Escaping Underground Nets: Extracellular DNases Degrade Plant Extracellular Traps and Contribute to Virulence of the Plant Pathogenic Bacterium Ralstonia solanacearum

Abstract

Plant root border cells have been recently recognized as an important physical defense against soil-borne pathogens. Root border cells produce an extracellular matrix of protein, polysaccharide and DNA that functions like animal neutrophil extracellular traps to immobilize pathogens. Exposing pea root border cells to the root-infecting bacterial wilt pathogen Ralstonia solanacearum triggered release of DNA-containing extracellular traps in a flagellin-dependent manner. These traps rapidly immobilized the pathogen and killed some cells, but most of the entangled bacteria eventually escaped. The R. solanacearum genome encodes two putative extracellular DNases (exDNases) that are expressed during pathogenesis, suggesting that these exDNases contribute to bacterial virulence by enabling the bacterium to degrade and escape root border cell traps. We tested this hypothesis with R. solanacearum deletion mutants lacking one or both of these nucleases, named NucA and NucB. Functional studies with purified proteins revealed that NucA and NucB are non-specific endonucleases and that NucA is membrane-associated and cation-dependent. Single ΔnucA and ΔnucB mutants and the ΔnucA/B double mutant all had reduced virulence on wilt-susceptible tomato plants in a naturalistic soil-soak inoculation assay. The ΔnucA/B mutant was out-competed by the wild-type strain in planta and was less able to stunt root growth or colonize plant stems. Further, the double nuclease mutant could not escape from root border cells in vitro and was defective in attachment to pea roots. Taken together, these results demonstrate that extracellular DNases are novel virulence factors that help R. solanacearum successfully overcome plant defenses to infect plant roots and cause bacterial wilt disease.

The Ralstonia solanacearum csp22 peptide, but not flagellin‐derived peptides, is perceived by plants from the Solanaceae family

“…Although this approach has been very helpful in the identification of the immunosuppression capacity of many microbial molecules, it raises the question of whether such microbes are able to suppress immunity activated by their own elicitors. The infection process by R. solanacearum most likely involves at least a quantitative suppression of elicitor‐triggered responses, mediated either by T3Es (Mukaihara et al ., ; Sang et al ., ) or other molecules (Tran et al ., ). From a practical point of view, our findings show that it is possible to use an actual R. solanacearum elicitor (csp22 Rsol ) to experimentally trigger immunity in Solanaceae , instead of employing a heterologous elicitor from Pseudomonas , such as flg22.…”

NETs in pneumonia: is just enough the right amount?

“…Since the first description of NETs in 2004 their role as a potent mechanism to deal with invading pathogens has been suggested [4], and the fact that this process is highly conserved in the animal [5] and plant kingdom [6] has served to underline this. However, the overall contribution of NETs to bacterial killing in humans, in vivo, remains unclear, particularly in comparison to other processes such as phagocytosis.…”

Salivary DNase II from Laodelphax striatellus acts as an effector that suppresses plant defence

“…Extracellular DNA released by plant border cells formed traps to immobilise the invading pathogens (Hawes et al ., ). The DNases secreted by pathogens degraded the extracellular DNA and escaped root border cell traps (Tran et al ., ). We presumed that the DNases might be prevalently used by different species in coping with plant defences.…”