The main driving force behind water transport in plants is the air's low water potential. In the presence of high humidity, the transpiration process is halted and water transport is mainly sustained by the root pressure. The surplus of water following the removal of essential components (e.g. salts) is excreted by the plant via guttation through the hydathodes. When guttation occurs, the plant surface is wetted. These are the conditions that will allow epiphytic living, motile bacteria to move and to eventually enter the plant's interior via the hydathodes. The question arose as to whether the plant has developed a protection mechanism against motile bacteria in the vicinity of the hydathodes. Such a protection mechanism could use the well known pathogenesis-related (PR) proteins. Indeed, an analysis of the guttation fluid using one- and two-dimensional electrophoresis showed a clustering of approximately 200 proteins, primarily with isoelectric points in the acidic pH. Proteins identified using electrospray ionization mass spectroscopic analysis and western blot analysis belong mostly to the family of PR-proteins suggesting a role in plant protection against invaders. The protein profile of the guttation fluid was remarkably modified by treating plants with methyl jasmonic acid suggesting that the protein composition of the guttation fluid is controlled by internal and/or external stimuli.