Seed storage proteins and approaches for improvement of their nutritional quality by genetic engineering

Seed storage proteins of grain crops meet the major dietary protein requirement of over half of the world population. However, seed proteins in general are deficient in some essential amino acids and hence are of poor nutritional quality. Therefore, intensive research is going on to isolate and characterize these proteins and their genes, and to produce transgenic crop plants with modified seed protein genes, with a view to improving their nutritive value as human food and animal feed. Many seed storage protein genes from cereals, legumes and oil seeds have been isolated, sequenced and their regulation has been studied by promoter deletion assay in transgenic plants. The amino acid composition of seed proteins of some transgenic crops has been marginally improved by modifying these genes for more methionine and lysine codons by sitedirected mutagenesis or by introducing heterologous genes. The synthesis, processing and targeting of the introduced gene products and their stable expression in successive generations of transgenic plants have also been studied. The potentiality of such genetic engineering approaches has been amply demonstrated. But problems of control over copy number and site of integration of the foreign gene and the position effect of the changes in the amino acid in the polypeptide chain on the final level of expression and deposition of the correctly processed storage protein remains unsolved. More basic research is needed before large-scale application of this approach for improvement of nutritional quality of seed proteins becomes possible.