Quorum Sensing in Pseudomonas savastanoi pv. savastanoi and Erwinia toletana: Role in Virulence and Interspecies Interactions in the Olive Knot

The olive knot disease (Olea europea L.) is caused by the bacterium Pseudomonas savastanoi pv. savastanoi. P. savastanoi pv. savastanoi in the olive knot undergoes interspecies interactions with the harmless endophyte Erwinia toletana; P. savastanoi pv. savastanoi and E. toletana colocalize and form a stable community, resulting in a more aggressive disease. P. savastanoi pv. savastanoi and E. toletana produce the same type of the N-acylhomoserine lactone (AHL) quorum sensing (QS) signal, and they share AHLs in planta. In this work, we have further studied the AHL QS systems of P. savastanoi pv. savastanoi and E. toletana in order to determine possible molecular mechanism(s) involved in this bacterial interspecies interaction/cooperation. The AHL QS regulons of P. savastanoi pv. savastanoi and E. toletana were determined, allowing the identification of several QS-regulated genes. Surprisingly, the P. savastanoi pv. savastanoi QS regulon consisted of only a few loci whereas in E. toletana many putative metabolic genes were regulated by QS, among which are several involved in carbohydrate metabolism. One of these loci was the aldolase-encoding gene garL, which was found to be essential for both colocalization of P. savastanoi pv. savastanoi and E. toletana cells inside olive knots as well as knot development. This study further highlighted that pathogens can cooperate with commensal members of the plant microbiome. IMPORTANCE This is a report on studies of the quorum sensing (QS) systems of the olive knot pathogen Pseudomonas savastanoi pv. savastanoi and olive knot cooperator Erwinia toletana. These two bacterial species form a stable community in the olive knot, share QS signals, and cooperate, resulting in a more aggressive disease. In this work we further studied the QS systems by determining their regulons as well as by studying QS-regulated genes which might play a role in this cooperation. This represents a unique in vivo interspecies bacterial virulence model and highlights the importance of bacterial interspecies interaction in disease.