Synergistic effects of arbuscular mycorrhizal fungi and melatonin on peanut tolerance to saline-alkali stress: integrated physiological and transcriptomic analysis

Soil salinization severely threatens global crop productivity. While arbuscular mycorrhizal fungi (AMF) and melatonin (MT) can individually enhance plant stress tolerance, the interplay between these factors and the associated molecular pathways has yet to be fully elucidated. Accordingly, we designed this study to explore the synergistic role of AMF ( Funneliformis mosseae ) inoculation and exogenous MT application in mitigating saline-alkali stress (SS) in peanut ( Arachis hypogaea L.) seedlings through an integrated physiological and transcriptomic approach. Our results demonstrated that SS severely inhibited growth, disrupted ion homeostasis, impaired photosynthesis, and induced oxidative stress. AMF inoculation alone significantly improved plant performance, while exogenous MT application effectively promoted AMF symbiosis, increasing colonization rates under SS. Critically, the combined + MT+AMF treatment exerted the most robust synergistic effects, resulting in maximum biomass accumulation and photosynthetic efficiency, alongside the lowest Na⁺/K⁺ ratio and the highest antioxidant enzyme activities. Transcriptomic profiling revealed that the combined treatment uniquely upregulated 18 key genes involved in ion transport (i.e., AhNHX2 , AhHKT1 ), antioxidant defense (i.e., AhMn-SOD , AhCAT2 , AhAPX5 ), and photosynthesis (i.e., AhPsbA , AhFBA3 ). qRT-PCR validation confirmed these expression patterns, showing upregulation of nine stress-responsive genes under the combined treatment. Spearman correlation analysis further established significant negative correlations between antioxidant module expression and MDA content, and a positive relationship linking photosynthesis module expression to the rate of net photosynthesis, providing quantitative evidence linking transcriptomic reprogramming to physiological improvement. Our findings elucidate that MT functions as a potent signaling molecule that primes AMF symbiosis, and their combination synergistically orchestrates a multi-layered defense system encompassing ion homeostasis, oxidative stress alleviation, and photosynthetic protection. This work provides a conceptual foundation for applying the AMF‑MT combination as an eco‑friendly strategy to improve peanut seedling performance under saline‑alkali stress. Further field trials are required to evaluate its practical potential for crop productivity in saline‑alkali soils.