CHARACTERIZATION OF THE WHEAT MR 15000 GRAIN-SOFTNESS PROTEIN AND ANALYSIS OF THE RELATIONSHIP BETWEEN ITS ACCUMULATION IN THE WHOLE SEED AND GRAIN SOFTNESS

The Mr 15000 protein associated with water-washed wheat starch granules from soft wheats was shown to be heterogeneous: it could be divided into a fraction containing one or more alpha-amylase inhibitor subunits and a fraction largely composed of a previously uncharacterised polypeptide(s) referred to as the ''grain-softness protein'' (GSP). The major N-terminal sequence and sequences of peptides derived from protease digests of GSP are reported. An antiserum specific for GSP was used to show that GSP accumulated in both hard and soft wheat grains, but the GSP in soft grains associated more strongly with starch granules than the GSP in hard grains. A positive correlation between grain softness and accumulation of GSP in the seed was demonstrated for a range of cultivars. This differs from the qualitative relationship, based on the isolated starch fraction, between GSP and grain softness that has already been reported. Analysis of wholemeal extracts with the antiserum demonstrated that the accumulation of GSP in the seed was dependent on the short arm of chromosome 5D, which also encodes the Ha locus. In addition, examination of near-isogenic lines differing in hardness indicated that the gene(s) controlling GSP was (were) linked with the Ha locus. The findings indicate that GSP may be the product of the Ha locus and thus be the major factor that determines the milling characteristics of bread wheats.