Agriculture Reference
In-Depth Information
these sectors sometimes crossed over between root and hypocotyl, it was concluded
that individual cells can change hypocotyl/root fate depending on yet unidentified
positional cues until the end of embryonic development. It was also determined
that the promeristem (the QC with the surrounding initials) is established during the
heart stage and maintained thereafter. Unlike the root-hypocotyl sectors, the lineage
of the hypophysis showed no variation and its descendants always give rise to the
QC and columella initials. The cells in the small part of the embryonic root that is
not a part of the meristem will give rise to the 'collet', the junction between root and
hypocotyl with typical epidermal morphology and early emerging root hairs. Clonal
and anatomical analyses also show that the most basal epidermal cell will become
the initial of the lateral root cap. This takes place at the heart stage of embryogenesis.
8.2.3
Development of secondary roots
In most plant species, secondary (such as lateral) roots are initiated from the per-
icycle cells that are associated with protoxylem. These particular pericycle cells
are distinguishable from the other pericycle cells by the increased number of cell
divisions in the axial plane, causing them to be shorter. When a secondary root is
initiated, the pericycle cells undergo a set of periclinal cell divisions, increasing cell
layer number to between 2 and 4. These cells then increase in size, causing the bud
to emerge from the root. The radial patterning of endodermis, cortex and epidermis
of the prospective lateral root is then already in place (Malamy & Benfey, 1997).
After the lateral root primordium emergences, cell division rate increases and a new
meristem is formed. Laskowski
et al.
(1995) showed by excision experiments that
a functional meristem has developed at the 3-5 cell layer stage. Auxin treatments
indicated that the lateral root formation is a two-step process where first a set of
cells is initiated to divide, followed by activation of those cells to form an organized
meristem. Dubrovsky
et al.
(2000) proposed that by maintaining cell proliferation
the root is prepared for environmental changes and can activate the formation of
secondary roots once the need for them is high. As the anatomy of a secondary root
is very similar to that of the primary root, the emerging picture is that secondary root
development recapitulates genetic processes controlling development of a primary
root.
8.3
Cell fate studies of the growing root
As the root meristem gives rise to clonal cell files, it was originally proposed that
clonal relationships are important for patterning of the root tissues. This view was
already challenged by the observation of the mitotically inactive QC in the focal
point of the clonal cell files by thymidine radiolabelling studies (Clowes, 1956).
Experimental studies on young
Arabidopsis
seedlings during 1990s resolved the
question of the clonal relationships, initial cells and the role of QC (van den Berg
et al.
, 1995, 1997; Berger
et al.
, 1998; Kidner
et al.
, 2000).
