Transcriptional Control of the Plant Cell Cycle 23
bidopsis cells. However, in yeast, high levels of cell cycle synchrony persist for
more than two cycles, whereas in plant cell cultures, including Arabidopsis,
tight synchrony does not persist for an entire cycle.
There were∼60 transcription factors amongst the cohort of∼1100 genes
that were cell cycle regulated or cell cycle associated, but only 14 of these
were clearly cell cycle regulated. With two likely exceptions (3RMYB genes,
see below), it is presently unclear whether any of these transcription factors
mediates the phase-specific expression of core cell cycle regulators. In this
scenario, they would be central to the transcription control cycle that is in-
terlocked with the CDK and proteolysis cycles. Alternatively, these factors
could be required to orchestrate the expression of effector genes associated
with phase-specific physiological processes such as DNA or cell wall synthe-
sis. Interestingly, eight of the cell cycle regulated factors showed only very
modest amplitudes (∼two- to threefold) of transcript oscillation. This prop-
erty would be expected for at least some transcription factors that target core
cell cycle regulators, as their activity, subcellular localization or stability is
likely to be regulated post-transcriptionally by CDK-dependent phosphoryla-
tion as a mechanism to interlock these regulatory cycles.
3
Cell Cycle Associated Transcription Factors in Plants
Three regulatory circuits involved in cell cycle phase-specific gene expression
are currently known in plants. These are at present only defined by thecis-
regulatory elements identified in promoters of some periodically expressed
genes and not yet conclusively by the cognate proteins that bind these elem-
ents. These are: (i) expression of histone genes in S-phase depends on octamer
(OCT) and hexamer (HEX) motifs (Chaubet et al. 1996; Taoka et al. 1999; Mi-
nami et al. 2000); (ii) the promoters of many genes that are up-regulated at
G1/S or in S-phase contain motifs known to bind E2F/DP factors (Ramirez-
Parra et al. 2003; Vandepoele et al. 2005); and (iii) expression of some genes at
G2/M depends on MSA elements, to which a class of transcription factors with
three repeats of the canonical Myb-type DNA binding motif bind (3RMybs)
(Ito et al. 1998, 2001). However, with the exception of the latter class of Myb-
type transcription factors (Ito et al. 2001; Haga et al. 2007), far too little is
known about the transcription factors that comprise the regulatory network
responsible for cell cycle phase-specific gene expression in plants.
3.1
Histone Expression in S-Phase
Histone gene expression is strongly stimulated in S-phase. Detailed promoter
analysis of histone genes in maize, Arabidopsis and wheat has revealed the