Ectomycorrhizal symbiosis shapes root exudation across discrete stages and phosphorus and nitrogen limitation in Pinus yunnanensis seedlings

Mycorrhiza, a mutualistic plant-fungal symbiosis, is essential for plant nutrients acquisition and stresses resistance, with root exudates playing crucial roles in these processes. Although ectomycorrhizal fungal colonization is known to influence root physiology, its specific effects on exudation profiles remain largely unexplored. In this study, Lactarius deliciosus - Pinus yunnanensis ectomycorrhizal seedlings were established using pouch and pot systems to compare root exudation profiles between mycorrhizal and non-mycorrhizal seedlings, track exudates across ectomycorrhizal formation stages (signal recognition, initial colonization, rapid development, maturity), and analyze exudative responses to early phosphorus and nitrogen deficiencies. The impacts of P-deficiency-induced ectomycorrhizal root exudates on phosphate-solubilizing bacteria proliferation were also assessed. Results showed L. deliciosus inoculation altered exudate compositions by introducing organohalogen and organometallic compounds, without changing total carbon exudation. Stage-specific biomarkers (His-Pro, trichloroacetic acid, sulfobacin b, γ-dodecalactone) were annotated during ectomycorrhizal development. Under early phosphorus/nitrogen deficiency, phenylpropanoids and polyketides were the main differential metabolites. Ectomycorrhizal colonization induced root exudates with antioxidant, antibacterial, and antifungal activities. Notably, P-limitation triggered trans-11-octadecenoic acid and FA20:1, which significantly stimulated phosphate-solubilizing bacteria growth. Trans-11-octadecenoic acid enriched Pseudomonas sp. NR6-04, without influencing its extracellular phosphatase or phytase activities. Our findings indicate that colonization by the ectomycorrhizal fungus L. deliciosus modified root exudation profiles and their responses to phosphorus and nitrogen limitation in P. yunnanensis seedlings. Fungal-symbiosis-associated metabolites significantly enhanced the enrichment of phosphate-mobilizing bacteria and demonstrated key eco-physiological functions. This study reveals new insights underlying the L. deliciosus - P. yunnanensis symbiosis originated by root exudates, highlighting their roles in ectomycorrhizal ecophysiology.