Identification and characterisation of a novel glutenin subunit in bread wheat (Triticum aestivum L.).

Abstract

Bread is one of the major constituents of the human diet and wheat (Triticum aestivum L.) is the most important cereal for bread making. The gluten proteins (glutenins and gliadins) are recognised as important components affecting the processing quality of wheat flour. Glu-B1al is an allele that includes a duplication of a gene encoding an x-type high-molecular-weight glutenin subunit, and is thought to increase dough strength through overexpression of that subunit. In this research, a particular glutenin subunit in an Australian cultivar, H45, was investigated. H45 seemed to carry Glu-B1al, but it has relatively low unextractable polymeric protein (UPP, an indicator of weak dough). Two Bx genes from H45 were cloned and sequenced. Their sequences differ from each other, and each differ by four single nucleotide polymorphisms (SNPs) from the sequence of the Bx genes of Glu-B1al in the Canadian wheat cultivar Glenlea. One of the SNPs leads to an extra cysteine residue in one of the subunits. The Glu-B1 allele of H45 was designated Glu-B1br. With a restriction digest assay designed to distinguish the Glu-B1br allele from other overexpression alleles, it was demonstrated that Glu-B1br is co-inherited with low UPP. Among accessions present in the pedigree of H45 and accessions carrying overexpression alleles, Glu-B1br was detected only in H45. Efforts were made to develop alternative markers for Glu-B1br. Potential polymorphic regions within or close to Glu-B1 locus were investigated, but no closely linked polymorphisms were found that could be targeted for marker design. Individual glutenin subunits encoded by overexpression alleles and a mutant gene (MutBx7.1) derived from the first gene (Bx7.1) of Glu-B1br were obtained by heterologous expression. Flour incorporation tests showed that the glutenin subunit with the extra cysteine residue (Bx7.1) affects flour and dough mixing properties differently from MutBx7.1 and from the Bx subunits encoded by other overexpression alleles. Given that Bx7.1 and MutBx7.1 differ only with respect to the additional cysteine in Bx7.1, the effects of Bx7.1 on the dough properties of H45 can be attributed directly to that cysteine, which may act by impeding polymerisation.