Bioengineering approaches to improve the nutritional values of seeds by increasing their methionine content

Methionine is a nutritionally essential, sulfur-containing amino acid found at low levels in plants and in their seeds. Methionine levels often limit the plant's value as a source of dietary protein for humans and animals. Despite recent accumulated knowledge of methionine metabolism in vegetative tissues, there is still little knowledge of methionine metabolism in seeds. In this review, we summarize the efforts made to increase the levels of methionine in seeds using genetic engineering methods. Two main approaches were tested: the first was the expression of methionine-rich storage proteins in a seed-specific manner, with the goal of trapping the soluble methionine into protein form and competing with the catabolism of methionine to its essential metabolites. However, in many cases this approach does not lead to a significant increase in total methionine content. The second approach aimed to increase the soluble content of methionine in seeds. Despite the nutritional significance of methionine, the factors regulating soluble methionine content in seeds are not fully known. Evidence shows that two biosynthetic pathways, the aspartate family pathway and the S-methylmethionine pathway, contribute to soluble methionine content in seeds. However, their roles in soluble methionine synthesis and accumulation are not fully understood. In recent years, combinations of these two approaches have been tested; however, they have not yet succeeded in elevating total methionine content in seeds. More emphasis should be applied to gaining knowledge of the biosynthesis pathways that could contribute to an increase in methionine content in seeds.