The plant hormone cytokinin plays a crucial role in plant development from germination to senescence. Nevertheless, the exact mechanisms of its influence on these several developmental processes remain unexplained. We report an attempt to reveal more information about the mode of action of cytokinin in the model plant Arabidopsis thaliana. For this purpose a classical proteome-based approach using two-dimensional electrophoretic separation followed by protein digestion and identification by mass spectrometry was established. Total proteins were extracted from whole Arabidopsis plantlets of following genotypes: wild-type ColO, amp1 mutant accumulating six-fold higher level of endogenous cytokinin (1) and pOp-ipt::LhGR transgenic line in which the production of endogenous cytokinin is inducible by dexamethasone (2). All plants were grown for 15 days under long-day conditions (16h light/8h dark) on MS media, in parallel with and without 2.5 mM dexamethasone. Image analysis of two-dimensional gels in triplicates using Progenesis Workstation software and subsequent statistical evaluation revealed both qualitative and quantitative changes in protein expression between plants with elevated cytokinin levels and controls. Out of these differently expressed protein spots so far 21 significant ones were in-gel digested with trypsine and the resulting peptides were subjected to MALDI-TOF MS analysis. Proteins corresponding to the acquired mass spectra were identified in 17 of the analysed spots. Elucidating the relationship of the identified proteins to cytokinin metabolism and action will be a longer-term task. A study of the kinetics of the proteome changes after dexamethasone treatment in pOp-ipt::LhGR plants represents the immediate challenge. This project was supported by the Ministry of Education of the Czech Republic (grant No. MSM 143100008) and by the scholarship IN-CO Czech 2002-2004 within the Leonardo da Vinci programme. References 1. Chaudhury A. M., Letham S., Craig S., and Dennis E.S.: Plant J. 4, 907916 (1993). 2. Moore I., Galweiler L., Grosskopf D., Schell J. and Palme K.: Proc Natl Acad Sci USA 95(1), 376-81 (1998).