Involvement of MAP kinase SIMK and actin cytoskeleton in the regulation of root hair tip growth.

Mitogen-activated protein kinases (MAPKs), a specific class of serine/threonine protein kinases, are involved in controlling many cellular functions in all eukaryotes. Signaling through MAPK cascades is involved in cell division, differentiation, and stress sensing. Recently, the stress induced MAPK (SIMK) (Munnik et al., 1999) and its upstream activator SIMKK (Kiegerl et al., 2000) have been characterized in Medicago sativa L. and shown to be inducible by osmotic stress and various fungal elicitors (Cardinale et al., 2000). In different plant species dense F-actin meshworks at the tip of root hairs were observed by immunolabeling with actin antibodies or in vivo using GFP fused to the F-actin binding domain of talin (Braun et al., 1999, Baluška and Volkmann, 2002, Baluška et al., 2000b). As MAPKs are involved in stress signaling to the actin cytoskeleton in yeast and animals, we have analyzed the function of the stress-activated alfalfa MAP kinase SIMK in root hairs