Salt-dependent expression of a nitrate transporter and two amino acid transporter genes in Mesembryanthemum crystallinum

Uptake and transport of inorganic nitrogen and allocation of amino acids are essential for plant growth and development. To study the effects of salinity on the regulation of transporters for nitrogenous compounds, we characterized the putative nitrate transporter McNRT1 and the amino acid transporters McAAT1 and McAAT2 from Mesembryanthemum crystallinum. By transcript analyses, McAAT1 was found in leaves, McAAT2 in roots, and McNRT1 in both tissues. By in situ PCR McNRT1 was localized, for example, to epidermal and vascular cells whereas McAAT2 was abundant in most cell types in mature roots and McAAT1 in the mesophyll and cells neighbouring xylem vessels in leaves. In response to salt stress, expression of McAAT2 and McNRT1 was stimulated in the root vasculature. In addition, McNRT1 and McAAT1 signals increased in the leaf phloem. Growth of yeast mutants deficient in histidine uptake was restored by McAAT2 whereas both McAAT1 and McAAT2 complemented a yeast mutant carrying a defect in proline uptake. The differential and cell-specific transcriptional activation of genes encoding nitrogen and amino acid transporters under salt stress suggest complex coordinated regulation of these transporter families to maintain uptake and distribution of nitrogenous compounds and amino acids under conditions of high salinity in plants.