Wheat grain hardness: multigenic basis and its implications

Abstract

The grain hardness (or texture) of common wheat (Triticum aestivum L.) is an important measure of grain quality and significantly influences the end-use of wheat and its commercial value. The two puroindoline genes, Pina-D1 and Pinb-D1, located at the Ha locus on chromosome 5D only of common wheat, are key genetic determinants of grain hardness. These encode small molecular weight(~13 kDa) proteins called puroindoline A and B (PINA and PINB),respectively, which are lipid-binding proteins with a distinct 10-cysteine backbone, a helicoid structure and a unique tryptophan-rich domain (TRD). When both PINs are in wild-type, they are proposed to lead to soft grain texture, while diverse mutations in either/both genes have been associated with grain hardness of common wheat and deletion of Pin genes in both genomes associated with the very hard texture of durum (reviewed in Bhave
& Morris, 2008).The recently reported Pinb-2 gene on chromosome 7 of common and durum wheat may also have a role in texture (Wilkinson et al., 2008; Chen et al., 2010). We aimed to analyse the Pinb-2 gene diversity in a number of Chinese land races as China is the secondary centre of origin of wheat. The alleles of Pina-D1 and Pinb-D1 were also analysed to get a complete Pin genotype, and durum wheats were included to test their Pinb2 genotypes. Synthetic peptides designed on the PINB-2 TRD were also tested for any antimicrobial activity. The genes were amplified from genomic DNAs using degenerate primers to allow amplification of variants, the PCR products cloned, and a number of clones sequenced to identify individual sequences. The Pinb-2 genes were identified in all wheats tested and comprised a small family with variations in the putative proteins. A new Pina-D1 allele, Pina-D1t,
with a tryptophan-to-stop codon mutation, was also identified. Taken together, the results suggest that the final texture of common wheat could be determined by three interactors: the PINA and PINB (in wild type or altered/null forms) and PINB2 variants, while the texture of durums could be influenced by PINB2 variants. In addition, the PINB2-based peptides exhibited antimicrobial activity, as we noted for PINA and PINB-based peptides, suggesting likely seed defence roles for the whole PIN family