Regulation of starch granule-bound starch synthase I gene expression by circadian clock and sucrose in the source tissue of sweet potato

Studies on regulations of transitory starch synthesis and degradation in the leaf tissue are important for understanding how carbons could be distributed effectively from the source to sink tissues. Therefore, expressions of starch granule-bound starch synthase I (GBSSI) gene in leaves of sweet potato were studied under different photoperiodic conditions and various sugar treatments. Results indicated that accumulations of GBSSI mRNA and its protein were controlled by an endogenous biological clock. Starch accumulations in leaves also showed a pattern characteristic of circadian rhythm. In addition to circadian clock, sucrose also played an important role in regulating GBSSI mRNA accumulations. Although sucrose stimulated the transcription of GBSSI, it had no effect on the rhythmic pattern of GBSSI gene expressions. Protein phosphorylation/dephosphorylation were involved in the sucrose-related signal transduction for GBSSI gene expressions. However, the sugar sensing for regulation of GBSSI was independent of the hexokinase-mediated pathway. In conclusion, the GBSSI gene expression in leaves of sweet potato appears to be regulated by two independent pathways. First, light is responsible for setting up biological clock(s) that control the circadian expression of GBSSI gene; and second, light plays an indirect signal to enhance GBSSI mRNA accumulations mediated by the photosynthetic product, sucrose.