Extracellular adenosine 5'-triphosphate (eATP) is emerging as an important plant signalling compound capable of mobilising intracellular second messengers such as Ca(2+), nitric oxide, and reactive oxygen species. However, the downstream molecular targets and the spectrum of physiological processes that eATP regulates are largely unknown. We used exogenous ATP and a non-hydrolysable analogue as probes to identify the molecular and physiological effects of eATP-mediated signalling in tobacco. 2-DE coupled with MS/MS analysis revealed differential protein expression in response to perturbation of eATP signalling. These proteins are in several functional classes that included photosynthesis, mitochondrial ATP synthesis, and defence against oxidative stress, but the biggest response was in the pathogen defence-related proteins. Consistent with this, impairment of eATP signalling induced resistance against the bacterial pathogen Erwinia carotovora subsp. carotovora. In addition, disease resistance activated by a fungal pathogen elicitor (xylanase from Trichoderma viride) was concomitant with eATP depletion. These results reveal several previously unknown putative molecular targets of eATP signalling, which pinpoint eATP as an important hub at which regulatory signals of some major primary metabolic pathways and defence responses are integrated.