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

Abstract

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 (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.

The Effect of the Amount of Nitrogen Fertilizer on Starch Metabolism in Leaf Sheath of Japonica and Indica Rice Varieties during the Heading Period

“…In rice plants, although the expression level of RBE4 was similar to that of RBE1 in the leaves, RBE1 was expressed more intensively in seeds than in leaves and stems, while RBE4 was expressed in the leaves as well as in the developing seeds Mizuno et al, 2001). In Mutator insertional maize mutant of BEIIa type starch branching enzyme, to which RBE4 belongs, short chains of amylopectin were much diminished in leaf starch, but there was no difference in the chainprofi le for amylopectin of kernel starch between the mutant and wild-type (Blauth et al, 2001). These results suggest that RBE4 may play a crutial role in the changes in the amount of starch accumulation and the SBE activity to the nitrogen conditions in the rice leaf sheath.…”

Starch accumulation, activities of key enzyme and gene expression in starch synthesis of wheat endosperm with different starch contents

“…And SSS is affected by temperature and heat shock treatment resulted in a significant decline in starch content (Asthir and Bhatia 2011). SBE is the only enzyme that can introduce α-(1, 6) glucosidic linkages intoα-polyglucans, and therefore, it plays a crucial role in the biosynthesis of amylopectin (Blauth et al 2001(Blauth et al , 2002Kim et al 1998). DBE catalyzes the hydrolysis of α-(1, 6) linkages.…”

A review of starch‐branching enzymes and their role in amylopectin biosynthesis

“…Maize SBEIIa mutants display a clear phenotype in leaf starch, but no measurable alteration in storage starch (37), suggesting a more important role for SBEIIa in transient starch synthesis in leaves but no critical role in endosperm amylopectin biosynthesis or one that is compensated by other SBEs. Studies with SBEIIa mutants of maize suggest that the isoform plays an important role in influencing starch structure to facilitate nocturnal degradation.…”

“…In developing wheat endosperm, SBEIIb is expressed at much lower levels than SBEIIa (36), whereas in maize endosperm, SBEIIb predominates, being expressed at 50 times the level of the IIa form (35). SBEIIb is the most abundant protein in maize endosperm amyloplasts, whereas in leaf chloroplasts, the major BE isoform is SBEIIa (37,38). The analysis of mutant plants deficient in BE activity has shed much light on the roles of the various isoforms of SBE in amylopectin synthesis.…”

“…Production of high-amylose wheat starch involves suppression of both genes encoding SBEIIa and SBEIIb, resulting in starches containing >70% amylose, as suppression of SBEIIb alone caused no alteration in amylose content (43), and in potato, a high-amylose starch was produced by downregulation of both SBE isoforms (44). Interestingly, loss of SBEI activity in monocots and dicots has minimal apparent effects on starch synthesis and composition (45), although in a study by Yao et al, loss of SBEI from a SBEIIbdeficient background in maize was shown to cause increased branching of amylopectin. Yao et al (46) postulated that this result was suggestive of a regulatory role for SBEI in influencing other SBE isoforms.…”