Summary WRKY transcription factors play a key role in modulating the plant defense transcriptome. Here we show that the Arabidopsis mutant wrky27-1, which lacks a functional WRKY27 transcription factor, showed delayed symptom development in response to the bacterial wilt pathogen Ralstonia solanacearum. Additionally, wrky27-1 plants did not express PR marker genes upon infection, as also observed in resistant Nd-1 plants. Spatial expression of WRKY27 correlated well with the route of bacterial infection and propagation in planta. Complementation experiments restored both the early wilting phenotype of wild-type Col-1 plants and activation of PR genes, not only when the WRKY27 cDNA is expressed under the control of the native promoter, but also when the SUC2 promoter was used, suggesting that WRKY27 exerts its function in phloem companion cells. Expression studies identified genes involved in nitrogen metabolism and nitric oxide (NO) generation as potential targets of negative regulation by WRKY27. Our results show that WRKY27 negatively influences symptom development of a vascular pathogen, possibly by affecting signaling or trafficking between the phloem and the xylem. © 2008 Blackwell Publishing Ltd.