Differential Regulation of Nitrate Reductase Induction in Roots and Shoots of Cotton Plants

Abstract

The induction of nitrate reductase activity in root tips of cotton (Gossypium hirsutum L.) was regulated by several amino acids and by ammonium. Glycine, glutamine, and asparagine strongly inhibited induction of activity by nitrate and also decreased growth of sterile-cultured roots on a nitrate medium. Methionine, serine, and alanine weakly inhibited induction, and 11 other amino acids had little or no effect. Ammoniuni also decreased induction in root tips, but was most effective only at pH 7 or higher. The optimum conditions for ammonium regulation of induction were identical to those for growth of sterile-cultured roots on ammonium as the sole nitrogen source. Aspartate and glutamate strongly stimulated induction, but several lines of evidence indicated that the mechanism of this response was different from that elicited by the other amino acids. The effects of amino acids on induction appeared to be independent of nitrate uptake.In green shoot tissues, all attempts to (lenionstrate regulation of induction by amino acids failed. The great difference in observed responses of root and shoot to amino acids suggests that their nitrate reductase activities are regulated differently. Differential regulation of this enzyvme is consistent with the responses of root and shoot nitrate reductase activity to nitrate.The enzyme nitrate reductase occupies a control point in the pathway of nitrate assimilation (1). Activity of the enzyme fluctuates widely in response to many environmental or physiological factors, such as the presence of reduced N in the growth medium. The various ways by which enzyme activity can be affected include de novo synthesis and turnover (12,30), allosteric modifications (24, 28), and activation and inactivation (16,27). In addition, cofactor availability can limit in vivo rates of nitrate reduction (5,13,18 by amino acids in higher plants, the conclusions are again contradictory. Filner (7) and Heimer and Filner (8,9), working with tobacco cell cultures, carefully separated and analyzed the regulation by amino acids of both NR activity and nitrate uptake. They found a complex pattern of control of both activities, including repression and derepression of enzyme synthesis. Stewart (26), working with Lemnna minzor, similarly found amino acid-induced decreases in NR activity that were independent of nitrate uptake. Sims et al. (24), also with L. m?iinor, showed limited evidence for a slight inhibition of NR, presumably allosteric, by certain amino acids. In contrast, Ingle et al. (11). and Schrader and Hageman (23) found no amino acid effects on NR activity in radish cotyledons and corn leaves, respectively. The present paper reports evidence for the regulation of NR by ammonium and amino acids in roots, but not in shoots, of cotton plants.MATERIALS AND METHODS Chemicals. L-Amino acids and chloramphenicol were purchased from Calbiochem.

Amino Acid Interactions in the Regulation of Nitrate Reductase Induction in Cotton Root Tips

“…Regardless, aspartic and glutamic acids apparently behaved differently from other amino acids. This conclusion is consistent with earlier studies (15).…”

“…Procedures were similar to those described earlier (15). Seeds of cotton (Gossypium hirsutum L. Table I presents a summation of many experiments in which amino acids were applied singly or in pairs to root tips during induction of NR activity.…”

Nitrate Reductase Activity of Sphagnum Species in the South Pennines

“…The situation is less clear in higher plants. Ammonium has heen reported to inhibit the induction of nitrate reductase in barley roots (Smith and Thom^pson, 1971a) and in roots, but not shoots of cotton plants (Radin, 1975). Ammonium inhihition of the formation of nitrate reductase activity has been well estahlished for Lemna minor L. (Joy, 1969;Orehamjo andStewart, 1974, 1975).…”

Effect of Mixed Amino Acids on Crop Growth

“…The results for the possible regulation of NR activity by amino acids for higher plants are contradictory. Many authors agree with that amino acids can inhibit the activity of NR in higher plants (Radin, 1975(Radin, , 1977Oaks et al, 1979;Ivashikian and Sokolov, 1997;Sivasankar et al, 1997;Aslam et al, 2001). But Aslam et al (2001) reported that inhibition did not occur when the concentration of NO 3 -in the external solutions had been increased to 10 mM.…”

“…In studies of the possible regulation of NR activity by amino acids in higher plants, the results have often been conflicting. For example, Radin (1975Radin ( , 1977 had shown that the reduction of nitrate to nitrite in cotton roots is inhibited by specific amino acids. On the other hand, Oaks (1977) had found using an in vitro assay those amino acids results in enhanced levels of NR and also cause only minor inhibitions in both intact and excised corn roots.…”