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quiescent center (QC) (Figure 1 C), which has a role organizing the meristem and is also
involved in the stem cell identity maintenance QC removal results in the de novo formation
of a new QC with adjacent initial cells and stem cells adjacent to the cortex and endodermal
stem cells yield to epidermal initial cells and the lateral root cap [20-22]. Directly upwards from
the QC the proximal meristem is located, as the distal meristem is located below, and within
the meristems the forward growth is carried on as cells divide and grow there at a steady rate.
When reaching certain distance from the meristem, in elongation area (EA)(Figure 1D) division
is arrested and the cells start to elongate. Elongated cells are associated with endoreplication,
a process of DNA replication without actual cell division which accumulates genome copies
in the cell and uses part of the machinery associated with cell cycle, and involves the inacti‐
vation of mitotic CYC-CDK (Cyclin- Cyclin Dependent Kinase) complexes [23-25]. Pericycle
and cambium cells, distanced from the root tip, maintain the potential to reenter division,
forming LRs or transitional cells at the meristem end, depending on localized auxin responses
[26] or oscillating gene expression [27].
3.1.1. Cell cycle
The cell cycle is a temporal regulator of proliferative cell division, and it is comprised of mitosis,
cytokinesis, post-mitotic interphase (G1), DNA synthetic phase (S) and post-synthetic inter‐
phase (G2)[28]. The conjunction of all these is the key force driving organogenesis and growth
in plants and other eukaryotes. The mitotic cycle is driven by the periodic activation of a
multicomponent system that relies on CDKs as key regulators. CDKs combine with different
CYCs to trigger the transition from the G1 to S phase and the G2 to M phase, and a wide variety
of components control the activity of these kinases, thus becoming part of a complex molecular
network that is still being studied [29-31]. In plants, a number of core cycle regulators have
been revealed to exist [32, 33] and what appears to be distinctive in plants is that they appear
to have many more CYCs and CDKs in comparison to animals and yeasts [21, 24]. The reason
of this abundance of putative function overlapping components can be the one suggested in
[34], postulating that that plants have evolved a combinatorial resource pool consisting of
around ninety different CDK-CYC complex variants, thus explaining to an extent the plasticity
of plant development regulation, as they provide with a strategy to recognize distinct stimuli
and environments, and thus promote different phases of the cell cycle. Cell cycle progression
and controlling mechanisms include transcriptional regulation, protein-protein interaction,
phosphorylation-dephosphorylation and protein degradation [29, 30, 35, 36]. As recently
reviewed [36], the evidence obtained from interaction studies suggests that Arabidopsis
CDKA;1 primarily binds to CYCDs to promote the G1/S transition and to CYCA3 to drive the
S phase progression while CDKA;1 pairs with CYCD3 to drive the M phase progression. In
contrast, CDKBs presumably interact preferentially with CYCA2 and CYCBs to promote the
G2/M transition and the M phase progression [37-39]. In Arabidopsis, the accumulation of the
CYCB1;1 transcript is correlated with meristematic tissues [40], activated from early S phase
in synchronized cells with no significantly increase during G2 phase [41]. Together with
environmental and hormonal stimuli, the coordination of the different cell cycle control
processes lead to a balance between cell division and expansion that ensures the correct
embryonic and post-embryonic development. As part of the extensive toolset that plants
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