4 F.-Y. Bouget et al.
Cells often divide with a period of 24 hours, leading sometimes to a con-
fusion between the circadian clock and the CDC. The rhythmicity of cell
division has been known for a long time in unicellular algae such as the
flagellate Euglena (Edmunds et al. 1982), the green algaChlamydomonas
(Bruce 1972; Goto and Johnson 1995) or the marine dinoflagellate Gonyaulax
(Sweeney 1958). These cells usually divide once per day or with a multiple of
24 hours. However, when they are allowed to divide more than once per day,
the rhythmicity of cell division is lost inEuglenaor inGonyaulax.Itwascon-
cluded, according to the “circadian/infradian” rule that circadian rhythms of
division exist only in cells dividing in a circadian or infradian way, that is,
once per day or less. According to this rule, both the circadian clock and the
CDC are tightly interconnected and cell division might even be controlled by
an autonomous clock that is completely independent of a circadian clock. Ex-
periments in the prokaryotic cyanobacteriaSynechococcuscontradicted this
rule, since cells dividing more than once per day still exhibit robust circadian
rhythms of cell division and gene expression (Kondo et al. 1997).
Plants are photosynthetic organisms, which heavily rely on light as a source
of energy and it is sometime difficult to discriminate between the effect of light
as a signal through activating specific signalization pathways and as a source of
energy that is a required for a cell to grow and divide when it reaches a critical
size. The molecular basis of both the cell cycle core machinery (Inze 2005) and
the circadian clock are well understood in plants (McClung 2006). However,
neither a direct regulation of cell division by light nor a circadian regulation
of the CDC have been demonstrated in higher plants. Recent results from our
group, using the firefly luciferase reporter protein fused toArabidopsisspe-
cific promoters indicate that the expression of cell cycle regulated genes such
ashistone H4and of central CDC genes such ascyclinBoccurs at the end of
the day and is under circadian control in theArabidopsisshoot apical meris-
tem (unpublished data). However, direct studies of the CDC remain difficult
to performin plantabecause only a limited number of cells divide in grow-
ing plants and cell division is restricted to meristems. Cell division studies
are much easier to perform in unicellular organisms, such as unicellular algae,
which can be naturally synchronized by the light/dark cycle. Circadian regu-
lation of cell division has been known for over 30 years in green algae such as
Chlorophyta chlamydomonas.Chlorophytabelong to the green lineage and are
a sister group toStreptophytawhich encompasses higher plants.
Recently, several important findings have started to unravel the molecular
basis of circadian regulation of cell division in animals (Fu et al. 2002; Matsuo
et al. 2003). In this review we will first focus on the circadian regulation of cell
division in unicellular photosynthetic eukaryotes, especially those evolution-
arily related to higher plants, that is green algae. We will also summarize the
actual knowledge of molecular basis of circadian regulation of the CDC based
on recent work in animals. Finally, we will speculate on what we can expect
from plants.