Mitogen activated protein kinases (MAPK) are proteins transducing external stimuli into cellular responses including remodeling of gene expression and subcellular structure. Arabidopsis mutants lacking a MAPK cascade component have usually severe developmental defects and changed tolerance to biotic or abiotic stresses. Using comparative studies on MAPK mutants, proteomics is powerful in identification of protein candidates responsible for various developmental or physiological processes regulated by MAPKs.