Volatile-mediated inhibitory activity of the biocontrol agent Lysobacter capsici AZ78 as a result of multiple factors interaction

Plant beneficial rhizobacteria are able to inhibit the growth of soilborne phytopathogenic microorganisms through the release of a relevant number of volatile compounds. Based on this, we investigated the ability of the biocontrol agent Lysobacter capsici AZ78 (AZ78) to produce volatile organic compounds (VOCs) that may contribute to its efficacy in controlling soilborne phytopathogenic microorganisms. AZ78 significantly reduced the growth of Pythium ultimum, Rhizoctonia solani and Sclerotinia minor in split Petri dish assays. The GC-MS analysis revealed that AZ78 produce 22 VOCs and most of them were putatively identified as mono- and dialkylated methoxypyrazines. Exposure to 2,5-dimethylpyrazine, 2-ethyl-3-methoxypyrazine and 2-isopropyl-3-methoxypyrazine determined a drastic reduction of Pythium ultimum, Rhizoctonia solani and Sclerotinia minor mycelium growth in split Petri dish assays. However, the discrepancy of the toxicity between the quantity of pyrazines and the AZ78 lead us to further investigate the volatile-mediated inhibitory activity of the biocontrol bacterium. Further experiments revealed the ability of AZ78 cells to produce ammonia that caused the alkalinization of the physically separated culture medium in split Petri dishes assays. Results achieved in this work clearly demonstrated that VOCs, ammonia and the alkalinization of growth medium contribute to the overall inhibitory activity of AZ78 against soilborne phytopathogenic microorganisms.