Inferring the Evolutionary History of the Plant Pathogen Pseudomonas syringae from Its Biogeography in Headwaters of Rivers in North America, Europe, and New Zealand

Abstract

Nonhost environmental reservoirs of pathogens play key roles in their evolutionary ecology and in particular in the evolution of pathogenicity. In light of recent reports of the plant pathogen Pseudomonas syringae in pristine waters outside agricultural regions and its dissemination via the water cycle, we have examined the genetic and phenotypic diversity, population structure, and biogeography of P. syringae from headwaters of rivers on three continents and their phylogenetic relationship to strains from crops. A collection of 236 strains from 11 sites in the United States, in France, and in New Zealand was characterized for genetic diversity based on housekeeping gene sequences and for phenotypic diversity based on measures of pathogenicity and ice nucleation activity. Phylogenetic analyses revealed several new genetic clades from water. The genetic structure of P. syringae populations was not influenced by geographic location or water chemistry, whereas the phenotypic structure was affected by these parameters. Comparison with strains from crops revealed that the metapopulation of P. syringae is structured into three genetic ecotypes: a crop-specific type, a water-specific type, and an abundant ecotype found in both habitats. Aggressiveness of strains was significantly and positively correlated with ice nucleation activity. Furthermore, the ubiquitous genotypes were the most aggressive, on average. The abundance and diversity in water relative to crops suggest that adaptation to the freshwater habitat has played a nonnegligible role in the evolutionary history of P. syringae. We discuss how adaptation to the water cycle is linked to the epidemiological success of this plant pathogen.

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

“…Six reference strains were also included. Samples were part of a survey for hypersensitive incompetent strains that included a range of environmental substrates collected from 2004 to 2010, mostly in alpine regions and outside of agricultural fields (Table 1) and processed as described previously (Morris et al, 2007(Morris et al, , 2010 to determine the density of P. syringae populations. Phenotypic traits that confirm identity as P. syringae were determined as described previously (absence of arginine dihydrolase and cytochrome oxidase).…”

“…Védrantais) was quantified by injecting 50 ml of bacterial suspensions (prepared as for hypersensitivity tests) into the junction of the cotyledons of 12 plants at the cotyledon stage (ca. 10 days after sowing) as described previously (Morris et al, 2010). Plants were incubated for 7 days at 25 1C during the daylight period (16 h) and 18 1C at night (8 h), and symptoms were observed daily after inoculation.…”

“…Gene sequencing and phylogenetic analysis From previous studies (Mohr et al, 2008;Clarke et al, 2010;Morris et al, 2010), the clade identity was known for 19 strains used here relative to the phylogenetic tree reported by Morris et al (2010), and based on four housekeeping genes (rpoD, gyrB, cts and gapA). For the remaining strains, assignment to clades was based on our previous results that strains could be accurately assigned to clades based on DNA similarity in the cts sequence using a threshold dissimilarity of 1.8% (Morris et al, 2010).…”

“…For the remaining strains, assignment to clades was based on our previous results that strains could be accurately assigned to clades based on DNA similarity in the cts sequence using a threshold dissimilarity of 1.8% (Morris et al, 2010). The sequence of the cts gene was determined as previously described (Morris et al, 2010).…”

“…Recent work on the ecology of P. syringae in nonagricultural habitats revealed the existence of nine new clades of this bacterium that had not been previously observed (Morris et al, 2010). All phylogenetic descriptions of this species before the study of nonagricultural habitats focused on strains from plants.…”