Starch branching enzyme IIb in wheat is expressed at low levels in the endosperm compared to other cereals and encoded at a non-syntenic locus

Abstract

Studies of maize starch branching enzyme mutants suggest that the amylose extender high amylose starch phenotype is a consequence of the lack of expression of the predominant starch branching enzyme II isoform expressed in the endosperm, SBEIIb. However, in wheat, the ratio of SBEIIb and SBEIIa expression are inversely related to the expression levels observed in maize and rice. Analysis of RNA at 15 days post anthesis suggests that there are about 4-fold more RNA for SBE IIa than for SBE IIb. The genes for SBE IIa and SBE IIb from wheat are distinguished in the size of the first three exons, allowing isoform-specific antibodies to be produced. These antibodies were used to demonstrate that in the soluble fraction, the amount of SBE IIa protein is two to three fold higher than SBIIb, whereas in the starch granule, there is two to three fold more SBE IIb protein amount than SBE IIa. In a further difference to maize and rice, the genes for SBE IIa and SBE IIb are both located on the long arm of chromosome 2 in wheat, in a position not expected from rice-maize-wheat synteny.

Physicochemical characterization of starch from hexaploid triticale (X TriticosecaleWittmack) genotypes

“…Low starch contents, such as those observed in substituted triticales although with similar amylose contents to the complete triticales, open the possibility that the bread wheat SBEIIb genes might have been silenced with no effect on amylose synthesis as has been observed in wheat (Regina et al, 2005). For years triticale has been successfully used as feed due to its high protein and carbohydrate content (Peña, 1996).…”

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

“…3g). Studies on wheat endosperm demonstrated that SBE-IIb expressed at much lower levels than SBE-IIa (Regina et al 2005), which is in contrast to maize endosperm, where SBEII-b is the predominant form of SBE-II, being expressed at approximately 50 times the level of SBE-IIa (Gao et al 1997). Our results indicated the similar expression profiles for SBE-IIa and SBE-IIb in endosperm, although SBE-IIa exhibited significantly higher expression levels than SBE-IIb.…”

Recent progress toward understanding the role of starch biosynthetic enzymes in the cereal endosperm

“…The catalytic activity of SBEII is regulated by protein phosphorylation, as inferred from wheat endosperm studies [71]. In the developing wheat endosperm, the expression of SBEIIb is at a much lower level than SBEIIa [72,73], in contrast to the maize endosperm [74]. In rice, SBEIIa is at the earliest expressed 3 days after flowering (DAF) and maximally at 5-7 DAF, while SBEI and SBEIIb are most abundant at 7-10 DAF [75,76].…”

“…Moreover, a rice SBEIIb mutant had a 50% reduction of SSI activity, suggesting that SBEIIb and SSI may interact in vivo [80]. However, high amylose starch was only developed by suppressing both SBEIIa and SBEIIb in wheat [73]. The amylopectin from these mutants appeared to have greatly elongated chains and fewer short chains of DP ≤ 17, especially in chains of DP 8-12, suggesting that SBEIIb plays an important role in the formation of amylopectin A chains.…”