Molecular cloning and characterization of a gene encoding soluble starch synthase III (SSSIII) in Lotus (Nelumbo nucifera)

Soluble starch synthase (SSS) is a key enzyme in the biosynthesis of plant amylopectin. To reveal the starch synthesis pathway in lotus, we cloned a gene named NnSSSIII from Nelumbo nucifera in this study. A full-length cDNA of soluble starch synthase (SSS), designated as NnSSSIII, which was 4070 bp (GenBank accession NO. KR062069) containing a 3696 bp open reading frame and encoding a protein of 1231 amino acid. Twenty SNPs of NnSSSIII were found from three breeds of lotus, which resulted in 13 coding amino acid changes. The complete genomic structure of NnSSSIII comprises 17 exons and 16 introns and covers a successive region of 70.6 kb. Expression pattern and enzyme activity assay showed that NnSSSIII gene is mostly expressed in leaves and rhizomes at the time between 8th to 14th weeks after germination. Our results indicated that NnSSSIII may paly an important role in transient starch synthesis of photosynthetic tissue and storage starch synthase of storage tissue at the total developmental stages. In addition, characterization of a genetic polymorphism of NnSSSIII cDNA contributed more molecular markers about the SSSIII gene in lotus. This study elucidates more molecular information about the NnSSSIII gene and provided guidance for future improvement of starch quality and content in lotus.