Effects of Nitrite, Chlorate, and Chlorite on Nitrate Uptake and Nitrate Reductase Activity

Abstract

Effects of N02-, C103-, and C102-on the induction of nitrate transport and nitrate reductase activity (NRA) as well as their effects on N03-influx into roots of intact barley (Hordeum vulgare cv Klondike) seedlings were investigated. A 24-h pretreatment with 0.1 mol m-3 N02-fully induced N03-transport but failed to induce NRA. Similar pretreatments with C103-and C102-induced neither N03-transport nor NRA. Net C103-uptake was induced by N03-but not by C103-itself, indicating that N03-and C103-transport occur via the N03-carrier. At the uptake step, N02-and C102-strongly inhibited N03-influx; the former exhibited classical competitive kinetics, whereas the latter exhibited complex mixed-type kinetics. C103-proved to be a weak inhibitor of N03-influx (Ki = 16 mol m-3) in a noncompetitive manner. The implications of these findings are discussed in the context of the suitability of these N03-analogs as screening agents for the isolation of mutants defective in N03-transport.Most high-affinity solute transport systems in plants appear to be derepressible; when their solutes are withheld, transport rates increase severalfold (14,22). Ion transport systems involved in the absorption of Cl-, S04, H2PO4-, K+, Na+ (see ref. 14 for review), and NH4' (34) conform to this this pattern.The HATS2 for N03-absorption is, therefore, exceptional in being substrate inducible as well as subject to negative feedback (6,24,31). Enzymes involved in nitrate assimilation, NR and NiR, are also substrate inducible.The absence of a readily available radiotracer for N03-has led to the widespread use of C103-as a N03-analog in studies of N03-uptake and assimilation in both plants and microorganisms (10). 36C03-, which can be generated by electrolysis of 36Cl-, has provided a useful tracer for N03-. However, at high concentrations (0.5 mol m-3 and above) and during prolonged exposures, C103-has proven to be toxic to most plants. Indeed, C103-was previously used extensively as a herbicide (16 In Vivo NRA In vivo NRA in roots was assayed by measuring NO2-production under anaerobic conditions (12). Briefly, excised roots were incubated in potassium phosphate buffer (pH 7.7) containing 100 mol m-3 NO3-, maintained at 250C for 20 min. Before the transfer of roots to anaerobic conditions, the buffer was purged with He for 5 min. At the end of the incubation period, the tubes were transferred to a boiling water bath for 10 min to extract NO2-from the roots. NO2 content was measured spectrophotometrically: the color re-

Comparative Induction of Nitrate and Nitrite Uptake and Reduction Systems by Ambient Nitrate and Nitrite in Intact Roots of Barley (Hordeum vulgare L.) Seedlings

“…5A). This is in conflict with the results of Siddiqi et al (1992), who found that NOZ-did not induce NR. In our study, the accumulation of NO3-in the roots supplied with higher concentrations of NOz-suggests that enzyme induction may have resulted from NO3-produced by the oxidation of absorbed NOz-.…”

“…In a preliminary report, we showed that NR was induced in barley roots by both NO3-and NOz-and also that NOS-appeared in roots after feeding with NOz- (Aslam et al, 1992b). In contrast, Siddiqi et al (1992) recently reported that 0.1 m NOZ-did not induce NR in barley roots during a 24-h induction period.…”

Nitrite Transport in Chloroplast Inner Envelope Vesicles (I. Direct Measurement of Proton-Linked Transport)

“…If it was assumed that all of the NO,-were localized in the cytoplasm, then the NO,-concentration of that compartment could be estimated based on the assumption that the cytoplasm occupies approximately 5% of the cell volume (Lee and Ratcliffe, 1983). Using this assumption Siddiqi et al (1992) estimated the cytoplasmic NO,-concentration to be 2.8 mM in roots and 0.16 to 0.2 mM in shoots. Although these levels are 2 to 3 times lower than the concentration of NO,-used in this study, the rate of proton-linked NO,-movement across the membrane clearly has the capacity to move higher concentrations of NO,-across the membrane very rapidly.…”

“…Therefore, the rate of proton-linked NO,-transport is approximately 5 to 80 times greater than the measured activity of NO,-reductase in chloroplasts. The NO,-content in shoot and root tissues of barley plants grown with NO,-as the sole nitrogen source has been measured (Siddiqi et al, 1992). If it was assumed that all of the NO,-were localized in the cytoplasm, then the NO,-concentration of that compartment could be estimated based on the assumption that the cytoplasm occupies approximately 5% of the cell volume (Lee and Ratcliffe, 1983).…”

Investigation of the Apparent Induction of Nitrate Uptake in Barley (Hordeum vulgare L.) Using NO3--Selective Microelectrodes (Modulation of Coarse Regulation of NO3- Uptake by Exogenous Application of Downstream Metabolites in the NO3- Assimilatory Pathway)

“…In support of this idea, Bloom and Sukrapanna (1990) observed high initial rates of Nos-uptake in plants grown with NH4+ as the sole nitrogen source. Similarly, Siddiqi et al (1992) demonstrated that pretreatments with 100 p~ NOzproduced barley plants with high initial rates of Nos-uptake, although in earlier investigations Tompkins et al (1978) found that NOz--pretreatments inhibited the induction of Nos-uptake.…”