The determinants of grain texture in cereals

Kernel hardness is an important agronomic trait that influences end-product properties. In wheat cultivars, this trait is determined by thePuroindoline a (Pina) andPuroindoline b (Pinb) genes, located in theHardness locus (Ha) on chromosome 5DS of the D genome. Wild type alleles code puroindoline a (PINA) and puroindoline b (PINB) proteins, which form a 15-kDa friabilin present on the surface of water-washed starch granules. Both the proteins are accumulated in the starch endosperm cells and aleurone of the mature kernels.Puroindoline-like genes coding puroindoline-like proteins in the starch endosperm occur in some of the genomes of Triticeae and Aveneae cereals. Orthologs are present in barley, rye and oats. However, some genomes of these diploid and polyploid cereals, like that ofTriticum turgidum var.durum (AABB) lack thepuroindoline genes, having a very hard kernel texture. The two wild type alleles in opposition (dominant loci) control the soft pheno-type. Mutation either inPina orPinb or in both leads to a medium-hard or hard kernel texture. The most frequent types ofPin mutations are point mutations within the coding sequence resulting in the substitution of a single amino acid or a null allele. The latter is the result of a frame shift determined by base deletion or insertion or a one-point mutation to the stop codon. The lipid-binding properties of the puroindolines affect not only the dough quality but also the plants’ resistance to pathogens. Genetic modification of cereals withPuroindoline genes and/or their promoters enable more detailed functional analyses and the production of plants with the desired characteristics.