Coordination of Cell Division and Differentiation 379
initiation and growth in plants is a post-embryonic and continuous process
that occurs over the entire lifespan of the organism.
In some cases, after completing one full genome replication cycle during
the S-phase, cells take the decision of exiting the normal cell cycle to enter the
endocycle, a different cycle in which repeated rounds of genome replication
occur in the absence of cytokinesis. Endoreplication, by which cells increase
exponentially their nuclear DNA content, occur in all eukaryotic cells (Edgar
and Orr-Weaver 2001), but in plants is a much more frequent event in many
cell types (Kondorosi et al. 2000; Larkins et al. 2001). Interestingly, a variety
of specific differentiation programs depend on the occurrence of endocycle
rounds.
Another important aspect in this context is the ability of plants to regener-
ate. By this process certain cells can dedifferentiate and revert to a totipotent
or pluripotent state, proliferate and take various cell fates to originate an or-
gan de novo. It is obvious that cell cycle control should have a crucial impact
in coordinated regeneration although the molecular details have not been
studied yet with sufficient detail.
Fig. 2 Different cellular pools can be distinguished in a developing plant. Cells that are
arrested or slow proliferating (some of these could be stem cells) respond to hormonal
or developmental signals to initiate an active proliferative phase. Later in development,
some cells exit the cell cycle and are recruited to enter the endocycle program before they
differentiate within particular organs. Some cell cycle regulatory components affect these
transitions. Cyclins (the acronym CYC has been omitted for simplicity) has been placed
separately, although they most likely exert their function together by activating specific
CDK which phosphorylate a variety of factors. The individual combinations of cyclins,
CDK and their targets in vivo are not known