380 C. Gutierrez
Therefore, in the plant organs, cellular pools including slow proliferating
and stem cells, proliferating cells and endoreplicating cells have to maintain
a strict balance for a proper homeostasis (Fig. 2). The control of cell prolif-
eration and differentiation during development depends, in most cases, on
the concerted action of plant hormones. Quite interestingly, hormones di-
rectly impinge on the availability of several cell cycle regulators either at the
level of transcriptional regulation or, later, on postranslational modifications,
typically, selective proteolysis (del Pozo et al. 2005).
2
Cell Cycle Control in a Developmental Context
Studies in single-cell model systems (yeast and mammalian cells in culture)
have provided during the last two decades most of our current knowledge
of molecular mechanisms regulating cell cycle transitions. Phylogenetic an-
alysis of cell cycle regulators, together with molecular, cellular and genetic
approaches, has revealed that the basic cell cycle machinery is also highly
conserved in all eukaryotes. The identification and further cloning of the first
plant cell cycle regulators (John et al. 1989, 1990; Feiler and Jacobs 1990;
Hata et al. 1990; Ferreira et al. 1991) showed a striking similarity with those
identified in yeast and human cells, just one or two years earlier (Lee and
Nurse 1987; Nurse, 1990). However, one possibility, based on the different
complexities and properties of animals and plants, was that cell cycle reg-
ulation could be relatively more similar to that occurring in yeast. Intense
efforts in the following years, highly helped by genomic information from
A. thaliana, revealed that this was not the case. In addition, plant-specific
genes and circuits have been identified and they are reviewed in other chap-
ters in this volume.
Detailed information of each group of cell cycle regulators has been pro-
vided in previous chapters. Studies in various model systems are revealing
unanticipated roles of cell cycle regulators in the context of a developing
or growing organism. It seems that cell cycle regulators are also the targets
to integrate cell proliferation with other cellular activities as well as with
organogenesis (Gutierrez 2005; de Jager et al. 2005). Here, I will present an
overview of cell proliferation control under a developmental perspective with
the aim of providing current evidence in support of the need to reassess the
role of cell cycle regulators in the context of a multicellular organism.
2.1
CDKs, Cyclins and CDK Inhibitors
Different types of CDKs have been identified inArabidopsis.TypeAisconsti-
tuted by CDKs containing the typical amino acid motif PSTAIRE. A variation