Stress activated MAPKs in plants

Plants are exposed to a wide variety of extracellular stimuli and employ a broad set of signaling pathways to give the appropriate response. M itogen a ctivated p rotein k inases (MAPKs) play an important role in the signal transduction of yeast and animals and increasing evidence suggests a similar role of MAPKs in the signal transduction of plants. MAPKs employ their function as part of protein kinase cascades, composed of a MAPK, a M AP Kk inase (MKK), and a M AP Kk inase k inase (MKKK). MKKKs activate MKKs by phosphorylation of conserved threonine or serine residues, and subsequently MKKs activate the MAPKs by phosphorylation of highly conserved tyrosine and threonine residues.The introduction gives a brief overview of the MAPK cascades employed by yeast and animals and a more extensive overview of our current knowledge about the function of MAPK cascades in plants. To get an overview of the plant MAPKs and their functions the known plant MAPKs were classified and the analyses of all isolated full-length plant MAPK sequences reveals that they can be divided into at least five distinct subfamilies (Chapter 2). For some of these groups it could be shown that MAPKs with similar sequences also perform similar functions. In addition, analysis of e xpressed s equence t ags (ESTs) and partial cDNAs coding for MAPKs revealed the existence of a new plant MAPK subfamily.The goal of the research described in this thesis was to provide insight in the role of several plant MAPKs in stress responses of plants. One of the most severe environmental stresses to which plants can be exposed is wounding. It can be the result of physical injury, herbivore or pathogen attack and induces a wide range of responses, in general involving the induction of genes active in healing and defense processes. In chapter 3 the involvement of a MAPK in the wound response of alfalfa is discussed. It could be shown that wounding activates the alfalfa s tress a ctivated M AP K (SAMK) both at the post