What do genome sequences of Xylella fastidiosa tell us on host adaptation and lifestyle of Xylella?

The extension of international trading and global changes affect the distribution of diseases posing new threats to food production and the preservation of some plant species in the environment. Thus an in-depth knowledge on how pathogens adapt to their environment is a prerequisite for further control and surveillance strategies. Xylella fastidiosa is one of the most dangerous plant-pathogenic bacterium that recently emerged in Europe. Following its discovery in Italy in 2013, it has been recorded in France, Germany and Spain. More than 350 plant species are listed as hosts of this bacterium. While X. fastidiosa is responsible for economically important diseases in some crops, ornamentals, and trees, in most host plant it does not cause any symptoms; these plants serving as unseen inoculum reservoir. However, some plant species fail to support X. fastidiosa growth in environmental conditions favorable to this bacterium. Moreover, X. fastidiosa is a genetically heterogeneous species with six subspecies and a number of clusters of strains that more or less group strains according to host plants and/or diseases. Finally, X. fastidiosa is prone to recombination and in several cases recombination was associated to host shift in field. These characteristics make X. fastidiosa a good model to look for genetic determinants responsible for host adaptation and lifestyle. We use typing methods, comparative genomics, and develop tools to identify sequences associated with specific traits. Analyses of core and accessory genomes of sets of strains differing by their taxonomic assignation to a subspecies, host and places of isolation highlighted key elements involved in host specificity that could also be used to develop novel identification tests.