Study of interactions between Xanthomonas translucens and barley

Bacterial Leaf Streak is a disease affecting small grain cereals like barley, wheat, rye, and triticale. This disease is characterized by water-soaked streaks on cereals’ leaves. The causal agent of BLS is the phytopathogenic bacteria, Xanthomonas translucens. This organism is characterized by a wide host range, causing yield losses up to 40% which poses a threat on agriculture. In order to provide an efficient tool for classification of X. translucens strains at the infra species and pathovar level and with the intention of linking phylogenetic information to pathogenicity, a Multi-Locus Sequence Analysis was performed. An approach adapted to all clade-1 Xanthomonas was developed and involved five housekeeping genes, atpD, dnaK, gyrB, rpoD and fyuA. The phylogenetic analysis grouped strains by pathovar; the clusters undulosa, translucens and cerealis being closely related to each other. Pathogenicity tests were conducted on an asset of differential cultivars of barley and wheat and yielded differences in virulence between strains. More surprisingly between cultivars of the same species for a given strain. A correlation between the phylogenetic position of strains and their pathogenicity profiles was proposed as well. The variations of pathogenicity and the imperfect correspondence between virulence and phylogeny could be explained by the repertoire-for-repertoire model, conceiving plant-pathogen interactions as the result of the combination of a whole range of genes from the pathogen and from the host. The number of elements involved in the equation could thus lead to a wide array of plant-pathogen interactions that are difficult to represent by amplifying five housekeeping genes as it was done with the MLSA approach developed. With the purpose of characterizing plant-pathogen interactions, virulence genes of Xanthomonas translucens were investigated. The type three secretion system, a needle-like protein assembly allowing bacteria to inject proteins into hosts’ cells was shown to be required for pathogenicity. Among the proteins, called effectors, secreted by Xanthomonas translucens, a family called Transcription Activator-Like Effectors, TALE, was proven to play a major role in pathogenicity. The impact of different Type three effectors on the Xanthomonas translucens strain UPB787 pathogenicity was studied. Knock-out mutations were generated for the TAL genes of a Xanthomonas translucens pv. translucens strain. Inoculations were performed on barley and wheat but did not show a significant decrease in virulence which could be explained by redundant actions of TALEs, or a cumulative contribution of each effector to pathogenicity or a too narrow range of cultivars chosen. Avirulence type three effectors were also previously shown to be involved in non-host hypersensitive reactions on pepper. In order to investigate the distribution of such avirulent type three effectors amongst Xanthomonas translucens, several strains of X. translucens belonging to different pathovars were screened on pepper ECW as well as on Nicotiana tabacum cv. Benthamiana and cv. Xanthi. Six strains were identified as HR positive on pepper as previously shown for X. translucens strains UPB787 and UPB820 on pepper ECW-10R carrying the Bs1 resistance genes (Pesce et al., 2017). Interestingly, the eight HR positive strains clustered mostly together and almost all belonged to the translucens pathovar.