Type Three Effector Gene Distribution and Sequence Analysis Provide New Insights into the Pathogenicity of Plant-Pathogenic Xanthomonas arboricola

Abstract

ABSTRACTXanthomonas arboricolais a complex bacterial species which mainly attacks fruit trees and is responsible for emerging diseases in Europe. It comprises seven pathovars (X. arboricolapv. pruni,X. arboricolapv. corylina,X. arboricolapv. juglandis,X. arboricolapv. populi,X. arboricolapv. poinsettiicola,X. arboricolapv. celebensis, andX. arboricolapv. fragariae), each exhibiting characteristic disease symptoms and distinct host specificities. To better understand the factors underlying this ecological trait, we first assessed the phylogenetic relationships among a worldwide collection ofX. arboricolastrains by sequencing the housekeeping generpoD. This analysis revealed that strains ofX. arboricolapathovar populi are divergent from the mainX. arboricolacluster formed by all other strains. Then, we investigated the distribution of 53 type III effector (T3E) genes in a collection of 57X. arboricolastrains that are representative of the mainX. arboricolacluster. Our results showed that T3E repertoires vary greatly betweenX. arboricolapathovars in terms of size. Indeed,X. arboricolapathovars pruni, corylina, and juglandis, which are responsible for economically important stone fruit and nut diseases in Europe, harbored the largest T3E repertoires, whereas pathovars poinsettiicola, celebensis, and fragariae harbored the smallest. We also identified several differences in T3E gene content betweenX. arboricolapathovars pruni, corylina, and juglandis which may account for their differing host specificities. Further, we examined the allelic diversity of eight T3E genes fromX. arboricolapathovars. This analysis revealed very limited allelic variations at the different loci. Altogether, the data presented here provide new insights into the evolution of pathogenicity and host range ofX. arboricolaand are discussed in terms of emergence of new diseases within this bacterial species.