In plants, amino acids fulfil a wide variety of functions. Their common role is to serve as building blocks of proteins, which exert manifold functions in plant metabolism, and as metabolites and precursors they are involved in plant defence, vitamin, nucleotide and hormone biosynthesis, and as precursors of a huge variety of secondary compounds. One way or the other, as active catalysts or as precursors, amino acids are essentially involved in all metabolic, regulatory, and physiological aspects of plant metabolism (for comprehensive reviews see, Miflin and Lea, 1990; Singh et al., 1992; Singh, 1999; Buchanan et al., 2000).
Uncovering plant specific aspects of amino acid biosynthesis contributed both to fundamental and applied research. The goals are to understand the biosynthetic pathways and their regulation and to understand the regulation of genes controlling growth-limiting processes (e.g., the assimilation of inorganic nitrogen into amino acids). A second huge field of investigations are the pathways and factors regulating the synthesis of secondary plant compounds derived from amino acid precursors. However, these areas are mainly covered in other chapters of this handbook. In addition to their obvious role in protein synthesis, amino acids perform essential functions in both primary and secondary plant metabolism. Some amino acids as glutamine and asparagine serve to assimilate and transport fixed nitrogen from sources to sinks. The aromatic amino acids serve as precursors to secondary products such as hormones and compounds involved in plant defence. Thus, the synthesis of amino acids controls directly or indirectly various aspects of plant growth and development. Such studies should also provide a framework for manipulating amino acid biosynthesis pathways in transgenic plants. For example, enzymes in several pathways have been identified as targets for herbicides. In some cases, the genes encoding these enzymes have been used for engineering herbicide resistance in plant of the first generation of transgenic plants. Eventually, future progress in amino acid biosynthesis research may provide enhanced crop resistance to osmotic stress and improved food protein composition. Therefore, the structural and regulatory genes controlling amino acid biosynthesis in plants are of interest not only to biochemists but also to agricultural biotechnologists, breeders and agrochemical industry to develop new transgenic plants with benefit to the consumer, sometimes befitted as the ‘second generation’ of genetically modified organisms (GMOs).
Beside the proteinogenic amino acids, many plants channel large amounts of nitrogen into amino acids that are not usually constituents of proteins (Herrmann, 1995; DellaPenna, 1999; Grusak and DellaPenna, 1999). These non-protein amino acids comprise a diverse and often complex group of compounds: several hundreds of them have been found in plants, most often in seeds (especially those of legumes), where they can accumulate to high levels. Non-protein amino acids are found in all plant tissues as intermediates in the synthesis of protein amino acids (e.g., homoserine, diaminopimelic acid, ornithine, citrulline), and in a more restricted range of plants as metabolic ‘end products’. The function of the latter one is often unclear, but they are frequently toxic to animals and found in seeds where they appear to serve both as a storage reserve of reduced nitrogen and as a feeding-deterrent to herbivores. The mode of toxicity varies, but is usually based on interference with regulation, transport or protein synthesis. One amino acid fairly common in legumes is canavanine A, which can account for up to 6% of the fresh weight of seeds of the Jack bean. It is very similar in structure to arginine and is thus able to interfere with arginine metabolism in animals that ingest those seeds. When metabolised, canavanine A causes a variety of toxic effects, including pupal malformation in insects and immune dysfunction in vertebrates. Other toxic arginine analogues include homoarginine and indospicine, also found in legumes.
Carbon and nitrogen are the principal constituents of amino acids. The carbon backbones are derived at different branching points from primary carbon metabolism while reduced nitrogen is transferred to these 2-oxo acids to form the common amino acid head group. The unique feature of this amino acid head group is the ability to form peptide bonds and thus polymerise to proteins. The side groups determine the chemical properties of the respective amino acids and later the derived protein structures (Table 27.1). These side groups are either directly derived from the carbon moiety or modified, e.g. through binding of reduced sulphur (cysteine and methionine), or through cyclisation (proline and aromatic amino acids). Actually amino acids are the entry port of the macronutrients N and S into plant metabolism. Nitrogen and sulphur assimilation first results in amino acids, i.e. glutamine and cysteine, respectively. These two amino acids serve as precursors of all further organic molecules containing any of these macronutrients.