Regulation of the absorption and release of nitrate by plant cells: A review of current ideas and methodology

Abstract

Ms. NM 1.1 H. Lambers, J. J. Neeteson and I. Swlen (Eds.) , Fundamental, ecological and agricultural aspects of nitrogen metabolism in higher plants. ISBN Abbreviations DES -Diethylstilboestrol FCCP -Carbonyl cyanide P-trifluoromethoxy· phenylhydrazone Summary In most circumstances nitrate uptake into the cells of roots is thermodynamically uphill and the process is severely curtailed by conditions which inhibit the synthesis of A TP and protein. The activity of the transport system can be augmented 2 to S-fold from a constitutive level to an induced level, by the provision of nitrate in the external solutiun to cells which have not previously received a nitrate supply. There is some evidence which suggests that the constitutive and the induced NO;; carriers may be distinct; they may turn over at different rates and the Km of the former is smaller. When cells are deprived of nitrate there is a loss of the induced carrier (= decay) but there may be an increase in the constitutive carrier. During N-starvation both Km and V max of net uptake and influx may vary.Evidence suggests that NO; influx is coupled to the proton motive force across the plasma membrane. NO; influx is usually accompanied by alkalinization of the external solution, but the effects of influx on the membrane potential are not always those to be expected for a 2H+: NO; symport. The insensitivity of NO;; influx to inhibition of proton pump activity by DES suggests that it may be premature to decide on the type of mechanism which carries NO;.The efflux of nitrate from the cytoplasmic pool is rapid and in many circumstances efflux may be a substantial proportion of the net uptake. Evidence is considered which suggests that net uptake is strongly controlled by the size of the efflux, influx being relatively constant. This view, however, does not take into account well-documented effects of the inhibition of uptake via the pool of amino compounds in the cytoplasm; the depression of uptake caused by NH; is probably indirect and mediated through the amino acid pool. Some evidence shows that NH; can promote rapid NO; efflux while having little or no short-term effect on influx.The net uptake of nitrate across the plasmalemma appears to be regulated jointly by the size of the cytoplasmic pool of NO;; -which influences efflux, and of amino acids -which influences influx. Fluxes across the tonoplast appear to be regulated, in unknown ways, by the vacuolar concentration of NO; + Cl-and perhaps other anions.The prospects for understanding regulation will be improved when it is possible to get better measurements of the metabolic/transport pool of NO;; in the cytoplasm. Some preliminary approaches to compartmentation using NMR spectroscopy are briefly described.

A stay‐green mutation of Lolium perenne affects NO3− uptake and translocation of N during prolonged N starvation

“…The day 0 values of K^^^, ranging from 9-7 to 12-3//M across genotypes, were lower than those reported for ryegrass by Laine et al (1993) (36//M) and Lycklama (1963) (33//,M), and fall at the lower end of the range recorded for high-affinity NO3" uptake systems in higher plants (Clarkson, 1986;Crawford, 1995). This is consistent with the fact that the plants were grown in flowing nutrient solutions (Wild, Jones & Macdufl, 1987).…”

“…The maximum uptake rates (day 0) were also lower than those reported by Laine et al (1993), although these authors used younger plants that were not preconditioned under a regime of N deprivation. We attribute the generally low I/,.,^ values in the present study to low demand for NO.j" resulting from the low growth rates (Clarkson, 1986), a consequence of the preconditioning period, when the supply of NO.5" was withdrawn.…”

The effects of light on induction, time courses, and kinetic patterns of net nitrate uptake in barley

“…Nitrate uptake occurs thermodynamically uphill and can be inhibited by processes which depress synthesis of ATP and proteins (for a review, see Clarkson 1986). The energy dependence of nitrate uptake is consistent with a NO 3 -: 2H + symport (Glass, Shaff & Kochian 1992).…”

Essential Arginine Residues in the Nitrate Uptake System from Corn Seedling Roots

“…It has been suggested that anion transport may be coupled to the proton gradient generated by the plasma membrane H+-ATPase (3,15,16,23). Others have questioned the involvement of H+-ATPase in anion transport.…”

“…In natural and agricultural soils, nitrate is the most prevalent form of inorganic nitrogen and is believed to be absorbed by plant roots through protein channels or carriers, which may be coupled to the proton gradient generated by the plasma membrane H+-ATPase (3,15,16,23). The capacity of nitrate uptake can be induced several fold from a constitutive low level by supplying nitrate to previously nitratestarved plants such as corn (8), wheat (9), and barley (19).…”