GENETIC IDENTIFICATION OF A GENE INVOLVED IN CONSTITUTIVE, HIGH-AFFINITY NITRATE TRANSPORT IN HIGHER-PLANTS

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 a bsence 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 cul ture (that is, within 30 min of initial exposure to nitrate), All othe r uptake activities measured (high-affinity phosphate and sulfate upta ke, inducible high-affinity nitrate uptake, and constitutive low-affin ity nitrate uptake) were present or nearly normal in the backcrossed m utant, Electrophysiological analysis of individual root cells showed t hat the nrt2 mutant showed little response to 0.25 mM of nitrate, wher eas NRT2 wild-type cells showed an initial depolarization followed by recovery. At 10 mM of nitrate both the mutant and wild-type cells disp layed similar, strong electrical responses, These results indicate tha t NRT2 is a critical and perhaps necessary gene for constitutive, high -affinity nitrate uptake in Arabidopsis, but not for inducible, high-a ffinity nor constitutive, low-affinity nitrate uptake, Thus, these sys tems are genetically distinct.