IX. CONCLUSIONS
Several major themes are emerging from the investigations of cell cycle regulatory mechanisms using dif-
ferent model systems ranging from simple eukaryotes (yeast and Aspergillus) to complex metazoans in-
cluding vertebrates, invertebrates, and plants. First, it is increasingly evident that a few key proteins are
critical in controlling the decision points in the cell cycle and these key proteins are highly conserved in
all eukaryotes, indicating the universality of these key components. Second, the activity of certain protein
kinases appears to play a key role in regulating the transition points between different phases of the cell
cycle. Third, the mode of regulation of these key proteins may vary across phylogenetically divergent
species. Finally, the regulatory mechanisms that control the cell cycle are far more complex in multicel-
lular organisms than in unicellular organisms.
Cell cycle research in plants is in its early stages but because of the developments and tremendous
progress made with fungi, vertebrates, and invertebrate systems and the highly conserved nature of some
of the key cell cycle regulatory proteins, progress has been expedited in finding similarities and differ-
ences in regulatory mechanisms between plants and other eukaryotic organisms. Some of the key proteins
known to be involved in yeast and mammalian systems have been identified in plants. Figure 4 is a model
of what has been discovered in plants and how they might be involved in cell cycle control. Because no
in vivo cyclin/Cdk partnerships have been shown and no in vivo substrates for Cdk phosphorylation have
been identified, much is left to be discovered about cell cycle control in plants.
Although the cell cycle is common to all eukaryotes, it is controlled by different hormones or growth
factors in plants and animals. Hence, although some key proteins are highly conserved across phyloge-
netically divergent species, it is likely that different regulatory mechanisms exist in plants and animals.
As cell division is so fundamental to growth and development, it is bound to be an exciting area of re-
search in plant biology. Advances in molecular and cell biology offer new approaches to investigate this
very complex process. Manipulation of cell cycle regulatory proteins in cultured cells and transgenic
plants should provide more insights into cell cycle regulation in plants as well as in plant development.
ACKNOWLEDGMENTS
Research in our laboratory is supported by grants from the National Science Foundation, Agricultural Ex-
periment Station, USDA, NASA, Plant Biotechnology Laboratory, Colorado Biotechnology Research In-
stitute, and Colorado RNA Center.
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