234 M.Sasabe·Y.Machida
MAPKKs is S/T-X 4 -S/T. Activated MAPKKs also activate MAPKs by phospho-
rylating the conserved amino acid motif TDY or TEY, which is structurally
similar to the typical motif in animals and yeast. The large number of MAP-
KKKs may be due to their role in processing a wide variety of various external
and internal stimuli.
Plant MAPK cascades are activated by a wide variety of stimuli, includ-
ing wounding, cold stress, and osmotic stress and participate in defensive
responses to pathogens, cellular responses to hormones, and regulation of the
cell cycle (Chang 2003; Chinnusamy et al. 2004; Guo and Ecker 2004;ˇSamaj
et al. 2004; Takahashi et al. 2004; Pedley and Martin 2005; Boudsocq and Lau-
riere 2005; Nakagami et al. 2005). Some MAPK cascades also appear to be
involved in the growth and differentiation of plant organs. For example, SIMK
MAPK is required for the formation of root hairs in alfalfa (ˇSamaj et al. 2002),
and YODA MAPKKK regulates the first cell fate decision and stomatal de-
velopment and patterning inArabidopsis(Lukowitz et al. 2004; Bergmann
et al. 2004). Further studies, however, are needed to identify all of the com-
ponents of the MAPK cascades and the mechanisms regulating these protein
kinases.
Here, we review our recent studies on a MAPK cascade that is involved
in the regulation of cytokinesis in plant cells. This MAPK cascade includes
NPK1 MAPKKK, NQK1 MAPKK, and NRK1 MAPK, and it is activated by
the NACK kinesin-like proteins (KLPs) in tobacco. All of the components
of this cascade are both structurally and functionally conserved in tobacco
andArabidopsis(Fig. 1; Hulskamp et al. 1997; Spielman et al. 1997; Strompen
et al. 2002; Nishihama et al. 1997, 2001, 2002; Soyano et al. 2003; Yang et al.
2003; Tanaka et al. 2004). In addition, we recently identified a microtubule-
associated protein (MAP), NtMAP65, as a downstream target of this cascade
in tobacco.
Fig. 1 The NACK-PQR pathway that controls plant cytokinesis. The components of the
NACK-PQR pathways in tobacco andArabidopsisare shown