Isolation, characterization, and expression analysis of starch synthase IIa cDNA from wheat (Triticum aestivum L.)

We characterized three near-full-length putative homoeologous cDNA (Ss2a-1, Ss2a-2, and Ss2a-3) in wheat endosperm most similar to the maize zSSIIa. P olypeptide sequences deduced from three Ss2a cDNA clones share a 95% overal l sequence similarity, and may thus have similar biochemical properties and may make identical contributions to starch biosynthesis in wheat endosperm . The accumulation of RNA transcripts corresponding to three Ss2a genes in developing endosperm varies among three cultivars studied, but usually peak s in young endosperm at about 10 days post anthesis (DPA). The polyclonal a ntibody for the SSIIa-1 recombinant protein strongly reacted to three previ ously identified granule-bound starch synthases of 100 to 115 kDa. The poly clonal antibody for the granule-bound starch synthases strongly reacted to the SSIIa-1 recombinant protein. Sequences of the N-terminal and an interna l peptide of these three granule-bound starch synthases match well with tho se of three predicted mature SSIIa polypeptides. These granule-bound starch synthases may therefore be SSIIa polypeptides. The antibodies also recogni zed a group of three polypeptides with the same gel mobility as the three g ranule-bound starch synthases, a polypeptide of 90 kDa, and a group of thre e polypeptides of about 80 to 82 kDa. Thus, the wheat SSIIa may exist in se veral functional forms in the stroma of amyloplasts.