XacFhaB adhesin, an important Xanthomonas citri ssp. citri virulence factor, is recognized as a pathogen‐associated molecular pattern

Abstract

Adhesion to host tissue is one of the key steps of the bacterial pathogenic process. Xanthomonas citri ssp. citri possesses a non-fimbrial adhesin protein, XacFhaB, required for bacterial attachment, which we have previously demonstrated to be an important virulence factor for the development of citrus canker. XacFhaB is a 4753-residue-long protein with a predicted β-helical fold structure, involved in bacterial aggregation, biofilm formation and adhesion to the host. In this work, to further characterize this protein and considering its large size, XacFhaB was dissected into three regions based on bioinformatic and structural analyses for functional studies. First, the capacity of these protein regions to aggregate bacterial cells was analysed. Two of these regions were able to form bacterial aggregates, with the most amino-terminal region being dispensable for this activity. Moreover, XacFhaB shows features resembling pathogen-associated molecular patterns (PAMPs), which are recognized by plants. As PAMPs activate plant basal immune responses, the role of the three XacFhaB regions as elicitors of these responses was investigated. All adhesin regions were able to induce basal immune responses in host and non-host plants, with a stronger activation by the carboxyl-terminal region. Furthermore, pre-infiltration of citrus leaves with XacFhaB regions impaired X. citri ssp. citri growth, confirming the induction of defence responses and restraint of citrus canker. This work reveals that adhesins from plant pathogens trigger plant defence responses, opening up new pathways for the development of protective strategies for disease control.

3-methylcrotonyl Coenzyme A (CoA) carboxylase complex is involved in the Xanthomonas citri subsp. citri lifestyle during citrus infection

“…Plants were grown in a growth chamber in incandescent light at 28°C with a photoperiod of 16 h. Bacteria were cultured in NB broth to an optical density at 600 nm (OD 600 ) of 1, harvested by centrifugation, and resuspended in 15 mM NaCl at 10 5 to 10 7 CFU ml -1 . For disease symptoms assays, bacterial suspensions were infiltrated into leaves with needleless syringes [ 80 ]. In planta growth assays were performed by grinding 0.8 cm diameter leaf discs from infiltrated leaves in 1 ml of 15 mM NaCl, followed by serial dilutions, and plating onto NB agar plates.…”

HrpE, the major component of the Xanthomonas type three protein secretion pilus, elicits plant immunity responses

“…2a ). In tomato leaves, the expression levels of defense response-related genes 12 , including SlPti5 , SlLrr22 and SlGras2 , and the transcription factor SlWrky28 , were analyzed. A significant induction of the expression of all these genes was observed 4 hpi with HrpE relative to the Trx control ( p < 0 .…”

“…It is noteworthy, that defense responses in citrus were weaker than in non-host plants. This can be attributed to the fact that citrus leaf is naturally more resistant to elicitors of defense responses than non-host plants 11 , 12 . Supporting a role for HrpE as an enhancer of defense responses, pre-infiltration of citrus and tomato leaves with HrpE impaired the virulence of Xcc and Xcv on these plants, respectively.…”

“…Examples of bacterial PAMPs include flagellin, elongation factor Tu, lipopolysaccharide, peptidoglycans and methylated DNA fragments 10 . In Xcc , a harpin protein 11 , a non-fimbrial adhesin 12 and the osmoprotectant sugar trehalose 13 , also have a role in the elicitation of plant defense responses. In view of the high degree of conservation of HrpE in the Xanthomonas genus and since HrpE is the major component of the HrpE pilus which is crucial for pathogenicity and is exposed on the bacterial surface, it had been previously hypothesized that HrpE would be a candidate to act as a PAMP 14 .…”

Xanthomonas citri subsp. citri: host interaction and control strategies

“…Such a response is thought to be triggered by multiple pathogen-associated molecular patterns (PAMPs) including the bacterial flagellin, adhesins, lipopolysaccharides and components of the Hrp (hypersensitive response and pathogenicity) pilus. This idea is supported by studies that show that, in addition to playing a role in cell adhesion, motility and biofilm formation, thus contributing to bacterial virulence, these molecules can also trigger defense responses in both host and non-host plants (Gottig et al 2009;Casabuono et al 2011;Petrocelli et al 2012;Di Lorenzo et al 2017;Garavaglia et al 2016;Yan et al 2012;Gottig et al 2018;Andrade et al 2014).…”