Amelioration of nitrate uptake under salt stress by ectomycorrhiza with and without a Hartig net

Salt stress is an important environmental cue impeding poplar nitrogen nutrition. Here, we characterized the impact of salinity on proton-driven nitrate fluxes in ectomycorrhizal roots and the importance of a Hartig net for nitrate uptake. We employed two Paxillus involutus strains for root colonization: MAJ, which forms typical ectomycorrhizal structures (mantle and Hartig net), and NAU, colonizing roots with a thin, loose hyphal sheath. Fungus-colonized and noncolonized Populus × canescens were exposed to sodium chloride and used to measure root surface pH, nitrate (NO₃^- ) flux and transcription of NO₃^- transporters (NRTs; PcNRT1.1, -1.2, -2.1), and plasmalemma proton ATPases (HAs; PcHA4, -8, -11). Paxillus colonization enhanced root NO₃^- uptake, decreased surface pH, and stimulated NRTs and HA4 of the host regardless the presence or absence of a Hartig net. Under salt stress, noncolonized roots exhibited strong net NO₃^- efflux, whereas beneficial effects of fungal colonization on surface pH and HAs prevented NO₃^- loss. Inhibition of HAs abolished NO₃^- influx under all conditions. We found that stimulation of HAs was crucial for the beneficial influence of ectomycorrhiza on NO₃^- uptake, whereas the presence of a Hartig net was not required for improved NO₃^- translocation. Mycorrhizas may contribute to host adaptation to salt-affected environments by keeping up NO₃^- nutrition.