3-Mercaptopropionic acid as a potential biocontrol factor contributes to plant protection efficacy of Pseudomonas protegens CHA0

<i>Pseudomonas protegens</i> CHA0 and other fluorescent pseudomonads suppress plant diseases by producing a number of metabolites with antibiotic activities in the rhizosphere. However, limited information is available on the physiological functions of most of these metabolites. We herein describe the detection of 3-mercaptopropionic acid (3-MPA), a thiol compound, based on the metabolome of the supernatant of strain CHA0. 3-MPA exhibited antibiotic activity against oomycete and fungal pathogens and suppressed <i>Pythium</i> damping-off and root rot in cucumber. 3-MPA also exhibited antibiotic activity without direct contact with pathogens, indicating the volatilization effect of this compound. We identified a homologous gene encoding 3-MPA dioxygenase in the CHA0 genome and the production level of 3-MPA was elevated in a <i>3mdo</i>-negative mutant. Growth inhibitory activity against oomycete and fungal pathogens and plant protection efficacy exhibited by the <i>3mdo</i>-negative mutant were stronger than those of wild-type CHA0. These results suggest that 3-MPA is biocontrol factor of strain CHA0.