Engineering Bacillus velezensis with high production of acetoin primes strong induced systemic resistance in Arabidopsis thaliana.

Many plant growth promoting rhizobacteria such as Bacillus velezensis GJ11 can produce acetoin to trigger induced systemic resistance (ISR) in plants. For improving acetoin production, the mutant strains were respectively constructed by knockout of the gene of bdh (2,3-butanediol dehydrogenase) and gdh (glycerol dehydrogenase) in GJ11, but only GJ11Δbdh produced a high level of acetoin triggering strong ISR against Pseudomonas syringae infection in plants. GJ11Δbdh could induce H₂O₂ accumulation in plants by producing a high level of acetoin. H₂O₂ was necessary for triggering ISR against the pathogen infection because after scavenging H₂O₂ with ascorbic acid or catalase, the inhibition role to pathogen infection induced by acetoin almost disappeared in plants. Further investigation found the plants treated with GJ11Δbdh in an obvious "priming" state, in which the mild immune response was observed such as a slight increase of H₂O₂ production, callose deposition, and enzymes activity related with defence response (e.g. POD, PAL and PPO). The plants in "priming" could rapidly respond to the pathogen infection accompanying with a significant increase of H₂O₂ production, callose deposition, and enzymes activity. Collectively, this study provides new insight into the role of acetoin as a strong elicitor of defense response, and ascribes a new approach to construct the mutant strains with high production of acetoin for triggering stronger ISR against pathogens infection in plants.