cDNA cloning, expression, and characterization of Taro SSII: a novel member of starch synthase II family.

A novel soluble starch synthase II (SSII) gene was isolated from taro (Colocasia esculenta var. esculenta) tubers. This 2939 bp SSII transcript encodes 804 amino acids with a putative transit peptide of 52 residues. It displays 58-63% identity and 63-69% similarity with SSIIs from other sources. Alignment and phylogenetic analyses showed that taro SSII is more closely related with dicot SSIIs than with the monocot ones, though taro is a monocot. The identification of taro SSII clone as starch synthase was confirmed by the expression of its enzymatic activity in Escherichia coli. Genomic DNA blot analysis revealed a single copy or low number copies of SSII in taro. Expression profile showed that more transcript and protein were accumulated in tubers of 597 +/- 37 g fresh weight, that is, a stage of rapid starch synthesis, than tubers of other stages. By Western blot analysis, SSII was found in both soluble and granule bound portions of tuber extracts, and more SSII protein was found in aged leaves than in leaves of other stages. These results suggest that taro SSII is a novel starch synthase for the synthesis of both transit and storage starch.