Paxillus involutus-Facilitated Cd2+ Influx through Plasma Membrane Ca2+-Permeable Channels Is Stimulated by H2O2 and H+-ATPase in Ectomycorrhizal Populus × canescens under Cadmium Stress

Yuhong Zhang, Gang Sa, Yinan Zhang, Zhimei Zhu, Shurong Deng, Jian Sun, Nianfei Li, Jing Li, Jun Yao, Nan Zhao, Rui Zhao, Xujun Ma, Andrea Polle, Shaoliang Chen

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 CdCl2. 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 (LaCl3, GdCl3, 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 H2O2 production. H2O2 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 H2O2 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 H2O2 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.