MAP Kinase Signaling During M Phase Progression 239
ing cytokinesis. Whether NACK proteins directly bind to MTs remains to be
determined, but it will be interesting to investigate whether the binding of
NACK1 to NPK1 and MTs is controlled by cyclin/CDK complexes (Fig. 2C).
Function of NACK proteins. We postulated that overexpression of a trun-
cated form of NACK1 (NACK:ST) that contained the stalk (ST) region might
have a phenotypic effect similar to overexpression of a kinase-defective NPK1
on cytokinesis if the direct binding of NACK1 is required for the localiza-
tion and activation of NPK1. As expected, both BY-2 cells and tobacco plants
overexpressing NACK:ST have many multinucleated cells with incomplete cell
plates (Fig. 4A; Nishihama et al. 2002). In these BY-2 cells, time-lapse ob-
servations of cytokinesis reveals that expansion of cell plates is markedly
suppressed (Nishihama et al. 2002). We therefore concluded that NACK1 reg-
ulates plant cytokinesis by activating NPK1 MAPKKK.
The homologs of theNACK1andNACK2genes inArabidopsisare desig-
natedAtNACK1andAtNACK2and are identical toHINKEL(HIK)andSTUD
Fig. 4The involvement of the NACK-PQR pathway in plant cytokinesis.AGeneration of
bi- or multi-nucleate stomata and pavement cells upon expression of a motor domain-less
NACK1 (+DEX) in tobacco plants (bottom panels).Bar, 10 μm. Seeds of a plant trans-
formed with the dexamethasone (DEX)-inducible NACK:ST construct were allowed to
germinate on solid medium with DEX (+ DEX) or without DEX (-DEX).Top panelsshow
the surface of cotyledons of 10-day-old seedlings.Bar, 10 mm.BEffects of the knock-
out of theANQ1gene on the development and cell division ofArabidopsis.a11-day-old
plants of wild-type (WT; Wassiliewskija;left),anq1(center)andatnack1-1(right).Bar,
1 cm.bSections of roots of 12-day-old plants of WT (left)andanq1(right)stained
with toluidineblue.Arrowsindicate incomplete cell walls.Arrowheadsindicate nuclei in
multinucleate cells.Bar, 50 μm