Phosphate application enhances the resistance of arbuscular mycorrhizae in clover plants to cadmium via polyphosphate accumulation in fungal hyphae

• The role of polyphosphate (polyP) in enhanced Cd resistance of AM is still unevidenced.
• AMF hyphae associated with clover plants were applied with different levels of Pi and Cd.
• Cd decreased hyphal biomass and polyP accumulation while Pi alleviated this effect.
• Fluorescent probes showed the overlapping of Cd signal and polyP signal, indicating their chelation.
• Chelation of Cd with polyP is alternative mechanism for AM hyphae to detoxify Cd.

Both phosphorus (P) application and arbuscular mycorrhizal (AM) fungal inoculation can enhance plant resistance to Cd toxicity, however, their interaction and the underlying mechanism are still unclear. The aims of this study were to investigate the effect of P application on AM resistance to Cd and to reveal the corresponding mechanism. We designed a compartmented system to separate extraradical hyphae (EH) of Rhizophagus irregularis from the roots of clover (Trifolium ripense L.). EH was applied with different levels of P and cadmium (Cd). The content and uptake of P and Cd were monitored. Polyphosphate (polyP) accumulation in EH was quantified. The localization of polyP and Cd was visualized using fluorescent staining. Results indicated that Cd greatly decreased EH biomass and polyP accumulation while P increased polyP accumulation. PolyP accumulation in EH enhanced AM resistance to Cd, and also reduced Cd uptake by plants. Fluorescent images indicated the overlapping of Cd signals and polyP signals. These data suggest that P application can enhance the resistance of AM to Cd via polyP chelation, and this represents another mechanism underlying the detoxification of Cd by AM hyphae other than the absorption by hyphae.