Molecular characterization and expression of starch granule-bound starch synthase in the sink and source tissues of sweet potato

In an effort to study regulation of starch synthesis in the source and sink tissues, a cDNA clone for starch granule-bound starch synthase (GBSSI), which encodes a 67-kDa protein, was isolated from a cDNA library prepared from tuberous roots of sweet potato (Ipomoea batatas Lam. cv. Tainong 62). This GBSSI protein contains a signal peptide of 77 amino acids, and the mature protein has a molecular mass of about 59 kDa. The mature protein shares 75–85% sequence identity with GBSSIs of dicotyledonous plants and about 70% identity with those of monocotyledons. The sequence of the signal peptide, on the other hand, differs substantially from those of the dicotyledons and monocotyledons studied, although their hydropathic profiles are all similar. This GBSSI gene was well expressed in tuberous roots, leaves, and stems, but not in roots. However, mechanisms involved in regulating the expression of this gene were different between tuberous roots and leaves. In tuberous roots, the synthesis of GBSSI transcript increased coordinately with tuberous root expansion; nevertheless, accumulation rates of GBSSI protein in starch granules remained constant regardless of tuberous root sizes, suggesting an involvement of post-transcriptional regulation for the synthesis of this protein. The levels of GBSSI transcript were investigated in photosynthetic tissues during diurnal cycles, and the results suggest that the transcription of the GBSSI gene in leaves is controlled by the endogenous circadian rhythm.