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 (NO (3) (−)) flux and transcription of NO(3) (−) transporters (NRTs; PcNRT1.1, ‐1.2, ‐2.1), and plasmalemma proton ATPases (HAs; PcHA4, ‐8, ‐11). Paxillus colonization enhanced root NO (3) (−) 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 NO (3) (−) efflux, whereas beneficial effects of fungal colonization on surface pH and HAs prevented NO (3) (−) loss. Inhibition of HAs abolished NO (3) (−) influx under all conditions. We found that stimulation of HAs was crucial for the beneficial influence of ectomycorrhiza on NO (3) (−) uptake, whereas the presence of a Hartig net was not required for improved NO (3) (−) translocation. Mycorrhizas may contribute to host adaptation to salt‐affected environments by keeping up NO (3) (−) nutrition.