Bacterial citrus canker: Molecular aspects of a compatible plant-microbe interaction

Canker is an important disease affecting citrus worldwide. It is caused by two phylogenetically distinct group of strains of Xanthomonas citri (Xc), with all citrus cultivars being susceptible to at least one Xc strain. It is known that canker causing xanthomonads carry at least one pathogenicity gene of the pthA (of Asiatic X. citri pv citri) gene family, which is required for causing canker on citrus. However little is known on the host molecular events leading to canker. The goal here was to understand host molecular mechanisms underlying disease development as well as identifying bacterial components related to phylogenic or pathogenic characteristics of canker-causing xanthomonads.; First we identified on plasmid pXcB of the South American strain X. citri pv aurantifolii B69, a pathogenicity island composed of previously identified pathogenicity gene pthB and a type IV secretion system (TFSS). This TFSS was shown to be required for self-mobilization of pXcB, which led us to propose that natural horizontal transfer of a pth host-specific pathogenicity gene may account for the two phylogenetically distinct groups of strains, the Asiatic and the South American group of strains, causing identical symptoms on citrus.; Second, we investigated the plant response to pthB using differential display PCR, suppressive subtractive hybridization and microscopy techniques. Forty-nine genes were identified as differentially regulated when RNA expression profiles of leaves inoculated with Xca B69 were compared to those of leaves inoculated with a B69 mutant lacking pthB. Among these were genes predicted to be involved in processes such as cell wall modifications vesicule trafficking and protein modifications. It was also shown that B69 was able to trigger cell division and cell expansion in citrus leaves.; Finally, we focused on one canker responsive gene with high similarity to the S&barbelow;mall U&barbelow;biquitin like M&barbelow;odifier (SUMO) from Arabidopsis. Analysis of B69 mutant strains lacking PthB or the type III secretion system (TTSS) component, HrpG, revealed PthB-dependent and TTSS dependent/PthB-independent changes in SUMO conjugation profiles associated with canker.; Genes and cellular processes identified by this study reflect the molecular events leading to disease development and contribute to the more general efforts aiming to understand the mechanisms underlying the variety of diseases caused by compatible interactions between xanthomonads and their host plants.