Functional analyses of cytosolic glucose-6-phosphate dehydrogenases and their contribution to seed oil accumulation in Arabidopsis.

Glucose-6-phosphate dehydrogenase (G6PDH) has been implicated in the supply of reduced nicotine amide cofactors for biochemical reactions and in modulating the redox state of cells. In plants, identification of its role is complicated due to the presence of several isoforms in the cytosol and plastids. Here we focus on G6PDHs in the cytosol of Arabidopsis (Arabidopsis thaliana) using single and double mutants disrupted in the two cytosolic G6PDHs. Only a single G6PDH isoform remained in the double mutant and was present in chloroplasts, consistent with a loss of cytosolic G6PDH activity. The activities of the cytosolic isoforms G6PD5 and G6PD6 were reciprocally increased in single mutants with no increase of their respective transcript levels. We hypothesized that G6PDH plays a role in supplying NADPH for oil accumulation in developing seeds in which photosynthesis may be light limited. G6PDH activity in seeds derived from G6PD6 and a plastid G6PDH isoform and showed a similar temporal activity pattern as oil accumulation. Seeds of the double mutant but not of the single mutants had higher oil content and increased weight compared to those of the wild type, with no alteration in the carbon to nitrogen ratio or fatty acid composition. A decrease in total G6PDH activity was observed only in the double mutant. These results suggest that loss of cytosolic G6PDH activity affects the metabolism of developing seeds by increasing carbon substrates for synthesis of storage compounds rather than by decreasing the NADPH supply specifically for fatty acid synthesis.