378 C. Gutierrez
Fig. 1Each proliferating cell develops different processes during its life. Some of them are
depicted in the figure. These cycles frequently communicate and their superimposition
gives rise to different functional stages that we define as the cell cycle
However, as expected in a system under a strict homeostasis, these dif-
ferent cycles do not run independently. Rather, they are coupled in vari-
ous ways, and complex crosstalks occur among them. In fact, the coordi-
nation between the cycles is crucial for proper cell cycle progression. For
example, cell growth signals accumulate to favor the accumulation of com-
ponents necessary to increase CDK activity late in G1. This allows the in-
activation of the RBR protein and the release of E2F activity that triggers
the G1/S transition (Fig. 1). Therefore, understanding the mechanisms that
regulate cell cycle progression requires the interplay between different pro-
cesses. In fact, the coordination among all of them is at the basis of the
large variety of genes that either directly or indirectly have an impact on cell
proliferation.
The coordinated activities that occur during cell cycle progression also
contribute to its unidirectionality. Thus, the regulated transition from one
functional stage to the next occurs in a manner where way back is not pos-
sible. The molecular basis for this is that the activity of specific cell cycle
drivers very frequently modifies their targets in such a way that once they ex-
ecute their function, they are irreversibly converted into an “inactive” form.
This may occur, for example, by changing their cellular localization, by target-
ing them for selective proteolysis, or by modifying the cellular transcriptional
program.
Plants, like animals, as multicellular organisms have evolved regulatory
mechanisms that allow the integration of processes at the cellular level with
those related to organogenesis. These coordination mechanisms regulate, in
the context of a developing or growing organism, the cell division potential,
both in proliferating cells and in stem cell populations, cell cycle arrest and
its withdrawal back to cell cycle, cell differentiation and cell death. Further-
more, plants and animals have evolved very different developmental strate-
gies. While organogenesis in animals occurs during embryogenesis, organ