22.4Root Growth and Branching
Germinated juvenile plants start full-scale growth through cell division by consuming stored substances. In the root, development occurs through cell division in the root apical meristem. The mechanism by which cell division is regulated in an orderly manner in the root apical meristem holds the key to understanding the morphogenesis and development of the root.
The root apical meristem is an undifferentiated cell population covered and protected by the root cap, located at the tip of the root. It is comprised of a small number of cells that do not undergo cell division, called the quiescent center, surrounded by a group of cells with high division activity. The vertical section image of the Arabidopsis root (Figure 22-4A) shows the orderly arrangement, from outside to inside, of the epidermis, cortex, endothelium, and central cylinder including the vascular bundle. These layers can be seen to converge towards the tip of the root in which the quiescent center is located. Surrounding the quiescent center are initial cells, which develop into the cells of the various tissues comprising the root. The quiescent center and initial cells are unique because they remain in the root apical meristem. Other cells, while dividing themselves, are eventually pushed out of the apical meristem by cells that proliferate in areas closer to the quiescent center. After separating from the apical meristem, cells acquire differentiated characteristics while elongating rapidly and eventually maturing.
Normally, there is no preprogrammed limit to root growth in organ development in plants. The root can go on growing for a very long time. For example, the tip of the root of woody plants sometimes continues growing for several hundred years. Such growth is called indeterminate growth, and vital to the indeterminate growth of roots is the structural stability of the root apical meristem. Apparently, the balance of two types of intercellular signals in respect to initial cells is involved in ensuring this stability. One signal is sent from the quiescent center and works to maintain the undifferentiated state of the initial cells, while the other is sent from tissues that have differentiated and works to induce differentiation (Figure 22-4B). In fact, when cells of the quiescent center are destroyed with a laser irradiation, the adjacent initial cells will start differentiating, or when communication is cut off in a tissue between zones with differentiated and undifferentiated cells, the undifferentiated cells will not differentiate.
Root branching is triggered by the formation of lateral roots. First, the cells of the pericycle of the taproot (tissue of the outermost layer of the central cylinder, in other words, the tissue of the layer just inside the endothelium) divide to form the rudiment of a lateral root. Cell division occurs throughout the whole rudiment during this period. After the rudiment has grown to a certain size, the root apical meristem formed in the rudiment is activated and initiates the growth of lateral roots. To start the formation of the lateral roots, auxins, transported via the central cylinder from the shoot by polar transport, are essential. Cytokinins, synthesized at the tip of the taproot, work to inhibit the formation of lateral roots. The balance between auxins and cytokinins forms the basis of the internal regulation of root branching.
Figure 22-4 Root apical meristem
A) Root apical meristem of Arabidopsis. B) Intercellular interaction in root apical meristem.
Root branching patterns are also influenced by external factors. Interestingly, when inorganic nutrients such as nitrogen and phosphorous are unevenly distributed in the soil, lateral roots start growing locally in areas rich with nutrients. Thus, lateral root growth corresponds remarkably to the concentration of nutrients to form a root system that maximizes the absorption of nutrients. In some cases, inorganic nutrients may affect only the growth of lateral root, and in other cases, they may affect both the start of lateral root formation and the growth of these lateral roots.
When exposed to a certain external stimulus, plant organs may respond by either moving towards the stimulus or by turning or bending to avoid the stimulus. This biological phenomenon is called tropism. Bending towards the stimulus is called positive tropism and bending away is called negative tropism. Types of tropism that are well known include negative gravitropism and positive phototropism in the stem, and positive gravitropism, negative phototropism, and positive hydrotropism in the root. In view of the functional roles of the stem and root, both positive and negative tropisms are movement responses that are important to plant growth.
Bending in the stem or the root is caused by differences in the cell elongation rate between the side facing the stimulus and the opposite side that lead to differential growth. In turn, the differences in cell elongation are caused by the uneven distribution of auxins. However, it is still not fully understood how stimuli are detected and how this changes the distribution of auxins. In the case of gravitational tropism in the root, the columella cells in the root cap detect gravity (Column Figure 22-3). Columella cells have well-developed amyloplasts which store large amounts of starch, and it is generally considered that these amyloplasts function as a type of statolith. In columella cells of roots that have been pushed over sideways, the amyloplasts move downwards, inducing changes in the localization of PIN proteins via some type of second messenger (most likely Ca2+). Auxins are then transported acropetally to the center of the root by polar transport, and then distributed radially at the root cap. At the cortex, auxins are sent back basipetally. This distribution of auxins at the rootcap is influenced by changes in the localization of the PIN proteins, which increases the rate of auxins returned to the cortex at the lower side of the root. As a result, the concentration of auxins in the cortex tissue in the elongation zone at the lower side of the root becomes too high, thereby inhibiting root cell elongation. Eventually, the root bends downwards due to the difference in cell elongation at the upper and lower sides of the root.