Deficiency of maize starch-branching enzyme i results in altered starch fine structure, decreased digestibility and reduced coleoptile growth during germination

Endosperm starch granules from the sbe1a mutant were more resistant to digestion by pancreatic α-amylase, and the sbe1a mutant starch had an altered branching pattern for amylopectin and amylose. When kernels were germinated, the sbe1a mutant was associated with shorter coleoptile length and higher residual starch content, suggesting that less efficient starch utilization may have impaired growth during germination.The present report documents for the first time a molecular phenotype due to the absence of SBEI, and suggests strongly that it is associated with altered physiological function of the starch in vivo. We believe that these results provide a plausible rationale for the conservation of SBEI in plants in both monocots and dicots, as greater seedling vigor would provide an important survival advantage when resources are limited.The starch granule is a highly-ordered structure with alternating crystalline and amorphous growth rings [1,2]. Starch molecules are biopolymers of anhydroglucose units linked by α-1,4 and α-1,6 glycosidic bonds. They are composed of two glucan polymers, the generally linear fraction, amylose, and the branched fraction, amylopectin. The constituent amylopectin chains can be mainly categorized into A chains (not bearing any branches) and B chains (bearing one or more branches) [3]. The main physiological functions of starch include high-density storage of energy and the controlled release of this energy during starch degradation.Starch-branching enzyme (SBE) plays an important role in starch biosynthesis by introducing branch points, the α-1,6 linkages in starch. Boyer and Preiss [4] identified three major SBE isoforms in developing maize kernels: SBEI, SBEIIa, and SBEIIb. The SBE isoforms have been shown to be encoded by different genes [5-8]. Phylogenetic analyses of SBE sequences from a number of plant species have shown that the SBEI and SBEII isoforms are conserved among most plants, and that SBEIIa and SBEIIb isoforms are conserve