Regulation of the Accumulation and Reduction of Nitrate by Nitrogen and Carbon Metabolites in Maize Seedlings

Abstract

lhe accumulation and reduction of nitrate in the presence of the nitrogen metabolites asparagine (Asn) and glutamine (Cln) and the carbon metabolite sucrose (Suc) were examined i n maize (Zea mays L.) seedlings in an attempt to separate their effects on the nitrate uptake system and the nitrate reduction system. After 8 h of exposure to nitrate in the presence of 1 mM Asn, tissue nitrate accumulation was reduced at 250 ~L M external nitrate, but not at 5 mM Asn.The induction of nitrate reductase (NR) activity was reduced at both external nitrate concentrations. In the presence of 1 mM Cln or 1 '/ O SUC, tissue nitrate concentration was not significantly altered, but the induction of root NR activity was reduced or enhanced, respectively. l h e induction of root nitrite reductase (NiR) activity was also reduced in the presence of Asn or Cln and enhanced i n the presence of SUC. Transcript levels of NR and NiR in roots were reduced in the presence of the amides and enhanced i n the presence of SUC. When Suc was present i n combination with either amide, there was complete relief from the inhibition of NiR transcription observed in the presence of amide alone. In the case of NR, however, this relief from inhibition was negligible. l h e inhibition of the induction of NR and NiR activities in the presence of Cln and Asn is a direct effect and is not the result of altered nitrate uptake in the presence of these metabolites.Nitrate, the primary nitrogen source for most land plants, is absorbed by a specific nitrate uptake permease(s) in plant roots, reduced to nitrite by NR in the cytoplasm, and then reduced to ammonium by NiR in the plastid. Therefore, nitrate uptake permease(s), NR, and NiR constitute the first three enzymes of the nitrate assimilatory pathway and are subject to regulation by severa1 endogenous and environmental stimuli, including nitrate, Gln, light, and SUC (for review, see Crawford and Arst, 1993; Oaks, 1994; Sivasankar and Oaks, 1996). Nitrate induces the expression of both the uptake and reduction systems, whereas SUC and light enhance this induction. Gln represses the induction of the nitrate assimilatory system. The regulation of NR by nitrate, SUC, and Gln occurs at the transcriptional level, whereas the regulation by light occurs at both the transcriptional and posttranslational levels (Melzer et al., 1989; Cheng et al., 1992; Vincentz et al., 1993; Li et al., 1995; MacKintosh et al., 1995). Because the effec- tors Gln and SUC influence both the uptake and reduction systems, and the presence of nitrate is required for the induction of NR and NiR, it is possible that any effect of Gln and SUC on NR and NiR is an indirect effect arising out of a direct effect on nitrate uptake. Separation of the effects of these metabolites on the uptake and reduction systems has not yet been attempted. Nitrate uptake is mediated by two or more classes of nitrate transporters, referred to as the low-and highaffinity classes, which are present at the plasmalemma of root cells. In barley at least three transp...

Effect of Mixed Amino Acids on Crop Growth

“…It has been suggested that glutamate inhibited NR activity in roots, but no inhibition in shoots (Ivashikian and Sokolov, 1997). Sivasankar et al (1997) observed that Gln and asparagine (Asn) inhibited the induction of NR activity in corn roots at both 250 M and 5 mM of external NO 3 -supply. They concluded that inhibition was not the result of altered NO 3 -uptake, and tissue nitrate accumulation was reduced at 250 M external nitrate in the presence of 1 mM Asn, but not at 5 mM Asn.…”

“…Li et al (1995) also demonstrated that 5 mM glutamine added together with NO 3 − resulted in reduced levels of NR mRNA in both root and shoot of maize. Sivasankar et al (1997) observed that Gln and asparagine (Asn) inhibited the induction of NR activity (NRA) in corn roots at an external supply of 250 M and 5 mM NO 3 − . They concluded that inhibition was not the result of altered NO 3 − uptake, and tissue nitrate accumulation was reduced at 250 M external nitrate in the presence of 1mM Asn, but not at 5mM Asn.…”

“…Several authors reported that free amino acids could down regulate NO 3 -uptake. It was found that exogenously supplied amino acids and amides could decrease the uptake of NO 3 -by soybean (Muller and Touraine, 1992); wheat (Rodgers and Barneix, 1993); maize (Ivashikian and Sokolov, 1997;Padgett and Leonard, 1996;Sivasankar et al, 1997); barley (Aslam et al, 2001). In this experiment, the effectiveness of the MAA treatments on NO 3 -uptake was similar to above references at low MAA treatment rate (0.3 mM MAA, Fig.…”

“…It was found that exogenously supplied amino acids and amides could decrease the uptake of nitrate by soybean (Muller and Touraine, 1992); wheat (Rodgers and Barneix, 1993); maize (Ivashikina and Sokolov, 1997;Leonard, 1993, 1996;Sivasankar et al, 1997); barley (Aslam et al, 2001) Table 1 . Plants appear to have multiple mechanisms for regulating nitrate uptake in addition to amino acids or Nstatus (Padgett and Leonard, 1993).…”

Transcriptional Regulation of Sorghum Defense Determinants against a Phloem-Feeding Aphid

“…Feeding directly from phloem sap allows aphids to get access to free amino acids that do not require protein digestion before assimilation. This withdrawal could signal depletion of the nitrogen metabolites Asn and Gln (Sivasankar et al, 1997), leading to the observed induction of NiR.…”