Isolation, identification, and biocontrol mechanisms of endophytic Burkholderia vietnamiensis C12 from Ficus tikoua Bur against Rhizoctonia solani

Rice sheath blight is a devastating fungal disease caused by Rhizoctonia solani. Biocontrol utilizing endogenous microorganisms has been considered a safe and sustainable strategy. To obtain a new type of antagonistic bacteria with a high biocontrol effect on rice sheath blight, over 50 endophytic bacteria were isolated from the root of the medicinal plant Ficus tikoua Bur and screened for their antifungal activity by dual culture assay. Among them, strain C12 showed the strongest inhibitory activity against R. solani (inhibition rate 94.78%), and exhibited a broad-spectrum antifungal activity. According to the phylogenetic tree based on 16S rDNA and recA genes, C12 was identified as Burkholderia vietnamiensis, belonging to the Burkholderia cepacia complex, and safe for plants and humans. Through the study on B. vietnamiensis C12, the strain could secrete cell wall degrading enzymes (protease and chitinase), produce IAA, and siderophores, fix nitrogen, and solubilize phosphorus. The bacterial culture filtrate (although treated at 121 °C) of strain C12 and its ethyl acetate extract significantly inhibited the growth of R solani and resulted in distorted hyphae. Moreover, when the extract was 50 μg/mL, 100 μg/mL, 200 μg/mL, and 500 μg/mL, the diameter of mycelium was decreased by 50.44%, 56.43%, 67.27%, and 74.80% respectively, the number of sclerotia was decreased by 85.45%, 90.42%, 96.83%, and 99.22% respectively. Multiple antifungal substances in extracts of C12 were detected through LC-MS. Strain C12 gave a 70.98% reduction in lesion length in a detached inoculated leaf assay. In greenhouse and field tests, strain C12 significantly reduced the disease index of rice sheath blight, and the control efficiency reached 68.75% and 66.52%, respectively. Furthermore, C12 enhanced the resistance of rice to R. solani by increasing the activities of defense enzymes SOD, PAL, POD, CAT, and PPO. The strain also exhibited plant-promoting abilities on rice. The results indicated that B. vietnamiensis C12 had great biocontrol potential and could be used as a candidate strain for green control of rice sheath blight.