A meristem is the tissue Tissue is a cellular organizational level intermediate between cells and a complete organism. Hence, a tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. Organs are then formed by the functional grouping together of multiple tissues in all plants Plants are living organisms belonging to the kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The scientific study of plants, known as botany, has identified about 350,000 extant species of plants, defined as seed plants, bryophytes, ferns and fern allies. As of 2004, consisting of undifferentiated cells (meristematic cells) and found in zones of the plant where growth can take place.

The term meristem was first used in 1858 by Karh Wilhelm von Nägeli (1817–1891) in his book Beiträge zur Wissenschaftlichen Botanik.^[1] It is derived from the Greek word merizein (μερίζειν), 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 Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly equal shares of these cellular components. Mitosis and 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 Stem cells are cells found in all multi cellular organisms. They are characterized by the ability to renew themselves through mitotic cell division and differentiate into a diverse range of specialized cell types. Research in the stem cell field grew out of findings by Ernest A. McCulloch and James E. Till at the University of Toronto in the 1960s in animals Animals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. All animals are also, are incompletely or not at all differentiated In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a single zygote to a complex system of tissues and cell types. Differentiation is a common, and are capable of continued cellular division (youthful). Furthermore, the cells are small and protoplasm Protoplasm is the living content of a cell that is surrounded by a plasma membrane. This term is not commonly used in modern cell biology. Protoplasm is composed of a mixture of small molecules such as ions, amino acids, monosaccharides and water, and macromolecules such as nucleic acids, proteins, lipids and polysaccharides. In eukaryotes the fills the cell completely. The vacuoles A vacuole is a membrane bound organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules including enzymes in solution, though in certain cases they may contain solids which have been are extremely small. The cytoplasm All the contents of the cells of prokaryote organisms are contained within the cytoplasm. Within the cells of eukaryote organisms the contents of the cell nucleus are separated from the cytoplasm, and are there called the nucleoplasm does not contain differentiated plastids Plastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell. Plastids often contain pigments used in photosynthesis, and the types of pigments present can change or determine the cell's colour (chloroplasts Chloroplasts are organelles found in plant cells and other eukaryotic organisms that conduct photosynthesis. Chloroplasts capture light energy to conserve free energy in the form of ATP and reduce NADP to NADPH through a complex set of processes called photosynthesis or chromoplasts Chromoplasts are plastids responsible for pigment synthesis and storage. They, like all other plastids , are organelles found in specific photosynthetic eukaryotic species), although they are present in rudimentary form (proplastids Plastids are major organelles found in the cells of plants and algae. Plastids are the site of manufacture and storage of important chemical compounds used by the cell. Plastids often contain pigments used in photosynthesis, and the types of pigments present can change or determine the cell's colour). 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.

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.

Meristems also are induced in the roots of legumes such as soybean The soybean or soya bean (UK) (Glycine max) is a species of legume native to East Asia. The plant is classed as an oilseed rather than a pulse. It is an annual plant that has been used in China for 5,000 years to primarily add nitrogen into the soil as part of crop rotation, Lotus japonicus, pea A pea is most commonly the small spherical seed or the seed-pod of the legume Pisum sativum. Each pod contains several peas. Peapods are botanically a fruit, since they contain seeds developed from the ovary of a flower. However, peas are considered to be a vegetable in cooking. The name is also used to describe other edible seeds from the, and Medicago truncatula after infection with soil bacteria commonly called Rhizobium Rhizobium is a genus of Gram-negative soil bacteria that fix nitrogen. Rhizobium forms an endosymbiotic nitrogen fixing association with roots of legumes. Cells of the inner or outer cortex in the so-called "window of nodulation" just behind the developing root tip are induced to divide. The critical signal substance is the lipo-oligosaccharide An oligosaccharide is a saccharide polymer containing a small number of component sugars, also known as simple sugars (monosaccharides). The name is derived from the Greek word oligos, meaning "a few", and from the Latin/Greek word sacchar which means "sugar". Oligosaccharides can have many functions; for example, they are Nod-factor, decorated with side groups to allow specificity of interaction. The Nod factor receptor proteins NFR1 and NFR5 were cloned from several legumes including Lotus japonicus, Medicago truncatula and soybean (Glycine max). Regulation of nodule meristems utilizes long distance regulation commonly called "Autoregulation of Nodulation" (AON). This process involves a leaf-vascular tissue located LRR receptor In biochemistry, a receptor is a protein molecule, embedded in either the plasma membrane or the cytoplasm of a cell, to which one or more specific kinds of signaling molecules may attach. A molecule which binds to a receptor is called a ligand, and may be a peptide (short protein) or other small molecule, such as a neurotransmitter, a hormone, a kinases In chemistry and biochemistry, a kinase, alternatively known as a phosphotransferase, is a type of enzyme that transfers phosphate groups from high-energy donor molecules, such as ATP, to specific substrates. The process is referred to as phosphorylation, not to be confused with phosphorolysis, which is carried out by phosphorylases (LjHAR1, GmNARK and MtSUNN), CLE peptide Peptides are short polymers of amino acids linked by peptide bonds. They have the same chemical structure as proteins, but are shorter in length signalling , and KAPP interaction, similar to that seen in the CLV1,2,3 system. LjKLAVIER also exhibits a nodule regulation phenotype A phenotype is any observable characteristic or trait of an organism: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior . Phenotypes result from the expression of an organism's genes as well as the influence of environmental factors and the interactions between the two though it is not yet known how this relates to the other AON receptor kinases

Apical meristems

The apical meristem, or growing tip, is a completely undifferentiated meristematic tissue found in the buds 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. Once formed, a bud may remain for some time in a dormant condition, or it may form a shoot immediately and growing tips of roots in plants Plants are living organisms belonging to the kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The scientific study of plants, known as botany, has identified about 350,000 extant species of plants, defined as seed plants, bryophytes, ferns and fern allies. As of 2004,. 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 In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating (growing up above the ground or especially above water). Furthermore, a stem normally occurring below ground is not exceptional either (see rhizome). So, it is or shoot Shoots are new plant growth, they can include stems, flowering stems with flower buds, leaves. The new growth from seed germination that grows upward is a shoot where leaves will develop. In the spring, perennial plant shoots are the new growth that grows from the ground in herbaceous plants or the new stem and/or flower growth that grows on woody behind itself, pushing itself forward. 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 Monocotyledons, also known as monocots, are one of two major groups of flowering plants that are traditionally recognized, the other being dicotyledons, or dicots. Monocot seedlings typically have one cotyledon (seed-leaf), in contrast to the two cotyledons typical of dicots. Monocots have been recognized at various taxonomic ranks, and under, the tunica determine the physical characteristics of the leaf edge and margin. In dicots The dicotyledons, also known as dicots, are a group of flowering plants whose seed typically has two embryonic leaves or cotyledons. There are around 199,350 species within this group. Flowering plants that are not dicotyledons are monocotyledons, typically having one embryonic leaf, layer two of the corpus determine the characteristics of the edge of the leaf. 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 an 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^{[citation needed]}.

Shoot apical meristems

The source of all above-ground organs. Cells at the shoot apical meristem 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 shoot apical meristem consists of 4 distinct cell groups: -.

• Stem cells Stem cells are cells found in all multi cellular organisms. They are characterized by the ability to renew themselves through mitotic cell division and differentiate into a diverse range of specialized cell types. Research in the stem cell field grew out of findings by Ernest A. McCulloch and James E. Till at the University of Toronto in the 1960s
• 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. In Arabidopsis thaliana Arabidopsis thaliana , is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, Arabidopsis is popular as a model organism in plant biology and genetics. Its genome is one of the smallest plant genomes and was the first plant genome to be sequenced. Arabidopsis is a popular , 3 interacting CLAVATA genes are required to regulate the size of the stem cell Stem cells are cells found in all multi cellular organisms. They are characterized by the ability to renew themselves through mitotic cell division and differentiate into a diverse range of specialized cell types. Research in the stem cell field grew out of findings by Ernest A. McCulloch and James E. Till at the University of Toronto in the 1960s reservoir in the shoot apical meristem by controlling the rate of cell division Cell division is the process by which a parent cell divides into two or more daughter cells. Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort of cell division in prokaryotes is known as binary.^[2] 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 and pharmacology, a ligand is a substance that is able to bind to and form a complex with a biomolecule to serve a biological purpose. In a narrower sense, it is a signal triggering molecule, binding to a site on a target protein.^[3][4][5] CLV3 shares some homology A homolog in chemistry refers to a chemical compound from a series of compounds that differ only in the number of repeated structural units. A homolog is a special case of an analog. Examples are alkanes and compounds with alkyl functional groups that differ in the length of their sidechain or a peptide with one amino acid replaced by another with the ESR proteins of maize Maize , known in many English-speaking countries as corn, is a grass domesticated by indigenous peoples in Mesoamerica in prehistoric times. The Aztecs and Mayans cultivated it in numerous varieties throughout central and southern Mexico, to cook or grind in a process called nixtamalization. Later the crop spread through much of the Americas, with a short 14 amino acid Amino acids are molecules containing an amine group, a carboxylic acid group and a side chain that varies between different amino acids. These molecules contain the key elements of carbon, hydrogen, oxygen, and nitrogen. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula region being conserved between the proteins.^[6][7] Proteins that contain these conserved regions have been grouped into the CLE family of proteins.^[6][7]

CLV1 has been shown to interact with several cytoplasmic All the contents of the cells of prokaryote organisms are contained within the cytoplasm. Within the cells of eukaryote organisms the contents of the cell nucleus are separated from the cytoplasm, and are there called the nucleoplasm proteins that are most likely involved in downstream signalling In biology, signal transduction is a mechanism that converts a mechanical or chemical stimulus to a cell into a specific cellular response. Signal transduction starts with a signal to a receptor, and ends with a change in cell function, for example the CLV complex has been found to be associated with Rho/Rac small GTPase related proteins GTPases are a large family of hydrolase enzymes that can bind and hydrolyze guanosine triphosphate (GTP). The GTP binding and hydrolysis takes place in the highly conserved G domain common to all GTPases.^[2] 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.^[8] KAPP is a kinase-associated protein phosphatase that has been shown to interact with CLV1.^[9] KAPP is thought to act as a negative regulator of CLV1 by dephosphorylating it.^[9]

Another important gene in plant meristem maintenance is WUSCHEL (shortened to WUS), which is a target of CLV signalling.^[10] WUS is expressed in the cells below the stem cells of the meristem and its presence prevents the differentiation In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a single zygote to a complex system of tissues and cell types. Differentiation is a common of the stem cells.^[10] CLV1 acts to promote cellular differentiation by repressing WUS activity outside of the central zone containing the stem cells.^[10] STM also acts to prevent the differentiation of stem cells by repressing the expression of Myb genes that are involved in cellular differentiation.^[2]

Root apical meristems

Unlike the shoot apical meristem, the root apical meristem 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 root cap is a section of tissue at the tip of a plant root. Root caps contain statoliths which are involved in gravity perception in plants. If the cap is carefully removed the root will grow randomly. The root cap protects the growing tip in plants. It secretes mucilage to ease the movement of the root through soil, and may also be involved. The center of the root apical meristem 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 root apical meristem. 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 Monocotyledons, also known as monocots, are one of two major groups of flowering plants that are traditionally recognized, the other being dicotyledons, or dicots. Monocot seedlings typically have one cotyledon (seed-leaf), in contrast to the two cotyledons typical of dicots. Monocots have been recognized at various taxonomic ranks, and under (particularly grass) stems at the base of nodes and leaf blades. Horsetails Equisetum (horsetail, snake grass, puzzlegrass) is the only living genus in the Equisetaceae, a family of vascular plants that reproduce by spores rather than seeds. ) also exhibit intercalary growth. 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 transition from shoot meristem to floral meristem requires floral meristem identity 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 The stamen is the male reproductive organ of a flower. Each stamen generally has a stalk called the filament (from Latin filum, meaning "thread"), and, on top of the filament, an anther (from Ancient Greek anthera, feminine of antheros "flowery," from anthos "flower"), and pollen sacs, called microsporangia. The and carpels The gynoecium is a term with several meanings in botanical usage. In reference to mosses; liverworts; and hornworts, gynoecium refers to a cluster of archegonia and any associated modified leaves or stems present on a gametophyte shoot. The more common use of gynoecium, however, is to refer to the ovule-producing part of a flower. The gynoecium in.^[2] 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.^[11] 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.^[12] 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.^[12] Once AG is activated it represses expression of WUS leading to the termination of the meristem.^[12]

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.

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