The Bacterial Wilt Reservoir Host Solanum dulcamara Shows Resistance to Ralstonia solanacearum Infection

Ralstonia solanacearum causes bacterial wilt, a devastating plant disease, responsible for serious losses on many crop plants. R. solanacearum phylotype II-B1 strains have caused important outbreaks in temperate regions, where the pathogen has been identified inside asymptomatic bittersweet (Solanum dulcamara) plants near rivers and in potato fields. S. dulcamara is a perennial species described as a reservoir host where R. solanacearum can overwinter, but their interaction remains uncharacterised. In this study, we have systematically analysed R. solanacearum infection in S. dulcamara, dissecting the behaviour of this plant compared with susceptible hosts such as tomato cv. Marmande, for which the interaction is well described. Compared with susceptible tomatoes, S. dulcamara plants (i) show delayed symptomatology and bacterial progression, (ii) restrict bacterial movement inside and between xylem vessels, (iii) limit bacterial root colonisation, and (iv) show constitutively higher lignification in the stem. Taken together, these results demonstrate that S. dulcamara behaves as partially resistant to bacterial wilt, a property that is enhanced at lower temperatures. This study proves that tolerance (i.e., the capacity to reduce the negative effects of infection) is not required for a wild plant to act as a reservoir host. We propose that inherent resistance (impediment to colonisation) and a perennial habit enable bittersweet plants to behave as reservoirs for R. solanacearum.

NtRNF217, Encoding a Putative RBR E3 Ligase Protein of Nicotiana tabacum, Plays an Important Role in the Regulation of Resistance to Ralstonia solanacearum Infection

E3 ubiquitin ligases, the most important part of the ubiquitination process, participate in various processes of plant immune response. RBR E3 ligase is one of the E3 family members, but its functions in plant immunity are still little known. NtRNF217 is a RBR E3 ligase in tobacco based on the sequence analysis. To assess roles of NtRNF217 in tobacco responding to Ralstonia solanacearum, overexpression experiments in Nicotiana tabacum (Yunyan 87, a susceptible cultivar) were performed. The results illuminated that NtRNF217-overexpressed tobacco significantly reduced multiplication of R. solanacearum and inhibited the development of disease symptoms compared with wild-type plants. The accumulation of H2O2 and O2− in NtRNF217-OE plants was significantly higher than that in WT-Yunyan87 plants after pathogen inoculation. The activities of CAT and SOD also increased rapidly in a short time after R. solanacearum inoculation in NtRNF217-OE plants. What is more, overexpression of NtRNF217 enhanced the transcript levels of defense-related marker genes, such as NtEFE26, NtACC Oxidase, NtHIN1, NtHSR201, and NtSOD1 in NtRNF217-OE plants after R. solanacearum inoculation. The results suggested that NtRNF217 played an important role in regulating the expression of defense-related genes and the antioxidant enzymes, which resulted in resistance to R. solanacearum infection.