Identification of Mutator Insertional Mutants of Starch-Branching Enzyme 2a in Corn

Starch-branching enzymes (SBE) break the α-1,4 linkage of starch, re-attaching the chain to a glucan chain by an α-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 (Zea mays), three isoforms of SBE 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 and sequenced. Plants homozygous for sbe2a::Mu have undetectable levels of Sbe2a transcripts and SBEIIa in their leaves. Characterization of leaf 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 for normal plant development

Why do gelatinized starch granules not dissolve completely? Roles for amylose, protein, and lipid in granule "Ghost" integrity

After gelatinization in water, starch granules persist in swollen hydrated forms known as ghosts. Three potential mechanisms for ghost formation are tested. Proteins and lipids on the granule surface are found to be a determinant of ghost robustness, but not ghost formation. Proteins inside pre-made maize or wheat starch ghosts are degraded extensively by proteases without any apparent change in ghost properties, making an internal protein cross-linking mechanism unlikely. Waxy maize mutants with a range of amylose contents have ghost integrities that correlate with (low) apparent amylose levels. It is hypothesized that ghost formation is due to cross-linking of polysaccharide chains within swollen granules, most likely involving double helices formed from polymer chains that become free to move following heat-induced granule swelling. The size and robustness of granule ghosts is proposed to be determined by the relative rates of swelling and cross-linking, modulated by surface non-polysaccharide components