Meristems

Plants have the impressive abilities to reproduce asexually and regenerate damaged parts. The secret to these abilities lies within a tissue type called meristem. Meristematic cells are fully developed and functional at maturity, but unlike other cells in the plant, they remain totipotent. This means that when induced, they can develop into any specific plant tissue at any point during the life of the plant. Other cells in the plant are fully differentiated (meaning that they are specialized in both form and function) and do not divide. Cells in the meristem, however, divide and produce all of the new cells in a plant.

While meristem tissue is the source of the regenerative potential of a plant, meristems also play a pivotal role in normal plant growth. Plants have the unique ability to continue to grow and develop new organs while functioning as a mature, reproducing organism. Plants grow larger via cell division and cell elongation. Simple plant growth is facilitated by meristem tissue because it is the primary site of cell division (mitosis) in the plant. Plants develop new organs (stems, leaves, flowers, roots) via cell division and cell differentiation. Because the source of all new cells in a plant is the meristem, this tissue plays an important role in organ development as well. While some of the cells of the apical meristem divide to generate new meristematic cells, most of the offspring cells differentiate into specialized cell types that stop dividing and function as a part of the organ in which they were generated.

Meristems and Simple Plant Growth

Plants have meristematic tissue in several locations. Both roots and shoots have meristematic tissue at their tips called apical meristems that are responsible for the lengthening of roots and shoots. The shoot apical meristem is formed during embryonic development, but after germination gives rise to the stem, leaves, and flowers. The root apical meristem is also formed during development, but during germination gives rise to the root system. Cell division and cell elongation in the apical meristem is called primary growth and results in an increase in plant height and root length. Increasing root length enables the plant to tap into the water and mineral resources of a new region or layer of soil. Increasing shoot length makes the plant taller, thus allowing it better access to sunlight for photosynthesis.

Many types of plants also increase the diameter of their roots and stems throughout their lifetime. This type of growth is called secondary growth and is the product of lateral meristem. Lateral meristem is called the vascular cambium in many of the plants in which it is found. Secondary growth gives a plant added stability that allows for the plant to grow taller. Lastly, some plants have intercalary meristem. These are areas of plants that help in the regeneration of parts of the plant that have been damaged by predators or the environment. Intercalary meristems produce growth at the base of grass blades, for instance.

Meristem tissue is not autonomous. Throughout the life of the plant, the rate of cell division and cell elongation in the meristems is regulated by plant hormones . For example, giberellins stimulate cell division in shoot apical meristem, causing the plant to grow taller. These hormones also cause cell elongation in intercalary meristem of grasses. Cytokinin and auxin are also important growth regulators. Auxin stimulates growth by inducing cell elongation, while cytokinins are thought to stimulate both cell division and cell elongation.

Apical Meristems and Pattern Formation

As the source for all new cells of the growing plant, the meristem plays an important role in the formation of new organs and in the correct placement of those organs within the plant body. The process by which this organization happens is called pattern formation and, in plants, is directed by the meristem. To accomplish this task, meristematic cells must be able to interpret their position in the plant and establish a certain fate.

For example, during the development of a new leaf, the dividing cells of the meristem must differentiate into several different functional types of epidermal cells and parenchyma cells. However, they do not need to differentiate into reproductive cells like those found in a flower. How is it that meristematic cells "know" what to become? The actively dividing cells of the apical meristem use positional cues such as hormones and cell-cell interactions as guides during differentiation. Moreover, these positional cues result in the activation of certain genes and the inactivation of other genes in a set of cells, thus initiating their specific differentiation pattern based on their spatial location in the plant. The specific genes that are initially activated in meristem cells during this process are called homeotic genes. These genes encode a family of transcription factors that, once activated, will determine the fate of a cell by activating and inactivating a whole host of other genes.

One mechanism of differential gene expression (the activation and inactivation of genes during differentiation and organ development) is binding of plant hormones to the developing cell's surface. Hormones such as cytokinins have been shown to affect ribonucleic acid (RNA) transcription and translation . It is thought that the presence of both cytokinins and another class of hormones, called auxins, are important for proper root and shoot development. In the laboratory, if a set of undifferentiated meristem cells are grown in culture, they will not develop into a plant embryo unless they are stimulated with auxin and cytokinin. A high cytokinin/auxin ratio will stimulate the meristematic cells to develop stems, leaves, and flower buds. On the other hand, a high auxin/cytokinin ratio will stimulate the meristematic cells to develop roots.

see also Anatomy of Plants; Differentiation in Plants; Genetic Control of Development; Hormones, Plant; Roots; Shoots; Water Movement in Plants

Susan T. Rouse

Bibliography

Fletcher J. C., and Elliott M. Meyerowitz. "Cell Signaling Within the Shoot Meristem." Current Opinions in Plant Biology 3 (2000): 23–30.

Meyerowitz, Elliott M. "The Genetics of Flower Development." Scientific American (November 1994): 56–65.

Moore, Randy, W. Dennis Clark, and Kingley R. Stern. Botany. Boston: William C. Brown Publishers, 1995.

Salisbury, Frank B., and Cleon Ross. Plant Physiology, 4th ed. Belmont, CA: Wadsworth, Inc., 1992.

Van Overbeek, J. "The Control of Plant Growth." Scientific American 219 (1968): 75–81.

Meristems

Meristems are regions of active cell division within a plant. In general there are two types of meristems: apical meristems and lateral meristems. Apical meristems are located at the tip (or apex) of the shoot and the root, as well as at the tips of their branches. These meristems occur in all plants and are responsible for growth in length. By contrast, lateral meristems are found mainly in plants that increase significantly in diameter, such as trees and woody shrubs. Lateral meristems are located along the sides of the stem, root, and their branches; are found just inside the outer layer; and are responsible for growth in diameter.

The term meristem comes from the Greek word meaning "divisible," which emphasizes the fundamental role played by mitotic cell division in these tissues. Meristematic cells are those that divide repeatedly and in a self-perpetuating manner; that is, when a meristematic cell divides, one of the daughter cells remains meristematic. Meristems, however, may not be constantly active. For example, in temperate climates meristematic cells stop dividing during the winter but then begin dividing again in the spring.

Apical Meristems

Both root and shoot apical meristems consist of a group of two types of cells: initials and their immediate derivatives. Initials are the true meristematic cells in that they divide almost continuously throughout the growing season. When an initial divides it forms two daughter cells, one a new initial and the other a derivative that soon stops dividing and eventually differentiates into part of the mature tissues of the plant. In many cases the older derivatives elongate, and it is this process that pushes the initials of the shoot apical meristem higher into the air and the initials of the root apical meristem deeper into the soil. All tissues produced by an apical meristem are called primary tissues.

In most plants the root apical meristem is covered by a protective root cap and consists of a group of relatively small, roughly spherical cells, each having a dense cytoplasm and a large nucleus but no apparent vacuole . The derivatives of certain apical initials give rise to additional root cap cells, thus replacing those that were lost as the root cap rubbed against soil particles. The derivatives of other initials give rise to the mature tissues of the main body of the root, such as xylem, phloem, cortex, and epidermis . In the center of the root apex is a cluster of cells that divides very infrequently. These cells comprise the quiescent center, whose apparent function is to serve as a source of cells should the initials become damaged.

In angiosperms , the shoot apical meristem is not covered by a protective cap and has additional features that distinguish it from the root apex. For example, the lateral appendages of the stem—the leaves and lateral buds—are produced at the shoot apex. Leaves arise as small protuberances (called leaf primordia ) slightly to the side of the apical-most cells. As they elongate, the resulting leaves cover and protect the apical meristem. Buds develop in the angle between the stem and each leaf primordium, a location called the leaf axil. In a plant growing vegetatively, these axillary (or lateral) buds contain meristems that can develop into branches. When the plant reproduces sexually, the shoot apical meristem produces flowers instead of leaves. The various flower parts—petals, sepals , stamens, and carpels —are modified leaves and are produced in a manner similar to that of leaf primordia.

The apical meristem of most angiosperms has a tunica-corpus arrangement of cells. The tunica consists of two or more layers of cells, and the corpus is a mass of cells underneath. Cells of the tunica and corpus give rise to the leaves, buds, and mature tissues of the stem.

Lateral Meristems

Two types of lateral meristems, also called cambia (singular: cambium), are found in plants: the vascular cambium and the cork cambium. Each type consists of a hollow, vertical cylinder of cells that contribute to the thickness of woody plants. As with apical meristems, lateral meristems consist of initials and their immediate derivatives. All tissues produced by a lateral meristem are called secondary tissues.

The vascular cambium contains two kinds of initials: fusiform initials and ray initials, both of which have large vacuoles. Each type of initial produces derivatives toward the inside that develop into xylem cells and derivatives toward the outside that develop into phloem cells. The fusiform initials are long, tapering cells that are vertically oriented. They give rise to xylem vessel elements and phloem sieve-tube members; these cells are involved in the vertical transport of materials through the plant. The ray initials are cube-shaped cells that give rise to xylem parenchyma and phloem parenchyma and together constitute the vascular rays. Rays are involved in the lateral transport of materials. Both the fusiform and ray initials produce many more xylem cells than phloem cells. The accumulating xylem cells push the vascular cambium increasingly farther away from the center of the root, stem, or branch, and as a result the organ increases in diameter.

In response to this increase in thickness the epidermis and other cells exterior to the vascular cambium stretch and eventually break. Before cracks occur, a cork cambium differentiates from cells of the cortex. The cork cambium (or phellogen) produces cork cells (phellem) toward the outside and phelloderm toward the inside. Together, these three tissues constitute the periderm. Cork cells have a flattened shape, and their walls become filled with suberin, a fatty material that makes these cells an impermeable barrier to water, gases, and pathogens . Although the cork cambium and phelloderm are alive at maturity, cork cells are dead. The cork thus provides an effective seal that replaces the epidermis. As the plant organ continues to increase in diameter, the cork cells themselves crack, and additional cork cambia differentiate from underlying tissues as replacements.

see also Anatomy of Plants; Bark; Cells, Specialized Types; Differentiation and Development; Germination and Growth; Tissues; Vascular Tissues.

Robert C. Evans

Bibliography

Esau, Katherine. Plant Anatomy, 2nd ed. New York: John Wiley & Sons, 1965.

Mauseth, James D. Plant Anatomy. Menlo Park, CA: Benjamin/Cummings Publishing Co., 1988.

Moore, Randy, W. Dennis Clark, and Darrell S. Vodopich. Botany, 2nd ed., New York: McGraw-Hill, 1998.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Company, 1999.