Accumulation of protein subunits and their polymers in developing grains of hexaploid wheats

Abstract

Technological properties of wheat flours are largely governed by the functional properties of storage proteins, comprising single chain gliadins and disulphide bonded polymeric proteins (glutenins, triticins, HMW albumins). This study was conducted (a) to measure the rate of accumulation of different classes of proteins, (b) to monitor the rate of polymerization of the polymeric subunits, and (c) to investigate the origin of differences in the degree of polymerization arising from allelic differences at the Glu-D1 locus of HMW glutenin subunits. Developing grains from hexaploid wheat cultivars were collected at 3-d intervals between 7 d and 40 d after anthesis (DAA). The accumulation behaviour of polymeric proteins was determined by size exclusion-HPLC and of the constituent polypeptides (HMW glutenin subunits, LMW glutenin subunits, HMW albumin subunits) by a combination of SDS-PAGE, reversed phase-HPLC and immunoblotting. The relative time for peak accumulation of the albumins/gobulins, gliadins and polymeric proteins occurred in that order during grain filling. Deposition of HMW glutenin subunits, LMW glutenin subunits and HMW albumin subunits (β-amylases) began as early as 7 d after anthesis and progressed until maturity. Disulphide-linked aggregation (polymerization) of these subunits also commenced at 7 DAA. However, significant changes in size distribution of the polymers occurred only during the late stages of seed development which coincided with a rapid increase in the amounts of the glutenin subunits. The effects of HMW albumin subunits and LMW glutenin subunits on polymer size were relatively smaller than that of the HMW glutenin subunits. The wheat biotype with HMW glutenin subunits 5+10 (Glu-D1 d allele) accumulated larger polymers more quickly than the biotype with allelic subunits 2+12 (Glu-D1 a allele), this difference becoming apparent at 28 DAA which could be attributed to variation in the accumulation rate of HMW glutenin subunits.