A plant root has hairs that originate from a specific file of epidermal cells, called the trichoblast, by tip growth. In the growing tip of root hairs, actin filaments are abundant and are involved in polar growth (Hepler et al. 2001; Sieberer et al. 2005). The actin cytoskeleton is speculated to serve as a guide rail for the polar transport of vesicles. SIMK, a MAPK of alfalfa (lucerne), has been found to localize to the root tip and peripheral spots, suggesting a role in root hair outgrowth (Samaj et al. 2002). This localization is strengthened by the pharmacological stabilization of an actin mesh network by jasplaki-nolide and weakened by the inhibition of MAPKK by UO126. Expression of a gain-of-function version of SIMK overcomes the inhibition of tip growth by pharmacological inactivation of MAPKK. Regulation of the actin cytoskele-ton by a MAP kinase pathway is also observed in animal cells. It is interesting that MAP kinase pathways regulate the cytoskeleton in both cytokinesis and tip growth, although the targets differ - microtubules for the former and actin filaments for the latter.
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