Quorum-sensing signaling is required for production of the antibiotic pyrrolnitrin in a rhizospheric biocontrol strain of Serratia plymuthica

One mechanism that bacteria have adopted to regulate the production of antimicrobial compounds is population-density-dependent LuxRI-type quorum sensing (QS), exploiting the production of N-acyl homoserine lactone (AHL) autoinducer signals. In biocontrol bacteria, most known cases involve the AHL control of phenazine antibiotics production by rhizospheric pseudomonads. This work is the first to demonstrate that phenazines are not the only group of biocontrol-related antibiotics whose production is regulated by QS systems. Strain HRO-C48 of Serratia plymuthica isolated from the rhizosphere of oilseed rape and described as a chitinolytic bacterium, which protects crops against Verticillium wilt, was also shown to produce wide-range antibiotic pyrrolnitrin and several AHLs, including N-butanoyl-HSL, N-hexanoyl-HSL and N-3-oxo-hexanoyl-HSL (OHHL). The genes splI and splR, which are analogues of luxI and luxR genes from other Gram-negative bacteria, were cloned and sequenced. The mutant AHL-4 (splI::miniTn5) was simultaneously deficient in the production of AHLs and pyrrolnitrin, as well as in its ability to suppress the growth of several fungal plant pathogens in vitro. However, pyrrolnitrin production could be restored in this mutant by introduction of the splIR genes cloned into a plasmid or by addition of the conditioned medium from strain C48 or OHHL standard to the growth medium.