The poplar NRT2 gene family of high affinity nitrate importers: Impact of nitrogen nutrition and ectomycorrhiza formation

Abstract As a result of natural or artificial disturbances, elevated nitrate concentrations are found in forest soils and could thus be important for tree nutrition. In an EST project of Populus tremula  ×  tremuloides / Amanita muscaria ectomycorrhizas three putative high-affinity nitrate transporters of the NRT2 gene family were discovered of which PttNRT2.4A and PttNRT2.5B were expressed highest in fine roots at comparable levels. Ectomycorrhiza formation resulted in a nitrate independent enhanced PttNRT2.5B gene expression. However, as characterization of transport properties by heterologous expression failed, PttNRT2.5B function still remains unclear. With a K M value of about 26 μM, as determined by heterologous expression in Xenopus laevis oocytes, PttNRT2.4A is a high affinity nitrate importer. In fine roots of nitrate depleted poplar plants PttNRT2.4A expression was very low. Transcript levels increased 75-fold when plants were exposed to low nitrate concentrations (10–100 μM). This increase was lower at elevated nitrate concentrations or in the presence of ammonium. By generating P. tremula  ×  tremuloides plants transformed with a PttNRT2.4A promoter GFP fusion construct, nitrate induced PttNRT2.4A expression was localized in root hairs but not in underlying cortex cells. In ectomycorrhizas of ammonium grown plants PttNRT2.4A expression decreased compared to non-mycorrhizal fine roots. However, nitrate induced PttNRT2.4A expression in ectomycorrhizas indicates nitrate permeability of the fungal mantle under certain conditions.