Nitrate: nutrient and signal for plant growth.

The mineral nutrient needed in greatest abundance by plants is nitrogen. Plants, however, must compete for nitrogen in the soil with abiotic and biotic processes such as erosion, leaching, and microbial consumption. Soil nitrogen is also lost when crops are harvested and plant material is removed from the soil. To be competitive, plants have evolved several mechanisms to acquire nitrogen at low concentrations and to use a variety of forms of nitrogen. Plants can assimilate inorganic forms, such as nitrate and ammonia, and organic forms, such as urea. Some plants, including legumes, can fix dinitrogen gas in association with symbiotic bacteria (see Mylona et al., 1995, this issue). This review focuses on the assimilation of nitrate, a transient yet critical form for plants. Two key questions arise when considering nitrate assimilation. First, how do plants acquire optimal quantities of nitrate when the soil nitrate concentration can vary from 10 pM to 100 mM? Second, how is nitrate assimilation integrated into the overall metabolic program of plants so that optimal amounts of energy and carbon are provided? What is known about nitrate assimilation at the cellular leve1 is summarized in Figure 1. Once taken up, nitrate is either stored in the vacuole or reduced to nitrite by nitrate reductase (NR). Nitrite enters the chloroplast (or plastid in the root) and is then reduced to ammonja by nitrite reductase (NiR). Ammonia can then be fixed into carbon by the action of glutamine synthetase (see Lam et al., 1995, this issue). Reducing energy is provided in the form of NAD(P)H for NR and reduced ferredoxin for NiR. Glutamate provides the immediate carbon skeleton. Because nitrate assimilation produces several hydroxide ions, organic acids are also required for pH homeostasis. Coordinating these steps is a regulatory network that is responsive to both interna1 and external signals, some of which are diagrammed in Figure 1. Ultimately, it is the understanding of this regulatory network that will answer the aforementioned key questions. Genetic analysis of nitrate assimilation has been possible because mutants blocked in the pathway can be rescued by providing ammonia as a source of nitrogen. Such mutants are obtained either by screening directly with an in vitro assay for plants defective in nitrate reduction or by selecting plants that are resistant to chlorate, the chlorine analog of nitrate. When chlorate is taken up and reduced by NR, toxic chlorite is produced. In higher plants, chlorate-resistant mutants are impaired in nitratelchlorate reduction because they have either a defective nitrate reductase structural gene (NIA) or a defective molybdenum cofactor gene (CNX). The one exception is the chll mutant of Arabidopsis, which is defective in nitrate/chlorate uptake. Only in fungi and algae have regulatory mutants been descri bed. There are many excellent recent reviews of the nitrate assimilation field (Pelsy and Caboche, 1992; Warner and Kleinhofs, 1992; Cove, 1993; Crawford and Arst, 1993; Marzluf, 1993; Crawford, 1994; Hoff et al., 1994; Huppe and Turpin, 1994). This review provides an update on recent molecular advances that have uncovered genes and mechanisms responsible for nitrate uptake, reduction, and regulation.