Root development

Primary roots; cell divisions in the apical meristem, expansion & differentiation

((a) Based on Clowes 1959; (b) based on Waisel et al. 1996)

จ500-600 cells

Separate cell divisions

at the leading edge

generate a root cap which

extends forward as a protective structure

The central cells of

the root cap are often

oriented in longitudinal arrays (columella)

The meristem may be

found 24 h later in a fully

differentiated structure (e.g. phloem)

Divisions can be in any of

three planes

- anticlinal (normal to the

root axis)

- periclinal (tangential to the

root axis) or - radial to the axis.

Cell cycle length in maize root tips. Measured by pulse labelling of

DNA with [3H]thymidine. With the exception of cortical cells

adjacent to the quiescent centre (QC), and the QC itself, all regions of the meristem have similar cell cycle times.

Cell location Duration of cell cycle (h)

QC

Cortex just outside QC

Cortex 1000 µm from QC

Stele just outside QC

Stele 1000 µm from QCRoot cap initials

170.0

42.8

18.6

17.0

17.4

14.0

(Barlow, 1973)

The root cap

and adjacent

epidermal cells are

covered by a layer

of mucigel

consisting of

polysaccharides

secreted by root cells

The location of primary tissues in an elongating root and relative amounts of absorption at various distances behind the apex.

Epidermis and Root Hairs

- Direct contact with the soil

- Epidermis composed of

relatively thin-walled, elongated

cells, which produce protrusions

from the epidermal cells, termed

root hairs.

older root hairs usually

are destroyed by

suberization of epidermal cells

The walls of endodermis often become conspicuously

thickened, plus strips of suberized tissue on the radial walls (Casparian strips)

Endodermis : the innermost layer of the cortex

Structure of endodermis, (A) Cross section of part of a morning

glory root (Convolvu-lus arvensis) showing location of endodermis.

(B) Diagram of endodermal cells, showing Casparian strips on radial

walls of endodermal cells.From Kramer (1983)

Xylem, Phloem

Multiple epidermis

Aerenchyma in aquatic plants

Honeycomb aerenchyma

Wheel shaped aerenchyma

Lateral roots

Lateral roots are a major component of root systems, and

their production is controlled both by environmental cues and by genetics.

http://www.plantcell.org/content/vol19/issue12/images/large/PC.055863.wc.f2.jpeg

Lateral roots are initiated from root pericycle cells adjacent to the protoxylem poles of the parent root.

Storage root of Sweet potato

Corn arial root (prop root)

Phloem parenchyma cells adjacent to the vascular cambium divided and produced adventitious roots.

Adventitious root formation in Marianna 2624 plum, a rootstock used in the production

of fruit trees. Note that the roots appear both at the very base of the cutting (A) and

higher up on the stem where no callus has formed. One of the roots appears to be coming

from the callus. Closer examination shows that the roots are emerging from an area near

the vascular ring and not from the callus itself (B).

Longitudinal section of a stem with a

root primordium. Note the proximity to

the vascular cylinder.

Cross section through a stem with an

adventitious root. Note origin of root

at vascular cylinder.

General process

1. Initiative phase - meristemoids forms. Cells form

from divisions of a few cells – either in callus area

or near vascular bundles. Is a meristematic locus -source of cells. These are the ROOT INITIALS.

2. The root initials then differentiate into ROOT

PRIMORDIA which are organized structures

characteristic of root. Have a root cap, apical meristem, and central cylinder.

3. Root elongate and emerge though the side of the stem or base of the stem.