Root colonization by Serendipita indica triggers resistance against stem rot in Arachis hypogaea L

Root colonization by the endophytic fungus Serendipita indica (Si) effectively inhibited Sclerotium rolfsii (Sr) and controlled peanut stem rot. In vitro and microscopic analyses confirmed the strong antifungal activity of S. indica against S. rolfsii. Pot experiments revealed that Si markedly reduced the disease index to 5.12 ± 0.49% in resistant 'Luhua11' (L) and 8.65 ± 0.53% in susceptible 'Huayu6103' (H), indicating > 90% protection. Furthermore, Si colonization increased the biomass and health of the two peanut cultivars. Antioxidant enzyme activities were significantly increased: peroxidase (POD) activity in roots increased by 39.28-110.8% (24-96 h), superoxide dismutase (SOD) activity was increased in 'LSi-Sr' and 'HSi-Sr' at 48-96 h, ascorbate peroxidase (APX) activity was increased in 'LSi-Sr' (≥ 48 h) and 'HSi-Sr' (≥ 72 h), phenylalanine ammonia-lyase (PAL) activity peaked at 72 h, and catalase (CAT) activity under 'Si-Sr' exceeded that under 'Sr'. Similarly, hydrogen peroxide (H2O2) was detected within 96 h after Sr infection, whereas the malondialdehyde (MDA) content decreased. Si significantly increased jasmonic acid (JA) (500-600 ^{pmol L⁻¹}) and salicylic acid (SA) (900-1100 ^{pmol L⁻¹}) accumulation in peanut roots, with stronger induction in the Huayu103 cultivar under Si + Sr and Si + Sr treatments, peaking at 72-96 h, whereas leaf levels remained comparatively low and less variable. Defense gene expression was activated, with greater induction of AhPAL, AhNPR1, and AhPR10 in the 'Luhua11' cultivar than in 'Huayu6103' at 96 h; AhCSD1 increased in 'LSi-Sr' at 24 h and 96 h and in 'HSi-Sr' at 96 h; AhAPX was upregulated in 'LSi-Sr' at 96 h; and AhCAT was greater in 'LSi-Sr' (72 h) and 'HSi-Sr' (96 h). This study highlights S. indica as a potent biocontrol agent for managing peanut stem rot caused by S. rolfsii.