Plasmid Profiles of Pseudomonas syringae pv. savastanoi.

Pseudomonas syringae pv. savastanoi (Psv) is the causal agent of olive knot disease. Forty nine bacterial isolates of Psv were isolated from knots on several hosts at the western area of Libya: 31 isolates from olive Olea europaea, 17 isolates from athel Tamarix aphylla (on which the disease is documented for the first time) and one isolate from retem Retama raetam. The isolates were identified on the basis of their morphological characteristics and LOPAT profile. They produced round, white creamy colonies on selective media (PAB and KB), from which 15 isolates produced fluorescent pigments. With the exception of other LOPAT analysis, all isolates were pectinolytic activity and arginine dihydrolase negative. some isolates were levan positive (10 isolates) and oxidase positive (12 isolates), while the rest of isolates were negative for both tests. Most of the isolates induced a hypersensitive reaction on tobacco and pepper leaves. Plasmid profile analysis of Psv strains indicated high genetic variability between the isolates of the same host or different hosts. Most of the olive isolates were classified according to their plasmid profile into five groups (A, B, C, D, F), however, the athel isolates were separated into three different groups designated as G, K, N, on the other hand, group E and H contained mixed isolates from different hosts: group H included two isolates from olive (OS25w and OS42w) and one isolate from retem (Ra1); only two strains OS6w and Ta5y from olive and athel respectively were classified within the same group designated as E. The remaining seven isolates from all hosts were unique. The total number of plasmids ranged from 1-4 for the strains tested, while the DNA content varied widely ranging from 540 to 13550 bp. No plasmid were detected in 14 isolates tested. Genome analysis based on plasmid profiles indicated the great potential of this technique to discriminate between the isolates of Psv from different hosts and geographical regions.

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

ABSTRACT 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.