Identification of Mutator insertional mutants of starch-branching enzyme 2a in corn

Starch-branching enzymes (SBE) break the alpha -1,4 linkage of starch, re-a ttaching the chain to a glucan chain by an alpha -1,6 bond, altering starch structure. SBEs also facilitate starch accumulation by increasing the numb er of non-reducing ends on the growing chain. In maize (Zen mays), three is oforms of SEE have been identified. To examine the function of the SBEIIa i soform, a reverse genetics polymerase chain reaction-based screen was used to identify a mutant line segregating for a Mutator transposon within Sbe2a . To locate the insertion within the second exon of Sbe2a, the genomic sequ ence of Sbe2a containing the promoter and 5 ' end was isolated anal sequenc ed. Plants homozygous for sbe2a::Mu have undetectable levels or Sbe2a trans cripts and SBEIIa in their leaves. Characterization of loaf starch from sbe 2a::Mu mutants shows reduced branching similar to yet more extreme than tha t seen in kernels lacking SBEIIB activity. Characterization of endosperm st arch from sbe2a::Mu mutants shows branching that is indistinguishable from wild-type controls. These mutant plants have a visible phenotype resembling accelerated senescence, which was correlated with the Mutator insertion wi thin Sbe2a. This correlation suggests a specific role for SBEIIa in leaves, which may be necessary far normal plant development.