Comprehensive evaluation and mechanisms of Bacillus velezensis AX22 against rice bacterial blight

Abstract

Rice bacterial blight (BB), caused by Xanthomonas oryzae pv. oryzae (Xoo) poses a serious threat to global rice production, leading to substantial yield losses. Although biocontrol strategies utilizing antagonistic microorganisms have shown potential, systematic evaluations of their comprehensive efficacy and underlying mechanisms remain limited. In this study, a promising biocontrol agent, AX22, was isolated from the rhizosphere soil of BB-infected rice and evaluated through a novel multi-index assessment framework that integrates direct antagonistic activity with indirect plant growth-promoting and systemic resistance-inducing traits, offering a more comprehensive evaluation compared to conventional single-trait or dual-trait screening methods. AX22 exhibited strong antagonistic activity, along with the ability to produce exopolysaccharides (EPS), siderophores (SID), indole-3-acetic acid (IAA), and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase. Pot experiments confirmed that AX22 significantly enhanced rice disease resistance, reducing the BB index by 63.53 %, promoting plant growth, and inducing systemic resistance. Whole-genome sequencing identified AX22 as Bacillus velezensis and revealed multiple biocontrol-related genes, particularly a key difficidin biosynthetic gene cluster. Antibacterial assays showed that AX22 cell-free supernatant and purified difficidin effectively disrupted Xoo cellular morphology, resulting in nucleic acid and protein leakage and suppressing pathogenic traits such as biofilm formation, EPS production, and extracellular enzyme activity. These findings establish AX22 as a potent biocontrol agent against Xoo, with difficidin playing a critical role in its antagonistic effects. This study presents a multi-parameter evaluation framework for biocontrol agents and provides a foundation for their application in sustainable rice bacterial blight management.