Lessons from the root apex - Intercellular Communication in Plants - page 204

In the classical study by van den Berg et al. (1995), each of the four cells of
the QC was laser ablated. This resulted in expansion and division of the cells in
the proximal vascular bundle to replace the dead cells. Furthermore, the study of
cell-specific markers showed that these invading cells expressed the root cap marker
instead of vascular marker in their new position (van den Berg et al. , 1995). This
was the first demonstration that cell fates in the root are not permanent but can be
changed based on their position in relation to other cells.
Although this study revealed the important principle of position-dependent cell
fates, the role of QC itself remained open. That was revealed by a subsequent study
in which only one of the QC cells was ablated at a time (van den Berg et al. , 1997).
Ablation of one QC cell resulted in cessation of cell division of the columella ini-
tials, which were in direct contact with the ablated cell but not in those that were
contacting the remaining intact QC cells. This demonstrated that contact with QC
cells keeps cells in an initial-specific, less differentiated state. Thus, central cells
control cell differentiation possibly in a cell-contact-dependent manner. Further-
more, examination of the fates of QC cells and initials by inducing clones in young
meristems using a heat-inducible transposon-based marker indicated that the central
cells (QC) divide at a low frequency and that their daughter cells end up in various
cell files in a relatively random fashion (Kidner et al. , 2000).
Positional information also determines cell fate in the radial plane. Pericycle
cells were able to replace ablated cortical initials, indicating that pericycle cells
switch fate when moved over radial clonal boundaries. Cortical cells also respond
to changes in position (van den Berg et al., 1995). When epidermal initials were
ablated, a neighbouring cortical cell occupied its position and took up its fate (van den
Berg et al., 1995, Berger et al. , 1998). This indicated that cell fate in the epidermis
is determined by position relative to the underlying cell layer, the cortex (Berger
et al. , 1998). Laser ablation studies indicated that all cell types can adopt position-
dependent fates (van den Berg et al., 1995). This was also shown by clonal analysis
studies (Berger et al. , 1998, Kidner et al. , 2000) in which a rare class of sectors
was observed that spanned the tissue radially, indicating that also in a growing root
sometimes cells that have a clonal origin end up in adjacent radial cell files.
In order to understand the direction of information flow underlying the radial
patterning in the root apex, van den Berg et al. (1995) uncoupled the initial cells
from the older tissue. Isolation of ground tissue initial cells from more mature ground
tissue cells by ablation of their apically located daughters interferes with the change
in orientation of the cell division plane required to form new endodermal and cortical
cells. Thus, positional signals can be perpetuated from more mature to initial cells
to guide the pattern of meristem cell differentiation (van den Berg et al., 1995). On
the other hand, since the QC delays differentiation or promotes stem cell fate, these
observations may also reflect an accumulation of stem cell promoting factors. Laser
ablation and genetic data have further shown that it is a balance between short-range
signals inhibiting differentiation and signals that reinforce cell fate decisions which
control pattern formation in the root meristem (van den Berg et al., 1997). Therefore,
root development involves cell-to-cell signalling.