Swarming by Pseudomonas syringae B728a requires gacS (lemA) and gacA but not the acyl-homoserine lactone biosynthetic gene ahlI

Pseudomonas syringae pv. syringae B728a, a causal agent of bacterial brown spot on snap beans, swarms with a characteristic dendritic pattern on semisolid (0.4%) agar plates. Filamentation of swarming cells of B728a was not observed. Mutations in either the gacS (formerly lemA) or gacA gene of B728a eliminate the ability of this P. syringae isolate to swarm without obvious effects on bacterial motility. Three field isolates showed a similar dependence on gacS for swarming. Since gacS and gacA mutants are known to be deficient in N-acyl-L-homoserine lactone (acyl-HSL) production, a mutant was constructed by disruption of the ahlI gene of B728a. This mutant did not make any acyl-HSL detectable by the Agrobacterium traG::lacZ reporter system, yet was unaffected in its ability to swarm. Other phenotypes of gacS and gacA mutations were similarly unaffected in the ahlI mutant.