The gene encoding starch synthase IIc exists in maize and wheat

Two starch synthase II (SSII) cDNAs, ZmSSIIc and TaSSIIc, each containing an open reading frame of 2328 and 2229 bp in length, were cloned from maize and wheat, respectively. Both ZmSSIIc and TaSSIIc contain the putative ADP-Glc binding motif of KXGGL, but lack the conserved motif of PLAGXNVMNX that was detected in other SSIIa and SSIIb proteins. The deduced amino acid sequences of the ZmSSIIc and TaSSIIc shared 49.8–88.9% identity with those of other SSIIs. Phylogenetic analysis indicated that genes encoding SSIIc were classified into a new SSII gene subfamily in higher plants, which is different from dicot SSII subfamily and monocot SSIIa and SSIIb subfamily. The C-terminal catalytic domain of both ZmSSIIc and TaSSIIc expressed in Escherichia coli, and the activity of their starch synthase was confirmed. RT-PCR revealed that ZmSSIIc and TaSSIIc were expressed in leaves, roots and endosperm, and their transcripts reached maximum level at the middle developmental age of endosperm after pollination, while declined gradually over time. The role of SSIIc in transitory and storage starch synthesis in Gramineae will be discussed.