Conceptual Overview

Mitosis & Cytokinesis

Plants grow by cell division and cell enlargement, both of which give rise to a diversity of cell, tissue, and organ structure. Division of their nuclei always precedes the origin of new cells, which occurs by cytoplasmic division, or cytokinesis. Unlike plastids and mitochondria, the nuclei never divide into two by simple constriction. Such a method could not provide equal distribution of chromosomes between sister nuclei. However, such an exact distribution is achieved by a complex and very precise mechanism known as mitosis, the universal form of nuclear division common to all eukaryotes. Four phases of mitosis are recognized: prophase, metaphase, anaphase and telophase. However, in plants the first sign of incipient mitosis is the clustering of cortical cytoplasmic microtubules in a pre-prophase band of microtubules. This band is oriented in a plane coinciding with that of the future cell plate and cytokinesis.

During prophase, chromatin starts to condense and chromosomes begin to appear. Near the beginning of prophase the pre-prophase band of microtubules and nucleolus disappear, and the nuclear envelope disintegrates into fragments of endoplasmic reticulum. A set of microtubules, the spindle apparatus is formed, beginning from the poles of the cell and extending to the center of the cell. Some of the spindle microtubules are attached to chromosomes. It becomes apparent that each chromosome is composed of a pair of identical threads, sister chromatids. At metaphase, chromosomes aggregate in the equatorial plane of the cell, a site determined earlier by the pre-prophase band of microtubules. At anaphase, chromatids separate from each other and move to opposite poles of the spindle. In addition to chromatids, it is also believed that some organelles may be segregated to the cell poles by the action of microtubules, either as a part of the spindle or separately. When the new chromatids (now called single-stranded chromosomes) reach the poles of the cell at telophase, they group together and begin to decondense. Each sister nucleus gets a set of chromosomes genetically identical to one another and to that of the parent nucleus. Finally, chromatids decondense and nucleoli and the nuclear envelope are restored. Interphase is the non-divisional part of the cell cycle and, during it, there is preparation for a new mitotic cycle.

Cytokinesis starts with the initiation of a cell plate (the developing cell wall) in the equatorial plane between the sister nuclei during telophase. The cell plate is disc-like and grows centrifugally towards the wall of the mother cell. Its growth occurs by the fusion of Golgi vesicles, and is associated with the phragmoplast, an aggregation of microtubules with their orientation perpendicular to the expanding cell plate. The microtubules of the phragmoplast are believed to direct the movement of secretory vesicles containing new wall materials toward the cell plate. Eventually, the cell plate grows to the wall of the mother cell and the separation of sister cells is complete. The phragmoplast then disappears. Although cell separation is technically complete, new cell wall material is then laid down all around the cell walls--not only at the cell plate.

Apical Meristems

Unlike animals, plants continue to grow and produce new organs and tissues throughout their lives. It is their adaptation to a stationary and autotrophic life habit. In order to maintain their supply of water and nutrients, plants must grow continuously, expanding the surface of their interaction with the environment. There are two principal types of plant growth, the growth in height and length (primary growth of stems, leaves and roots), and the subsequent growth in thickness (secondary growth). Each type of growth takes place by both cell division and cell enlargement. The cell divisions are primarily concentrated in specific zones called meristems. Primary growth is achieved mainly through the activity of apical meristems consisting of undifferentiated cells that are structurally similar. There are shoot apical meristems occupying the tips of the shoots, and root apical meristems found in root-tips. In addition, intercalary primary growth frequently occurs due to the activity of intercalary meristems. Meristematic activity including both cell divisions and cell growth continues to a lesser extent in the zones adjacent to apical meristems.

Apical meristems originate during embryogenesis at the two poles of the embryo axis as shoot and root apical meristems (see unit on "Reproduction: Megasporogenesis & Embryology"). The shoot apical meristem gives rise to all above-ground organs of the plant such as organs of the shoot (stem and leaves), and also reproductive organs (flower parts, fruits and seeds). In addition, underground organs such as tubers, bulbs and rhizomes are derivatives of the shoot apical meristem. While the characteristic feature of the shoot apical meristem is its ability to initiate the lateral organs such as leaves, axillary buds and flower parts, the initiation of lateral organs is not the function of the root apical meristem, and the lateral roots and the whole root system of the plant are not formed as the direct result of its activity. In addition, an important difference between the root apical meristem and the shoot apical meristem is the ability of the former to produce the rootcap.

Apical meristems, also called promeristems, consist of cells û ranging from initials to primary meristems. The primary meristems divide repeatedly to form primary permanent tissues. The following primary meristems can be distinguished, namely:

protoderm from which the epidermis differentiates,

ground meristem which gives rise to the cortex and pith in stems, cortex in roots, and to mesophyll in leaves,

procambium in which vascular tissues develop and,

calyptrogen which is found in roots of monocotyledons and from which the rootcap, or calyptra, differentiates.

Secondary growth is the result of the activity of lateral meristems, primarily vascular cambium and, to a much lesser extent, cork cambium. These topics are treated in separate units of study.