Salmonella is one of the most prominent causes of food poisoning and increasing evidence indicates that contaminated fruits and vegetables are an increasing concern for human health. Successful infection demands the suppression of the host immune system, which is often achieved via injection of bacterial effector proteins into host cells. In this report we present for the first time a function of Salmonella effector protein in plant cell, supporting the new concept of trans-kingdom competency of this bacteria. We identified several Salmonella Typhimurium effector proteins with a function in plants. Among these, the phosphothreonine lyase SpvC was found to attenuate the induction of resistance relevant genes when expressed in plant cells. Using in vitro and in vivo systems we show that this effector protein actively interacts with and dephosphorylates activated Arabidopsis mitogen-activated protein kinase 6 (MPK6), thereby inhibiting defense signaling. The requirement of Salmonella SpvC for full virulence toward plants was demonstrated by the decreased pathogenicity of the L1spvC mutant. These results suggest that some Salmonella effector proteins have conserved role during animal and plant infection. The fact that Salmonella and other Enterobacteriaceae can actively use plants as hosts strongly suggests that plants might represent a much larger reservoir of animal pathogens than so far estimated.