Transcriptional Control of the Plant Cell Cycle 19
mediated proteolysis complements and reinforces the CDK-dependent switch
mechanism: the degradation of CDK inhibitors prevents the cell cycle from
dithering or going backwards. Similarly, at the onset of M-phase with the
breakdown of the nuclear membrane and at the metaphase–anaphase transi-
tion, the APC complex is activated, directly or indirectly, by CDK-dependent
phosphorylation. APC fulfills an essential role in destroying the proteins that
hold the sister chromatids joined until the start of anaphase, and in destroy-
ing mitotic cyclins at M/G1, without which cells could not exit from mitosis.
In these functions, the exquisite balance between ubiquitination and de-
ubiquitination activities is critical for the precise timing of cell cycle progress
(Stegmeier et al. 2007), while CDK-dependent phosphorylation of a regu-
latory subunit of APC is required for its inactivation late in G1 to allow
subsequent accumulation of mitotic cyclins. Taken together, proteolysis acts
to complement, reinforce and re-set cyclin–CDK regulation. Its activity is
closely coupled to CDK regulation.
1.1.4
Other Oscillating Parameters in the Plant Cell Cycle
Proliferation of plant cells requires growth regulators such as auxins and
cytokinins. However, the requirement for cytokinin supplements is not ab-
solute, as some cell cultures, including the tobacco BY2 cell line, are “habit-
uated”, which means that endogenous cytokinin production is adequate to
sustain proliferation.
Interestingly, recent work indicates that cytokinin synthesis, degradation
and perception are exquisitely regulated in the course of cell cycle progres-
sion: Peak transcript accumulation of one of the three cytokinin receptors in
Arabidopsis,CRE1(also known asAHK4,orWOL1)occursinG1(Menges
et al. 2003). In cells treated with low concentrations of lovastatin, which spe-
cifically suppresses synthesis of cytokinin isoprenoid side-chains, only zeatin
supplements can rescue cells from the ensuing cell cycle arrest (Laureys et al.
1998). Cytokinin levels, specifically oftrans-zeatin, increase in a very intrigu-
ing pattern in the course of cell cycle progression: a broad peak is observed
in S-phase and a very transient, and high peak is observed at the G2/M
transition (Dobrev et al. 2002). Other reports find additional peaks of cy-
tokinin accumulation associated with all cell cycle phase transitions in the
tobacco BY2 cell cycle (Hartig and Beck 2005). Furthermore, these strik-
ing dynamics of cytokinin accumulation are complemented by fluctuations
of cytokinin-degrading cytokinin oxidase activity (Dobrev et al. 2002; Har-
tig and Beck 2005), suggesting that the observed sharp transients are the
net result of waves of cytokinin synthesis and degradation. These observa-
tions raise the intriguing possibility that fluctuations in cytokinin abundance
and perception are an additional oscillating mechanism contributing to over-
all robustness in plant cell cycle control. Although cytokinins have been