EXPRESSION STUDIES OF NRT2-1NP, A PUTATIVE HIGH-AFFINITY NITRATE TRANSPORTER - EVIDENCE FOR ITS ROLE IN NITRATE UPTAKE

Expression of the gene Nrt2Np, which encodes a putative high-affinity nitrate transporter of Nicotiana plumbaginifolia was studied under var iable physiological conditions. Nrt2Np is rapidly induced by very low nitrate concentrations and repressed by reduced nitrogen metabolites. Furthermore, Nrt2Np is expressed in coordination with other genes invo lved in nitrate assimilation (Nia, Nii). A deficiency in nitrate reduc tase activity, which is accompanied by high internal nitrate concentra tion and low levels of nitrogen metabolites, e.g, glutamine, leads to an overexpression of Nrt2Np, showing that high nitrate concentration p er se does not repress Nrt2Np expression. By investigating plants with altered nitrate uptake properties, we showed a correlation between Nr t2 mRNA accumulation and 15N nitrate influx rates, providing the first evidence that the expression of Nrt2 correlates with the rate of nitr ate uptake. In situ hybridization revealed a tissue-specific expressio n pattern. Nrt2Np mRNA accumulation is localized throughout all layers of the root tip, being highest in epidermal and endodermal cells. How ever, in mature root tissue, Nrt2 expression was detected mainly in th e lateral root primordia and in the epidermis.