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The meristem is a type of
tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dubitata'', a species of geometer mot ...
found in plants. It consists of undifferentiated cells (meristematic cells) capable of
cell division Cell division is the process by which a parent cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious ...

cell division
. Cells in the meristem can develop into all the other tissues and organs that occur in plants. These cells continue to divide until a time when they get differentiated and then lose the ability to divide. Differentiated plant cells generally cannot divide or produce cells of a different type. Meristematic cells are undifferentiated or incompletely differentiated. They are
totipotent Cell potency is a cell's ability to differentiate into other cell types. The more cell types a cell can differentiate into, the greater its potency. Potency is also described as the gene activation potential within a cell, which like a continuum, ...
and capable of continued
cell division Cell division is the process by which a parent cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious ...

cell division
. Division of meristematic cells provides new cells for expansion and differentiation of tissues and the initiation of new organs, providing the basic structure of the plant body. The cells are small, with no or small vacuoles and
protoplasmProtoplasm (/prəʊtə(ʊ)ˌplaz(ə)m/, plural protoplasms) is the living part of a cell that is surrounded by a plasma membrane cell membrane vs. Prokaryotes The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, an ...

protoplasm
fills the cell completely. The
plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cells of plants, algae, and some other eukaryotic organisms. They are considered to be intracelluar endosymbiotic Cy ...
s (
chloroplast Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy-storage molecules Adenosine triphosphate, ATP and NADPH while fr ...

chloroplast
s or
chromoplast is controlled by a specialized organelle in plant cells called a chromoplast. Chromoplasts are plastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cells of plants ...
s), are undifferentiated, but are present in rudimentary form (
proplastid The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cells of plants, algae, and some other eukaryotic organisms. They are considered to be intracelluar endosymbiotic Cy ...
s). Meristematic cells are packed closely together without intercellular spaces. The cell wall is a very thin primary
cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane cell membrane vs. Prokaryotes The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to a ...
. The term ''meristem'' was first used in 1858 by Carl Wilhelm von Nägeli (1817–1891) in his book ''Beiträge zur Wissenschaftlichen Botanik'' ("Contributions to Scientific Botany"). It is derived from the Greek word ''merizein'' (μερίζειν), meaning to divide, in recognition of its inherent function. There are three types of meristematic tissues: apical (at the tips), intercalary or basal (in the middle), and lateral (at the sides). 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

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. There are two types of apical meristem tissue: shoot apical meristem (SAM), which gives rise to organs like the leaves and flowers, and root apical meristem (RAM), which provides the meristematic cells for future root growth. SAM and RAM cells divide rapidly and are considered indeterminate, in that they do not possess any defined end status. In that sense, the meristematic cells are frequently compared to the
stem cells In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type ...
in animals, which have an analogous behavior and function. The apical meristems are layered where 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
monocot Monocotyledons (), commonly referred to as monocots, (Lilianae ''sensu'' Chase & Reveal) are grass and grass-like flowering plants (angiosperms), the seeds of which typically contain only one Embryo#Plant embryos, embryonic leaf, or cotyledon. The ...
s, the tunica determines the physical characteristics of the leaf edge and margin. In
dicot 200px, Young castor oil plant showing its prominent two embryonic leaves (cotyledons), that differ from the adult leaves. The dicotyledons, also known as dicots (or more rarely dicotyls), are one of the two groups into which all the flowering ...
s, layer two of the corpus determines 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.


Shoot apical meristems

Shoot apical meristems are the source of all above-ground organs, such as leaves and flowers. 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 Image:Root primordia.JPG, 250px, Root primordia (brown spots) as seen on the butt of a freshly cut pineapple crown intended for vegetative reproduction. A primordium (; plural: primordia; synonym: anlage) in embryology, is an Organ (anatomy), orga ...
of leaves, sepals, petals, stamens, and ovaries are initiated here at the rate of one every time interval, called a
plastochronAs the tip of a plant shoot grows, new leaves are produced at regular time intervals if temperature is held constant. This time interval is termed the plastochron (or plastochrone). The plastochrone index and the leaf plastochron index are ways of me ...
. 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 auxiliary 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 four distinct cell groups: *
Stem cell In multicellular organisms Multicellular organisms are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties o ...
s * The immediate daughter cells of the stem cells * A subjacent organizing center *. Founder cells for organ initiation in surrounding regions These four distinct zones are maintained by a complex signalling pathway. In ''
Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant The flowering plants, also known as Angiospermae (), or Magnoliophyta (), are the most diverse group of Embryophyte, land plants, with 64 Orde ...

Arabidopsis thaliana
'', 3 interacting '' CLAVATA'' genes are required to regulate the size of the
stem cell In multicellular organisms Multicellular organisms are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties o ...
reservoir in the shoot apical meristem by controlling the rate of
cell division Cell division is the process by which a parent cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious ...
. CLV1 and CLV2 are predicted to form a receptor complex (of the
LRR receptor-like kinaseLRR may refer to: * Laminated root rot, a root disease in conifers *Leucine-rich repeat, a type of protein domain *LoadingReadyRun, a Canadian comedy troupe * Long Range Radar * ''Long River Review'', a literary magazine of the University of Connecti ...
family) to which CLV3 is a
ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electro ...
. CLV3 shares some homology with the ESR proteins of maize, with a short 14
amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. Due to carbon's ability to Catenation, c ...
region being conserved between the proteins. Proteins that contain these conserved regions have been grouped into the CLE family of proteins. CLV1 has been shown to interact with several
cytoplasm In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
ic 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. These proteins may act as an intermediate between the CLV complex and a
mitogen-activated protein kinase A mitogen-activated protein kinase (MAPK or MAP kinase) is a type of protein kinase that is specific to the amino acids serine and threonine (i.e., a serine/threonine-specific protein kinase). MAPKs are involved in directing cellular responses to a ...
(MAPK), which is often involved in signalling cascades. KAPP is a kinase-associated protein phosphatase that has been shown to interact with CLV1. KAPP is thought to act as a negative regulator of CLV1 by dephosphorylating it. Another important gene in plant meristem maintenance is '' WUSCHEL'' (shortened to ''WUS''), which is a target of CLV signaling in addition to positively regulating CLV, thus forming a feedback loop. ''WUS'' is expressed in the cells below the stem cells of the meristem and its presence prevents the
differentiation Differentiation may refer to: Business * Differentiation (economics), the process of making a product different from other similar products * Product differentiation, in marketing * Differentiated service, a service that varies with the identity o ...
of the stem cells. CLV1 acts to promote cellular differentiation by repressing ''WUS'' activity outside of the central zone containing the stem cells. The function of ''WUS'' in the shoot apical meristem is linked to the phytohormone
cytokinin 122px, The cytokinin Zea''.">Teosinte.html" ;"title="zeatin is named after the genus of corn, ''Teosinte">Zea''. Cytokinins (CK) are a class of plant growth substances (phytohormones) that promote cell division, or cytokinesis, in plant roots and ...
. Cytokinin activates
histidine kinase Histidine kinases (HK) are multifunctional, and in non-animal kingdoms, typically transmembrane Image:Polytopic membrane protein.png, 400px, Schematic representation of transmembrane proteins: 1) a single transmembrane α-helix (bitopic membrane pr ...
s which then
phosphorylate In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they unde ...

phosphorylate
histidine phosphotransfer proteins. Subsequently, the phosphate groups are transferred onto two types of Arabidopsis response regulators (ARRs): Type-B ARRS and Type-A ARRs. Type-B ARRs work as transcription factors to activate genes downstream of
cytokinin 122px, The cytokinin Zea''.">Teosinte.html" ;"title="zeatin is named after the genus of corn, ''Teosinte">Zea''. Cytokinins (CK) are a class of plant growth substances (phytohormones) that promote cell division, or cytokinesis, in plant roots and ...
, including A-ARRs. A-ARRs are similar to B-ARRs in structure; however, A-ARRs do not contain the DNA binding domains that B-ARRs have, and which are required to function as transcription factors. Therefore, A-ARRs do not contribute to the activation of transcription, and by competing for phosphates from phosphotransfer proteins, inhibit B-ARRs function. In the SAM, B-ARRs induce the expression of ''WUS'' which induces stem cell identity. ''WUS'' then suppresses A-ARRs. As a result, B-ARRs are no longer inhibited, causing sustained cytokinin signaling in the center of the shoot apical meristem. Altogether with CLAVATA signaling, this system works as a negative feedback loop. Cytokinin signaling is positively reinforced by WUS to prevent the inhibition of cytokinin signaling, while WUS promotes its own inhibitor in the form of CLV3, which ultimately keeps WUS and cytokinin signaling in check.


Root apical meristem

Unlike the shoot apical meristem, the root apical meristem produces cells in two dimensions. It harbors two pools of
stem cells In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type ...
around an organizing center called the quiescent center (QC) cells and together produces most of the cells in an adult root. At its apex, the root meristem is covered by the root cap, which protects and guides its growth trajectory. Cells are continuously sloughed off the outer surface of the
root cap Root tip magnified 100×. 1. Meristem 2. Columellae (statocytes with statolithes) 3. Lateral part of the tip 4. Dead cells 5. Elongation zone The root cap is a type of tissue at the tip of a plant root In vascular plants, the roots are the plant ...
. The QC cells are characterized by their low mitotic activity. Evidence suggests that the QC maintains the surrounding stem cells by preventing their differentiation, via signal(s) that are yet to be discovered. This allows a constant supply of new cells in the meristem required for continuous root growth. Recent findings indicate that QC can also act as a reservoir of stem cells to replenish whatever is lost or damaged. Root apical meristem and tissue patterns become established in the embryo in the case of the primary root, and in the new lateral root primordium in the case of secondary roots.


Intercalary meristem

In angiosperms, intercalary (sometimes called basal) meristems occur in
monocot Monocotyledons (), commonly referred to as monocots, (Lilianae ''sensu'' Chase & Reveal) are grass and grass-like flowering plants (angiosperms), the seeds of which typically contain only one Embryo#Plant embryos, embryonic leaf, or cotyledon. The ...
(in particular,
grass Poaceae () or Gramineae () is a large and nearly ubiquitous family of monocotyledonous flowering plants known as grasses. It includes the cereal grasses, bamboo Bamboos are a diverse group of evergreen perennial plant, perennial flower ...

grass
) stems at the base of nodes and leaf blades.
Horsetails ''Equisetum'' (; horsetail, snake grass, puzzlegrass) is the only living genus in Equisetaceae, a family (biology), family of vascular plants that reproduce by spores rather than seeds. ''Equisetum'' is a "living fossil", the only living genus o ...
and ''
Welwitschia ''Welwitschia'' is a monotypic taxon, monotypic gymnosperm genus, comprising solely the distinctive ''Welwitschia mirabilis'', Endemism, endemic to the Namib desert within Namibia and Angola. The plant is commonly known simply as welwitschia in En ...

Welwitschia
'' also exhibit intercalary growth. Intercalary meristems are capable of cell division, and they 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.


Floral meristem

When plants begin flowering, the shoot apical meristem is transformed into an inflorescence meristem, which goes on to produce the floral meristem, which produces the sepals, petals, stamens, and carpels of the flower. In contrast to vegetative apical meristems and some efflorescence meristems, floral meristems cannot continue to grow indefinitely. Their growth is limited to the flower with 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
stamen The stamen ( plural ''stamina'' or ''stamens'') is the pollen-producing reproductive organ of a flower. Collectively the stamens form the androecium., p. 10 Morphology and terminology A stamen typically consists of a stalk called the filament ...
s and
carpel Gynoecium (; ) is most commonly used as a collective term for the parts of a flower A flower, sometimes known as a bloom or blossom, is the reproduction, reproductive structure found in flowering plants (plants of the division Magnoliophyta, ...
s. ''AG'' is necessary to prevent the conversion of floral meristems to inflorescence shoot meristems, but is identity gene ''
LEAFY LEAFY (abbreviated LFY) is a plant gene that causes groups of undifferentiated Cell (biology), cells called meristems to develop into flowers instead of leaves with associated shoots. ''LEAFY'' is involved in floral meristem identity. ''LEAFY'' e ...
'' (''LFY'') and ''WUS'' and is restricted to the centre of the floral meristem or the inner two whorls.Lohmann, J. U. et al. (2001) A Molecular Link between Stem Cell Regulation and Floral Patterning in Arabidopsis Cell 105: 793-803 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. Once AG is activated it represses expression of WUS leading to the termination of the meristem. Through the years, scientists have manipulated floral meristems for economic 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 that it grows much faster than other tobacco plants.


Apical dominance

Apical dominance show particularly strong apical dominance, strongest of all being in the family Araucariaceae Araucariaceae – also known as araucarians – is a very ancient family of coniferous trees. The family achieved its maximum diversity during the Juras ...
is 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 shoot meristem. Therefore, the tip of the trunk grows rapidly 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 behavior after the removal of apical meristem, leading to a bushy growth. The mechanism of apical dominance is based on
auxin Auxins (plural of auxin ) are a class of plant hormones (or plant-growth regulators) with some morphogen-like characteristics. Auxins play a cardinal role in coordination of many growth and behavioral processes in plant life cycles and are essentia ...

auxin
s, types of plant growth regulators. These are produced in the apical meristem and transported towards the roots in the
cambium A cambium (plural cambia or cambiums), in plants, is a tissue layer that provides partially undifferentiated cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic ...
. If apical dominance is complete, they prevent any branches from forming as long as the apical meristem is active. If the dominance is incomplete, side branches will develop. Recent investigations into apical dominance and the control of branching have revealed a new plant hormone family termed
strigolactoneStrigolactones are a group of chemical compounds produced by a plant's roots. Due to their mechanism of action, these molecules have been classified as plant hormone Plant hormones (also known as phytohormones) are signal molecules, produced withi ...
s. These compounds were previously known to be involved in seed germination and communication with
mycorrhizal fungi A mycorrhiza (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 mi ...
and are now shown to be involved in inhibition of branching.


Diversity in meristem architectures

The SAM contains a population of
stem cells In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type ...
that also produce the lateral meristems while the stem elongates. It turns out that the mechanism of regulation of the stem cell number might be evolutionarily conserved. The ''CLAVATA'' gene ''CLV2'' responsible for maintaining the stem cell population in ''
Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant The flowering plants, also known as Angiospermae (), or Magnoliophyta (), are the most diverse group of Embryophyte, land plants, with 64 Orde ...

Arabidopsis thaliana
'' is very closely related to the
maize Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn ( North American and Australian English), is a cereal grain first domesticated by indigenous peoples Indigenous peoples, also referred to as F ...

maize
gene ''FASCIATED EAR 2''(''FEA2'') also involved in the same function. Similarly, in rice, the ''FON1-FON2'' system seems to bear a close relationship with the CLV signaling system in ''
Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant The flowering plants, also known as Angiospermae (), or Magnoliophyta (), are the most diverse group of Embryophyte, land plants, with 64 Orde ...

Arabidopsis thaliana
''. These studies suggest that the regulation of stem cell number, identity and differentiation might be an evolutionarily conserved mechanism in
monocots Monocotyledons (), commonly referred to as monocots, (Lilianae ''sensu'' Chase & Reveal) are grass and grass-like flowering plants (angiosperms), the seeds of which typically contain only one Embryo#Plant embryos, embryonic leaf, or cotyledon. The ...

monocots
, if not in
angiosperms Flowering plants include multiple members of the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλά ...
. Rice also contains another genetic system distinct from ''FON1-FON2'', that is involved in regulating
stem cell In multicellular organisms Multicellular organisms are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties o ...
number. This example underlines the
innovation 190px, Thomas Edison with phonograph. Edison was one of the most prolific inventors in history, holding List of Edison patents, 1,093 U.S. patents in his name. Innovation is the practical implementation of ideas that result in the introduction ...

innovation
that goes about in the living world all the time.


Role of the KNOX-family genes

Genetic screens A genetic screen or mutagenesis screen is an experimental technique used to identify and select for individuals who possess a phenotype of interest in a mutagenized population. Hence a genetic screen is a type of phenotypic screen. Genetic screen ...
have identified genes belonging to the KNOX family in this function. These genes essentially maintain the stem cells in an undifferentiated state. The KNOX family has undergone quite a bit of evolutionary diversification while keeping the overall mechanism more or less similar. Members of the KNOX family have been found in plants as diverse as
Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant The flowering plants, also known as Angiospermae (), or Magnoliophyta (), are the most diverse group of Embryophyte, land plants, with 64 Orde ...

Arabidopsis thaliana
, rice,
barley Barley (''Hordeum vulgare''), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains, particularly in Eurasia Eurasia () is the largest continental area on E ...

barley
and tomato. KNOX-like genes are also present in some
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cel ...
, mosses, ferns and
gymnosperms The gymnosperms, also known as Acrogymnospermae, are a group of seed-producing plants that includes conifers, cycads, '' Ginkgo'', and gnetophytes. The term "gymnosperm" comes from the composite word in el, γυμνόσπερμος ( el, γ ...
. Misexpression of these genes leads to the formation of interesting morphological features. For example, among members of ''
Antirrhineae The Antirrhineae are one of the 12 Tribe (biology), tribes of the family (biology), family Plantaginaceae. It contains the toadflax relatives, such as snapdragons. They are probably most closely related to the turtlehead tribe (Cheloneae) and/or a ...

Antirrhineae
'', only the species of the genus
Antirrhinum ''Antirrhinum'' is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer ...
lack a structure called
spur A spur is a metal tool designed to be worn in pairs on the heels of riding boot A riding boot is a boot made to be used for horse riding. The classic boot comes high enough up the leg to prevent the leathers of the saddle The saddle ...

spur
in the floral region. A spur is considered an evolutionary
innovation 190px, Thomas Edison with phonograph. Edison was one of the most prolific inventors in history, holding List of Edison patents, 1,093 U.S. patents in his name. Innovation is the practical implementation of ideas that result in the introduction ...

innovation
because it defines
pollinator A pollinator is an animal that moves pollen File:Pollen Tube.svg, Pollen Tube Diagram Pollen is a powdery substance consisting of pollen grains which are Sporophyte, microsporophytes of spermatophyta, seed plants, which produce male gametes ...

pollinator
specificity and attraction. Researchers carried out
transposon A transposable element (TE, transposon, or jumping gene) is a DNA sequence that can change its position within a genome, sometimes creating or reversing mutations and altering the cell's genetic identity and genome size. Transposition often resu ...
mutagenesis in ''Antirrhinum majus'', and saw that some insertions led to formation of spurs that were very similar to the other members of ''
Antirrhineae The Antirrhineae are one of the 12 Tribe (biology), tribes of the family (biology), family Plantaginaceae. It contains the toadflax relatives, such as snapdragons. They are probably most closely related to the turtlehead tribe (Cheloneae) and/or a ...

Antirrhineae
'', indicating that the loss of spur in wild ''Antirrhinum majus'' populations could probably be an evolutionary innovation. The KNOX family has also been implicated in
leaf A leaf (plural leaves) is the principal lateral appendage of the vascular plant plant stem, stem, usually borne above ground and specialized for photosynthesis. The leaves, stem, flower and fruit together form the shoot system. Leaves are ...

leaf
shape evolution ''(See below for a more detailed discussion)''. One study looked at the pattern of KNOX gene expression in '' A. thaliana'', that has simple leaves and ''
Cardamine hirsuta ''Cardamine hirsuta'', commonly called hairy bittercress, is an annual or biennial species of plant in the family Brassicaceae, and is edible as a salad green. It is common in moist areas around the world. Description Depending on the climate ...

Cardamine hirsuta
'', a plant having complex leaves. In ''A. thaliana'', the KNOX genes are completely turned off in leaves, but in ''C.hirsuta'', the expression continued, generating complex leaves. Also, it has been proposed that the mechanism of KNOX gene action is conserved across all
vascular plants Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from the Greek ''trācheia''), form a large group of plants ( 300,000 accepted known species) that are defined as land plants with lignified tissue ...
, because there is a tight
correlation In statistics Statistics is the discipline that concerns the collection, organization, analysis, interpretation, and presentation of data. In applying statistics to a scientific, industrial, or social problem, it is conventional to begin wi ...
between KNOX expression and a complex leaf morphology.


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 outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Oth ...
. * Procambium: lies just inside of the protoderm and develops into primary
xylem Xylem is one of the two types of transport tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North Amer ...

xylem
and primary
phloem Phloem (, ) is the living biological tissue, tissue in vascular plants that transports the soluble Organic chemistry, organic Chemical substance, compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose ...

phloem
. It also produces the
vascular cambium The vascular cambium is the main growth tissue in the stems and roots of many plants, specifically in dicots such as buttercups and oak trees, gymnosperm The gymnosperms, also known as Acrogymnospermae, are a group of seed-producing plants th ...
, and
cork cambium Cork cambium (pl. cambia or cambiums) is a biological tissue, tissue found in many vascular plants as a part of the epidermis. It is one of the many layers of Bark (botany), bark, between the cork and primary phloem. The cork cambium is a lateral m ...
, secondary meristems. The cork cambium further differentiates into the phelloderm (to the inside) and the phellem, or cork (to the outside). All three of these layers (cork cambium, phellem, and phelloderm) constitute the
periderm Bark is the outermost layers of Plant stem, stems and roots of woody plants. Plants with bark include trees, woody vines, and shrubs. Bark refers to all the tissues outside the vascular cambium and is a nontechnical term. It overlays the wood a ...
. In roots, the procambium can also give rise to the pericycle, which produces lateral roots in eudicots.Evert, Ray, and Susan Eichhorn. Raven Biology of Plants. New York: W. H. Freeman and Company, 2013. Print. * Ground meristem: develops into the
cortex Cortex or cortical may refer to: Science Anatomy * Cortex (anatomy), the outermost or superficial layer of an organ * Cortex (hair), the middle layer of a strand of hair * Adrenal cortex, the portion of the adrenal gland that produces cortisol and ...
and the
pith shoot cut longitudinally to show the broad, solid pith (rough-textured, white) inside the wood (smooth, yellow-tinged). Scale in mm. shoot cut longitudinally to show the chambered pith found in this genus. Scale in mm. Image:Taxus wood.jpg, 250 ...
. Composed of
parenchyma Parenchyma () is the bulk of functional substance in an animal organ or structure such as a tumour. In zoology Zoology ()The pronunciation of zoology as is typically regarded as nonstandard, though it is not uncommon. is the branch of biolo ...

parenchyma
,
collenchyma The ground tissue of plants includes all tissues that are neither dermal The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily ...
and
sclerenchyma The ground tissue of plants includes all tissues that are neither dermal nor vascular. It can be divided into three types based on the nature of the cell walls. # Parenchyma cells have thin primary walls and usually remain alive after they bec ...
cells. 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 The vascular cambium is the main growth tissue in the stems and roots of many plants, specifically in dicots such as buttercups and oak trees, gymnosperm The gymnosperms, also known as Acrogymnospermae, are a group of seed-producing plants th ...
, which produces secondary xylem and secondary phloem. This is a process that may continue throughout the life of the plant. This is what gives rise to wood in plants. Such plants are called
arboraceous A woody plant is a plant Plants are mainly multicellular organisms, predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, ...
. This does not occur in plants that do not go through secondary growth (known as
herbaceous Herbaceous plants are vascular plant Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from the Greek ''trācheia''), form a large group of plants ( 300,000 accepted known species) that are defined ...
plants). *
Cork cambium Cork cambium (pl. cambia or cambiums) is a biological tissue, tissue found in many vascular plants as a part of the epidermis. It is one of the many layers of Bark (botany), bark, between the cork and primary phloem. The cork cambium is a lateral m ...
, which gives rise to the periderm, which replaces the epidermis.


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. The growth of nitrogen-fixing
root nodule Root nodules are found on the root In vascular plants, the roots are the plant organ, organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller ...
s on legume plants such as soybean and pea is either determinate or indeterminate. Thus, soybean (or bean and Lotus japonicus) produce determinate nodules (spherical), with a branched vascular system surrounding the central infected zone. Often, Rhizobium infected cells have only small vacuoles. In contrast, nodules on pea, clovers, and Medicago truncatula are indeterminate, to maintain (at least for some time) an active meristem that yields new cells for Rhizobium infection. Thus zones of maturity exist in the nodule. Infected cells usually possess a large vacuole. The plant vascular system is branched and peripheral.


Cloning

Under appropriate conditions, each shoot meristem can develop into a complete, new plant or
clone Clone or Clones or Cloning or The Clone may refer to: Places * Clones, County Fermanagh * Clones, County Monaghan, a town in Ireland Biology * Clone (B-cell), a lymphocyte clone, the massive presence of which may indicate a pathological conditio ...
. 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 '' Bryophyllum daigremontianum'' produces plantlets along the margins of its leaves. When they are mature enough, they drop off and root in any suitable soil beneath. Vegetative reproduction (also known as vegetative propagation, vegetative ...
and is widely practiced in horticulture to mass-produce plants of a desirable
genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is one of two, or more, forms of a given gene ...
. This process is also known as mericloning. Propagating through cuttings is another form of vegetative propagation that initiates root or shoot production from secondary meristematic cambial cells. This explains why basal 'wounding' of shoot-borne cuttings often aids root formation.


Induced meristems

Meristems may also be induced in the roots of
legume A legume () is a plant in the family Fabaceae (or Leguminosae), or the fruit or seed of such a plant. When used as a dry grain, the seed is also called a pulse. Legumes are grown agriculturally, primarily for human consumption, for livestock forag ...

legume
s such as
soybean The soybean, soy bean, or soya bean (''Glycine max'') is a species of legume native to East Asia, widely grown for its edible bean, which has numerous uses. Traditional unfermented food uses of soybeans include soy milk, from which tofu and t ...

soybean
, ''
Lotus japonicus ''Lotus japonicus'' is a wild legume that belongs to family Fabaceae The Fabaceae or Leguminosae,
'',
pea The pea is most commonly the small spherical seed A seed is an embryonic plant enclosed in a protective outer covering. The formation of the seed is part of the process of reproduction Reproduction (or procreation or breeding) is ...

pea
, and '' Medicago truncatula'' after infection with soil bacteria commonly called
Rhizobia '' bacteria Rhizobia are diazotrophic bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a ...

Rhizobia
. 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 (/ˌɑlɪgoʊˈsækəˌɹaɪd/; from the Greek wikt:ὀλίγος#Ancient Greek, ὀλίγος ''olígos'', "a few", and σάκχαρ ''sácchar'', "sugar") is a carbohydrate, saccharide polymer containing a small number (typicall ...
Nod factor 300px, The structure of the major Nod factor produced by '' Sinorhizobium meliloti''. Nod factors (nodulation factors or NF), are signaling molecule In biology, cell signaling (cell signalling in British English), or cell-cell communication, gov ...
, 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 known as the autoregulation of nodulation (AON). This process involves a leaf-vascular tissue located LRR receptor
kinase In biochemistry, a kinase is an enzyme that catalysis, catalyzes the transfer of phosphate groups from High-energy phosphate, high-energy, phosphate-donating molecules to specific Substrate (biochemistry), substrates. This process is known as pho ...
s (LjHAR1, GmNARK and MtSUNN), CLE
peptide Peptides (from Greek language πεπτός, ''peptós'' "digested"; derived from πέσσειν, ''péssein'' "to digest") are short chains of amino acids linked by peptide bonds. Chains of fewer than ten or fifteen amino acids are called oligopep ...
signalling, and KAPP interaction, similar to that seen in the CLV1,2,3 system. LjKLAVIER also exhibits a nodule regulation
phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of petal color in pea plants. The letters B and b represent genes for color, and the pictures show the resultant phenotypes. Thi ...

phenotype
though it is not yet known how this relates to the other AON receptor kinases.


See also

*
Secondary growth In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Anc ...
*
Stem cell In multicellular organisms Multicellular organisms are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties o ...
*
Thallus Thallus (plural: thalli), from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the po ...
* Tissues


References


Footnotes

* Plant Anatomy Laboratory from
University of Texas The University of Texas at Austin, shortened to UT Austin, UT, or Texas, is a public In public relations and communication science, publics are groups of individual people, and the public (a.k.a. the general public) is the totality of s ...
; the lab of JD Mauseth
Micrographs of plant cells and tissues, with explanatory text.
* * Scofield and Murray (2006). The evolving concept of the meristem. Plant Molecular Biology 60:v–vii
Meristemania.org
Research on meristems {{Botany Plant anatomy Plant physiology