Objective This study aimed to investigate the induction mechanism of defense responses in Camellia oleifera induced by different application methods (leaf spraying and root irrigation) of Bacillus tequilensis.

Method B. tequilensis YYC155, which exhibits strong inhibitory effects against Colletotrichum fructicola (the causal agent of anthracnose on C. oleifera), was employed to induce defense responses in C. oleifera seedlings through leaf spraying and root irrigation treatments. Subsequently, the activities of defense-related enzymes and the contents of substance compounds were determined, and transcriptomic analysis was conducted to elucidate the underlying regulatory mechanisms.

Result Compared with the control group, both treatments significantly enhanced the activities of defense-related enzymes in C. oleifera, including phenylalanine ammonia-lyase (PAL), cinnamate-4-hydroxylase (C4H), 4-coumarate coenzyme A ligase (4CL), and chalcone isomerase (CHS). Additionally, these treatments promoted the accumulation of total phenols and flavonoids. Transcriptomic analysis revealed that there were 652 differentially expressed genes (DEGs) identified between leaf spraying samples (YYC155P) and root irrigation samples (YYC155G), which were enriched in 66 KEGG pathways. Among these pathways, 72.41% of the DEGs in the four antioxidant system-related pathways were upregulated, while 47.37% of the DEGs in the eight secondary metabolism-related pathways were upregulated.

Conclusion Leaf spraying treatment primarily activates defense responses in C. oleifera through secondary metabolic pathways, whereas root irrigation treatment relies more on the enhancement of the antioxidant system.