Pseudomonas syringae naturally lacking the canonical type III secretion system are ubiquitous in nonagricultural habitats, are phylogenetically diverse and can be pathogenic

Abstract

The type III secretion system (T3SS) is an important virulence factor of pathogenic bacteria, but the natural occurrence of variants of bacterial plant pathogens with deficiencies in their T3SS raises questions about the significance of the T3SS for fitness. Previous work on T3SS-deficient plant pathogenic bacteria has focused on strains from plants or plant debris. Here we have characterized T3SS-deficient strains of Pseudomonas syringae from plant and nonplant substrates in pristine nonagricultural contexts, many of which represent recently described clades not yet found associated with crop plants. Strains incapable of inducing a hypersensitive reaction (HR À ) in tobacco were detected in 65% of 126 samples from headwaters of rivers (mountain creeks and lakes), snowpack, epilithic biofilms, wild plants and leaf litter and constituted 2 to 100% of the P. syringae population associated with each sample. All HR À strains lacked at least one gene in the canonical hrp/hrc locus or the associated conserved effector locus, but most lacked all six of the genes tested (hrcC, hrpL, hrpK1, avrE1 and hrpW1) and represented several disparate phylogenetic clades. Although most HR À strains were incapable of causing symptoms on cantaloupe seedlings as expected, strains in the recently described TA-002 clade caused severe symptoms in spite of the absence of any of the six conserved genes of the canonical T3SS according to PCR and Southern blot assays. The phylogenetic context of the T3SS variants we observed provides insight into the evolutionary history of P. syringae as a pathogen and as an environmental saprophyte.

Comparative Genomics of Pathogenic and Nonpathogenic Strains of Xanthomonas arboricola Unveil Molecular and Evolutionary Events Linked to Pathoadaptation

“…These genes compose the type1 Restriction-Modification system known in Escherichia coli to be involved in phage defense mechanism (Makarova et al, 2013 ). A BLAST research with alternative T3SSs observed in other bacteria did not lead to the identification of other T3SS (Araki et al, 2006 ; Diallo et al, 2012 ). In CFBP 7651, the hrp / hrc cluster was followed by a specific region of about 16.7 kb.…”

Inheritance of Pantoea type III secretion systems through both vertical and horizontal transfer

“…If this is the case, the relatively limited distribution of the Hrc 1 system would be suggestive of widespread T3SS loss, possibly early in the divergence of the different species. In P. syringae, the T3SS was present in the species before diversification, such that a T3SS gene tree is congruent with the P. syringae species tree (Sawada et al 1999;Sarkar and Guttman 2004); however, a more recent, broad-scale survey showed that many strains appear to have lost their typical T3SS (Diallo et al 2012), and acquired an atypical T3SS (Clarke et al 2010). This may explain what we observe in extant Pantoea strains, with loss of the ancestral PSI-1-like system, and gain of the PSI-1a and PSI-1b systems later from other members of the Enterobacteriaceae.…”

Light Regulation of Swarming Motility in Pseudomonas syringae Integrates Signaling Pathways Mediated by a Bacteriophytochrome and a LOV Protein

“…The species Pseudomonas syringae is found in several habitats that are dominated by light, including leaves where it can function as a foliar pathogen, surface soils, mountain creeks and lakes, and snowpack (7). The genome sequences of several strains of this species indicate the presence of multiple photosensory proteins.…”