Three orthologs in rice, Arabidopsis, and Populus encoding starch branching enzymes (SBEs) are different from other SBE gene families in plants

Starch branching enzymes (SBEs) play important roles in plant starch synthesis. Three orthologs encoding SBEs in rice, Arabidopsis thaliana, and Populus trichocarpa are described. Putative amino acid sequences of these three SBE genes show ∼ 30% identity to those of SBEI and SBEII from plants such as maize, barley, and wheat. More interestingly, they share ∼ 31% amino acid sequence identity with those of glycogen-branching enzymes from such animals as mouse, horse, and monkey. The three genes have similar genomic structures, but their structural features are quite different from those of genes of both SBEI and SBEII families in plants. Based on phylogenetic analysis and genomic structure comparison, it is proposed that the three SBE genes represent a new family of SBEs.