![Méristème couches](https://upload.wikimedia.org/wikipedia/commons/7/70/M%C3%A9rist%C3%A8me_couches.png)
The meristem is a type of
tissue found in plants. It consists of undifferentiated cells (meristematic cells) capable of
cell division
Cell division is the process by which a parent cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there ar ...
. 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 Pluripotency: These are the cells that can generate into any of the three Germ layers which imply Endodermal, Mesodermal, and Ectodermal cells except tissues like the placenta.
According to Latin terms, Pluripotentia means the ability for many thin ...
and capable of continued
cell division
Cell division is the process by which a parent cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there ar ...
. 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
protoplasm fills the cell completely. The
plastids (
chloroplasts or
chromoplast
Chromoplasts are plastids, heterogeneous organelles responsible for pigment synthesis and storage in specific photosynthetic eukaryotes. It is thought that like all other plastids including chloroplasts and leucoplasts they are descended from sy ...
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 intracellular endosymbiotic cyanob ...
s). Meristematic cells are packed closely together without intercellular spaces. The cell wall is a very thin primary
cell wall.
The term ''meristem'' was first used in 1858 by
Carl Wilhelm von Nägeli Carl may refer to:
*Carl, Georgia, city in USA
*Carl, West Virginia, an unincorporated community
* Carl (name), includes info about the name, variations of the name, and a list of people with the name
*Carl², a TV series
* "Carl", an episode of te ...
(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.
![Méristème coupe zones chiffres](https://upload.wikimedia.org/wikipedia/commons/d/d7/M%C3%A9rist%C3%A8me_coupe_zones_chiffres.png)
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 of ...
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. Th ...
s, the tunica determines the physical characteristics of the leaf edge and margin. In
dicot
The dicotyledons, also known as dicots (or, more rarely, dicotyls), are one of the two groups into which all the flowering plants (angiosperms) were formerly divided. The name refers to one of the typical characteristics of the group: namely, t ...
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
![Apical Meristems in Crassula ovata](https://upload.wikimedia.org/wikipedia/commons/d/d6/Apical_Meristems_in_Crassula_ovata.png)
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
A primordium (; plural: primordia; synonym: anlage) in embryology, is an organ or tissue in its earliest recognizable stage of development. Cells of the primordium are called primordial cells. A primordium is the simplest set of cells capable o ...
of leaves, sepals, petals, stamens, and ovaries are initiated here at the rate of one every time interval, called a
plastochron As 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 ...
. 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 cells
* 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'', 3 interacting ''
CLAVATA'' genes are required to regulate the size of the
stem cell 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 daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there ar ...
.
CLV1 and CLV2 are predicted to form a receptor complex (of the
LRR receptor-like kinase LRR 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 elec ...
. CLV3 shares some
homology
Homology may refer to:
Sciences
Biology
*Homology (biology), any characteristic of biological organisms that is derived from a common ancestor
* Sequence homology, biological homology between DNA, RNA, or protein sequences
*Homologous chrom ...
with the ESR proteins of maize, with a short 14
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...
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, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...
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 (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 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
Plant hormone (or phytohormones) are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, from embryogenesis, the regulation of organ size, ...
cytokinin
Cytokinins (CK) are a class of plant hormones that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and le ...
. Cytokinin activates
histidine kinase
Histidine kinases (HK) are multifunctional, and in non-animal kingdoms, typically transmembrane, proteins of the transferase class of enzymes that play a role in signal transduction across the cellular membrane. The vast majority of HKs are hom ...
s which then
phosphorylate
In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, ...
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
Cytokinins (CK) are a class of plant hormones that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and le ...
, 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
![Root-tip-tag](https://upload.wikimedia.org/wikipedia/commons/b/be/Root-tip-tag.png)
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 of ...
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
The root cap is a type of tissue at the tip of a plant root. It is also called calyptra. Root caps contain statocytes which are involved in gravity perception in plants. If the cap is carefully removed the root will grow randomly. The root cap ...
. 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. Th ...
(in particular,
grass
Poaceae () or Gramineae () is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos and the grasses of natural grassland and species cultivated in lawns a ...
) stems at the base of nodes and leaf blades.
Horsetails
''Equisetum'' (; horsetail, snake grass, puzzlegrass) is the only living genus in Equisetaceae, a family of ferns, which reproduce by spores rather than seeds.
''Equisetum'' is a " living fossil", the only living genus of the entire subclass ...
and ''
Welwitschia
''Welwitschia'' is a monotypic gymnosperm genus, comprising solely the distinctive ''Welwitschia mirabilis'', endemic to the Namib desert within Namibia and Angola. ''Welwitschia'' is the only living genus of the family Welwitschiaceae and orde ...
'' 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
stamens and
carpel
Gynoecium (; ) is most commonly used as a collective term for the parts of a flower that produce ovules and ultimately develop into the fruit and seeds. The gynoecium is the innermost whorl of a flower; it consists of (one or more) '' pistils' ...
s.
''AG'' is necessary to prevent the conversion of floral meristems to inflorescence shoot meristems, but is identity gene ''
LEAFY'' (''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 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
auxins, 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 cells for plant growth. It is found in the area between xylem and phloem. A cambium can also be defined as a cellular plant tissue from w ...
. 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
strigolactone Strigolactones 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 hormones or phytohormones. So far, strigolactones have been identified to be responsible f ...
s. These compounds were previously known to be involved in seed germination and communication with
mycorrhizal fungi
A mycorrhiza (from Greek μύκης ', "fungus", and ῥίζα ', "root"; pl. mycorrhizae, mycorrhiza or mycorrhizas) is a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the pla ...
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 of ...
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'' 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 in southern Mexico about 10,000 years ago. The ...
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''.
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 embryonic leaf, or cotyledon. They constitute one of ...
, if not in
angiosperms
Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants ...
. Rice also contains another genetic system distinct from ''FON1-FON2'', that is involved in regulating
stem cell number.
This example underlines the
innovation
Innovation is the practical implementation of ideas that result in the introduction of new goods or services or improvement in offering goods or services. ISO TC 279 in the standard ISO 56000:2020 defines innovation as "a new or changed entit ...
that goes about in the living world all the time.
Role of the KNOX-family genes
Genetic screens 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, 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 as early as 10,000 years ago. Globally 70% of barley p ...
and tomato. KNOX-like genes are also present in some
algae, mosses, ferns and
gymnosperms. Misexpression of these genes leads to the formation of interesting morphological features. For example, among members of ''
Antirrhineae'', only the species of the genus
Antirrhinum
''Antirrhinum'' is a genus of plants commonly known as dragon flowers, snapdragons and dog flower because of the flowers' fancied resemblance to the face of a dragon that opens and closes its mouth when laterally squeezed. They are native t ...
lack a structure called
spur
A spur is a metal tool designed to be worn in pairs on the heels of riding boots for the purpose of directing a horse or other animal to move forward or laterally while riding. It is usually used to refine the riding aids (commands) and to ba ...
in the floral region. A spur is considered an evolutionary
innovation
Innovation is the practical implementation of ideas that result in the introduction of new goods or services or improvement in offering goods or services. ISO TC 279 in the standard ISO 56000:2020 defines innovation as "a new or changed entit ...
because it defines
pollinator specificity and attraction. Researchers carried out
transposon
A transposable element (TE, transposon, or jumping gene) is a nucleic acid sequence in DNA that can change its position within a genome, sometimes creating or reversing mutations and altering the cell's genetic identity and genome size. Tra ...
mutagenesis in ''Antirrhinum majus'', and saw that some insertions led to formation of spurs that were very similar to the other members of ''
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 ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, ste ...
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'', 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 (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They ...
, because there is a tight
correlation 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.
* Procambium: lies just inside of the protoderm and develops into primary
xylem
Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...
and primary
phloem
Phloem (, ) is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This transport process is c ...
. 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, gymnosperms such as pine trees, as well as in certain other vascular plants. It produces secondary xy ...
, and
cork cambium
Cork cambium (pl. cambia or cambiums) is a tissue found in many vascular plants as a part of the epidermis. It is one of the many layers of bark, between the cork and primary phloem. The cork cambium is a lateral meristem and is responsible for ...
, 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 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 and consist ...
. 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:
Biology
* Cortex (anatomy), the outermost layer of an organ
** Cerebral cortex, the outer layer of the vertebrate cerebrum, part of which is the ''forebrain''
*** Motor cortex, the regions of the cerebral cortex i ...
and the
pith
Pith, or medulla, is a tissue in the stems of vascular plants. Pith is composed of soft, spongy parenchyma cells, which in some cases can store starch. In eudicotyledons, pith is located in the center of the stem. In monocotyledons, it ext ...
. Composed of
parenchyma,
collenchyma
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 ...
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 beco ...
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, gymnosperms such as pine trees, as well as in certain other vascular plants. It produces secondary xy ...
, 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. This does not occur in plants that do not go through secondary growth (known as
herbaceous plants).
*
Cork cambium
Cork cambium (pl. cambia or cambiums) is a tissue found in many vascular plants as a part of the epidermis. It is one of the many layers of bark, between the cork and primary phloem. The cork cambium is a lateral meristem and is responsible for ...
, 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 nodules 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. 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
Vegetative reproduction (also known as vegetative propagation, vegetative multiplication or cloning) is any form of asexual reproduction occurring in plants in which a new plant grows from a fragment or cutting of the parent plant or spec ...
and is widely practiced in horticulture to mass-produce plants of a desirable
genotype. 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
legumes 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 a ...
, ''
Lotus japonicus'',
pea
The pea is most commonly the small spherical seed or the seed-pod of the flowering plant species ''Pisum sativum''. Each pod contains several peas, which can be green or yellow. Botanically, pea pods are fruit, since they contain seeds and d ...
, and ''
Medicago truncatula
''Medicago truncatula'', the barrelclover, strong-spined medick, barrel medic, or barrel medick, is a small annual legume native to the Mediterranean region that is used in genomic research. It is a low-growing, clover-like plant tall with trifol ...
'' after infection with soil bacteria commonly called
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 ὀλίγος ''olígos'', "a few", and σάκχαρ ''sácchar'', "sugar") is a saccharide polymer containing a small number (typically two to ten) of monosaccharides (simple sug ...
Nod factor
Nod factors (nodulation factors or NF), are signaling molecules produced by soil bacteria known as rhizobia in response to flavonoid exudation from plants under nitrogen limited conditions. Nod factors initiate the establishment of a symbiotic rel ...
, 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
Receptor may refer to:
* Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse
*Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a ...
kinases (LjHAR1, GmNARK and MtSUNN), CLE
peptide
Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides.
...
signalling, and KAPP interaction, similar to that seen in the CLV1,2,3 system. LjKLAVIER also exhibits a nodule regulation
phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological pr ...
though it is not yet known how this relates to the other AON receptor kinases.
See also
*
Secondary growth
In botany, secondary growth is the growth that results from cell division in the cambia or lateral meristems and that causes the stems and roots to thicken, while primary growth is growth that occurs as a result of cell division at the tips of ...
*
Stem cell
*
Thallus
Thallus (plural: thalli), from Latinized Greek (), meaning "a green shoot" or "twig", is the vegetative tissue of some organisms in diverse groups such as algae, fungi, some liverworts, lichens, and the Myxogastria. Many of these organisms ...
*
Tissues
References
Footnotes
* Plant Anatomy Laboratory from
University of Texas
The University of Texas at Austin (UT Austin, UT, or Texas) is a public research university in Austin, Texas. It was founded in 1883 and is the oldest institution in the University of Texas System. With 40,916 undergraduate students, 11,075 ...
; 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.orgResearch on meristems
{{Botany
Plant anatomy
Plant physiology