Genetic Engineering of Amino Acid Metabolism in Plants

Abstract
Amino acids are not only building blocks of proteins but also participate in many metabolic networks that control growth and adaptation to the environment. In young plants, amino acid biosynthesis is regulated by a compound metabolic network that links nitrogen assimilation with carbon metabolism. This network is strongly regulated by the metabolism of four central amino acids, namely glutamine, glutamate, aspartate, and asparagine (Gln, Glu, Asp, and Asn), which are then converted into all other amino acids by various biochemical processes. Amino acids also serve as major transport molecules of nitrogen between source and sink tissues, including transport of nitrogen from vegetative to reproductive tissues. Amino acid metabolism is subject to a concerted regulation by physiological, developmental, and hormonal signals. This regulation also appears to be different between source and sink tissues. The importance of amino acids in plants does not only stem from being central regulators of plant growth and responses to environmental signals, but amino acids are also effectors of the nutritional quality of human foods and animal feeds. Since mammals cannot synthesize about half of the 20-amino acid building blocks of proteins, they rely on obtaining them from foods and feeds. Yet, the major crop plants contain limited amounts of some of these so-called ‘‘essential amino acids,’’ which decreases nutritional value. Recent genetic engineering and more recently genomic approaches have significantly boosted our understanding of the regulation of amino acid metabolism in plants and their participation in growth, stress response, and reproduction. In addition, genetic engineering approaches have improved the content of essential amino acids in plants, particularly the contents of lysine and methionine, which are often most limiting.

Key Words: Transgenic plants, Genetic engineering, Amino acids, Essential amino acids, Biosynthesis, Catabolism, Metabolism, Seeds, Amide amino acids, Metabolic networks, Carbon/nitrogen partition, Nitrogen assimilation, Transport, Glutamate synthase, Glutamine synthase, Glutamate dehydrogenase, Glutamate, Glutamine, Aspartate, Asparagine, Aspartate family pathway, Lysine, Threonine, Methionine, Aspartate kinase, Dihydrodipicolinate synthase, Lysine-ketoglutarate reductase, Cystathionine γ-synthase, Threonine synthase, Lysine overproduction, Methionine overproduction, Lysine-rich proteins, Sulfur-rich storage proteins, Vegetative storage proteins, Nutritional quality, Ruminant animals, Nonruminant animals, Light, Signal, Sucrose, Stress, Development, Food, Feed.