Genetic diversity of Xanthomonas species pathogenic to citrus based on avr and leucine rich protein genes

The genus Xanthomonas includes a group of phytopathogenic bacteria that have great economic importance around the world. Citrus canker, caused by Xanthomonas citri subsp. citri, is one of the main threats to citrus crops worldwide. Different forms of citrus canker have been described based on pathogen specificity and geographical origin, and are caused by other two Xanthomonas species: X. alfalfae subsp. citrumelonis and X. fuscans subsp. aurantifolii. In the present work we developed DNA primers and probes based on pathogenicity genes in order to study the genetic diversity of these three Xanthomonas species, specially a Brazilian population of citri strains. The presence of avrXacE1, avrXacE2, avrXacE3 and leucine rich protein (lrp) genes were observed in almost all citri and aurantifolii strains, but avrXacE2 an avrXacE3 genes were absent in citrumelonis strains. The profiles generated by PCR-RFLP of the avr genes did not display any polymorphism after agarose gel electrophoresis. However the use of avr genes as Southern blot probes proved to be effective to differentiate Xanthomonas species pathogenic on citrus and to identify polymorphisms between strains belonging to the same species from Brazil. With the exception of the probe corresponding to lrp gene, the number of haplotypes identified (22) varied according to the gene/endonuclease used on Southern blot. The strains of subspecies citri possess a higher number of copies of the avr genes than subspecies aurantifolii and citrumelonis. Relatively high genetic diversity was observed in citri and aurantifolii Brazilian populations, independently of the citrus canker control method applied in the area of origin of the strains. (Texte integral)