Résumé
Numerous genes involved or putatively involved in pathogenicity of plant pathogenic bacteria have been identified. These genes code for different secretion system constitutive proteins, exopolysaccharides, virulence factors, toxins, plant cell-wall degrading enzymes, cell mobility and motility factors or adhesion factors. In contrast to most plant bacterial pathogens, no hrp or avr genes were found in Xanthomonas albilineans, the causal agent of sugarcane leaf scald. This pathogen produces, however, a pathotoxin called albicidin that is responsible for foliar disease symptoms. Recently, all genes involved in albicidin biosynthesis of strain Xa23R1 from Florida were cloned and sequenced. Variation in albicidin biosynthetic genes was, however, not correlated with variation in pathogenicity of X. albilineans. In this study, we attempted to identify new genes involved in pathogenicity of X. albilineans using several approaches and 19 strains of the pathogen differing in disease severity and stalk colonization in Guadeloupe. The in vitro production of albicidin varied among strains of X. albilineans, but all strains showed the same RFLP (restriction fragment length polymorphism) pattern with albicidin biosynthetic genes. Similarly, no variation was found among strains by PFGE (pulse-field gel electrophoresis). In contrast, variation among strains was found by AFLP (amplified fragment length polymorphism) with 16 selective primer combinations, after enzymatic digestion of total genomic DNA with Saci and Mspl. No relationship between this genetic variation and variation in pathogenicity was, however, identified. A total of 40 primer sets were then designed to amplify by PCR (polymerase chain reaction) 40 genes involved in pathogenicity of bacterial species closely related to X. albilineans, and particularly X. campestris pv. campestris. Only one gene, pilB, could be amplified from total genomic DNA of nine strains of X. albilineans differing in disease severity and stalk colonization in Guadeloupe. Nucleotide sequence identity was 100% identical among the strains of the pathogen and a phylogenetic study with this sequence confirmed that X. albilineans belongs to the genus Xanthomonas. Absence of amplification with 39 primer sets suggested that genes involved in pathogenicity of X. albilineans differ significantly from those of other closely related pathogens. Sequencing of the whole genome of X. albilineans will be a great step in unraveling pathogenicity of this sugarcane pathogen. (Texte intégral)