20 P. Doerner
shown to promote de-phosphorylation of the phospho-tyrosine inhibitory for
CDK activity in animal systems, and inNicotiana plumbaginifolia(Zhang
et al. 1996, 2005), there is no evidence that this plays an important role
in “normal” cell cycle progression in Arabidopsis (De Schutter et al. 2007).
However, cytokinins play an important role in plant source–sink relation-
ships, transducing environmental cues (Miyawaki et al. 2004), balancing the
activities of shoot and root meristems (Werner et al. 2001, 2003), and de-
termining meristem size and growth rate (Dello Ioio et al. 2007); all func-
tions related to growth control of the organism. It will be interesting to
more precisely identify the mechanisms of cytokinin action during cell cycle
progression.
1.2
Conclusions
Taken together, the currently available data indicates that the canonical cell
cycle mechanisms are not hierarchically organized but rather are equally im-
portant, specialized and complementary functions required for robust control
of the eukaryotic cell cycle. While each of these mechanisms is a closed cycle
inasmuch as their activities oscillate in a phase-specific manner, and early
steps in each cycle promote the activation of later steps, they are not indepen-
dent of each other. The canonical mechanisms are coupled through regulatory
interactions that allow them to continually entrain each other. However, the
coupling mechanisms can vary between model species. In a scenario of eq-
uitable but specialized functions, the transcriptional regulatory network has
the role of propelling cell cycle progression; while the CDK-dependent phos-
phorylation cascade provides the switches to move from one phase to the next
and the complementary ubiquitin-dependent proteolysis machinery insures
the directionality of the cycle.
2
Cell Cycle Regulated Gene Expression in Plants
Comprehensive studies on cell cycle regulated gene expression have been
conducted on two systems: the readily synchronized tobacco Bright Yellow 2
(BY2) cell culture system (Nagata et al. 1992), and a recently developed syn-
chronized Arabidopsis cell culture system (Menges and Murray 2002). The
degree of synchrony achievable with the Arabidopsis system is not quite as
high and persistent as with the BY2 system. However, it has the important
advantage of representing a completely sequenced genome and many array
tools for gene expression analysis are available in Arabidopsis. These tools
greatly facilitate the unequivocal identification of genes with dynamic expres-
sion patterns in the course of the cell cycle.