Amino acids are essential constituents of all cells. In addition to their role in protein synthesis, they participate in both primary and secondary metabolic processes associated with plant development and in responses to stress. For example, glutamine, glutamate, aspartate, and asparagine serve as pools and transport forms of nitrogen, as well as in balancing the carbon/nitrogen ratio. Other amino acids such as tryptophan, methionine, proline, and arginine contribute to the tolerance of plants against biotic and abiotic stresses either directly or indirectly by serving as precursors to secondary products and hormones. Apart from their biological roles in plant growth, some amino acids, termed ‘‘essential amino acids,’’ are also important for the nutritional quality of plants as foods and feeds. This is because humans, as well as most livestock, cannot synthesize all amino acids and therefore depend on their diets for obtaining them. Among the essential amino acids, lysine, methionine, threonine, and tryptophan are considered especially important because they are generally present in low or extremely low amounts in the major plant foods.
Studies on amino acid metabolism in plants have always benefited from the more advanced understanding of amino acid metabolismin microorganisms. Combined genetic, biochemical, molecular, and more recently genomics approaches, coupled with administration and metabolism of various precursors as major donors of carbon, nitrogen, and sulfur, have provided detailed identification of flux controls of amino acid metabolism in microorganisms (Stephanopoulos, 1999). These studies also clearly illustrated that amino acid metabolismin microorganisms is regulated by complex networks ofmetabolic fluxes,which are affected bymultiple factors. Although the regulation of amino acid metabolism in higher plants may be analogous to that in microorganisms, the multicellular and multiorgan nature of higher plants presents additional levels of complexity that render metabolic fluxes and regulatory metabolic networks in plants much more sophisticated than in microorganisms. Plant seeds and fruits, most important organs as food sources, or as a source for the production of specific compounds like oils and carbohydrates, represent an exciting example to illustrate the higher complexity of metabolic regulation in plants compared to microorganisms. Seed metabolism is regulated not only by internal metabolic fluxes but also by the availability of precursor metabolites that depend in turn on metabolic process operating in vegetative tissues and on the efficiency of transport of these metabolites from the source to developing seeds. Thus, the regulation of seed metabolism in plants may be significantly different, responding to different signals than vegetative metabolism.
Due to space limitation, it is impossible to discuss in detail all aspects of amino acid metabolism in this chapter. We will therefore focus on relatively recent studies employing molecular/biochemical approaches, as well as tailor-made genetic engineering, metabolic engineering, and gene knockout approaches to study the regulation of amino acid metabolism in plants. Most recent studies employing these approaches have focused on the metabolism of glutamine, glutamate, aspartate, and asparagine, as well as on the essential amino acids lysine, threonine, and methionine. Hence, this chapter will focus mainly on these amino acids. We make the case that the regulatory principles that emerged from studies of these amino acids will also be valid for explaining the metabolism of other amino acids. For discussion of the metabolism of other amino acids, readers are directed to the recent book edited by B. J. Singh (1999) and several reviews (Coruzzi and Last, 2000; Morot-Gaudry et al., 2001). Since improved understanding of plant amino acid metabolism enjoys significant biotechnological importance, we will also address this aspect focusing on metabolic engineering of the essential amino acids, lysine and methionine, for feeding ruminant and nonruminant animals. We then discuss future goals in studying plant amino acid metabolism.
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