Root Development and Abiotic Stress Adaptation - Abiotic Stress: Plant Responses and Applications in Agriculture - page 140

possess in order to finely tune the mitotic and endoreplicative cycles, the phase-specific
activation of CYC-CDK complexes via temporal transcription is a mechanism that is evidently
used but not completely understood in plants. In synchronized Arabidopsis cell cultures many
cell cycle genes present highly specific expression windows during the mitotic cell cycle [41,
42]. For example, the expression of several CYCAs is dramatically upregulated at the G1/S
transition and S phase, while others are accumulated at G2/M transition, as well as all CYCBs.
Most of CYCDs are expressed during G1 and S phases, with the exception of a few ones, like
CYCD3:1, expressed during G2-M. In the case of CDKs, CDKA;1 is expressed throughout the
cell cycle, with constant transcript levels, the CDKB1s are present from S to M phase, and
CDKB2s are detected specifically from late G2 to M phase.
3.1.2. Cell cycle control in root meristem
The expression windows of cell-cycle control genes can be extrapolated to their expression in
the actively dividing cells of the root meristems. In these and all dividing cells, The G1/S
transition is generally controlled by the E2F-DP-RBR (E2F-Dimerisation Partner-Retinoblas‐
toma Related) pathway. One of the three Arabidopsis-encoded E2F transcription factors forms
dimers with one of the two DP proteins to bind to certain promoter sites in the transcriptional
target genes to promote the G1/S transition, including those required for DNA replication and
repair. In G1, CYCD-CDKA complexes phosphorylate RBR, releasing the E2F-DP dimers to
allow them to bind to the transcriptional activation sites [43-46]. In the other hand, E2Fc-DPb
dimers act as transcriptional repressors with yet unknown target genes, although their
repressing mechanism appear to be independent from the RBR pathway [47]. Meanwhile,
genes expressed during G2 and M phases contain M phase-specific activator (MSA) elements
in their promoters, recognized by three Myb repeats (MYB3R) transcription factors, discovered
for the first time in tobacco [48]. The Arabidopsis genome encodes five MYB3R proteins
(MYB3R1-5), from which MYB3R1 and MYB3R4 are the closest homologs of the G2/M specific
transcriptional activators NtMYBA1 and NtMYBA2, with the first having a stable transcript
level through the cell cycle, and the latter having an expression peak during G2/M transition,
suggesting that MYB3R1 is post-translationally regulated. The expression of many G2 to M
specific genes possessing MSA elements in their promoters is visibly down-regulated in the
myb3r1 myb3r4 double mutant, but not completely abolished [49] suggesting an alternative
mechanism controlling the transcription of G2 and M phase genes. Additionally to E2F and
MYBs, there are other transcription factors that control cell cycle phase-specific gene expres‐
sion, as the DNA-binding with one finger (DOF) transcription factor, OBP1, whose overex‐
pression shortens the cell cycle with elevated expression of many other cell cycle genes, and
that normally upregulates the expression of replication-specific transcription factors and
CYCD3;3 [50]. Another form of controlling the activity of CYC-CDK complexes is through
post-translational mechanisms, and among them, the ubiquitin-mediated degradation of cell
cycle proteins is the most determinant for the correct timing in the progression of the cell cycle
[51-53]. A number of ubiquitin-dependent degradation pathways have been associated with
the mitotic cell cycle, and the E3 ubiquitin ligases participate in all cases, marking target
proteins by polyubiquitination and subsequent proteolysis. The Skp-cullin1-F-Box (SCF) E3
ligase regulates primarily the G1/S transition while the Anaphase Promoting Complex/