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.
