Tag Archives: pseudomonas

Local adaptation and the accessory genome in an endemic plant-pathogen

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Genetic variation is fodder for evolution, and microbial plant-pathogens have it in spades. The Pseudomonas syringae genome is characterized by many rare “accessory” genes that co-occur with “core” genes found in all individuals. In fact, accessory genes outnumber core genes 2:1, even though accessory genes are not essential for survival. Moreover, there is tremendous variation in the gene content of P. syringae; isolates from different crop species, for example, differ in gene content by ~32% (Karasov et al. 2017). Whether these strain-specific genes have adaptive potential remains unknown; they may simply be a consequence of high rates of mutation and lateral gene transfer, even if purifying selection to remove deleterious variants is strong. Another, not mutually exclusive possibility is that accessory genes are maintained by positive selection as pathogens adapt to alternative hosts. Indeed, local adaptation has been hypothesized to explain the presence of rare alleles in P. syringae, which causes major agricultural loss in multiple crop species each year. To address these hypotheses, I have paired a set of P. syringae isolates with their original hosts of isolation. I first test for local adaptation by comparing the in planta fitness of each isolate in its own, and in each other’s, native host. Next, I ask to what degree strain-specific genes influence adaptive patterns by using Tn-seq to track the in planta gene frequencies of each pathogen over the course of infection in each host. From this combination of experiments, we will learn to what extent host ecology influences genome evolution and virulence in P. syringae; this is important not only to inform our understanding of the selective process, but also to fields concerned with the emergence and spread of infectious disease. 

Fitness mechanism of Pseudomonas syringae in crops

Pseudomonas syringae is multi host generalist pathogen, it can infect more than 100 families plants. It has a complex life history, including pathogenic, epiphytic and saprophytic phases. The mechanisms of pathogen virulence and host resistance have been well characterized in several model systems. But knowledge about genetic dynamics in ecology is limited. Tn-seq high-throughput parallel sequencing will be used to elucidate the fitness mechanism of Pseudomonas syringae in crops.

Evolution of pathogenicity and intraspecific interactions in Pseudomonas syringae

The high selective pressures involved in the “arms race” between plants and their pathogens drives rapid evolution of genes involved in immunity on the host side and virulence on the pathogen side (Alcázar et al., 2011). However, plants are not typically infected by individual pathogens: they interact with a community of inter- and intraspecifically diverse microbes that also experience competitive pressures from one another. How these interactions among microbes affect their ability to cause disease and how the host plant influences the microbial community it harbors remain open questions for investigation.

Researchers have observed that P. syringae is a common natural pathogen of A. thaliana and that resistance to P. syringae infection varies among different A. thaliana accessions (Jakob et al., 2002). Recent work has shown that  P. syringae strains isolated from A. thaliana leaf tissue are not only genetically diverse but also differ in their degree of virulence: many isolates harbor a polymorphism in the type three secretion system (T3SS), losing the ability to cause disease (Barrett et al., 2011; Kniskern et al., 2011). Such strains show increased growth in plant tissue when co-inoculated with other P. syringae isolates harboring an intact T3SS. This result suggests a model where non-pathogenic strains engage in “cheating” through taking advantage of the nutrients released from host cells infected by pathogenic strains (Barrett et al., 2011).

Works cited

Alcázar, R., Reymond, M., Schmitz, G., and de Meaux, J. (2011). Genetic and evolutionary perspectives on the interplay between plant immunity and development. Curr. Opin. Plant Biol. 14, 378–384.

Barrett, L.G., Bell, T., Dwyer, G., and Bergelson, J. (2011). Cheating, trade-offs and the evolution of aggressiveness in a natural pathogen population. Ecol. Lett. 14, 1149–1157.

Jakob, K., Goss, E.M., Araki, H., Van, T., Kreitman, M., and Bergelson, J. (2002). Pseudomonas viridiflava and P. syringae–natural pathogens of Arabidopsis thaliana. Mol. Plant Microbe Interact. 15, 1195–1203.

Kniskern, J.M., Barrett, L.G., and Bergelson, J. (2011). Maladaptation in wild populations of the generalist plant pathogen Pseudomonas syringae. Evolution 65, 818–830.