Ralstonia solanacearum is a known bacterial pathogen of eucalypt and potato plants in Africa. A survey was undertaken to detect this pathogen in eucalypt plantations in South Africa, the Democratic Republic of Congo, and Uganda. Numerous bacterial strains were isolated from trees with symptoms typical of bacterial wilt, but only seven were positively identified as R. solanacearum. A polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) technique, based on the hrp (hypersensitive response and pathogenicity) gene region was used to determine and group the biovars of these R. solanacearum strains. The eucalypt isolates and one potato isolate formed a biovar 3 cluster, whereas the two other potato isolates formed a cluster that corresponded to biovar 2. Amplified fragment length polymorphism (AFLP) analysis confirmed these clusters. Therefore, PCR-RFLP can be used as a reliable diagnostic technique to enable researchers to rapidly identify the pathogen.

Bacterial wilt caused by Ralstonia solanacearum is a disease of widespread economic importance that affects numerous plant species, including Arabidopsis thaliana. We describe a pathosystem between A. thaliana and biovar 3 phylotype I strain BCCF402 of R. solanacearum isolated from Eucalyptus trees. A. thaliana accession Be-0 was susceptible and accession Kil-0 was tolerant. Kil-0 exhibited no wilting symptoms and no significant reduction in fitness (biomass, seed yield, and germination efficiency) after inoculation with R. solanacearum BCCF402, despite high bacterial numbers in planta. This was in contrast to the well-characterized resistance response in the accession Nd-1, which limits bacterial multiplication at early stages of infection and does not wilt. R. solanacearum BCCF402 was highly virulent because the susceptible accession Be-0 was completely wilted after inoculation. Genetic analyses, allelism studies with Nd-1, and RRS1 cleaved amplified polymorphic sequence marker analysis showed that the tolerance phenotype in Kil-0 was dependent upon the resistance gene RRS1. Knockout and complementation studies of the R. solanacearum BCCF402 effector PopP2 confirmed that the tolerance response in Kil-0 was dependent upon the RRS1-PopP2 interaction. Our data indicate that the gene-for-gene interaction between RRS1 and PopP2 can contribute to tolerance, as well as resistance, which makes it a useful model system for evolutionary studies of the arms race between plants and bacterial pathogens. In addition, the results alert biotechnologists to the risk that deployment of RRS1 in transgenic crops may result in persistence of the pathogen in the field.

Summary Ralstonia solanacearum, the causal agent of bacterial wilt, has been reported from Eucalyptus plantations in at least three countries in Africa. The lack of genomics resources in Eucalyptus species led us to develop and study a pathosystem between a previously characterized South African isolate and the model plant Arabidopsis thaliana. Ralstonia solanacearum BCCF401 isolated from a Eucalyptus grandis × Eucalyptus camaldulensis hybrid was shown to cause disease on A. thaliana ecotype Col‐5. Arabidopsis genomics tools were exploited to investigate gene expression changes during wilt disease development, and thereby develop hypotheses that can be tested in Eucalyptus once genomics resources are available. Transcriptome analysis using 5000 A. thaliana ESTs was performed and revealed 141 genes that were differentially regulated by R. solanacearum infection (at a significance threshold of p < 0.03; Bonferroni corrected). A software tool ‘Rank Correlation Comparer’ was developed to compare expression profiles with Arabidopsis Affymetrix NASCArray data. High correlations were observed between the response of Arabidopsis plants to both Eucalyptus (BCCF401) and tomato (GMI1000) isolates of R. solanacearum, as well as to Pseudomonas syringae, Botrytis cinerea and treatment with abscisic acid. Basal defence responses in Col‐5 in response to R. solanacearum were investigated by comparing the expression data following R. solanacearum infection to data after treatment with the Pathogen Associated Molecular Patterns (PAMP) flg22 and lipopolysaccharide, and the Type Three Secretion System deficient Pseudomonas syringae pv. tomato hrp− mutant. A subset of the genes which were induced by PAMPs were repressed by R. solanacearum infection, and vice versa, suggesting that these genes may be repressed or induced, respectively, by specific R. solanacearum effectors. We hypothesize that these genes represent targets of R. solanacearum effectors. The pending release of the Eucalyptus genome sequence will enable orthologues to be identified and these hypotheses to be tested in Eucalyptus trees.