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

• With a KM value of 26 μM poplar PttNRT2.4 is a high affinity nitrate importer.
• PttNRT2.4 expression is nitrate induced in root hairs of non-mycorrhizal poplars and ectomycorrhizas.
• Nitrate-based PttNRT2.4 induction in mycorrhizas point toward nitrate diffusion through the fungal mantle.
• As result, soil nitrate is taken up by mycorrhized host plants as extra nitrogen source in presence of reduced nitrogen.

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 KM 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.