Differentiated plant cells generally cannot divide or produce cells of a different type. Therefore, cell division in the meristem is required to provide new cells for expansion and differentiation of tissues and initiation of new organs, providing the basic structure of the plant body.
Meristematic cells are analogous in function to stem cells in animals, are incompletely or not at all differentiated, and are capable of continued cellular division (youthful). Furthermore, the cells are small and protoplasm fills the cell completely. The vacuoles are extremely small. The cytoplasm does not contain differentiated plastids (chloroplasts or chromoplasts), although they are present in rudimentary form (proplastids). Meristematic cells are packed closely together without intercellular cavities. The cell wall is a very thin primary cell wall.
Maintenance of the cells requires a balance between two antagonistic processes: organ initiation and stem cell population renewal.
Apical meristems are the completely undifferentiated (indeterminate) meristems in a plant. These differentiate into three kinds of primary meristems. The primary meristems in turn produce the two secondary meristem types. These secondary meristems are also known as lateral meristems because they are involved in lateral growth.
Meristems located at a bud on a branch or shoot are known as a node. Tissue between nodes is known as the internode.
The most general form of meristem is the apical meristem (also called terminal meristem). These are found in buds at the tips of shoots and at the root tip, they are responsible for shoot and root growth respectively. At the tip (apex) of the root the apical meristem is covered and protected by a root cap of differentiated cells. Buds can be naked (with the growing leaves visible), protected by non-overlapping scales (valvate buds) or by overlapping scales (imbricated buds).
Apical meristems are completely undifferentiated (indeterminate). They consist of 4 distinct cell groups: -
- Stem Cells
- The immediate daughter cells of the stem cells
- A subjacent organising centre
- Founder cells for organ initiation in surronding regions
The four distinct zones mentioned above are maintained by a complex signalling pathway. The organisation centre expresses WUS proteins which maintains the stem cell identity of the overlying cells. The stem cells signal back with CLAVATA3 (CLE3) which is assumed to be a ligand for the CLE1 receptor kinase. When CLV1 interacts with CLV3 it initiates a signalling pathway that results in the repression of the expression of wus. This controls the size of the organising centre.
Apical meristems may differentiate into three kinds of primary meristem:
- Protoderm - lies around the outside of the stem and develops into the epidermis.
- Procambium - lies just inside of the protoderm and develops into primary xylem and primary phloem. It also produces the vascular cambium, a secondary meristem.
- Ground meristem develops into the pith. It produces the cork cambium, another secondary meristem.
There are two types of secondary meristems:
- Vascular cambium - produces secondary xylem and secondary phloem, this is a process which may continue throughout the life of the plant. This is what gives rise to wood in plants. Such plants are called arborescent. This does not occur in plants which do not go through secondary growth (known as herbaceous plants).
Cork cambium - gives rise to the bark of a tree.
These are also called the lateral meristems because they surround the established stem of a plant and cause it to grow laterally (i.e. larger in diameter).
Indeterminate Growth of Meristems
Though each plant grows according to a certain set of rules, each new root and shoot meristem can go on growing for as long as it is alive; In many plants meristematic growth is potentially indeterminate, making the overall shape of the plant not determinate in advance. This is the primary growth.
Under appropriate conditions, each shoot meristem can develop into a complete new plant or clone. Such new plants can be grown from shoot cuttings that contain an apical meristem. Root apical meristems are not readily cloned, however. This cloning is called asexual reproduction or vegetative reproduction and is widely practiced in horticulture to mass-produce plants of a desirable genotype. This process is also known as mericloning.
- Neil A. Campbell and Jane B. Reece Biology, 6th edition. Benjamin Cummings.
- Schoof et al. The stem cell population of Arabidopsis shoot meristems is maintained by a regulatory loop between CLAVATA and WUSCHEL genes. Cell 100: 635-644.
- Research on meristems: meristemania.org