Metabolic adaptations of Pinus massoniana to low phosphorus and acidic aluminium with or without ectomycorrhization

Phosphorus (P) deficiency and aluminium toxicity often coexist and are the major constraints hampering plant growth in acidic soils. Plants have evolved several strategies to cope with these adverse environments, including the recruitment of beneficial microbes, especially mycorrhizal fungi. By employing microscopic observation, histochemical staining, and UPLC-MS/MS techniques, we analyzed the aluminium deposition and cell viability of root, symbiotic process with ECM fungus Suillus bovinus, as well as the metabolic adjustments in Pinus massoniana under low P and/or acidic aluminium conditions. The formation process of mycorrhizae between P. massoniana and S. bovinus was not affected by low P and acidic aluminium treatments, but low P postponed the development of mantle and Hartig nets. The colonization rate increased in the presence of acidic aluminium, and S. bovinus inoculation improved root cell viability and promoted the growth of P. massoniana under low P and/or acidic aluminium conditions. The metabolic responses of nonmycorrhizal and mycorrhizal P. massoniana to low P and/or acidic aluminium were different. Low P and/or acidic aluminium significantly increased the accumulation of phenolic acids and altered starch and sucrose metabolism in roots of nonmycorrhizal P. massoniana. However, for mycorrhizal P. massoniana, low P and/or acidic aluminium treatments increased the organic acids content of shared differentially accumulated metabolites (DAMs). Furthermore, acidic aluminium mainly altered arginine and proline metabolism, and phenylalanine metabolism was affected under low P conditions, linoleic acid metabolism was changed under the combined treatment of low P and acidic aluminium in mycorrhizal P. massoniana. These findings enhance our comprehension of how ECM fungi assist P. massoniana in coping with P deficiency and aluminium toxicity in acidic environments and provide a theoretical basis for ECM fungal application during seedling cultivation in acidic soils.