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Meristem

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This article is about a plant tissue. For other uses, seeMeristem (disambiguation).
Tunica-corpus modelof the apical meristem (growing tip). The epidermal (L1) and subepidermal (L2) layers form the outer layers called thetunica.The inner L3 layer is called the corpus. Cells in the L1 and L2 layers divide in a sideways fashion, which keeps these layers distinct, whereas the L3 layer divides in a more random fashion.

Incell biology,themeristemis a type oftissuefound in plants. It consists ofundifferentiated cells(meristematic cells) capable ofcell division.Cells in the meristem can develop into all the other tissues andorgansthat occur in plants. These cells continue to divide until they become differentiated and lose the ability to divide.

Differentiatedplant cells generally cannot divide or produce cells of a different type. Meristematic cells are undifferentiated or incompletely differentiated. They aretotipotentand capable of continuedcell 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 smallvacuolesor none, andprotoplasmfilling the cell completely. Theplastids(chloroplastsorchromoplasts) are undifferentiated, but are present in rudimentary form (proplastids). Meristematic cells are packed closely together without intercellular spaces. Thecell wallis a very thin primary cell wall.

The termmeristemwas first used in 1858 by Swiss botanistCarl Wilhelm von Nägeli(1817–1891) in his bookBeiträge zur Wissenschaftlichen Botanik( "Contributions to Scientific Botany" ).[1]It is derived fromGreekμερίζειν(merizein)'to divide', in recognition of its inherent function.[citation needed]

There are three types of meristematic tissues: apical (at the tips), intercalary or basal (in the middle), and lateral (at the sides also known as cambium). 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.

Primary meristems

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Apical meristems give rise to the primary plant body and are responsible forprimary growth,or an increase in length or height.[2][3]Apical meristems may differentiate into three kinds of primary meristem:

  • Protoderm:lies around the outside of the stem and develops into theepidermis.
  • Procambium:lies just inside of the protoderm and develops into primaryxylemand primaryphloem.It also produces thevascular cambium,andcork cambium(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 theperiderm.In roots, the procambium can also give rise to the pericycle, which produces lateral roots in eudicots.[4]
  • Ground meristem:Composed ofparenchyma,collenchymaandsclerenchymacells[4]that develop into thecortexand thepith.

Secondary meristems

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After the primary growth, lateral meristems develop as secondary plant growth. This growth adds to the plant in diameter from the established stem but not all plants exhibit secondary growth. There are two types of secondary meristems: the vascular cambium and the cork cambium.

  • Vascular cambium,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 calledarboraceous.This does not occur in plants that do not go through secondary growth (known asherbaceousplants).
  • Cork cambium,which gives rise to the periderm, which replaces the epidermis.

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 as they are involved in lateral growth.

Organisation of an apical meristem (growing tip)
  1. Central zone
  2. Peripheral zone
  3. Medullary (i.e. central) meristem
  4. Medullary tissue

There are two types of apical meristem tissue:shoot apical meristem(SAM), which gives rise to organs like the leaves and flowers, androot 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 thestem cellsin 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 thetunicawhile the innermost layers are thecorpus.Inmonocots,the tunica determines the physical characteristics of the leaf edge and margin. Indicots,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.[citation needed]

Shoot Apical Meristems

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Shoot apical meristems ofCrassula ovata(left). Fourteen days later, leaves have developed (right).

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.[citation needed]Primordiaof leaves, sepals, petals, stamens, and ovaries are initiated here at the rate of one every time interval, called aplastochron.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. InArabidopsis thaliana,3 interactingCLAVATAgenes are required to regulate the size of thestem cellreservoir in the shoot apical meristem by controlling the rate ofcell division.[5]CLV1and CLV2 are predicted to form a receptor complex (of theLRR receptor-like kinasefamily) to which CLV3 is aligand.[6][7][8]CLV3 shares somehomologywith the ESR proteins of maize, with a short 14amino acidregion beingconservedbetween the proteins.[9][10]Proteins that contain these conserved regions have been grouped into the CLE family of proteins.[9][10]

CLV1 has been shown to interact with severalcytoplasmicproteins that are most likely involved indownstream signalling.For example, the CLV complex has been found to be associated withRho/Rac small GTPase-related proteins.[5]These proteins may act as an intermediate between the CLV complex and amitogen-activated protein kinase(MAPK), which is often involved in signalling cascades.[11]KAPP is akinase-associated protein phosphatasethat has been shown to interact with CLV1.[12]KAPP is thought to act as a negative regulator of CLV1 by dephosphorylating it.[12]

Another important gene in plant meristem maintenance isWUSCHEL(shortened toWUS), which is a target of CLV signaling in addition to positively regulating CLV, thus forming a feedback loop.[13]WUSis expressed in the cells below the stem cells of the meristem and its presence prevents thedifferentiationof the stem cells.[13]CLV1 acts to promote cellular differentiation by repressingWUSactivity outside of the central zone containing the stem cells.[5]

The function ofWUSin the shoot apical meristem is linked to thephytohormonecytokinin.Cytokinin activateshistidine kinaseswhich thenphosphorylatehistidine phosphotransfer proteins.[14]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 ofcytokinin,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.[15]Therefore, A-ARRs do not contribute to the activation of transcription, and by competing for phosphates from phosphotransfer proteins, inhibit B-ARRs function.[16]In the SAM, B-ARRs induce the expression ofWUSwhich induces stem cell identity.[17]WUSthen suppresses A-ARRs.[18]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 anegative feedbackloop. 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.[19]

Root apical meristem

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10x microscope image of root tip with meristem
  1. quiescent center
  2. calyptrogen (live rootcap cells)
  3. rootcap
  4. sloughed off dead rootcap cells
  5. procambium

Unlike the shoot apical meristem, the root apical meristem produces cells in two dimensions. It harbors two pools ofstem cellsaround an organizing center called the quiescent center (QC) cells and together produces most of the cells in an adult root.[20][21]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 theroot 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.[22]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

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In angiosperms, intercalary (sometimes called basal) meristems occur inmonocot(in particular,grass) stems at the base of nodes and leaf blades.HorsetailsandWelwitschiaalso 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 and/or wildfires.

Floral meristem

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Further information:ABC model of flower development

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 thestamensandcarpels.[5]AGis necessary to prevent the conversion of floral meristems to inflorescence shoot meristems, but is identity geneLEAFY(LFY) andWUSand is restricted to the centre of the floral meristem or the inner two whorls.[23]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.[23]Once AG is activated it represses expression of WUS leading to the termination of the meristem.[23]

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

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Apical dominanceis 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 onauxins,types of plant growth regulators. These are produced in the apical meristem and transported towards the roots in thecambium.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.[citation needed]

Recent investigations into apical dominance and the control of branching have revealed a new plant hormone family termedstrigolactones.These compounds were previously known to be involved in seed germination and communication withmycorrhizal fungiand are now shown to be involved in inhibition of branching.[24]

Diversity in meristem architectures

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The SAM contains a population ofstem cellsthat 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. TheCLAVATAgeneCLV2responsible for maintaining the stem cell population inArabidopsis thalianais very closely related to themaizegeneFASCIATED EAR 2(FEA2) also involved in the same function.[25]Similarly, in rice, theFON1-FON2system seems to bear a close relationship with the CLV signaling system inArabidopsis thaliana.[26]These studies suggest that the regulation of stem cell number, identity and differentiation might be an evolutionarily conserved mechanism inmonocots,if not inangiosperms.Rice also contains another genetic system distinct fromFON1-FON2,that is involved in regulatingstem cellnumber.[26]This example underlines theinnovationthat goes about in the living world all the time.

Role of the KNOX-family genes

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Note the long spur of the above flower. Spurs attract pollinators and confer pollinator specificity.(Flower: Linaria dalmatica)
Complex leaves ofCardamine hirsutaresult from KNOX gene expression

Genetic screenshave identified genes belonging to theKNOXfamily 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 asArabidopsis thaliana,rice,barleyand tomato. KNOX-like genes are also present in somealgae,mosses, ferns andgymnosperms.Misexpression of these genes leads to the formation of interesting morphological features. For example, among members ofAntirrhineae,only the species of the genusAntirrhinumlack a structure calledspurin the floral region. A spur is considered an evolutionaryinnovationbecause it definespollinatorspecificity and attraction. Researchers carried outtransposonmutagenesis inAntirrhinum majus,and saw that some insertions led to formation of spurs that were very similar to the other members ofAntirrhineae,[27]indicating that the loss of spur in wildAntirrhinum majuspopulations could probably be an evolutionary innovation.

The KNOX family has also been implicated inleafshape evolution(See below for a more detailed discussion).One study looked at the pattern of KNOX gene expression inA. thaliana,that has simple leaves andCardamine hirsuta,a plant havingcomplex leaves.InA. thaliana,the KNOX genes are completely turned off in leaves, but inC.hirsuta,the expression continued, generating complex leaves.[28]Also, it has been proposed that the mechanism of KNOX gene action is conserved across allvascular plants,because there is a tightcorrelationbetween KNOX expression and acomplex leafmorphology.[29]

Indeterminate growth of meristems

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Further information:Root nodule

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 potentiallyindeterminate,making the overall shape of the plant not determinate in advance. This is theprimary 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-fi xingroot noduleson 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, andMedicago truncatulaare 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

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Under appropriate conditions, each shoot meristem can develop into a complete, new plant orclone.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 calledasexual reproductionorvegetative reproductionand is widely practiced in horticulture to mass-produce plants of a desirablegenotype.This process known as mericloning, has been shown to reduce or eliminate viruses present in the parent plant in multiple species of plants.[30][31]

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.[32]

Induced meristems

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Meristems may also be induced in the roots oflegumessuch assoybean,Lotus japonicus,pea,andMedicago truncatulaafter infection with soil bacteria commonly calledRhizobia.[citation needed]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-oligosaccharideNod factor,decorated with side groups to allow specificity of interaction. The Nod factor receptor proteins NFR1 and NFR5 were cloned from several legumes includingLotus japonicus,Medicago truncatulaand soybean (Glycine max). Regulation of nodule meristems utilizes long-distance regulation known as theautoregulation of nodulation(AON). This process involves a leaf-vascular tissue locatedLRRreceptorkinases(LjHAR1, GmNARK and MtSUNN), CLEpeptidesignalling, and KAPP interaction, similar to that seen in the CLV1,2,3 system. LjKLAVIER also exhibits a nodule regulationphenotypethough it is not yet known how this relates to the other AON receptor kinases.

Lateral Meristems

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Lateral meristems, the form of secondary plant growth, add growth to the plants in their diameter. This is primarily observed in perennial dicots that survive from year to year. There are two types of lateral meristems: vascular cambium and cork cambium.

In vascular cambium, the primary phloem and xylem are produced by the apical meristem. After this initial development, secondary phloem and xylem are produced by the lateral meristem. The two are connected through a thin layer of parenchymal cells which are differentiated into the fascicular cambium. The fascicular cambium divides to create the new secondary phloem and xylem. Following this the cortical parenchyma between vascular cylinders differentiates interfascicular cambium. This process repeats for indeterminate growth.[33]

Cork cambium creates a protective covering around the outside of a plant. This occurs after the secondary xylem and phloem has expanded already. Cortical parenchymal cells differentiate into cork cambium near the epidermis which lays down new cells called phelloderm and cork cells. These cork cells are impermeable to water and gases because of a substance called suberin that coats them.[34]

See also

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References

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Sources

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