A Eucalyptus bacterial wilt isolate from South Africa is pathogenic on Arabidopsis and manipulates host defences

Abstract

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.