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slightly proximally to the promeristem. In serial sections of the primary root meris-
tem, Mahonen
et al.
(2000) and Baum
et al.
(2002) observed that xylem cell lineages
form an axis composed of four to five cell files very close to the underlying QC.
Two domains of initials that give rise to the phloem cell lineages and to the undiffer-
entiated procambial cell lineages through asymmetric cell divisions flank this axis.
The number and exact pattern of these procambial divisions show some variability
between individual seedlings, which is in contrast to the invariant pattern of cell
lineages in the endodermis and outer layers (Scheres
et al.
, 1994).
Upon germination, cells of the meristem begin to divide and the root expands
axially as a result of cell expansion. As the root continues to grow, the number of
cells in the meristem increases and the rate of cell production increases. Cells are
organized along the longitudinal axis in the root in distinct developmental zones
(Dolan
et al.
, 1993). Cells are produced at the basal region of the meristem, the
meristematic/division zone, which is overlaid by the root cap. Proximal to this zone
is the elongation zone. The next zone is the differentiation zone, in which elongated
cells from different tissues mature into fully differentiated cells. Because root growth
is indeterminate, these processes are continual, resulting in all developmental stages
being present at all times. The radial symmetry of the root combined with a lack of
cell movement means that clonally related cells are frequently found in cell files.
8.2.2 Cellular organization of the root is established during
embryonic development
The basic layout of the mature plant, including root, is established during embryo-
genesis. In
Arabidopsis
, the zygote elongates approximately threefold and subse-
quently divides asymmetrically, resulting in a large basal and a smaller apical cell
(Mansfield & Briarty, 1991). The octant stage (Fig. 8.1A) is reached after three
rounds of cell division of the apical cell. At this stage the embryo constitutes of
the upper and lower tiers, which will produce the apical and basal areas of the later
stages of embryo and seedling respectively. Meanwhile, the basal cell divides only
horizontally and forms the suspensor structure; only the top cell (hypophysis) of the
suspensor will be part of the later seedling as the QC and the columella root cap
(Scheres
et al.
, 1994).
Subsequent tangential cell divisions of the octant result in the formation of the
epidermal (protoderm) layer: dermatogen stage (Fig. 8.1B). Succeeding cell divi-
sions lead to the globular stage (Figs. 8.2C and 8.2D), where the inner cell divisions
are oriented in the apical-basal plane, and axis formation can be seen. After the next
set of divisions, clear polarity of the embryo is established when in the lower tiers
the procambium is formed. In the upper tiers, the cell divisions start the formation
of two cotyledon primordia, which leads to the triangular shape embryo (Fig. 8.1E).
Cell number is now increased to more than a hundred and in the lower tiers the
ground tissue, epidermis and vascular initials are visible. The hypophysis also starts
to divide at this stage.
At the heart stage (Fig. 8.1F), most tissue types are present, cotyledon primordia
start to grow and the shoot apical meristem is initiated. Also the QC and columella
