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Walnut (Juglans regiaL.), an important economic forest worldwide, grows in mountainous areas prone to soil drought, which reduces its growth and yield. This study aimed to analyze whether Serendipita indica, a culturable endophytic fungus, colonizes walnut roots, hence improving walnut growth and resistance to drought stress. A high density of chlamydospores was observed within the cortical cells of S. indica-colonized roots, with root colonization ranged from 58.3 to 79.1%, as well as a significant inhibition under drought versus non-drought. Drought treatment suppressed aboveground performance and root morphology variables, while S. indica significantly boosted them. Inoculation with S. indica also significantly increased leaf nitrogen balance index and betaine levels under drought. In leaf mitochondria, drought treatment led to increased levels of hydrogen peroxide (H2O2) and superoxide anion radicals (O-2(center dot-)), with greater increases observed in uninoculated plants than in inoculated plants. However, inoculation with S. indica significantly decreased H2O2 and O-2(center dot-) levels in walnut leaves, with inoculated plants experiencing a greater substantial decrease than uninoculated plants. Drought treatment and S. indica collectively significantly increased leaf superoxide dismutase and catalase activities in mitochondria, allowing inoculated plants to maintain low oxidative damage by decreasing malondialdehyde levels. S. indica also boosted leaf juglone levels by promoting beta-glucosidase activity under drought conditions. In conclusion, S. indica is a helpful endophytic fungus for increasing walnut growth and tolerance to drought by activating antioxidant defenses in mitochondria, which presents a promising pathway for sustainable agroforestry practices.

来源平台：PLANT GROWTH REGULATION