Involvement of bacterial TonB-dependent signaling in the generation of an oligogalacturonide damage-associated molecular pattern from plant cell walls exposed to Xanthomonas campestris pv. campestris pectate lyases

Abstract

BackgroundEfficient perception of attacking pathogens is essential for plants. Plant defense is evoked by molecules termed elicitors. Endogenous elicitors or damage-associated molecular patterns (DAMPs) originate from plant materials upon injury or pathogen activity. While there are comparably well-characterized examples for DAMPs, often oligogalacturonides (OGAs), generated by the activity of fungal pathogens, endogenous elicitors evoked by bacterial pathogens have been rarely described. In particular, the signal perception and transduction processes involved in DAMP generation are poorly characterized.ResultsA mutant strain of the phytopathogenic bacterium Xanthomonas campestris pv. campestris deficient in exbD2, which encodes a component of its unusual elaborate TonB system, had impaired pectate lyase activity and caused no visible symptoms for defense on the non-host plant pepper (Capsicum annuum). A co-incubation of X. campestris pv. campestris with isolated cell wall material from C. annuum led to the release of compounds which induced an oxidative burst in cell suspension cultures of the non-host plant. Lipopolysaccharides and proteins were ruled out as elicitors by polymyxin B and heat treatment, respectively. After hydrolysis with trifluoroacetic acid and subsequent HPAE chromatography, the elicitor preparation contained galacturonic acid, the monosaccharide constituent of pectate. OGAs were isolated from this crude elicitor preparation by HPAEC and tested for their biological activity. While small OGAs were unable to induce an oxidative burst, the elicitor activity in cell suspension cultures of the non-host plants tobacco and pepper increased with the degree of polymerization (DP). Maximal elicitor activity was observed for DPs exceeding 8. In contrast to the X. campestris pv. campestris wild type B100, the exbD2 mutant was unable to generate elicitor activity from plant cell wall material or from pectin.ConclusionsTo our knowledge, this is the second report on a DAMP generated by bacterial features. The generation of the OGA elicitor is embedded in a complex exchange of signals within the framework of the plant-microbe interaction of C. annuum and X. campestris pv. campestris. The bacterial TonB-system is essential for the substrate-induced generation of extracellular pectate lyase activity. This is the first demonstration that a TonB-system is involved in bacterial trans-envelope signaling in the context of a pathogenic interaction with a plant.

Using Ecology, Physiology, and Genomics to Understand Host Specificity inXanthomonas

“…Interestingly, several CWDEs are associated with TonB-dependent transporters (TBDTs) forming specific carbohydrate utilization (CUT) systems (14,20,34,39). The pectate lyase activity of Xcc depends on the exbD2 gene, which belongs to the TonB-ExbBD system and is required for the induction of HR on pepper through production of pectin-derived oligogalacturonides acting as DAMPs (139). This association may represent a specific adaptation of xanthomonads to their host plants.…”

Recently emerged and diverse lineages of Xanthomonas perforans have independently evolved through plasmid acquisition and homologous recombination originating from multiple Xanthomonas species

“…One gene, exbD2 , distinguished the TonB operon found in Xanthomonas spp. from that of most other bacterial genera and is essential for pathogenicity and induction of bacterial pectate lyase activity (34, 35). Interestingly, we found that in addition to the TonB system, the entire Xps type II secretion cluster was recombinant among the pepper pathogenic strains of SC3.…”

Hemin transported protein of Xanthomonas axonopodis pv. glycines functions on leaf colonization and virulence on soybean

“…In Xanthomonas campestris pv. campestris, the strain deficient in exbD2, which encodes a component of its unusual elaborate TonB system, had impaired pectate lyase activity and caused no visible symptoms for defense on the non-host plant pepper (Vorhölter et al, 2012). It seems possible that extracellular enzyme productions of Xag may be regulated by other iron uptake pathway.…”