Approaches to improving the nutritional quality of barley seed proteins

The poor nutritional quality of barley grain is determined by the low level of lysine in the prolamin storage proteins (hordein). These account for between 35 to 50% of the total grain nitrogen, depending on the nutritional status of the plant. There is a reduced proportion of hordein in mutant high lysine lines but these also have reduced synthesis of storage carbohydrates and hence low yields. Three strategies for improvement are discussed. Increases in the lysine content of hordein may be difficult to achieve because of the presence of complex families of structural genes. It would also be necessary to insert a large number of additional lysine residues. Two more promising approaches are to increase the level of expression of genes coding for lysine-rich globulin storage proteins and to increase the pool of free lysine by selecting mutant lines with relaxed feedback regulation of lysine synthesis. (author)