C. Endoreduplication Cycle
The endoreduplication cycle, during which consecutive doublings of the genomic DNA occur in the ab-
sence of chromatin segregation and cytokinesis, is common in plants. Both local (endopolyploidy re-
stricted to specialized cell types such as endosperm) and systemic somatic polyploidy have been reported
[238–242]. Jacqmard et al. [243] investigated the presence of cell cycle–related genes in mitotically di-
viding cells and endoreduplicating tissues of Arabidopsis. They found that Cdks CDC2aAt and CDC2bAt
and cyclin Arath;CycB1;1 were present only in mitotically dividing cells while CKS1At (see Sec. III.B.3)
was present in both mitotic cells and endoreduplicative cells, suggesting that CKS1At may play a role in
both the mitotic and endoreduplication cycle. An H1K activity in maize endosperm was associated with
a maize cdc2-related protein precipitated with GST (glutathione-S-transferase) fusions to E2F-1 and E1A.
The H1K activity was higher during the period of endoreduplication [220].
A plant Rb protein, ZmRb, undergoes changes in level and phosphorylation state concomitant with
endoreduplication and it is phosphorylated in vitro by an S-phase kinase from endoreduplicating en-
dosperm cells [228]. Another cell cycle–related gene from maize, ZmWee1, is highly expressed during
endoreduplication, suggesting a possible role in this process [184].
Endoreduplication requires exit from the mitotic cycle and transformation of the cell cycle to the en-
docycle. A homologue of CCS52, a protein involved in mitotic cyclin degradation, was isolated from
Medicago sativaroot nodules [244]. Overexpression of CCS52 in yeast triggered mitotic cyclin degrada-
tion, cell division arrest, endoreduplication, and cell enlargement. Expression of CCS52 in Medicagowas
enhanced in differentiating cells undergoing endoreduplication [244].
V. ROLE OF CALCIUM AND CALMODULIN IN CELL CYCLE
REGULATIONCalcium, a key intracellular messenger in both plants and animals, has been shown to regulate many dif-
ferent processes in plants [245–247]. Calmodulin, a calcium-binding protein found in all eukaryotes, is
one of the primary mediators of calcium action (see Chapter 35 for more information on calmodulin). For
over a decade, calcium and calmodulin have been implicated in controlling cell proliferation in eukary-
otic cells including plants [246,248–252]. Calcium is essential for the growth of all eukaryotic cells. It has
been shown that cells require the presence of millimolar levels of extracellular calcium to proliferate
[253,254].
Progression of normal cells through the cell cycle is found to be associated with transient changes in
intracellular calcium concentration [248,249,251,255]. Neoplastic cells, which can proliferate in the ab-
sence of external calcium, contain a higher level of intracellular calcium than normal cells [256]. Manip-
ulation of cytosolic calcium concentration has been shown to affect cell cycle events [257–259]. By de-
termining the level of intracellular calcium during different stages of the cell cycle, it has been
demonstrated that rapid and transient increases in intracellular calcium occur at specific stages of the cell
cycle in plant and animal cells [260–263]. Calcium transients are observed at the awakening from quies-
cence, G 2 /M transition, as the cells completed mitosis, and both sides of G 1 /S boundary [249,252]. Mi-
totic events such as breakdown of nuclear envelope, chromatin condensation, and onset of anaphase have
been correlated with a transient increase in intracellular calcium [251,259,260]. Furthermore, these mi-
totic events could be induced prematurely by artificially elevating cytosolic calcium, whereas chelation
of intracellular calcium by calcium chelating agents blocked the nuclear envelope breakdown and the
metaphase/anaphase transition, suggesting that an increase in cytosolic calcium is required for these mi-
totic events to take place [257–259]. Blocking of intracellular calcium prior to the G 1 /S boundary results
in inhibition of DNA synthesis [249]. This suggests that calcium transients are critical for the progression
of cells from G 1 to S phase of the cell cycle.
Studies with both plant and animal tissues have revealed a higher level of calmodulin in dividing
cells as compared with nondividing cells [245,249,264]. An increased level of calmodulin mRNA, pro-
tein, and activity is observed in meristematic tissues of the plants [245,265,266]. In vertebrates and lower
eukaryotic cells, a twofold increase in the intracellular calmodulin concentration is observed at the G 1 /S
boundary [267–269]. Stimulation of quiescent cells to reenter the proliferative state elevated the amount
of calmodulin. Furthermore, transformed mammalian cell lines have been shown to contain elevated lev-
els of calmodulin [270,271]. To study the effect of altered levels of calmodulin on the cell cycle, Ras-
CELL CYCLE REGULATION IN PLANTS 245