ABSTRACT
I. INTRODUCTION In plants. all inorganic nitrogen is first reduced to ammonia before it is incorporated into organic fonn (Crawford and Arst 1993; Hoff et aI. 1994). Ammonia is then assimilated into glutamine and glutamate, which play a key role in primary and recycled nitrogen assimilation, including the reduced nitrogen from symbiotic nitrogen fixation (Lam et aI. ]996). These two amino acids serve as nitrogen donors in many cellular reactions including the biosynthesis of proline and arginine. Glutamate contributes not only the nitrogen atom, but also most of the carbon atoms for the synthesis of proline and arginine. Proline accumulation has been correlated in many organisms to adaptation to osmotic stress (Delauney and Venna ]993; also see Chap. ]2). In addition to its role in osmoproteetion, proline accumulation in plants under stress conditions may function in the storage of energy, amino nitrogen. and reducing power (Blum and Ebercon ]976). Proline acts as the primary energy source for supporting the rapid growth during ponen tube elongation (Hong-gu et aJ. 1982). In soybean nodules, proline metabolism results in the transfer of cytoplasmic reducing equivalents from the host plant to its symbiotic partner (Rhizobium), sustaining energy flow to fuel the process of nitrogen fixation in bacteria (Kohl et aI. 1988). Proline degradation produces glutamate, which acts as a source of nitrogen for the synthesis of other amino acids. Arginine fonns a major store of nitrogen because each arginine molecule has four nitrogen atoms. Most seeds contain ]()..4()% of their nitrogen as arginine (Van Etten et aI. ]967), and the release of this nitrogen is important for seed gennination. Nitrogen release from arginine during senescence of plant tissue is mobilized to other parts of the plant (Beevers 1976). Availabi]ity of nitrogen to the plant may regulate the synthesis of proline and arginine (Delauney et aI. 1993). In the following sections, we discuss the biosynthesis and degradation of proline and arginine, regulation of the respective pathways, and the related processes that occur during different stages of plant development.
