin) hydroxyl groups within the B-ring is almost equal. We isolated two dihydroflavonol 4-reductase (DFR) cDNA clones from strawberry fruits, which show 82% sequence similarity. The encoded enzymes revealed a high variability in substrate specificity. One enzyme variant did not accept DHK (with one hydroxyl group present in the B-ring), whereas the other strongly preferred DHK as a substrate. This appears to be an uncharacterized DFR variant with novel substrate specificity. Both DFRs were expressed in the receptacle and the achenes of both Fragaria species and the DFR2 expression profile showed a pronounced dependence on fruit development, whereas DFR1 expression remained relatively stable. There were, however, significant differences in their relative rates of expression. The DFR1/DFR2 expression ratio was much higher in the Fragaria×ananassa and enzyme preparations from F.×ananassa receptacles showed higher capability to convert DHK than preparations from F. vesca. Anthocyanin concentrations in the F.×ananassa cultivar were more than twofold higher and the cyanidin:pelargonidin ratio was only 0.05 compared to 0.51 in the F. vesca cultivar. The differences in the fruit colour of the two Fragaria species can be explained by the higher expression of DFR1 in F.×ananassa as compared to F. vesca, a higher enzyme efficiency (K cat/K m values) of DFR1 combined with the loss of F3’H activity late in fruit development of F.×ananassa.
Highlights
The strawberry is an appealing plant model for studying flavonoid metabolism during fruit development, as there is a change in the flavonoid classes and in their B-ring hydroxylation patterns
To investigate whether dihydroflavonol 4reductases (DFR) substrate specificity could contribute to the establishment of strawberry fruit anthocyanin hydroxylation patterns, we studied DFR in two species of Fragaria
Specific primers were designed for the two DFR variant and used for the isolation of cDNA clones from early and late developmental stages of fruits of F.6ananassa cv
Summary
The strawberry is an appealing plant model for studying flavonoid metabolism during fruit development, as there is a change in the flavonoid classes and in their B-ring hydroxylation patterns. In the F. vesca the ratio of anthocyanins possessing one (pelargonidin type) and two (cyanidin type) hydroxyl groups in the B-ring is almost equal, whereas pelargonidin type anthocyanins are prevalent in the F.6ananassa. This is frequently reflected in fruit colouration (Figure S1 in File S1) [1,4]. To investigate whether DFR substrate specificity could contribute to the establishment of strawberry fruit anthocyanin hydroxylation patterns, we studied DFR in two species of Fragaria
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