A
meristem is the
tissueTissue 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...
in all
plantPlants 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,...
s consisting of undifferentiated cells (
meristematic cells) and found in zones of the plant where growth can take place.
The term “meristem” was first used by
Karl Wilhelm von NägeliKarl Wilhelm von Nägeli was a Swiss botanist. He discovered what would later become known as chromosomes and apparently discouraged Gregor Mendel from further work on genetics.-Birth and education:...
(1817-1891) from his book “Beiträge zur Wissenschaftlichen Botanik” in 1858. It is derived from the Greek word “merizein”, meaning to divide in recognition of its inherent function.
Differentiated plant cells generally cannot divide or produce cells of a different type. Therefore,
cell divisionMitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two daughter nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two daughter cells containing...
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 cellStem cells are cells found in most, if not all, multi-cellular organisms. They are characterized by the ability to renew themselves through mitotic cell division and differentiating into a diverse range of specialized cell types. Research in the stem cell field grew out of findings by Canadian...
s in
animalAnimals 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...
s, are incompletely or not at all
differentiatedIn 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...
, and are capable of continued cellular division (youthful). Furthermore, the cells are small and
protoplasmProtoplasm is the living contents of a cell that are 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,...
fills the cell completely. The
vacuolethumb|400px|Plant cell structurethumb|400px|Animal cell structureA vacuole is a membrane organelle which is present in all plant and fungal cells and some protist, animal and bacterial cells...
s are extremely small. The
cytoplasmThe cytoplasm is the part of a cell that is enclosed within the cell membrane. In eukaryotic cells, the cytoplasm contains organelles, such as mitochondria, which are filled with liquid that is kept separate from the rest of the cytoplasm by biological membranes. The contents of the cell nucleus...
does not contain differentiated
plastidPlastids 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...
s (
chloroplastChloroplasts 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.The word chloroplast is...
s or
chromoplastChromoplasts are plastids responsible for pigment synthesis and storage. They, like all other plastids , are organelles found in specific photosynthetic eukaryotic species....
s), although they are present in rudimentary form (proplastids). Meristematic cells are packed closely together without intercellular cavities. The cell wall is a very thin
primary cell wall.
Maintenance of the cells requires a balance between two antagonistic processes: organ initiation and stem cell population renewal.
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, Lotus japonicus, pea, and Medicago truncatula after infection with soil bacteria commonly called Rhizobium. Cells of the inner or outer cortex in the socalled "window of nodulation" just behind the developing root tip are induced to divide. The critical signal substance is the lipo-oligosaccharide 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". This process involves a leaf-vascular tissue located
LRRA leucine-rich repeat is a protein structural motif that forms an α/β horseshoe fold. It is composed of repeating 20-30 amino acid stretches that are unusually rich in the hydrophobic amino acid leucine...
receptor kinases (LjHAR1, GmNARK, MtSUNN and LjKLAVIER), CLE peptide signalling , and KAPP interaction, similar to that seen in the CLV1,2,3 system.
Apical meristems
The
apical meristem, or growing tip, is a completely
undifferentiated-Biology:* Stem cells, undifferentiated cells* Cellular differentiation, the process by which stem cells develop specific roles-Economics:* Commodities, undifferentiated products* Product differentiation, the process by which products are made unique...
meristematic tissue found in the
budthumb|right|Flower buds have not yet bloomed into a full-size flower.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...
s and growing tips of roots in
plantPlants 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,...
s. 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
rootIn 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 . Furthermore, a stem normally occurring below ground is not exceptional either...
or
shootShoots 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...
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, the tunica determine the physical characteristics of the leaf edge and margin. In dicots, 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.
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 Cell
Stem cells are cells found in most, if not all, multi-cellular organisms. They are characterized by the ability to renew themselves through mitotic cell division and differentiating into a diverse range of specialized cell types. Research in the stem cell field grew out of findings by Canadian...
s
- 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 thalianaArabidopsis 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...
, 3 interacting
CLAVATA genes are required to regulate the size of the
stem cellStem cells are cells found in most, if not all, multi-cellular organisms. They are characterized by the ability to renew themselves through mitotic cell division and differentiating into a diverse range of specialized cell types. Research in the stem cell field grew out of findings by Canadian...
reservoir in the shoot apical meristem by controlling the rate of
cell divisionCell division is a process by which a cell, called the parent cell, divides into two or more cells, called 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...
. CLV1 and CLV2 are predicted to form a receptor complex (of the LRR receptor like kinase family) to which CLV3 is a
ligandIn 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.The binding occurs by intermolecular forces, such as...
. CLV3 shares some
homologyA homology 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...
with the ESR proteins of
maizeMaize , is a herbaceous plant domesticated in Mesoamerica and subsequently spread throughout the American continents...
, with a short 14
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and one of the twenty R-groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H
2NCHRCOOH, where R is an organic substituent...
region being
conservedConservation may refer to:* Conservation genetics - "an interdisciplinary science that aims to apply genetic methods to the conservation and restoration of biodiversity."...
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
cytoplasmThe cytoplasm is the part of a cell that is enclosed within the cell membrane. In eukaryotic cells, the cytoplasm contains organelles, such as mitochondria, which are filled with liquid that is kept separate from the rest of the cytoplasm by biological membranes. The contents of the cell nucleus...
ic proteins that are most likely involved in
downstream signallingIn biology, signal transduction refers to any process by which a cell converts one kind of signal or stimulus into another. Most processes of signal transduction involve ordered sequences of biochemical reactions inside the cell, which are carried out by enzymes and activated by second messengers,...
, for example the CLV complex has been found to be associated with
Rho/Rac small GTPase related proteinsGTPases are a large family of hydrolase enzymes that can bind and hydrolyze guanosine triphosphate . The GTP binding and hydrolysis takes place in the highly conserved G domain common to all GTPases.-Functions:...
. These proteins may act as an intermediate between the CLV complex and a
mitogen-activated protein kinaseMitogen-activated protein kinases are serine/threonine-specific protein kinases that respond to extracellular stimuli and regulate various cellular activities, such as gene expression, mitosis, differentiation, proliferation, and cell survival/apoptosis.-Function:MAPKs are involved in the...
(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 signalling.
WUS is expressed in the cells below the stem cells of the meristem and its presence prevents the
differentiationIn 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...
of the stem cells. CLV1 acts to promote cellular differentiation by repressing
WUS activity outside of the central zone containing the stem cells.
STM also acts to prevent the differentiation of stem cells by repressing the expression of Myb genes that are involved in cellular differentiation.
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 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 (particularly grass) stems at the base of nodes and leaf blades. Horsetails 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
stamenThe stamen is the male organ of a flower. Each stamen generally has a stalk called the filament , and, on top of the filament, an anther , and pollen sacs,...
s and carpels.
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. 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. 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 economics reasons. An example is the mutant tobacco plant "Maryland Mammoth" In 1936 the department of agriculture of Switzerland performed several scientific tests with this plant. "Maryland Mammoth" is peculiar in this sense that it grows much faster than other tobacco plants.
Apical dominance
Apical dominance is phenomenon where one meristem prevents or inhibits the growth of other meristems. As a result the plant will have one clearly defined main trunk. For example, in trees the tip of the main trunk bears the dominant meristem. Therefore the tip of the trunk grows 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 horizontal. Over the years the branch may begin to look more and more like an extension of the main trunk. Often several branches will exhibit this behaviour after the removal of apical meristem, leading to a bushy growth.
The mechanism of apical dominance is based on the plant hormone
auxinAuxins are a class of plant growth substance and morphogens . Auxins have an essential role in coordination of many growth and behavioral processes in the plant life cycle...
. It is produced in the apical meristem and transported towards the roots in the
cambiumThe vascular cambium is a lateral meristem in the vascular tissue of plants. The vascular cambium is the source of both the secondary xylem and the secondary phloem , and is located between these tissues in the stem and root...
. If apical dominance is complete, it prevents any branches from forming as long as apical meristem is active. If the dominance is incomplete, side branches will develop.
Recent investigations into apical dominance and the control of branching have revealed a new plant hormone family termed
strigolactoneStrigolactones are plant hormones that have been implicated in inhibition of shoot branching. Strigolactones are carotenoid-derived and trigger germination of parasitic plant seeds and stimulate symbiotic mycorrhizal fungi...
s. These compounds were previously known to be involved in seed germination and communication with mycorrhizal fungi and are now shown to be involved in inhibition of branching.
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 a single-layered group of cells that covers plants leaves, flowers, roots and stems. It forms a boundary between the plant and the external world. The epidermis serves several functions, it protects against water loss, regulates gas exchange, secretes metabolic compounds, and ...
.
- Procambium - lies just inside of the protoderm and develops into primary xylem
In vascular plants, xylem is one of the two types of transport tissue, phloem being the other. The word "xylem" is derived from classical Greek ξυλον , "wood", and indeed the best-known xylem tissue is wood, though it is found throughout the plant...
and primary phloemIn vascular plants, phloem is the living tissue that carries organic nutrients , particularly sucrose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...
. It also produces the vascular cambiumThe vascular cambium is a lateral meristem in the vascular tissue of plants. The vascular cambium is the source of both the secondary xylem and the secondary phloem , and is located between these tissues in the stem and root...
, a secondary meristem.
- Ground meristem develops into the pith
Pith is a substance that is found in vascular plants. It consists of soft, spongy parenchyma cells, and is located in the center of the stem in eudicots and in the center of the roots in monocots. It is encircled by a ring of xylem , and outside that, a ring of phloem . In some plants the pith is...
. It produces the cork cambiumCork cambium is a tissue found in many vascular plants as part of the periderm. The cork cambium is a lateral meristem and is responsible for secondary growth that replaces the epidermis in roots and stems...
, another secondary meristem.
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 a lateral meristem in the vascular tissue of plants. The vascular cambium is the source of both the secondary xylem and the secondary phloem , and is located between these tissues in the stem and root...
- produces secondary xylem and secondary phloem, this is a process which may continue throughout the life of the plant. This is what gives rise to woodWood is an organic material; in the strict sense wood is produced as secondary xylem in the stems of trees . In a living tree it transfers water and nutrients to the leaves and other growing tissues, and has a support function, enabling woody plants to reach large sizes or to stand up for themselves...
in plants. Such plants are called arborescentArborescent is a term used by the French thinkers Deleuze and Guattari to characterize thinking marked by insistence on totalizing principles, binarism and dualism...
. This does not occur in plants which do not go through secondary growth (known as herbaceousA herbaceous plant is a plant that has leaves and stems that die down at the end of the growing season to the soil level. They have no persistent woody stem above ground...
plants).
- Cork cambium
Cork cambium is a tissue found in many vascular plants as part of the periderm. The cork cambium is a lateral meristem and is responsible for secondary growth that replaces the epidermis in roots and stems...
- gives rise to the bark of a tree.
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 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
cloneCloning in biology is the process of producing populations of genetically-identical individuals that occurs in nature when organisms such as bacteria, insects or plants reproduce asexually. Cloning in biotechnology refers to processes used to create copies of DNA fragments , cells , or organisms...
. 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 reproductionVegetative reproduction is a type of asexual reproduction for plants, and is also called vegetative propagation, vegetative multiplication, or vegetative cloning. It is a process by which new plant "individuals" arise or are obtained without production of seeds or spores...
and is widely practiced in
horticultureHorticulture is the industry and science of plant cultivation. Some would say that horticulture is the process of preparing soil for the planting of seeds, tubers, or cuttings. Horticulturists work and conduct research in the disciplines of plant propagation and cultivation, crop production, plant...
to mass-produce plants of a desirable
genotypeThe genotype is the genetic constitution of a cell, an organism, or an individual usually with reference to a specific character under consideration...
. This process is also known as mericloning.