Lactococcus lactis subsp. lactis as a biocontrol agent against phytopathogens causing rice bacterial leaf blight

Bacterial leaf blight (BLB) disease in rice caused by Xanthomonas oryzae pv. oryzae (Xoo) and Pantoea ananatis (Pan), is a widespread problem that results in substantial economic losses in rice-growing countries, including Malaysia. Farmers commonly rely on chemical controls to treat this disease, which are not safe for human health and the environment. Alternatively, Lactic acid bacteria (LAB) have gained attention for their potential in sustainable agricultural practices, benefitting human health and the environment. This study hypothesized that LAB, specifically Lactococcus lactis subsp. lactis (RBX7), have antimicrobial activity against phytopathogens, namely Xoo and Pan. The research aims to determine the antimicrobial activity of RBX7 and identify the antimicrobial compounds likely responsible for disease suppression. The antimicrobial cell-free supernatant (CFS) of RBX7 was obtained and tested for antimicrobial activity against Xoo and Pan using multiple methods, i.e. agar well diffusion, time-kill assays, detached leaf assays, and visualized using scanning electron microscopy (SEM). Metabolomic profiling was conducted to identify potential antimicrobial metabolites using LC-MS/MS-QTOF. Finally, the antibacterial effects of LAB were confirmed by in planta experiment. The detached leaf assay showed no lesions following inoculation with RBX7 live culture and RBX7 CFS, in line with the results from agar well diffusion assays. SEM analysis showed Pan cell rupture and Xoo cell shrinkage when treated with RBX7. Metabolomic analysis of RBX7 identified antimicrobial compounds such as cefixime, cinchonidine, antimycin A, and bufalin. Finally, in planta assays revealed that both RBX7 CFS and the live culture effectively suppressed BLB symptoms in rice seedlings. This study highlights a safer and more sustainable approach for controlling BLB in rice by utilizing RBX7 as a biocontrol agent, advancing efforts toward achieving sustainable development goals in rice farming.