XacFhaB, a non-fimbrial adhesin of Xanthomonas axonopodis pv. citri is involved in citrus canker disease development

GOTTIG N; ORELLANO EG; OTTADO J

Rosario, Argentina.

Congreso; V Congreso Argentino de Microbiología General; 2008

Sociedad Argentina de Microbiología General

Bacterial attachment to host tissues by various adhesins is a first step in the pathogenesis of many animal pathogens, but a role for attachment in plant pathogenesis is less clear. In Bordetella pertussis, an animal pathogen, the non-fimbrial adhesin FhaB is secreted in a Sec-dependent manner by the two-partner secretion system, which involves an outer membrane-associated accessory protein (FhaC ‘‘partner’’ protein).  Xanthomonas axonopodis pv. citri (Xac) is the casual agent of citrus canker, which affects most commercial citrus cultivars resulting in significant losses worldwide. This phytopathogen encodes FhaB (XacFhaB) and FhaC (XacFhaC) homologs, in this work we analyzed the role of these proteins in the pathogenicity process. We observed an increase of XacFhaB mRNA levels in a medium that mimics the intercellular space of leaves and also in infected leaves, suggesting a role for XacFhaB in pathogenicity. To evaluate the participation of XacFhaB and XacFhaC in this process, we constructed deletion mutants of these genes (DXacFhaB and DXacFhaC). These mutants and Xac wild type (WT) were infiltrated in citrus leaves and we observed that at low bacterial concentrations (104 cfu/ml) despite infiltrated areas were similar, canker lesions produced by DXacFhaB mutant were smaller and appeared in a more dispersed fashion than typical lesion observed with WT bacteria. The bacterial number of DXacFhaB mutant recovered from these infected leaves was significantly fewer than that of Xac WT. The lesions and the bacterial growth in citrus leaves observed for DXacFhaC mutant were similar to the Xac WT. The virulence of DXacFhaB and DXacFhaC was also determined spraying it on leaves surface, a method that resembles the natural infection. In this case, we observed that DXacFhaB produced less number of cankers than Xac WT and the epiphytic fitness of DXacFhaB was also affected. The number of cankers and epiphytic fitness of DXacFhaC mutant was also impaired but in a less extension than for DXacFhaB mutant. Collectively these findings demonstrate that XacFhaB has an important role in X. axonopodis pv. citri pathogenicity and that XacFhaB protein could be partially transported by XacFhaC protein, nevertheless XacFhaC is not the principal secretion partner of XacFhaB. Having establish that XacFhaB is necessary for X. axonopodis pv. citri virulence in citrus leaves, we next analyzed at which of the different stages of the pathogenic process XacFhaB is involved and observed that this protein is necessary for bacterial adherence to leaf surface and for cell to cell aggregation involved in biofilm formation. In addition, we observed by swarm plate analysis that DXacFhaB mutant has a higher cell mobility which may explain the canker dispersed phenotype produced by this strain. In conclusion this work demonstrated that non-fimbrial adhesins have an important role in plant pathogen adherence and pathogenicity.