The virulence and avirulence activities of members of the Pseudomonas syringae HopAB family of effectors and AvrPto were examined in bean, tomato and Arabidopsis. Proteins were delivered by the RW60 strain of P. syringae pv. phaseolicola. RW60 causes a hypersensitive reaction (HR) in bean and tomato but is restricted without the HR in Arabidopsis. Dual avirulence and virulence functions in tomato and bean, respectively, were identified in virPphA homologues but only avrPtoB strongly enhanced virulence to Arabidopsis, overcoming basal defences operating against RW60. Virulence activity in both bean and Arabidopsis required regions of the C-terminus of the AvrPtoB protein, whereas elicitation of the rapid HR in tomato, with the matching Pto resistance gene, did not. The effect of AvrPtoB on Arabidopsis was accession-specific; most obvious in Wassilewskija (Ws-3), intermediate in Columbia and not detectable in Niedersenz (Nd-1) after inoculation with RW60 + avrPtoB. Analysis of crosses between Ws-3 and Nd-1 indicated co-segregation for the AvrPtoB virulence function with the absence of the Nd-1 FLS2 gene which mediates recognition of bacterial flagellin. In planta expression of AvrPtoB did not prevent the HR activated by P. syringae pv. tomato DC3000 + avrB, avrRpm1, avrRps4 or avrRpt2, but suppressed cell wall alterations, including callose deposition, characteristic of basal defence and was associated with reprogramming of the plant's transcriptional response. The success or failure of AvrPtoB in suppressing basal defences in Nd-1 depended on the timing of exposure of plant cells to the effector and the flagellin flg22 peptide.