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-attaching the chain to a glucan chain by an alpha -1,6 bond, altering starch structure. SBEs also facilitate starch accumulation by increasing the number of non-reducing ends on the growing chain. In maize (Zen mays), three isoforms of SEE have been identified. To examine the function of the SBEIIa isoform, 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 sequence of Sbe2a containing the promoter and 5 ' end was isolated anal sequenced. Plants homozygous for sbe2a::Mu have undetectable levels or Sbe2a transcripts and SBEIIa in their leaves. Characterization of loaf starch from sbe2a::Mu mutants shows reduced branching similar to yet more extreme than that seen in kernels lacking SBEIIB activity. Characterization of endosperm starch 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 within Sbe2a. This correlation suggests a specific role for SBEIIa in leaves, which may be necessary far normal plant development.