Root Stem Cells Generate Longitudinal Files of Cells

Meristems are populations of dividing cells, but not all cells in the meristematic region divide at the same rate or with the same frequency. Typically, the central cells divide much more slowly than the surrounding cells. These rarely dividing cells are called the quiescent center of the root meristem (see Figure 16.17).

Cells are more sensitive to ionizing radiation when they are dividing. This is the basis of the use of radiation as a treatment for cancer in humans. As a result, the rapidly dividing cells of the meristem can be killed by doses of radiation that nondividing and slowly dividing cells, such as those of the quiescent center, can survive. If the rapidly dividing cells of the root are killed by ionizing radiation, in many cases the root can regenerate from the cells of the quiescent center. This ability suggests that quiescent-center cells are important for the patterning involved in forming a root.

The most striking structural feature of the root tip, when viewed in longitudinal section, is the presence of the long files of clonally related cells. Most cell divisions in the root tip are transverse, or anticlinal, with the plane of cytokinesis oriented at right angles to the axis of the root (such divisions tend to increase root length). There are relatively few periclinal divisions, in which the plane of division is parallel to the root axis (such divisions tend to increase root diameter).

Stage 1 Stage 2 Stage 3

Stage 4

Stage 5

Stage 6

Stage 1 Stage 2 Stage 3

Stage 4

Stage 5

FIGURE 16.18 Model for lateral root formation in Arabidopsis. Six major stages are shown in the development of the primordium. The different tissue types are designated by colors. By stage 6, all tissues found in the primary root are present in the typical radial pattern of the branch root. (From Malamy and Benfey 1997.)

Epidermis Cortex Vasculature Pericycle Endodermis

FIGURE 16.18 Model for lateral root formation in Arabidopsis. Six major stages are shown in the development of the primordium. The different tissue types are designated by colors. By stage 6, all tissues found in the primary root are present in the typical radial pattern of the branch root. (From Malamy and Benfey 1997.)

Root cap-epidermal stem cell

Quiescent center Root cap

Cortical-endodermal stem cell

Root cap-epidermal stem cell

Quiescent center Root cap

Periclinal divisions occur mostly near the root tip and establish new files of cells. As a result, the ultimate origin of any particular mature cell can be traced back to one or a few cells in the meristem. These are the stem cells of a particular file. In Arabidopsis, the stem cells surround the quiescent center, but they are not part of the quiescent center. The stem cells ultimately may be derived from quiescent-center cells, but this origin must occur during embryogenesis, since the quiescent-center cells do not divide after germination in normal development. Analysis of the cell division patterns in the roots of the water fern Azolla have contributed to our detailed understanding of meristem function. (For a discussion of this work, see Web Topic 16.3.)

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