Characteristics of biological control and mechanisms of Pseudomonas chlororaphis zm-1 against peanut stem rot

Background: Peanut stem rot is a serious plant disease that causes great economic losses. At present, there are no effective measures to prevent or control the occurrence of this plant disease. Biological control is one of the most promising plant disease control measures. In this study, Pseudomonas chlororaphis subsp. aurantiaca strain zm-1, a bacterial strain with potential biocontrol properties isolated by our team from the rhizosphere soil of Anemarrhena asphodeloides , was studied to control this plant disease. Methods: We prepared extracts of P. chlororaphis zm-1 extracellular antibacterial compounds (PECEs), determined their antifungal activities by confrontation assay, and identified their components by UPLC-MS/MS. The gene knockout strains were constructed by homologous recombination, and the biocontrol efficacy of P. chlororaphis zm-1 and its mutant strains were evaluated by pot experiments under greenhouse conditions and plot experiments, respectively. Results: P. chlororaphis zm-1 could produce extracellular antifungal substances and inhibit the growth of Sclerotium rolfsii , the main pathogenic fungus causing peanut stem rot. The components of PECEs identified by UPLC-MS/MS showed that three kinds of phenazine compounds, i.e., 1-hydroxyphenazine, phenazine-1-carboxylic acid (PCA), and the core phenazine, were the principal components. In particular, 1-hydroxyphenazine produced by P. chlororaphis zm-1 showed antifungal activities against S. rolfsii , but the previously reported 2-hydroxyphenazine did not. The extracellular compounds of two mutant strains, Δ phzH and Δ phzE , was analysed and showed that Δ phzE did not produce any phenazine compounds, and Δ phzH no longer produced 1-hydroxyphenazine but could still produce PCA and phenazine. Furthermore, the antagonistic ability of Δ phzH declined, and that of Δ phzE was almost completely abolished. According to the results of pot experiments under greenhouse conditions, the biocontrol efficacy of Δ phzH dramatically declined to 47.21% compared with that of wild-type P. chlororaphis zm-1 (75.63%). Moreover, Δ phzE almost completely lost its ability to inhibit S. rolfsii (its biocontrol efficacy was reduced to 6.19%). The results of the larger plot experiments were also consistent with these results. Conclusions: P. chlororaphis zm-1 has the potential to prevent and control peanut stem rot disease. Phenazines produced and secreted by P. chlororaphis zm-1 play a key role in the control of peanut stem rot caused by S. rolfsii . These findings provide a new idea for the effective prevention and treatment of peanut stem rot.