Integrative Omics Analyses of Ectomycorrhizal Fungi Modulate Root Growth and Defense for Mitigating Aluminum Toxicity in Pinus massoniana

Abstract Aluminum (Al) toxicity is a critical environmental factor limiting plant productivity in acidic soils. Some ectomycorrhizal fungi (ECMF) can mitigate Al-induced stress and promote root growth in Pinus massoniana, however, the underlying molecular and metabolic mechanisms have not yet been fully elucidated. In this study, P. massoniana seedlings inoculated with Lactarius deliciosus (Ld) were subjected to acidic Al stress (pH 3.8) at Al3+ concentrations of 0.0 and 1.0 mM. Root growth, transcriptomic, metabolic and hormonal characteristics of the mycorrhizal symbionts were determined and analyzed. The study aimed to identify the key metabolites and metabolic pathways involved in ECMF-enhanced Al stress tolerance of P. massoniana, and to further elucidate on the underlying mechanism of ECMF in improving the Al resistance of P. massoniana from molecular and metabolic-physiological perspectives. Results showed that Ld inoculation significantly enhanced Al tolerance and promoted root growth and branching in P. massoniana. Specifically, it activated the phenylpropanoid-lignin biosynthesis pathway in mycorrhizal symbionts, downregulated carbon metabolism pathways, and reduced intracellular accumulation of citric acid and specific amino acids (L-proline, L-threonine, serine). Furthermore, Ld elevated salicylic acid and gibberellin levels, decreased jasmonic acid content, upregulated growth-promoting genes (MYC2, GH3, TCH4), and downregulated inhibitory genes (ARF9/19, DELLA). This study further refines and clarifies the mechanism underlying ECMF-enhanced Al resistance in P. massoniana, and provides a theoretical basis for the application of ECMF in the ecological restoration of P. massoniana forest areas affected by Al toxicity.