Characterizing the mode of action of Brevibacillus laterosporus B4 for control of bacterial brown strip of rice caused by A. avenae subsp. avenae RS-1.

Biological control efficacy of Brevibacillus laterosporus B4 associated with rice rhizosphere was assessed against bacterial brown stripe of rice caused by Acidovorex avenae subsp. avenae. A biochemical bactericide (chitosan) was used as positive control in this experiment. Result of in vitro analysis indicated that B. laterosporus B4 and its culture filtrates (70%; v/v) exhibited low inhibitory effects than chitosan (5 mg/ml). However, culture suspension of B. laterosporus B4 prepared in 1% saline solution presented significant ability to control bacterial brown stripe in vivo. Bacterization of rice seeds for 24 h yielded a greater response (71.9%) for controlling brown stripe in vivo than chitosan (56%). Studies on mechanisms revealed that B. laterosporus B4 suppressed the biofilm formation and severely disrupted cell membrane integrity of A. avenae subsp. avenae, causing the leakage of intracellular substances. In addition, the expression level of virulence-related genes in pathogen recovered from biocontrol-agent-treated plants showed that the genes responsible for biofilm formation, motility, niche adaptation, membrane functionality and virulence of A. avenae subsp. avenae were down-regulated by B. laterosporus B4 treatment. The biocontrol activity of B. laterosporus B4 was attributed to a substance with protein nature. This protein nature was shown by using ammonium sulfate precipitation and subsequent treatment with protease. The results obtained from this study showed the potential effectiveness of B. laterosporus B4 as biocontrol agent in control of bacterial brown stripe of rice.