Accumulation and changes in molecular size distribution of polymeric proteins in developing grains of hexaploid wheats: role of the desiccation phase

Two varieties of wheat differing in high molecular weight glutenin subunit composition (Soissons, 5+10, Glu-D1a allele; Thesee, 2+12, Glu-D1a allele) were examined to follow the accumulation of polymeric proteins and the changes in molecular size distribution of these proteins during grain filling. The accumulation behaviour of polymeric proteins was determined by size-exclusion-HPLC, multistacking SDS-PAGE and the constituent polypeptides (high molecular weight and low molecular weight glutenin subunits) by reversed-phase-HPLC. For both cultivars, the accumulation of each class of protein was highly asynchronous, especially between the early deposition of SDS-soluble polymers and the late deposition of SDS-insoluble polymers, such that the average molecular size of polymeric protein increased in the period from 30 to 45 days after anthesis in natural conditions. By applying premature grain desiccations during the cell enlargement phase, it was demonstrated that the SDS-insoluble polymers formation was closely related with the process of water loss from the grain. Moreover, the rapid accumulation of SDS-insoluble polymers coincided with a rapid decrease in mass of both SDS-soluble polymers and monomers, suggesting an aggregative mechanism. Over the same period, the molecular size distribution of the polymers which can be used to differentiate the two genotypes studied, is highly correlated with the percentage of high molecular weight glutenin subunits in glutenins present in kernels when desiccation occurred. The formation of SDS-insoluble fraction is discussed in connection with the specific contribution of high molecular weight glutenin subunits to the formation of polymers (subunits linked by disulfide bonds).