Tunica-Corpus model of the apical meristem (growing tip). The epidermal (L1) and subepidermal (L2) layers form the outer layers called the tunica. The inner L3 layer is called the corpus. Cells in the L1 and L2 layers divide in a sideways fashion which keeps these layers distinct, while the L3 layer divides in a more random fashion.

A meristem is a tissue in all plants consisting of undifferentiated cells (meristematic cells) and found in zones of the plant where growth can take place. Tissue is a cellular organizational level intermediate between cells and a complete organism Plants are living Organisms belonging to the kingdom Plantae.

The term “meristem” was first used by Karl Wilhelm von Nägeli (1817-1891) from his book “Beiträge zur Wissenschaftlichen Botanik” in 1858. Karl Wilhelm von Nägeli ( March 27, 1817 – May 11, 1891) was a Swiss botanist. It is derived from the Greek word “merizein”, meaning to divide in recognition of its inherent function.

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. Mitosis is the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei

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). Stem cells are cells found in most if not all multi-cellular Organisms. In Developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized Cell type. Furthermore, the cells are small and protoplasm fills the cell completely. Protoplasm is the living contents of a cell that are surrounded by a Plasma membrane. The vacuoles are extremely small. In general vacuole functions include Removing unwanted structural debris Isolating materials that might be harmful or a threat to the cell Containing The cytoplasm does not contain differentiated plastids (chloroplasts or chromoplasts), although they are present in rudimentary form (proplastids). The cytoplasm is the contents of a cell that is enclosed within the Plasma membrane. Plastids are major Organelles found in plants and algae Plastids often contain pigments used in photosynthesis and the types of pigments present can change Chloroplasts are Organelles found in Plant cells and eukaryotic Algae that conduct Photosynthesis. Chromoplasts are Plastids responsible for Pigment synthesis and storage. Plastids are major Organelles found in plants and algae Plastids often contain pigments used in photosynthesis and the types of pigments present can change 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. An antagonist (from Greek ανταγωνιστής - antagonistes, "opponent competitor rival" is a character or

Meristematic zones

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.

At the meristem summit there is a small group of slowly dividing cells which is commonly called the central zone. Cells of this zone have a stem cell function and are essential for meristem maintenance. The proliferation and growth rates at the meristem summit usually differ considerably from those at the periphery.

Apical meristems

Organisation of an apical meristem (growing tip)
1 - Central zone
2 - Peripheral zone
3 - Médullaire meristem
4 - tissus médullaires

The apical meristem, or growing tip, is a completely undifferentiated meristematic tissue found in the buds and growing tips of roots in plants. In Botany, a bud is an undeveloped or embryonic Shoot and normally occurs in the Axil of a Leaf or at the tip of the stem Plants are living Organisms belonging to the kingdom Plantae. Its main function is to begin growth of new cells in young seedlings at the tips of roots and shoots (forming buds, among other things). Specifically, an active apical meristem lays down a growing root or shoot behind itself, pushing itself forward. ROOT is an object-oriented program and library developed by CERN. Shoots are new plant growth they can include stems, flowering stems with flower buds leaves Apical meristems are very small, compared to the cylinder-shaped lateral meristems (see 'Secondary Meristems' below).

Apical meristems are composed of several layers. The number of layers varies according to plant type. In general the outermost layer is called the tunica while the innermost layers are the corpus. In monocots, the tunica determine the physical characteristics of the leaf edge and margin. Monocotyledons or monocots are one of two major groups of Flowering plants (angiosperms that are traditionally recognised the other being Dicotyledons In dicots, layer two of the corpus determine the characteristics of the edge of the leaf. Dicotyledons, or "dicots", is a name for a group of Flowering plants whose Seed typically has two embryonic leaves or Cotyledons There The corpus and tunica play a critical part of the plant physical appearance as all plant cells are formed from the meristems. Apical meristems are found in two locations: the root and the stem. Some arctic plants have a apical meristem in the lower/middle parts of the plant. It is thought that this kind of meristem evolved because it is advantageous in arctic conditions.

Shoot apical meristems

The source of all above-ground organs. Cells at the SAM summit serve as stem cells to the surrounding peripheral region, where they proliferate rapidly and are incorporated into differentiating leaf or flower primordia.

The shoot apical meristem is the site of most of the embryogenesis in flowering plants. Primordia of leaves, sepals, petals, stamens and ovaries are initiated here at the rate of one every time interval, called a plastochron. It is where the first indications that flower development has been evoked are manifested. One of these indications might be the loss of apical dominance and the release of otherwise dormant cells to develop as axillary shoot meristems, in some species in axils of primordia as close as two or three away from the apical dome. The SAM consists 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 surrounding regions

The four distinct zones mentioned above are maintained by a complex signalling pathway. Stem cells are cells found in most if not all multi-cellular Organisms. In Arabidopsis thaliana , 3 interacting CLAVATA genes are required to regulate the size of the stem cell reservoir in the SAM by controlling the rate of cell division. Arabidopsis thaliana ( A-ra-bi-dóp-sis tha-li-á-na; thale cress, mouse-ear cress or Arabidopsis) is a small Stem cells are cells found in most if not all multi-cellular Organisms. Cell division is a process by which a cell, called the parent cell divides into two or more cells called daughter cells. ^[1] CLV1 and CLV2 are predicted to form a receptor complex (of the LRR receptor like kinase family) to which CLV3 is a ligand. In Biochemistry, a ligand ( latin ligare = to bind is a substance that is able to bind to and form a complex with a Biomolecule ^[2][3][4] CLV3 shares some homology with the ESR proteins of maize, with a short 14 amino acid region being conserved between the proteins. Maize (ˈmeɪz ( Zea mays L. ssp mays) known as corn in some countries is a cereal grain domesticated in Mesoamerica In Chemistry, an amino acid is a Molecule containing both Amine and Carboxyl Functional groups In Biochemistry, this Conservation refers to a high degree of similarity in orthologous DNA sequences protein sequences, or Protein structures amongst various ^[5] Proteins that contain these conserved regions have been grouped into the CLE family of proteins. ^[5]

CLV1 has been shown to interact with several cytoplasmic proteins that are most likely involved in downstream signalling, for example the CLV complex has been found to be associated with Rho/Rac small GTPase related proteins. The cytoplasm is the contents of a cell that is enclosed within the Plasma membrane. In Biology, signal transduction refers to any process by which a cell converts one kind of signal or stimulus into another GTPases (singular GTPase are a large family of Hydrolase Enzymes that can bind and hydrolyze guanosine triphosphate (GTP. ^[1] These proteins may act as an intermediate between the CLV complex and a mitogen-activated protein kinase (MAPK) which is often involved in signalling cascades. Mitogen-activated protein (MAP kinases ( are Serine/threonine-specific protein kinases that respond to extracellular stimuli ( Mitogens and regulate various cellular ^[6] KAPP is a kinase-associated protein phosphatase that has been shown to interact with CLV1. ^[7] KAPP is thought to act as a negative regulator of CLV1 by dephosphorylating it. ^[7]

Another important gene in plant meristem maintenance is WUSCHEL (shortened to WUS), which is a target of CLV signalling. ^[8] WUS is expressed in the cells below the stem cells of the meristem and its presence prevents the differentiation of the stem cells. In Developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized Cell type. ^[8] CLV1 acts to promote cellular differentiation by repressing WUS activity outside of the central zone containing the stem cells. ^[8] STM also acts to prevent the differentiation of stem cells by repressing the expression of Myb genes that are involved in cellular differentiation. ^[1]

Root apical meristems

10x microscope image of root tip with meristem
1 - calyptrogen
2 - quiescent centre
3 - epidermis
4 - rootcap
5 - vascular cylinder

Unlike the SAM, the root apical meristem (RAM) produces cells in two directions. It is covered by the root cap, which protects the apical meristem from the rocks, dirt and pathogens. Cells are continuously sloughed off the outer surface of the root cap. The center of the RAM is occupied by a quiescent center which has low mitotic activity. Evidence suggests the quiescent center does function as the zone of initials. Infrequent division of initial cells in the quiescent center is the source of cells for the RAM. These initial cells and tissue patterns become established in the embryo in the case of the primary root and in the new lateral meristems in the case of secondary roots.

Intercalary meristem

In angiosperms, intercalary meristems occur only in monocot (particularly grass) stems at the base of nodes and leaf blades. Monocotyledons or monocots are one of two major groups of Flowering plants (angiosperms that are traditionally recognised the other being Dicotyledons Horsetails also exhibit intercalary growth. Equisetum is a genus of Vascular plants that reproduce by spores rather than seeds Intercalary meristems are capable of cell division and allow for rapid growth and regrowth of many monocots. Intercalary meristems at the nodes of bamboo allow for rapid stem elongation, while those at the base of most grass leaf blades allow damaged leaves to rapidly regrow. This leaf regrowth in grasses evolved in response to damage by grazing herbivores, but is more familiar to us in response to lawnmowers.

Floral meristem

When plants begin the developmental process known as flowering, the shoot apical meristem is transformed into an inflorescence meristem which goes on to produce the floral meristem which produces the familiar sepals, petals, stamens, and carpels of the flower.

In contrast to vegetative apical meristems and some exflorescence meristems, floral meristems are responsible for determinate growth, the limited growth of the flower to a particular size and form. The transistion from shoot meristem to floral meristem requires floral meristem idenity genes, that both specify the floral organs and cause the termination of the production of stem cells. AGAMOUS (AG) is a floral homeotic gene required for floral meristem termination and necessary for proper development of the stamens and carpals. The stamen ( Plural stamina or stamens, from Latin stamen meaning "thread of the warp " is the male In Tetrapods the carpus is the sole cluster of the Bones in the Wrist between the radius and Ulna and the Metacarpus ^[1] AG is necessary to prevent the conversion of floral meristems to inflorescence shoot meristems, but is not involved in the transition from shoot to floral meristem. ^[9] AG is turned on by the floral meristem identity gene LEAFY (LFY) and WUS and is restricted to the centre of the floral meristem or the inner two whorls. ^[10] This way floral identity and region specificity is achieved. WUS activates AG by binding to a consensus sequence in the AG’s second intron and LFY binds to adjacent recognition sites^[10]. Once AG is activated it represses expression of WUS leading to the termination of the meristem^[10].

Through the years scientists have manipulated floral meristems for economics reasons. An example is the mutant tobacco plant "Maryland Mammoth" In 1936 the department of agriculture of Switzerland performed several scientific tests with this plant. "Maryland Mammoth" is peculiar in this sense that it grows much faster than other tobacco plants.

Apical dominance

Apical dominance is phenomenon where one meristem prevents or inhibits the growth of other meristems. As a result the plant will have one clearly defined main trunk. For example, in trees the tip of the main trunk bears the dominant meristem. Therefore the tip of the trunk grows fast and is not shadowed by branches. If the dominant meristem is cut off, one or more branch tips will assume dominance. The branch will start growing faster and the new growth will be vertical. Over the years the branch may begin to look more and more like an extension of the main trunk. Often several branches will exhibit this behaviour after the removal of apical meristem, leading to a bushy growth.

The mechanism of apical dominance is based on the plant hormone auxin. Auxins are a class of Plant growth substance (often called Phytohormone or Plant hormone) It is produced in the apical meristem and transported towards the roots in the cambium. The vascular cambium is a Lateral meristem in the Vascular tissue of plants If apical dominance is complete, it prevents any branches from forming as long as apical meristem is active. If the dominance is incomplete, side branches will develop.

Primary meristems

Apical meristems may differentiate into three kinds of primary meristem:

• Protoderm - lies around the outside of the stem and develops into the epidermis. The epidermis is the outer single-layered group of cells covering a Plant, especially the Leaf and young tissues of a Vascular plant including stems
• Procambium - lies just inside of the protoderm and develops into primary xylem and primary phloem. In Vascular plants xylem is one of the two types of transport tissue Phloem being the other In Vascular plants phloem is the living tissue that carries organic Nutrients (known as photosynthate particularly Sucrose, a sugar to It also produces the vascular cambium, a secondary meristem. The vascular cambium is a Lateral meristem in the Vascular tissue of plants
• Ground meristem develops into the pith. Pith is a light substance that is found in Vascular plants It consists of soft spongy Parenchyma cells and is located in the center of the stem. It produces the cork cambium, another secondary meristem. Cork cambium is a tissue found in many vascular plants as part of the Periderm.

These meristems are responsible for primary growth, or an increase in length or height which were discovered by scientist Joseph D. Carr of North Carolina in 1943.

Secondary meristems

There are two types of secondary meristems, 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).

• Vascular cambium - produces secondary xylem and secondary phloem, this is a process which may continue throughout the life of the plant. The vascular cambium is a Lateral meristem in the Vascular tissue of plants This is what gives rise to wood in plants. Wood is hard fibrous lignified structural tissue produced as secondary Xylem in the stems of Woody plants notably trees but also shrubs Such plants are called arborescent. Arborescent is a term used by the French thinkers Deleuze and Guattari to characterize thinking marked by insistence on totalizing principles Binarism This does not occur in plants which do not go through secondary growth (known as herbaceous plants). A herbaceous plant (or in botanical use a Herb) is a Plant that has leaves and stems that die down at the end of
• Cork cambium - gives rise to the bark of a tree. Cork cambium is a tissue found in many vascular plants as part of the Periderm.

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. Primary growth leads to lengthening of the plant body and organ formation. All plant organs arise ultimately from cell divisions in the apical meristems, followed by cell expansion and differentiation. Primary growth gives rise to the apical part of many plants.

Cloning

Under appropriate conditions, each shoot meristem can develop into a complete new plant or clone. Cloning in Biology is the process of producing populations of genetically-identical individuals that occurs in nature when organisms such as Bacteria, Insects 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. Horticulture is the art and science of plant cultivation Horticulturists (or horticuluralists) work and conduct research in the fields of Plant propagation The genotype is the genetic constitution of a cell an organism or an individual (i This process is also known as mericloning.

Footnotes

References

• Mauseth J.D.
• 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.
• Scofield and Murray (2006). The evolving concept of the meristem. Plant Molecular Biology 60:v–vii
• Research on meristems: meristemania. org [1]