Arbuscular mycorrhizae in metalliferous and aluminum-rich soils

Metalliferous and aluminium (Al)-rich soils represent restrictive terrestrial environments where high concentrations of potentially toxic elements act as edaphic filters on plant communities. Since arbuscular mycorrhizal (AM) symbiosis can alleviate several environmental constraints, it may play an important role in plant performance in metalliferous and Al-rich soils. Here, we review current knowledge on the mechanisms and ecological relevance of AM symbiosis in naturally metalliferous and Al-rich ecosystems worldwide. Available evidence indicates that AM fungi (AMF) mitigate both trace metal and Al phytotoxicity through mechanisms including element sequestration, glomalin production, modulation of rhizosphere processes, and improvements in plant nutritional status and antioxidant responses. Cross-referencing global records of hyperaccumulator plants with genus-level mycorrhizal classifications further revealed that most trace metal- and Al-hyperaccumulating genera are potentially mycorrhizal, with AM being predominant in both groups. However, direct confirmation of these associations under natural conditions remains limited. The expression and effectiveness of AM symbiosis are also context dependent and vary with fungal identity, ecotypic differentiation, host-fungal compatibility, and environmental conditions. Current studies also indicate that metalliferous and Al-rich soils influence AMF community composition, although evidence for adaptive differentiation remains much more developed for metalliferous systems than for Al-rich systems. We conclude that identifying gaps in the knowledge of AMF diversity and function in metal- and Al-accumulating plants will benefit conservation and restoration efforts under environmental change.