Paxillus involutus-Facilitated Cd(2+) Influx through Plasma Membrane Ca(2+)-Permeable Channels Is Stimulated by H(2)O(2) and H(+)-ATPase in Ectomycorrhizal Populus × canescens under Cadmium Stress

Using a Non-invasive Micro-test Technique, flux profiles of Cd(2+), Ca(2+), and H(+) were investigated in axenically grown cultures of two strains of Paxillus involutus (MAJ and NAU), ectomycorrhizae formed by these fungi with the woody Cd(2+)-hyperaccumulator, Populus × canescens, and non-mycorrhizal (NM) roots. The influx of Cd(2+) increased in fungal mycelia, NM and ectomycorrhizal (EM) roots upon a 40-min shock, after short-term (ST, 24 h), or long-term (LT, 7 days) exposure to a hydroponic environment of 50 μM CdCl(2). Cd(2+) treatments (shock, ST, and LT) decreased Ca(2+) influx in NM and EM roots but led to an enhanced influx of Ca(2+) in axenically grown EM cultures of the two P. involutus isolates. The susceptibility of Cd(2+) flux to typical Ca(2+) channel blockers (LaCl(3), GdCl(3), verapamil, and TEA) in fungal mycelia and poplar roots indicated that the Cd(2+) entry occurred mainly through Ca(2+)-permeable channels in the plasma membrane (PM). Cd(2+) treatment resulted in H(2)O(2) production. H(2)O(2) exposure accelerated the entry of Cd(2+) and Ca(2+) in NM and EM roots. Cd(2+) further stimulated H(+) pumping activity benefiting NM and EM roots to maintain an acidic environment, which favored the entry of Cd(2+) across the PM. A scavenger of reactive oxygen species, DMTU, and an inhibitor of PM H(+)-ATPase, orthovanadate, decreased Ca(2+) and Cd(2+) influx in NM and EM roots, suggesting that the entry of Cd(2+) through Ca(2+)-permeable channels is stimulated by H(2)O(2) and H(+) pumps. Compared to NM roots, EM roots exhibited higher Cd(2+)-fluxes under shock, ST, and LT Cd(2+) treatments. We conclude that ectomycorrhizal P. × canescens roots retained a pronounced H(2)O(2) production and a high H(+)-pumping activity, which activated PM Ca(2+) channels and thus facilitated a high influx of Cd(2+) under Cd(2+) stress.