Abstract
The majority of bacterial foliar plant pathogens must invade the apoplast of host plants through points of ingress, such as stomata or wounds, to replicate to high population density and cause disease. How pathogens navigate plant surfaces to locate invasion sites remains poorly understood. Many bacteria use chemical-directed regulation of flagellar rotation, a process known as chemotaxis, to move towards favorable environmental conditions. Chemotactic sensing of the plant surface is a potential mechanism through which foliar plant pathogens home in on wounds or stomata, but chemotactic systems in foliar plant pathogens are not well characterized. Comparative genomics of the plant pathogen Pseudomonas syringae pathovar tomato (Pto) implicated annotated chemotaxis genes in the recent adaptations of one Pto lineage. We therefore characterized the chemosensory system of Pto. The Pto genome contains two primary chemotaxis gene clusters, che1 and che2. The che2 cluster is flanked by flagellar biosynthesis genes and similar to the canonical chemotaxis gene clusters of other bacteria based on sequence and synteny. Disruption of the primary phosphorelay kinase gene of the che2 cluster, cheA2, eliminated all swimming and surface motility at 21 °C but not 28 °C for Pto. The che1 cluster is located next to Type IV pili biosynthesis genes but disruption of cheA1 has no observable effect on twitching motility for Pto. Disruption of cheA2 also alters in planta fitness of the pathogen with strains lacking functional cheA2 being less fit in host plants but more fit in a non-host interaction.
Highlights
Pseudomonas syringae pv. tomato (Pto) is a common bacterial pathogen adapted to live in both agricultural and non-agricultural environments
These results suggest that swarming motility is favored at lower temperatures and twitching motility favored at higher temperatures for pathovar tomato (Pto)
During infection of non-host plants, the microbe benefits from remaining epiphytic, because it is ill-equipped to suppress the strong plant immune responses activated during endophytic invasion. Experimental evidence for both the attenuated elicitation of plant immune responses and extended epiphytic lifestyle of cheA2 mutants will greatly strengthen confidence in this model. These results demonstrate the importance of the chemotactic systems of Pto for bacterial motility and pathogenicity in plants
Summary
Pseudomonas syringae pv. tomato (Pto) is a common bacterial pathogen adapted to live in both agricultural and non-agricultural environments. How to cite this article Clarke et al (2016), Comparative genomics of Pseudomonas syringae pathovar tomato reveals novel chemotaxis pathways associated with motility and plant pathogenicity. The PtoT1 lineage (which includes the well-studied eponymous member PtoT1 (Almeida et al, 2009) has dominated the population for the last 60 years in North America and Europe (Cai et al, 2011). The PtoJL1065 and PtoDC3000 lineages were likely the dominant field populations (Cai et al, 2011). PtoDC3000 is more closely related to pathogens of Brassicaceae than to PtoJL1065 and PtoT1 and its host range includes members of the Brassicaceae family (Yan et al, 2008). Strains in the PtoT1 lineage are specialists in tomato (Cai et al, 2011) but can infect other Solanaceae (Clarke et al, 2014)
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