Plant Defense Stimulation by Natural Isolates of Bacillus Depends on Efficient Surfactin Production

Some plant-associated Bacillus strains produce induced systemic resistance (ISR) in the host, which contributes to their protective effect against phytopathogens. Little is known about the variety of elicitors responsible for ISR that are produced by Bacillus strains. Working with a particular strain, we have previously identified the surfactin lipopeptide as a main compound stimulating plant immune-related responses. However, with the perspective of developing Bacillus strains as biocontrol agents, it is important to establish whether a central role of surfactin is generally true for isolates belonging to the B. subtilis/amyloliquefaciens complex. To that end, we set up a comparative study involving a range of natural strains. Their secretomes were first tested for triggering early defense events in cultured tobacco cells. Six isolates with contrasting activities were further evaluated for ISR in plants, based both on macroscopic disease reduction and on stimulation of the oxylipin pathway as defense mechanism. A strong correlation was found between defense-inducing activity and the amount of surfactin produced by the isolates. These results support the idea of a widespread role for surfactin as a nonvolatile elicitor formed by B. subtilis/amyloliquefaciens, and screening for strong surfactin producers among strains naturally secreting multiple antibiotics could be an efficient approach to select good candidates as biopesticides.