COMPARTMENTATION OF NITROGEN ASSIMILATION IN HIGHER-PLANTS

This chapter describes important reactions of nitrogen assimilation and initiatives leading to new insights in the roles of several reactions involved in nitrogen metabolism. The chapter discusses the localization of these reactions and the advantage of such compartmentation to an organism and emphasizes leaf metabolism and the roles of chloroplasts, peroxisomes, and mitochondria in utilizing carbon and nitrogen intermediates involved in photorespiration. NO3- and NH4+ are the common forms of nitrogen added to the soil. NO3- taken up from the soil is either converted to NH4+ in the roots by the action of nitrate (NR) and nitrite (NiR) reductases or is transported to the shoot before assimilation. The conversion of NO3- to NH4+ involves three proteins that are induced by NO3-: (1) a permease that permits the selective uptake of NO3- from the medium (soil), (2) NR reductase, and (3) NiR reductase. Depending on the level of NO3- administered to a system, NO3- can be stored in the root, transferred to the leaf to be stored in the vacuole, or be reduced in the roots or leaves. End products of NO3- assimilation—NH4+ and amino acids— inhibit the induction of NR in Neurospora crassa. © 1992 Academic Press Inc.