Dual role of HCN produced by the endophyte Pseudomonas urmiensis A32 in biocontrol and the defence response of pepper.

Abstract

Plant-associated Pseudomonas species are widely recognised for their ecological versatility and potential as sustainable biocontrol and plant growth-promoting agents. Among their diverse metabolites, hydrogen cyanide (HCN) is traditionally considered a potent antimicrobial compound, yet its broader role in plant-microbe interactions remains underexplored. This study investigated Pseudomonas urmiensis A32, an endophytic bacterium previously shown to enhance drought tolerance and disease resistance in pepper (Capsicum annuum). Whole-genome analysis revealed genes involved in nutrient acquisition, stress protection, and competitive exclusion, including secondary metabolite clusters for carotenoid and HCN biosynthesis. Wild-type A32 showed increased tolerance to oxidative stress, highlighting a redox-active role for HCN. Seed inoculation with A32 reduced disease symptoms and pathogen load in pepper plants challenged with Xanthomonas euvesicatoria, indicating that HCN contributes not only to direct pathogen antagonism but also to modulation of host immune responses. Biochemical analyses showed decreased activity of antioxidant enzymes and increased phenylalanine ammonia-lyase activity. Gene expression analyses revealed upregulation of PR1 and the antimicrobial peptide gene DEF1, indicating enhanced activation of defence genes. These findings demonstrate that HCN acts as a multifunctional metabolite that fine-tunes the balance between reactive oxygen species detoxification and signalling, and integrates microbial biocontrol with host immune priming. In summary, this work highlights the ecological adaptability of P. urmiensis A32 and its potential as an effective bioinoculant for sustainable crop protection.