Swarming by Pseudomonas syringae B728a Requires gacS ( lemA ) and gacA but Not the Acyl-Homoserine Lactone Biosynthetic Gene ahlI

Abstract

ABSTRACT 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.