FIGURE 12.

Schematic models showing Cd²⁺ influx through plasma membrane (PM) Ca²⁺ channels that stimulated by H₂O₂ and H⁺-ATPase in Paxillus involutus-ectomycorrhizal (MAJ and NAU) and non-mycorrhizal (NM) Populus × canescens roots under Cd²⁺ stress. High external Cd²⁺ facilitates the rapid movement of Cd²⁺ along its electrochemical gradient into fungal and plant cells. Cd²⁺ ions penetrated the ectomycorrhizal fungal hyphae and poplar roots through PM Ca²⁺ channels and other metal transporters or channels. The PM Ca²⁺ channels mediate the entry of Ca²⁺ in the absence of Cd²⁺ (-Cd) while allow the entry of Cd²⁺ in the presence of Cd²⁺ ions (+Cd). The Cd²⁺-permeable Ca²⁺ channels were activated by H₂O₂ and H⁺-pumping activity. Thus the Cd²⁺-elicited H₂O₂ and active H⁺-pumps favored the Cd²⁺ influx through Ca²⁺ channels in NM roots and P. involutus-ectomycorrhizas. In ectomycorrhizas, Cd²⁺ enriched in hyphae is thought to be delivered to the host roots. Moreover, the colonization of P. × canescens roots with the fungal strains MAJ and NAU stimulates H₂O₂ production and increases H⁺-pumping activity, and thus accelerates Cd²⁺ entry through Ca²⁺ channels under excessive Cd²⁺. Cd²⁺ ions competitively enter Ca²⁺ channels, and thus diminish the entry of Ca²⁺, leading to a marked Cd²⁺ enrichment in ectomycorrhizal roots under Cd²⁺ stress.