242 M.Sasabe·Y.Machida
results indicate the NACK-PQR pathway is conserved and promotes cytokine-
sis in tobacco andArabidopsis.InArabidopsis, the pathway includes AtNACK1
and AtNACK2; ANP1, ANP2, and ANP3; and ANQ1/AtMKK6 (Fig. 1).
3
Factors Downstream of the NACK-PQR Pathway
In eukaryotes, the last stage of cell division is cytokinesis, which leads to
the appropriate distribution and separation of the sets of chromosomes and
cell components. Plant cytokinesis occurs at the late anaphase step of cell
division in the phragmoplast, which is a cytokinetic apparatus composed
mainly of MTs and actin filaments. Cytokinesis consists of four main pro-
cesses: (1) formation of the phragmoplast; (2) transport, accumulation, and
fusion of Golgi-derived vesicles (see the works by Sanderfoot, Nebenführ,
in this volume); (3) synthesis and maturation of cell walls (see the work by
Verma and Hong, in this volume); and (4) expansion of the phragmoplast (see
the work by Seguí-Simarro, Otegui, Austin and Staehelin, in this volume). The
NACK1/NPK1 complex and the subsequent MAPK cascade (the NACK-PQR
pathway) appears to control cytokinesis of plant cells by regulating phragmo-
plast expansion, which, as described above, is the final step; however, what
kind of molecules and how they are regulated downstream of the NACK-PQR
pathway has not been established. Recently, a MAP was identified as a sub-
strate of NRK1/NTF6 MAPK (Sasabe et al. 2006). This supports the hypothesis
that the NACK-PQR pathway might regulate an event related to the dynamics
of phragmoplast MTs. We propose a novel regulatory mechanism for the pro-
gression of cytokinesis, namely, via phragmoplast expansion directed by the
NACK-PQR pathway.
3.1
The Molecular Roles of the NACK-PQR Pathway during Cytokinesis
Cytokinesis is executed by the cytokinesis-specific apparatus, the central
spindle, or midbody in animals and the phragmoplast in plants (Nishihama
and Machida 2001; Otegui et al. 2005). These structures develop from late
anaphase to telophase between the two daughter nuclei and consist of two
bundles of antiparallel MTs. As cytokinesis proceeds, the central spindle
becomes compacted in animal cells, and in plant cells, the phragmoplast
expands centrifugally. In addition, new membranes and/or cell walls are
generated inside or outside the midzone of the central spindle or the phrag-
moplast. These dynamic processes appear to be mediated by the turnover of
MTs, which involves the depolymerization of MTs and the polymerization of
tubulins at the plus-ends (Shelden and Wadsworth 1990; Asada et al. 1991;
Hush et al. 1994; Straight and Field 2000). Thus, despite the opposite direc-