Light regulation of sucrose-phosphate synthase activity in the freezing-tolerant grass Deschampsia antarctica.

Deschampsia antarctica, a freezing-tolerant grass that has colonized the Maritime Antarctic, has an unusually high content of sucrose (Suc) in leaves, reaching up to 36% of dry weight. Suc accumulation has often been linked with increased activity of sucrose phosphate synthase (SPS; EC: 2.4.1.1.14). SPS, a key enzyme in sucrose biosynthesis, is controlled by an intricate hierarchy of regulatory mechanisms including allosteric modulators, reversible covalent modification in response to illumination, and transcriptional regulation. We hypothesized that during long day conditions in the Antarctic summer D. antarctica can maintain high SPS activity longer by indirect light regulation, thereby leading to a high sucrose accumulation in the leaves. The objectives of this study were to investigate a possible indirect light regulation of SPS activity and the effect of cold and day length on transcriptional and protein level of SPS in D. antarctica. Although SPS activity did not display an endogenous rhythm of activity in continuous light, activation of SPS at the end of the dark period was observed in D. antarctica. This activation of SPS is possibly controlled by covalent modification, because it was inhibited by okadaic acid while the SPS protein level did not significantly change. The highest SPS activity increase was observed after 21 days of cold-acclimation under long day conditions. This increased activity was not related to an increase in SPS gene expression or protein content. High SPS activity in cold long days leading to hyper accumulation of Suc appears to be among the features that permit D. antarctica to survive in the harsh Antarctic conditions.