Expression profiling of the host response to bacterial infection: the transition from basal to induced defence responses in RPM1-mediated resistance.

Changes in transcription in leaves of Arabidopsis thaliana were characterised following challenge with strains of Pseudomonas syringae pv. tomato DC3000 allowing differentiation of basal resistance (hrpA mutants), gene-specific resistance (RPM1-specified interactions) and susceptibility (wild-type pathogen). In planta avirulence gene induction, changes in host [Ca2+]cyt and leaf collapse were used to delineate the transition from infection to induced resistance. The plant responds rapidly, dynamically and discriminately to infection by phytopathogenic bacteria. Within the first 2 h host transcriptional changes are common to all challenges indicating that Type III effector function does not contribute to early events in host transcriptome re-programming. The timing of induction for specific transcripts was reproducible, hierarchical and modulated at least in part through EDS1 function. R gene-specific transcripts were not observed until 3 h after inoculation. Intriguingly, the R gene-specific response proteins are expected to localise to diverse cellular addresses indicative of a global impact on cellular homeostasis. The altered transcriptional response rapidly manifests into initial symptoms of leaf collapse within 2 h, although establishment of the full macroscopic HR occurs significantly later.