How does phosphorus influence Cd tolerance strategy in arbuscular mycorrhizal-Phragmites australis symbiotic system?

To clarify how phosphorus (P) influences arbuscular mycorrhizal fungi (AMF) interactions with host plants, we measured the effects of variation in environmental P levels and AMF colonization on photosynthesis, element absorption, ultrastructure, antioxidant capacity, and transcription mechanisms in Phragmites australis (P. australis) under cadmium (Cd) stress. AMF maintained photosynthetic stability, element balance, subcellular integrity and enhanced antioxidant capacity by upregulating antioxidant gene expression. Specifically, AMF overcame Cd-induced stomatal limitation, and mycorrhizal dependence peaked in the high Cd-moderate P treatment (156.08%). Antioxidants and compatible solutes responded to P-level changes: the primary driving forces of removing reactive oxygen species (ROS) and maintaining osmotic balance were superoxide dismutase, catalase, and sugars at limited P levels and total polyphenol, flavonoid, peroxidase, and proline at abundant P levels, we refer to this phenomenon as "functional link." AMF and phosphorus enhanced Cd tolerance in P. australis, but the regulation of AMF was P-dependent. Phosphorus prevented increases in total glutathione content and AMF-induced GSH/GSSG ratio (reduced to oxidized glutathione ratio) by inhibiting the expression of assimilatory sulfate reduction and glutathione reductase genes. The AMF-induced flavonoid synthesis pathway was regulated by P, and AMF activated Cd-tolerance mechanisms by inducing P-dependent signaling.