Below the QC lies thecolumnella root cap initialcells, which give rise to the root cap, a
protective structure. Above the QC are the epidermal initials, which will form the epidermis
and lateral root cap. The cortical and endodermal initial cells give rise to the cortex and
endodermis; the final layer is the vascular tissue. The initial cell that gives rise to either
endoderm or cortex divides anticlinally once and then periclinally once before these iden-
tities are laid down. The portion of the root enclosed by the endodermis is often referred to
as thestele.
Cell divisions from these initial cells follows a strict pattern of progressive differen-
tiation resulting in an expansion (elongation zone) and a differentiation (maturation
zone) to build a regular arrangement of cell files within the root body. It is not surprising
that expression domains of regulatory genes are responsible for cell fate patterning in the
RAM. For example, theshort root(Shr) andscarecrow(Scr) genes help specify the endo-
dermis and cortical identities of cells, respectively. SHR and SCR proteins function in a
novel signaling pathway to determine radial patterning in the root. The SHR protein is
translated in the stele and then moves to the adjacent cell layer, where it activates SCR
transcription and initiates endodermal specification. The SCR protein is then thought to
regulate the asymmetric cell division that results in the formation of cortex and
endodermis.
The plant hormoneauxin, or indole acetic acid, is required for formation of the embryo-
nic root, lateral roots, and maintenance of the cellular organization around the initials of
the seedling root. Auxin moves through the plant from the shoot, where it is synthesized,
to the root using a system of influx and efflux carriers localized asymmetrically in the
cells of the vascular tissues. It has been shown that the family of auxin transporters
encoded by the Pingenes are the auxin efflux carriers and that PIN1 localization
becomes progressively polarized in developing embryos. By the globular stage, PIN
expression is confined to the basal portion of the embryo, and as embryogenesis proceeds,
PIN becomes further localized to the developing vasculature. The effects of auxin on root
patterning can be visualized in transgenic plants containing five copies of an auxin respon-
sive gene promoter element to drive expression of the GUS (b-glucoronidase) reporter
gene. When expressed in transgenicArabidopsis, one can visualize auxin content by utiliz-
ing an assay that detects GUS activity. The results show that there exists an expression
maxima in the root initial cells, supporting the role of auxin in root patterning. Root mer-
istems are the focus of much research (Campilho et al. 2006; Costa and Dolan 2000).
The formation of lateral and adventitious roots also requires auxin. Lateral or secondary
roots originate from the percicyle, a specific cell type contained within the stele of the root.
Cells within the pericycle undergo cell division, and then further cell division and cell
expansion results in the formation of a lateral root. These cells begin cell division in
response to auxin and environmental cues and must establish a connection to the vascular
trace of the primary root. Adventitious roots can develop from the stems of some plants
when placed under inducing conditions. Tomato, for example, can develop many adventi-
tious roots from a cut stem when placed under humid conditions.
Root hairsare another type of cell contributing to the overall root function of absorp-
tion of water and minerals. The outer, epidermal layer of the root gives rise to root hairs.
Root hair formation occurs within a specific region of the root, a short distance above the
region of root elongation. Root hairs are short and short-lived and develop on both
primary and secondary roots. Interestingly, a root hair is a single cell that consists of a
thin cell wall, a thin lining of cytoplasm that contains the nucleus, and a large vacuole-
containing cell sap.
4.3. MERISTEMS 95