The Plant Pathogen Pantoea ananatis Produces N -Acylhomoserine Lactone and Causes Center Rot Disease of Onion by Quorum Sensing

A number of gram-negative bacteria have a quorum-sensing system and produceN -acyl-l -homoserine lactone (AHL) that they use them as a quorum-sensing signal molecule.Pantoea ananatis is reported as a common colonist of wheat heads at ripening and causes center rot of onion. In this study, we demonstrated thatP. ananatis SK-1 produced two AHLs,N -hexanoyl-l -homoserine lactone (C6-HSL) andN -(3-oxohexanoyl)-l -homoserine lactone (3-oxo-C6-HSL). We cloned the AHL-synthase gene (eanI ) and AHL-receptor gene (eanR ) and revealed that the deduced amino acid sequence of EanI/EanR showed high identity to those of EsaI/EsaR fromP. stewartii . EanR repressed theean box sequence and the addition of AHLs resulted in derepression ofean box. Inactivation of the chromosomaleanI gene in SK-1 caused disruption of exopolysaccharide (EPS) biosynthesis, biofilm formation, and infection of onion leaves, which were recovered by adding exogenous 3-oxo-C6-HSL. These results demonstrated that the quorum-sensing system involved the biosynthesis of EPS, biofilm formation, and infection of onion leaves inP. ananatis SK-1.