Genetic identification of a gene involved in constitutive, high-affinity nitrate transport in higher plants.

Abstract

Two mutations have been found in a gene (NRT2) of Arabidopsis thaliana that specifically impair constitutive, high-affinity nitrate uptake. These mutants were selected for resistance to 0.1 mM chlorate in the absence of nitrate. Progeny from one of the backcrossed mutants showed no constitutive uptake of nitrate below 0.5 mM at pH 7.0 in liquid culture (that is, within 30 min of initial exposure to nitrate). All other uptake activities measured (high-affinity phosphate and sulfate uptake, inducible high-affinity nitrate uptake, and constitutive low-affinity nitrate uptake) were present or nearly normal in the backcrossed mutant. Electrophysiological analysis of individual root cells showed that the nrt2 mutant showed little response to 0.25 mM of nitrate, whereas NRT2 wild-type cells showed an initial depolarization followed by recovery. At 10 mM of nitrate both the mutant and wild-type cells displayed similar, strong electrical responses. These results indicate that NRT2 is a critical and perhaps necessary gene for constitutive, high-affinity nitrate uptake in Arabidopsis, but not for inducible, high-affinity nor constitutive, low-affinity nitrate uptake. Thus, these systems are genetically distinct.The uptake of mineral nutrients by plants influences human and biological activity on a global scale. Nutrient uptake affects crop yield and vigor as well as the resistance of plants to salt and heavy metals. To provide for the nutrients needed by crops, massive quantities of fertilizers are applied to soils. For example, 23 billion pounds of nitrogen fertilizer was applied to United States farm land in 1995 (1). Some of the excess nitrogen that is not taken up by plants or microbes leaches from the soil as nitrate into water supplies (2-4). As a result, nitrate has contaminated drinking water supplies for millions of people in the United States and caused the closure of more drinking water wells in California than any other contaminant (1,5). If the mechanisms that mediate nitrate uptake in plants were better understood, it might be possible to modify the uptake systems of plants to reduce the amount of excess nitrate in the soil.The uptake of ions by higher plants is mediated by carriers that use the energy stored in the proton gradient created by the plasma membrane H+-ATPase (reviewed in refs. 6-10). The electrical gradient across the plasma membrane is often between -100 and -250 mV (negative inside the cell). The uptake of nitrate anion by root epidermal cells involves active transport up this electrical gradient (reviewed in refs. 11-14). Exposure of root cells to nitrate produces an electrogenic response; the plasma membrane potential initially depolarizes becoming more positive inside the cell (15)(16)(17)(18)(19). This depolarization is attributed to the cotransport of two or more protons with each nitrate ion into the cell and is observed over a broad concentration range of nitrate.Nitrate uptake displays kinetics that can be resolved into distinct kinetic classes, some of which are regulated and ...

Nitrate transporters and peptide transporters

“…The severe defect of cHATS in the nar2.1 mutant [65], but not the nrt2.1 nrt2.2 double mutant [28], suggested that another NRT2 gene(s) was probably responsible for cHATS. A mutant defective in cHATS, chl8, was isolated by chlorate selection at low concentration [66], but the gene mutated in chl8 has not been identified. It will be interesting to determine whether one of the NRT2 genes, particularly NRT2.6 or NRT2.7, both of which are constitutively expressed in root [67,68], is responsible for the chl8 phenotype.…”

Expression of Rice (Oryza sativa L.) Genes Involved in High-Affinity Nitrate Transport during the Period of Nitrate Induction

“…HATS consists of two systems, the nitrateinducible HATS (iHATS) and the constitutive HATS (cHATS) (Wang and Crawford 1996, Crawford and Glass 1998, Forde 2000. In Arabidopsis thaliana, seven genes were isolated as NRT2 family gene members (Orsel et al 2002b), suggesting that NRT2 in higher plants is composed of several genes.…”

Plant mineral accumulation, use and transport during the life cycle of plants: A review

“…Nitrogen uptake is affected by changes in the N content or composition in the media (Wang and Crawford 1996;Morgan and Jackson 1988;Foulkes et al 1998) and carbohydrate (Champigni and Tolouizte 1986) within the root as well as by N demand of the shoot (Imsande and Touraine 1994). Nitrogen uptake in plants i.e.…”