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Pathogenesis is based on the ability of the pathogen to evade host defences and adapt to its environment. Using a high-throughput method, we characterised the genes involved in the growth and adaptation of a phytopathogenic bacterium Xanthomonas during infection of a cauliflower leaf. The bacteria infect organs called hydathodes located at the leaf margin and used for guttation. These organs provide rapid access to the xylem vessels which are the second site of proliferation of the bacteria. We were able to show that the hydathode openings do not represent a limiting barrier for bacterial entry. We also identified 181 Xanthomonas genes important for multiplication in this organ. In particular, we characterised a new regulator important for the virulence of the bacterium and its adaptation to the plant environment.

Context and issues

Crop losses due to plant diseases have a significant economic and societal impact (1, 2). Brassicaceae cultivation is a multi-billion dollar a year industry worldwide. The development of black venation symptoms on cabbage leaves due to Xanthomonas campestris colonisation results in reduced food quality and yield losses that can exceed 50% in hot, humid climates. The impact of this disease is particularly important for the seed industry, as 0.03% of infected seed can be sufficient to cause an epidemic of black rot (3). Expanding our knowledge of plant pathology is a major economic and ecological challenge in order to deploy sustainable and effective tolerance against these diseases in plants of agronomic interest. The genes contributing to the adaptation of vascular pathogens to their plant niches represent new potential targets for pathogen control.

Results and perspectives

This work made it possible to define a list of genes essential for the growth of Xanthomonas campestris in vitro. The detailed study of the early stage of infection of the hydathode highlighted the genes that are important for efficient multiplication of the bacteria in this organ. This gene list reveals the importance of certain metabolic pathways, as well as the fitness cost of the pathways involved in virulence for the pathogen. This unprejudiced study also identified new virulence factors among genes whose function was previously unknown.

Beyond the publication described above, this work opens the way to numerous works in the team and has enabled the development of several local, national and international collaborations. In addition, obtaining a list of essential genes makes it possible to propose potential targets for the control of this pathogenic bacterium.