Agriculture Reference
In-Depth Information
8
Lessons from the root apex
Martin Bonke, Sari Tahtiharju and Yka Helariutta
8.1
Introduction
The Arabidopsis root with its simple structure serves as an excellent model for un-
derstanding plant organogenesis. In this chapter, we shall review the recent progress
in understanding intercellular communication underlying pattern formation, cell
division and cell differentiation during root development.
8.2
Organization of the root
8.2.1 Anatomy of the root meristem and procambium in the apex
of a growing root
The principal tissues of Arabidopsis primary root are arranged simply, and in con-
centric layers (see Plate 8.1, following page 146). In the mature region of the root,
single layers of epidermis (surrounded by the lateral root cap at the most distal
region of the root tip), cortex, endodermis and pericycle surround a small number
of vascular cells (Dolan et al. , 1993; Scheres et al. , 1994). All these layers of the
Arabidopsis root arise from files of cells that originate from a small number of initial
cells at the tip of the root, defined as a meristem. Internal to and contacting all the
initials are a small number of central cells that are mitotically inactive and are known
as the quiescent centre (QC) (Clowes, 1956; Dolan et al. , 1993). On the proximal
(shoot) side of the QC, a tier of cells comprises the pericycle initials and the vascular
tissue initials, the latter dividing to form the youngest xylem and phloem cells. At
the radial flanks are the initials that give rise to the endodermal and cortical layers.
At the distal (root apex) side of the QC are the initials for both the columella and
lateral root cap as well as the epidermis, with the lateral root cap and epidermis
appearing to originate from the same initials (Dolan et al. , 1993). All initial cells
together are defined as the promeristem, the minimal group of cells that is capable of
making all tissues by ordered divisions. With each division the initials add one cell
to the plant body, while the initial cell retains its position within the meristem. This
division of initials can be either solely anticlinal (resulting in a single file of cells) or
first periclinal then anticlinal (resulting in two or more cell layers). The columella
initials divide only anticlinally and their progeny undergo rapid cell expansion and
then differentiate. The other three types of initials undergo both periclinal and an-
ticlinal divisions, resulting in cell lineages that acquire different identities (Dolan
et al. , 1993). In contrast to other cell lineages, vascular cell lineages appear to arise
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