Ion fluxes in Paxillus involutus-inoculated roots of Populus x canescens under saline stress

The Paxillus involutus strains MAJ and NAU improve ion homeostasis under salt stress although only strain MAJ forms a functional ectomycorrhiza with poplar roots, whereas strain NAU induces defense reactions. The goal of the present study was to determine whether beneficial effects of MA) and NAU on root nutrient element fluxes are induced during early stages of fungal-root interaction or require long-term co-culture. Salt-induced flux profiles of H+, Na+, K+, and Ca2+ were examined in the salt-sensitive poplar Populus x canescens after inoculation for 10 d and 20 d with the P. involutus strains, MA) and NAU and after short-term (24h) and long-term (7 d) salt stress. Inoculation with P. involutus for 10 d and 20 d increased the capacity of P. x canescens roots to retain K+ after short and long-term exposure to NaCl stress (100 mM). P. involutus-inoculated plants reduced the influx of Na+, especially in the NAU-inoculated roots. The salt-elicited Na+ efflux corresponded with an apparent decline of H+ extrusion in NAU-inoculated roots, which was presumably the result of increased Na+/H+ exchange activity across the plasma membrane. After 10 days of fungal inoculation, P. x canescens roots exhibited an enhanced Ca2+ uptake ability upon salt treatments, whereas a prolonged inoculation time of 20 d caused a marked Ca2+ efflux from P. x canescens roots. The P. involutus-elicited Ca2+ enrichment was probably replaced by Na+ at the later stage of fungal colonization. Ca2+ enrichment is known to mediate Na+/H+ homeostasis in poplar roots under salt stress and therefore both NAU- and MAJ-impeded Na+ accumulation compared with non-inoculated roots. NAU provided greater benefit to the inoculated roots to the maintenance of the K+/Na+ homeostasis because of the pronounced Na+ extrusion during the early stage of fungal colonization when the Ca2+ enrichment was greater than in MAJ-inoculated roots. In accordance with flux data, the whole-plant assessment revealed that inoculation with P. involutus attenuated NaCl-induced leaf damage in P. x canescens. Overall, our results support that the formation of a mature ectomycorrhiza is not required for the amelioration of the protection from salinity stress. (C) 2013 Elsevier B.V. All rights reserved.