Inoculation with Paxillus involutus enhances lead translocation from the roots to the aerial tissues by promoting pectin degradation in the roots of Populus ×canescens

Paxillus involutus (Batsch) Fr. inoculation can improve Pb translocation from the roots to the aboveground tissues of poplars, but the underlying mechanisms remain unknown. In this study, we inoculated Populus ×canescens with or without Pa. involutus and then exposed the non-mycorrhizal and mycorrhizal poplars to 0 or 8 mM PbCl2. Inoculation with Pa. involutus alleviated the Pb-induced chlorosis and reductions in CO2 assimilation rates of the leaves. Meanwhile, Pb concentrations in the roots remained relatively constant, while they significantly increased in the aboveground tissues of mycorrhizal compared to non-mycorrhizal poplars. Accordingly, Pb translocation from the roots to the aboveground parts and Pb accumulation in the aerial tissues were enhanced in mycorrhizal poplars. Further investigation revealed that Pa. involutus inoculation improved radial Pb transport from the epidermis to the stele by reducing pectin-immobilized Pb rather than altering the expression of Pb transport-related genes in the roots. Specifically, Pa. involutus decreased pectin contents rather than enhancing pectin methyl-esterification degrees to reduce Pb immobilization by pectin. The reductions in pectin contents were associated with pectin degradation but not biosynthesis. Interestingly, Pa. involutus enhanced pectin degradation by up-regulating the expression of host genes encoding pectate lyases and polygalacturonases rather than its own. These results suggest that Pa. involutus inoculation promotes Pb translocation by enhancing pectin degradation in the roots of Po. ×canescens.