The "Repertoire for Repertoire" Hypothesis of type three effectors may explain host specificity in Xanthomonas

Abstract : Host range of many plant pathogenic bacteria is restricted to one or a few host plant species reflecting a tight adaptation of bacteria to specific hosts. Two hypotheses could explain host specificity: either it can be explained by the phylogenetic position of the strains, or, alternatively, by the association of virulence genes enabling a pathological convergence of phylogenetically distant strains. In this latter hypothesis, host specificity would result from the interaction between repertoires of bacterial virulence genes and repertoires of genes involved in host surveillance systems and defences (a "repertoire for repertoire" hypothesis). To challenge these two hypotheses, we selected 132 Xanthomonas axonopodis strains representative of 18 different pathovars which display different host range. First, the phylogenetic position of each strain was determined by sequencing the housekeeping gene rpoD. This study showed that many pathovars of Xanthomonas axonopodis are polyphyletic. We thus investigated the distribution of 37 T3E genes in these strains by both PCR and hybridization methods. Our study revealed that T3E repertoires are highly variable between these strains, both in terms of T3E presence and of size of repertoires. T3E repertoires comprise both core and flexible gene suites that likely have distinct roles in pathogenicity and different evolutionary histories. Our results showed a correspondence between composition of T3E repertoires and pathovars of Xanthomonas axonopodis. For polyphyletic pathovars, this strongly suggests that T3E genes explain a pathological convergence of phylogenetically distant strains. Finally, our results support the "repertoire for repertoire" hypothesis as the molecular basis of host specificity for plant pathogenic bacteria.