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Probing the Adaptation Strategies in the Genus Xanthomonas




Liyanapathiranage, Prabha Dhananjani

Type of Degree

PhD Dissertation


Entomology and Plant Pathology


Bacterial species belonging to the genus Xanthomonas can infect and cause severe diseases in a wide range of host plants including many economically important crops throughout the world. While the majority of studies investigating the mechanistic basis of pathogenesis of Xanthomonas have broadly looked at the contribution of virulence factors towards successful apoplastic growth of the pathogen, mechanisms by which pathogens successfully adapts to the phyllosphere and maintains epiphytic asymptomatic colonization are understudied. Identification of these adaptation factors that contribute to host-pathogen interactions and understanding the evolutionary history and distribution patterns of the bacterial fitness determinants are important to predict the emergence of novel pathogens and mitigate proper disease management strategies. In this study, we explored the adaptation strategies in the genus Xanthomonas, using two approaches. As the first step, we studied the recently identified type VI secretion system for its role in crucial points of the Xanthomonas life cycle using bacterial leaf spot causing pathogen, Xanthomonas perforans as our model system. Findings from this study highlighted the contribution of functional T6SS-i3* towards early events of epiphytic colonization, adaptation in response to epiphytic stress and crucial stages of Xanthomonas perforans life cycle, that are considered to be important for mitigation of the pathogen. Since the role of the T6SS in the genus Xanthomonas has not been fully understood yet, we studied all the available Xanthomonas genomes to describe the evolutionary patterns and distribution of the T6 clusters across the genus. Our findings revealed a possible acquisition of the T6SS by group 2 Xanthomonas spp. through an ancient horizontal gene transfer event followed by subsequent losses of the clusters in some of the Xanthomonas spp., thus indicating a possible role of T6SS during niche adaptation onto various hosts as pathovars evolved. Furthermore, through bioinformatic analysis we were able to recognize several candidate T6SS‐dependent effectors that are not limited to toxins, but rather an adaptation to the host environment, avoiding host defenses and in nutrient acquisition. As the second step, towards understanding the adaptation strategies in the genus Xanthomonas, we conducted an experimental evolution study, using BLS causing Xanthomonas perforans as our model system. We were able to reveal a route of host range expansion in the Xp4B passaged on the non-host pepper plants via the loss of the hypersensitive response in resistant pepper variety. Additionally, our phenotypic analysis showed the importance of avoiding the repeated exposure of susceptible as well as resistant host plants to bacterial pathogen, due to the possibility of the emergence of more virulent or novel pathogenic races. Future works combining experimental evolution with genome sequencing will provide useful information that can lead to the identification of specific genetic changes that occurred during the continuous passaging. Overall, these findings provide useful insight into pathogen adaptation strategies, that were not well understood before and understanding these adaptive processes in pathogens are important in understanding risk of emergence of novel or virulent pathogens.