Understanding host specificity of Pseudomonas syringae on the Prunus species cherry and plum

PhD project- Michelle Hulin

Pseudomonas syringae is globally important plant pathogen and model organism in the study of plant and microbe interactions. It has a broad host range, infecting over 180 plant species, which include many high-value crops.  The species can be divided into pathovars, which are groups of strains specialised to cause disease on particular hosts, for example pathovar (pv.) morsprunorum infects cherry/plum. The genetic basis of host specificity within P. syringae has not been fully elucidated. It is believed that pathogenicity factors such as effectors and toxins have been adapted to maximise virulence on a particular host, so may be involved in host specificity. Although individual P. syringae strains are highly specialised, there appears to have been many host jumps within the species where the pathogen evolves to be able to infect a distantly related plant species. This represents a paradox, as during coevolution with the host you would expect a strains repertoire of virulence factors to become fine-tuned towards a particular host and therefore reduce its capability to infect other plant species.

My PhD will explore the genetic basis of host specificity by using strains that cause bacterial canker on Prunus species. Three highly divergent groups of strains infect sweet cherry and plum (pv morsprunorum race 1, race 2 and pv. syringae). I will use comparative genomics to compare the effector repertoires and other pathogenicity factors of these strains to try and find genes important for infecting Prunus species. I will then also sequence the genome of a closely related pathogen isolated from Aquilegia vulgaris that may represent a host jump to or from cherry. Comparative genomics will be used to find the genetic changes that may play a role in a host jump. Once candidate effectors have been identified, approaches such as gene shuffling will then be used to attempt a host jump within the laboratory.

Bacterial canker of cherry. Arrows show necrotic lesion on trunk and exuded gum. Picture taken at East Malling Research (2014).

Bacterial canker of cherry. Arrows show necrotic lesion on trunk and exuded gum. Picture taken at East Malling Research (2014).

A greater understanding of pathogen host specificity and host jumps is important as in recent decades emerging diseases have become a major threat to agriculture and forests. Factors such as increased trade and global warming introduce pathogens to new environments and hosts, driving the evolution of host range expansion.  Therefore, studying host specificity will not only reveal more about pathogen evolution but could also allow us to predict potential future hosts as well as develop management strategies to prevent such rapid pathogen evolution.