Nutritional control of storage protein synthesis in developing grain of wheat and barley

The availability of nitrogen and sulphur have major effects on the synthesis of prolamin storage proteins in developing endosperms of wheat and barley. A high level of available nitrogen results in an increased proportion of prolamin storage proteins. However, changes in the storage protein composition occur if additional sulphur is not also provided, with increased proportions of sulphur-poor prolamins and HMW prolamins and decreased proportions of sulphur-rich prolamins. In the case of wheat, this results in increased resistance and decreased extensibility of dough for bread-making, with consequences for the end-use quality. Further limitation in the availability of sulphur results in decreased total prolamin synthesis and an increase in free aspartic acid/asparagine in the grain. Recent studies of the structure and regulation of prolamin genes indicate the presence of regulatory elements in the promoter regions of genes for S-rich and S-poor prolamins, which could respond to nitrogen levels, although the sensing and signal transduction mechanisms are not understood. Such elements have not so far been identified in genes for HMW prolamins. Similarly, there is no information on how the availability of sulphur could modulate prolamin gene expression.