Preface
The molecular mechanisms controlling cell cycle progression are conserved
in essentially the same form in all eukaryotes studied. Among the best-
characterized examples of the regulators involved here are the cyclin-dependent
kinases, CDKs, which were originally identified as cell cycle-related protein ki-
nases. These enzymes are activated at appropriate times in the cell cycle by
association with a phase-specific cyclin to form a cyclin-dependent kinase
complex that promotes progression through successive G1, S, and M phases.
Although plants employ the same cell-cycle regulatory proteins as other
eukaryotes, they have also evolved unique molecular mechanisms that allow
integration of environmental, physiological, and developmental signals into
networks to control proper cell division and expansion. Our understanding in
these fields has come, and could only have come, from experimental obser-
vations on plants, although studies on other model eukaryotes have provided
very useful knowledge in the control of cell cycle in general.
The circadian rhythmicity of cell division in unicellular algae has been
known for a long time and research into the circadian clock and cell cycle
control in higher plants is just beginning to emerge. Whereas the biological
clocks of plants and animals share common features in their organization
and functionality, most clock genes are different between these kingdoms.
Furthermore, since the contribution made by each plant cell to the shape of
the surrounding tissue is determined solely by the processes of cytokinesis
and differential cell expansion, cell-to-cell communication plays a critical role
in forcing division in a given plane, either periclinally or anticlinally. Whereas
unicellular organisms constantly face the divide-or-die challenge, higher plants
have mainly relied on the cell division in meristems to grow. They have evolved
regulatory mechanisms that allow the integration of processes at the cellular
level with those related to organogenesis. Such a coordination between cell
division and differentiation is required in order to develop a functional plant.
While organogenesis in animals occurs during embryogenesis, organ initiation
and growth in plants is a post-embryonic and continuous process that occurs
over the entire lifespan of the organism.
Cytokinesis in plants, involving the establishment of the cell plate, is mor-
phologically distinct from cytokinesis in yeast and animals. Significant progress
in plant cytokinesis research has been achieved primarily due to the discovery