Bacillus siamensis strain B30 as a biocontrol agent for enhancing systemic resistance and mitigating bean yellow mosaic virus infestation in faba bean plants

Bacillus siamensis strain B30 was isolated from the rhizosphere of faba bean and molecularly identified using the 16S rRNA (Acc# OQ878652) and rpoB (Acc# OR061338) genes. Under greenhouse conditions, the application of B30, whether through soil inoculum or foliar application of its culture filtrate, significantly promoted growth and triggered systemic resistance against bean yellow mosaic virus (BYMV) infection in faba bean. As a result, the disease severity and viral accumulation levels were significantly reduced, by about 64% and 88%, respectively, for foliar application treatment. Furthermore, B30 treatments resulted in significant elevations in antioxidant enzymes such as catalase (CAT), ascorbate peroxidase (APX), polyphenol oxidase (PPO), and superoxide dismutase (SOD), as well as total chlorophyll content and 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity. A significant decrease in oxidative stress markers, namely hydrogen peroxide (H₂O₂) and malondialdehyde (MDA), was observed. These changes were accompanied by an upregulation of the transcriptional levels of pathogenesis-related genes. Furthermore, HPLC analysis revealed that most polyphenolic compounds were induced and accumulated in plants undergoing B30 treatments. GC–MS analysis revealed that the ethyl acetate extract of B30 culture filtrate contains significant amounts of 3,3-dimethylbutan-1,2-dione oxime; 3,4-dihdro-2H-1,5-(3"-T-butyl) benzodioxepine;1-nonadecene; 1-dodecanol, 3,7,11-trimethyl-; 1-docosene; 3,7,11-trimethyl-; pyrrolo[1,2-a] pyrazine-1,4-dione, hexahydro-3-(2-methylpropyl)-; 2,2-dideutero octadecanal and á-sitosterol. The capability of B. siamensis B30 to boost faba bean growth and induce systemic resistance against BYMV infection implies that it could be utilized as a biocontrol agent, a viable and environmental approach to protecting faba bean plants from BYMV infection. This is the first time B. siamensis has been reported to have antiviral activity against plant viral infections.