The complete genome sequence of Xanthomonas albilineans provides insights into pathogenicity of this sugarcane pathogen and allows further assessments of the large diversity within this species

Objectives: Xanthomonas albilineans is a xylem-invading pathogen that causes leaf scald, one of the major diseases of sugarcane. Previous studies revealed that i/ two important genomic features differentiate X. albilineans from other sequenced species of Xanthomonas: genome erosion and the presence of a type III secretion system (T3SS) of the SPI-1 family (1-2), and ii/ large variability exists among strains of X. albilineans and all strains involved in outbreaks of sugarcane leaf scald disease since the late 1980s belong to the same genetic group called PFGE-B (3). In the present study, we used the genome sequence of X. albilineans strain GPE PC73 to describe all pathogenicity-related features either shared with all species of Xanthomonas or specific to X. albilineans, and to further investigate the large diversity of this species. Results Among the major virulence factors described so far in Xanthomonads, most of them are conserved in X. albilineans, except the T3SS of the Hrp family and the gum gene clusters, and Hrp T3SS effectors. The genome of X. albilineans also encodes specific pathogenicity-related factors including twelve non ribosomal peptide synthetases and five enzymes harboring a specific cellulose binding domain. Several DNA fragments present in PFGE-B strains and absent in other strains of the pathogen were isolated by suppression subtractive hybridization (SSH). Additionally, occurrence of methylation of genomic DNA by a specific Dam methyltransferase in PFGE-B strains, but not in other strains, was experimentally demonstrated. Large inter-strain variability in X. albilineans was confirmed using multi locus sequence analysis (MLSA), clustered regularly interspaced short palindromic repeats (CRISPR) and SSH markers. Conclusions This study allowed us to identify several new candidate pathogenicity genes. In-depth functional analyses are now necessary to explore the role of these genes in the successful invasion of the sugarcane xylem vessels by X. albilineans. (Resume d'auteur)