Editing Meristem

{{short description|Type of plant tissue involved in cell proliferation}}
{{About|a plant tissue}}
[[File:Méristème couches.png|thumb|Tunica-corpus model of the apical meristem (growing tip). The epidermal (L1) and subepidermal (L2) layers form the outer layers called the [[Tunica (biology)|tunica]]. The corpus (L3) will form the vascular and stem tissues. Cells in the outer layers divide in a sideways fashion relative to each other, which keeps these layers distinct, whereas the lower layer divides in a more random fashion in all directions.<ref name=":0">{{Cite journal |last1=Lindsay |first1=Penelope |last2=Swentowsky |first2=Kyle W. |last3=Jackson |first3=David |date=January 2024 |title=Cultivating potential: Harnessing plant stem cells for agricultural crop improvement |url=https://annas-archive.org/scidb/10.1016/j.molp.2023.12.014 |journal=Molecular Plant |language=en |volume=17 |issue=1 |pages=50–74 |doi=10.1016/j.molp.2023.12.014 |pmid=38130059 |bibcode=2024MPlan..17...50L |issn=1674-2052}}</ref>|right]]

In [[cell biology]], the '''meristem''' is a structure composed of specialized [[biological tissue|tissue]] found in plants, consisting of [[Stem cell|stem cells]], known as '''meristematic cells''', which are undifferentiated cells capable of continuous [[cell division|cellular division]]. These meristematic cells play a fundamental role in [[plant growth]], [[Regeneration (biology)|regeneration]], and [[acclimatization]], as they serve as the source of all [[Cellular differentiation|differentiated]] plant tissues and [[Plant organ|organs]]. They contribute to the formation of structures such as fruits, leaves, and seeds, as well as supportive tissues like stems and roots.<ref name=":0" />

Meristematic cells are [[totipotent]], meaning they have  the ability to differentiate into any [[plant cell]] type. As they divide, they generate new cells, some of which remain meristematic cells while others differentiate into specialized cells that typically lose the ability to divide or produce new cell types. Due to their active division and undifferentiated nature, meristematic cells form the foundation for the formation of new plant organs and the continuous expansion of the plant body throughout the plant's life cycle.

Meristematic cells are small cells, with thin [[Primary cell wall|primary cell walls]], and small or no [[vacuoles]]. Their [[protoplasm]] is dense, filling the entire cell, and they lack intercellular spaces. Instead of mature [[plastids]] such as [[chloroplasts]] or [[chromoplasts]], they contain [[proplastids]], which later develop into fully functional plastids.

Meristematic tissues are classified into three main types based on their location and function: ''apical'' meristems, found at the tips of roots and shoots; ''intercalary'' or ''basal'' meristems, located in the middle regions of stems or leaves, enabling [[Plant regrowth|regrowth]]; and ''lateral'' meristems or [[cambium]], responsible for [[secondary growth]] in [[woody plants]]. At the summit of the meristem, a small group of slowly dividing cells, known as the central zone, acts as a reservoir of stem cells, essential for maintaining meristem activity. The growth and proliferation rates of cells vary within the meristem, with higher activity at the periphery compared to the central region.

The term ''meristem'' was first used in 1858 by Swiss botanist [[Carl Wilhelm von Nägeli]] (1817–1891) in his book {{lang|de|Beiträge zur Wissenschaftlichen Botanik}} ("Contributions to Scientific Botany").<ref>Galun, Esra (2007). [https://www.worldcat.org/oclc/137324936 ''Plant Patterning: Structural and Molecular Genetic Aspects'']. World Scientific Publishing Company. p. 333. {{ISBN|9789812704085}}</ref> It is derived {{ety|el|''μερίζειν'' (merizein)|to divide}}, in recognition of its inherent function.{{Citation needed|date=February 2022}}

==Primary meristems==
Apical meristems, also known as the primary meristem, give rise to the primary plant body and are responsible for [[primary growth]], or an increase in length or height.<ref name=":1">{{Cite journal |last1=Baucher |first1=Marie |last2=AlmJaziri |first2=Mondher |last3=Vandeputte |first3=Olivier |title=From primary to secondary growth: origin and development of the vascular system |url=https://academic.oup.com/jxb/article/58/13/3485/492345 |access-date=2023-03-18 |journal=Journal of Experimental Botany|date=2007 |volume=58 |issue=13 |pages=3485–3501 |doi=10.1093/jxb/erm185 |pmid=17898423 }}</ref><ref name=":2">{{Cite journal |last1=Tognetti |first1=Vanesa B. |last2=Bielach |first2=Agnieszka |last3=Hrtyan |first3=Mónika |date=October 2017 |title=Redox regulation at the site of primary growth: auxin, cytokinin and ROS crosstalk: Apical meristems plasticity in response to stress |journal=Plant, Cell & Environment |language=en |volume=40 |issue=11 |pages=2586–2605 |doi=10.1111/pce.13021 |pmid=28708264 |doi-access=free}}</ref> Apical meristems may differentiate into three kinds of primary meristem:
* Protoderm: lies around the outside of the stem and develops into the [[Epidermis (botany)|epidermis]].
* Procambium: lies just inside of the protoderm and develops into primary [[xylem]] and primary [[phloem]]. It also produces the [[vascular cambium]], and [[cork cambium]] (part of the secondary meristems but descendants of apical meristematic cells). The cork cambium further differentiates into the [[phelloderm]], or bark, (to the inside) and the [[phellem]], or cork (to the outside). All three of these layers (cork cambium, phellem, and phelloderm) constitute the [[periderm]]. In roots, the procambium can also give rise to the [[pericycle]], which produces [[Lateral root|lateral roots]] in [[eudicots]].<ref name="Evert, Ray 2013">Evert, Ray, and Susan Eichhorn. Raven Biology of Plants. New York: W. H. Freeman and Company, 2013. Print.</ref>
* Ground meristem: Composed of [[Ground tissue#Parenchyma|ground tissue parenchyma]], [[collenchyma]] and [[sclerenchyma]] cells<ref name="Evert, Ray 2013" /> that develop into the [[Cortex (botany)|cortex]] and the [[pith]].

==Secondary meristems==
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 called [[arboraceous]]. This does not occur in plants that do not go through secondary growth, known as [[herbaceous]] plants.
* [[Cork cambium]], which gives rise to the periderm, which replaces the epidermis with bark and cork for example.
<!-- Is this even remotely true?

===Basal meristems===
As the name implies, this type of meristem is not found at the tip of a root or shoot but near the base. This type of meristem allows for primary growth even after the apex of the shoot has been severed. For example, the presence of basal meristem is the reason grass can continue growing after mowing.
-->==Apical meristems==
[[File:Méristème coupe zones chiffres.png|thumb|Organisation of an apical meristem (growing tip){{ordered list|Central zone|Peripheral zone|Medullary (i.e. central) meristem|Medullary tissue}}|upright=0.8]]
Apical meristems are the completely undifferentiated (indeterminate) meristems of a plant. They give rise to primary growth, enabling the elongation of shoots and roots. Apical meristems give rise to three types of primary meristems, which later develop into secondary or lateral meristems, contributing to the plant's lateral expansion. 

There are two main types of apical meristems: ''shoot'' apical meristem (SAM) and ''root'' apical meristem (RAM). The SAM is located at the tips of shoots and produces leaves, stems, and flowers, while the RAM is found at the tips of roots and generates new root tissues. Both types consist of rapidly-dividing cells that remain indeterminate, meaning they continuously produce new cells without a predefined final state, similar to [[stem cells]] in animals, which have an analogous behavior and function.

Structurally, apical meristems are organized into distinct zones. The central zone serves as a reservoir of undifferentiated cells, while the peripheral zone generates new organs and tissues. The medullary meristem contributes to vascular development, forming the medullary tissue, which makes up the plant's central structure. The meristem layers also vary depending on the plant type. The outermost layer, called the ''[[Tunica (biology)|tunica]]'', determines the leaf edge and margin in [[monocots]], whereas in [[dicots]], the second layer of the ''corpus'' influences leaf characteristics.

Apical meristems are generally found at the tips of roots and stems, but in some [[arctic plants]], they are located in the lower or middle parts of the plant. This [[Adaptation (biology)|adaptation]] is believed to provide advantages in extreme environmental conditions.{{Citation needed|date=February 2008}}

===Shoot Apical Meristems===
[[File:Apical Meristems in Crassula ovata.png|thumb|upright=1.6|Shoot apical meristems of ''[[Crassula ovata]]'' (left). Fourteen days later, leaves have developed (right).]]
[[File:Arabidopsis flat SAM.jpg|thumb|[[Microscopy|Microscopic image]] of a shoot apical meristem surrounded by leaf [[primordia]] of ''[[Arabidopsis thaliana]]''.]]
[[File:Dichotomy of Lycopodium shoot apex.png|thumb|A microscopic image of shoot apical meristems containing multiple stem cells during [[dichotomy]] in  ''[[Lycopodium clavatum]]'' (bar = 100 μm).]]
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|date=September 2018}} [[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 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]]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]]'', 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]].<ref name="Fletcher 2002">{{cite journal |author=Fletcher, J. C. |date=2002 |title=Shoot and Floral Meristem Maintenance in Arabidopsis |journal=[[Annu. Rev. Plant Biol.]] |volume=53|issue=1 |pages=45–66 |doi=10.1146/annurev.arplant.53.092701.143332 |pmid=12221985 |bibcode=2002AnRPB..53...45F }}</ref> [[CLV1]] and CLV2 are predicted to form a receptor complex (of the [[LRR receptor-like kinase]] family) to which CLV3 is a [[Ligand (biochemistry)|ligand]].<ref>{{cite journal | last1 = Clark | first1 = SE | last2 = Williams | first2 = RW | last3 = Meyerowitz | first3 = EM. |name-list-style=vanc | year = 1997 | title = The CLAVATA1 gene encodes a putative receptor kinase that controls shoot and floral meristem size in Arabidopsis | journal = Cell | volume = 89 | issue = 4| pages = 575–85 | doi = 10.1016/S0092-8674(00)80239-1 | pmid = 9160749 | s2cid = 15360609 | doi-access = free }}</ref><ref>{{cite journal | last1 = Jeong | first1 = S | last2 = Trotochaud | first2 = AE | last3 = Clark | first3 = SE. |name-list-style=vanc | year = 1999 | title = The Arabidopsis CLAVATA2 gene encodes a receptor-like protein required for the stability of the CLAVATA1 receptor-like kinase | journal = Plant Cell | volume = 11 | issue = 10| pages = 1925–33 | doi = 10.1105/tpc.11.10.1925 | pmid = 10521522 | pmc = 144110 | bibcode = 1999PlanC..11.1925J }}</ref><ref>{{cite journal | last1 = Fletcher | first1 = JC | last2 = Brand | first2 = U | last3 = Running | first3 = MP | last4 = Simon | first4 = R | last5 = Meyerowitz | first5 = EM |name-list-style=vanc | year = 1999 | title = Signaling of cell fate decisions by CLAVATA3 in Arabidopsis shoot meristems | journal = Science | volume = 283 | issue = 5409| pages = 1911–14 | doi = 10.1126/science.283.5409.1911 | pmid = 10082464 | bibcode = 1999Sci...283.1911F }}</ref> CLV3 shares some [[Homologous series|homology]] with the ESR proteins of maize, with a short 14 [[amino acid]] region being [[Conservation (genetics)|conserved]] between the proteins.<ref name="cock et al.">{{cite journal |author1=J. Mark Cock |author2=Sheila McCormick |title=A Large Family of Genes That Share Homology with CLAVATA3 |journal=Plant Physiology |date=July 2001 |volume=126 |issue=3 |pages=939–942 |pmid=11457943 |pmc=1540125 |doi=10.1104/pp.126.3.939}}</ref><ref name="Oelkers et al.">{{cite journal |author=Karsten Oelkers, Nicolas Goffard, Georg F Weiller, Peter M Gresshoff, [[Ulrike Mathesius]] and Tancred Frickey |title=Bioinformatic Analysis of the CLE signalling peptide family |journal=[[BMC Plant Biology]] |volume=8 |page=1 |date=3 January 2008 |issue=1 |pmid=18171480 |pmc=2254619 |doi=10.1186/1471-2229-8-1 |doi-access=free |bibcode=2008BMCPB...8....1O }}</ref> Proteins that contain these conserved regions have been grouped into the CLE family of proteins.<ref name="cock et al."/><ref name="Oelkers et al."/>

CLV1 has been shown to interact with several [[cytoplasm]]ic proteins that are most likely involved in [[Signal transduction|downstream signalling]]. For example, the CLV complex has been found to be associated with [[GTPase|Rho/Rac small GTPase-related proteins]].<ref name="Fletcher 2002"/> 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.<ref>{{cite journal |author=Valster, A. H. |date=2000 |title=Plant GTPases: the Rhos in bloom |journal=Trends in Cell Biology |volume=10 |issue=4 |pages=141–146 |display-authors=etal |doi=10.1016/s0962-8924(00)01728-1|pmid=10740268 }}</ref> KAPP is a [[kinase-associated protein phosphatase]] that has been shown to interact with CLV1.<ref name="KAPP">{{cite journal |author=Stone, J. M. |date=1998 |title=Control of meristem development by CLAVATA1 receptor kinase and kinase-associated protein phosphatase interactions |journal=Plant Physiology |volume=117 |issue=4 |pages=1217–1225 |pmid=9701578 |pmc=34886 |display-authors=etal |doi=10.1104/pp.117.4.1217}}</ref> KAPP is thought to act as a negative regulator of CLV1 by dephosphorylating it.<ref name="KAPP"/>

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.<ref name="WUS">{{cite journal |author=Mayer, K. F. X |date=1998 |title=Role of WUSCHEL in Regulating Stem Cell Fate in the Arabidopsis Shoot Meristem |journal=Cell |volume=95 |issue=6 |pages=805–815 |pmid=9865698  |doi=10.1016/S0092-8674(00)81703-1 |s2cid=18995751 |display-authors=etal|doi-access=free }}</ref> ''WUS'' is expressed in the cells below the stem cells of the meristem and its presence prevents the [[Cellular differentiation|differentiation]] of the stem cells.<ref name="WUS"/> CLV1 acts to promote cellular differentiation by repressing ''WUS'' activity outside of the central zone containing the stem cells.<ref name="Fletcher 2002"/>

The function of ''WUS'' in the shoot apical meristem is linked to the [[Plant hormone|phytohormone]] [[cytokinin]]. Cytokinin activates [[histidine kinase]]s which then [[Phosphorylation|phosphorylate]] histidine phosphotransfer proteins.<ref>{{Cite journal|author-link1=Jen Sheen|first1=Jen |last1=Sheen|last2=Hwang|first2=Ildoo|date=September 2001|title=Two-component circuitry in Arabidopsis cytokinin signal transduction|journal=Nature|volume=413|issue=6854|pages=383–389|doi=10.1038/35096500|pmid=11574878 |bibcode=2001Natur.413..383H |s2cid=4418158 |issn=1476-4687}}</ref> 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 signaling and response regulator protein|cytokinin]], 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.<ref>{{Cite journal|last1=Lohmann|first1=Jan U.|last2=Kieber|first2=Joseph J.|last3=Demar|first3=Monika|last4=Andreas Kehle|last5=Stehling|first5=Sandra|last6=Busch|first6=Wolfgang|last7=To|first7=Jennifer P. C.|last8=Leibfried|first8=Andrea|date=December 2005|title=WUSCHEL controls meristem function by direct regulation of cytokinin-inducible response regulators|journal=Nature|volume=438|issue=7071|pages=1172–1175|doi=10.1038/nature04270|pmid=16372013|bibcode=2005Natur.438.1172L|s2cid=2401801|issn=1476-4687}}</ref> Therefore, A-ARRs do not contribute to the activation of transcription, and by competing for phosphates from phosphotransfer proteins, inhibit B-ARRs function.<ref>{{Cite journal|last1=Kieber|first1=Joseph J.|last2=Ecker|first2=Joseph R.|last3=Alonso|first3=Jose M.|last4=Schaller|first4=G. Eric|last5=Mason|first5=Michael G.|last6=Deruère|first6=Jean|last7=Ferreira|first7=Fernando J.|last8=Haberer|first8=Georg|last9=To|first9=Jennifer P. C.|date=2004-03-01|title=Type-A Arabidopsis Response Regulators Are Partially Redundant Negative Regulators of Cytokinin Signaling|journal=The Plant Cell|volume=16|issue=3|pages=658–671|doi=10.1105/tpc.018978|issn=1040-4651|pmid=14973166|pmc=385279|bibcode=2004PlanC..16..658T }}</ref> In the SAM, B-ARRs induce the expression of ''WUS'' which induces stem cell identity.<ref>{{Cite journal|last1=Jurgens|first1=G.|last2=Berger|first2=J.|last3=Mayer|first3=K. F.|last4=Laux|first4=T.|date=1996-01-01|title=The WUSCHEL gene is required for shoot and floral meristem integrity in Arabidopsis|url=https://dev.biologists.org/content/122/1/87|journal=Development|volume=122|issue=1|pages=87–96|doi=10.1242/dev.122.1.87|issn=0950-1991|pmid=8565856|url-access=subscription}}</ref> ''WUS'' then suppresses A-ARRs.<ref>{{Cite journal|last1=Jackson|first1=David|last2=Simon|first2=Rüdiger|last3=Je|first3=Byoung Il|last4=Somssich|first4=Marc|date=2016-09-15|title=CLAVATA-WUSCHEL signaling in the shoot meristem|journal=Development|volume=143|issue=18|pages=3238–3248|doi=10.1242/dev.133645|issn=0950-1991|pmid=27624829|doi-access=free}}</ref> 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.<ref>{{Cite journal|last1=Gordon|first1=S. P.|last2=Chickarmane|first2=V. S.|last3=Ohno|first3=C.|last4=Meyerowitz|first4=E. M.|date=2009-08-26|title=Multiple feedback loops through cytokinin signaling control stem cell number within the Arabidopsis shoot meristem|journal=Proceedings of the National Academy of Sciences|volume=106|issue=38|pages=16529–16534|doi=10.1073/pnas.0908122106|pmid=19717465|pmc=2752578|bibcode=2009PNAS..10616529G|issn=0027-8424|doi-access=free}}</ref>

===Root apical meristem===
{{Multiple image
| direction         = vertical
| image1            = Light microscopy of root apical meristem.png
| image2 = Root-tip-tag.png
| caption2 = 10× microscope image of root tip with meristem{{ordered list|quiescent center|calyptrogen (live rootcap cells)|rootcap|sloughed off dead rootcap cells|procambium}}
}}
Unlike the shoot apical meristem, the root apical meristem produces cells in two dimensions. It harbors two pools of [[stem cells]] around an organizing center called the quiescent center (QC) cells and together produces most of the cells in an adult root.<ref name=" Jose Sebastian 2013">{{cite journal | doi = 10.1002/9780470015902.a0020121.pub2 | title=Root Apical Meristems | journal=eLS | last1 = Sebastian | first1 = Jose | last2 = Lee | first2 = Ji-Young| year=2013 | isbn=978-0470016176 }}</ref><ref name="Bennett, T">{{cite journal | last1 = Bennett | first1 = Tom | last2 = Scheres | first2 = Ben | year = 2010 | title = Root development-two meristems for the price of one? | journal = Current Topics in Developmental Biology | volume =  91| pages =  67–102| doi = 10.1016/S0070-2153(10)91003-X | pmid = 20705179 | isbn = 9780123809100 }}</ref> 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 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.<ref name=" Heidstra, R">{{cite journal | last1 = Heidstra | first1 = Renze | last2 = Sabatini | first2 = Sabrina | year = 2014 | title = Plant and animal stem cells: similar yet different | journal = Nature Reviews Molecular Cell Biology | volume = 15 | issue = 5| pages = 301–12 | doi = 10.1038/nrm3790 | pmid = 24755933 | s2cid = 34386672 }}</ref> 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{{anchor|intercalary}}===
In angiosperms, intercalary (sometimes called basal) meristems occur in [[monocot]] (in particular, [[Poaceae|grass]]) stems at the base of nodes and leaf blades. [[Horsetails]] and ''[[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 and/or wildfires.

===Floral meristem===
{{Further|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 the [[stamen]]s and [[carpel]]s.<ref name=" Fletcher 2002"/> ''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.<ref name="wus">Lohmann, J. U. et al. (2001) A Molecular Link between Stem Cell Regulation and Floral Patterning in Arabidopsis Cell 105: 793-803</ref> 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.<ref name="wus" /> Once AG is activated it represses expression of WUS leading to the termination of the meristem.<ref name="wus" />

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 [[auxin]]s, types of plant growth regulators. These are produced in the apical meristem and transported towards the roots in the [[vascular cambium|cambium]]. 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|date=September 2018}}

Recent investigations into apical dominance and the control of branching have revealed a new plant hormone family termed [[strigolactone]]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.<ref>{{cite journal|journal=[[Nature (journal)|Nature]]|title=Branching out: new class of plant hormones inhibits branch formation|url=http://www.nature.com/nature/journal/v455/n7210/edsumm/e080911-01.html|date=2008-09-11|access-date=2009-04-30|volume=455|issue=7210}}</ref>

===Diversity in meristem architectures===
The SAM contains a population of [[stem cells]] 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]] gene ''FASCIATED EAR 2''(''FEA2'') also involved in the same function.<ref>{{cite journal | author=Taguchi-Shiobara | title = The fasciated ear2 gene encodes a leucine-rich repeat receptor-like protein that regulates shoot meristem proliferation in maize | journal=Genes & Development | volume=15 | issue=20 | pages=2755–2766 | year=2001 | doi = 10.1101/gad.208501 | pmid=11641280 | last2=Yuan | first2=Z | last3=Hake | first3=S | last4=Jackson | first4=D | pmc=312812 |display-authors=etal}}</ref> Similarly, in rice, the ''FON1-FON2'' system seems to bear a close relationship with the CLV signaling system in ''[[Arabidopsis thaliana]]''.<ref name="Suzaki T. 2006 1591–1602">{{cite journal | author=Suzaki T. | title = Conservation and Diversification of Meristem Maintenance Mechanism in Oryza sativa: Function of the FLORAL ORGAN NUMBER2 Gene | journal=Plant and Cell Physiol. | volume=47 | issue=12 | pages=1591–1602 | year=2006 | doi = 10.1093/pcp/pcl025 | pmid=17056620 | last2=Toriba | first2=T | last3=Fujimoto | first3=M | last4=Tsutsumi | first4=N | last5=Kitano | first5=H | last6=Hirano | first6=HY| doi-access=free }}</ref> These studies suggest that the regulation of stem cell number, identity and differentiation might be an evolutionarily conserved mechanism in [[monocots]], if not in [[angiosperms]]. Rice also contains another genetic system distinct from ''FON1-FON2'', that is involved in regulating [[stem cell]] number.<ref name="Suzaki T. 2006 1591–1602"/> This example underlines the [[innovation]] that goes about in the living world all the time.

===Role of the KNOX-family genes===
[[File: Linaria spur.jpg|thumb | left |Note the long spur of the above flower. Spurs attract pollinators and confer pollinator specificity. ''(Flower: Linaria dalmatica)'']]
[[File:Cardamine hirsuta.jpg| thumb | upright | right |Complex leaves of ''[[Cardamine hirsuta]]'' result from KNOX gene expression]]
[[Genetic screens]] have identified genes belonging to the [[KNOX (genes)|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]] 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]] lack a structure called [[spur]] in the floral region. A spur is considered an evolutionary [[innovation]] because it defines [[pollinator]] specificity and attraction. Researchers carried out [[transposon]] mutagenesis in ''Antirrhinum majus'', and saw that some insertions led to formation of spurs that were very similar to the other members of ''[[Antirrhineae]]'',<ref>{{cite journal | author=Golz J.F. | title = Spontaneous Mutations in KNOX Genes Give Rise to a Novel Floral Structure in Antirrhinum | journal=Curr. Biol. | volume=12 | issue=7 | pages=515–522 | year=2002 | doi = 10.1016/S0960-9822(02)00721-2 | pmid = 11937019 | last2=Keck | first2=Emma J. | last3=Hudson | first3=Andrew| s2cid = 14469173 | doi-access=free | bibcode = 2002CBio...12..515G }}</ref> 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]] 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.<ref>{{cite journal | author=Hay and Tsiantis | title = The genetic basis for differences in leaf form between ''Arabidopsis thaliana'' and its wild relative ''Cardamine hirsuta'' | journal=Nat. Genet. | volume=38 | pages=942–947 | year=2006 | doi = 10.1038/ng1835 | pmid=16823378 | last2=Tsiantis | first2=M | issue=8 | s2cid = 5775104 }}</ref> Also, it has been proposed that the mechanism of KNOX gene action is conserved across all [[vascular plants]], because there is a tight [[correlation]] between KNOX expression and a [[complex leaf]] morphology.<ref>{{cite journal  |vauthors=Bharathan G, etal | title = Homologies in Leaf Form Inferred from KNOXI Gene Expression During Development | journal=Science | volume=296 | issue=5574 | pages=1858–1860 | year=2002 | doi = 10.1126/science.1070343 | pmid=12052958 | bibcode = 2002Sci...296.1858B | s2cid = 45069635 }}</ref>

==Indeterminate growth of meristems==
{{Further|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 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]]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 [[cloning|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]]''' and is widely practiced in horticulture to mass-produce plants of a desirable [[genotype]]. This process known as mericloning, has been shown to reduce or eliminate viruses present in the parent plant in multiple species of plants.<ref>{{cite journal |last1=Adams |first1=Alexa |title=Elimination of viruses from the hop (Humulus lupulus) by heat therapy and meristem culture |journal=Journal of Horticultural Science |date=April 2013 |volume=50 |issue=2 |pages=151–160 |doi=10.1080/00221589.1975.11514616 |url=https://www.tandfonline.com/doi/abs/10.1080/00221589.1975.11514616 |access-date=24 January 2023|url-access=subscription }}</ref><ref>{{cite journal |last1=Alam |first1=I |last2=Sharmin |first2=SA |last3=Naher |first3=MK |last4=Alam |first4=MJ |last5=Anisuzzaman |first5=M |last6=Alam |first6=MF |title=Elimination and detection of viruses in meristem-derived plantlets of sweetpotato as a low-cost option toward commercialization |journal=3 Biotech |date=April 2013 |volume=3 |issue=2 |pages=53–164 |doi=10.1007/s13205-012-0080-6 |pmid=8324570 |pmc=3597136 }}</ref>

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.<ref>{{cite journal | last1 = Mackenzie | first1 = K.A.D | last2 = Howard | first2 = B.H | year = 1986 | title = The Anatomical Relationship Between Cambial Regeneration and Root Initiation in Wounded Winter Cuttings of the Apple Rootstock M.26 | journal = Annals of Botany | volume = 58 | issue = 5| pages = 649–661 | doi = 10.1093/oxfordjournals.aob.a087228 }}</ref>

== Induced meristems ==
Meristems may also be induced in the roots of [[legume]]s such as [[soybean]], ''[[Lotus japonicus]]'', [[pea]], and ''[[Medicago truncatula]]'' after infection with soil bacteria commonly called [[Rhizobia]].{{Citation needed|date=June 2018}} 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]] [[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 known as the [[Root nodule#Nodulation|autoregulation of nodulation]]  (AON). This process involves a leaf-vascular tissue located [[Leucine-rich repeat|LRR]] [[receptor (biochemistry)|receptor]] [[kinase]]s (LjHAR1, GmNARK and MtSUNN), CLE [[peptide]] signalling, and KAPP interaction, similar to that seen in the CLV1,2,3 system. LjKLAVIER also exhibits a nodule regulation [[phenotype]] though it is not yet known how this relates to the other AON receptor kinases.

== Lateral Meristems ==
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.<ref>{{Cite journal |last1=Nieminen |first1=Kaisa |last2=Blomster |first2=Tiina |last3=Helariutta |first3=Ykä |last4=Mähönen |first4=Ari Pekka |date=January 2015 |title=Vascular Cambium Development |journal=The Arabidopsis Book |language=en |volume=13 |pages=e0177 |doi=10.1199/tab.0177 |issn=1543-8120 |pmc=4463761 |pmid=26078728}}</ref>

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.<ref>{{Cite web |title=Plant Development II: Primary and Secondary Growth {{!}} Organismal Biology |url=https://organismalbio.biosci.gatech.edu/growth-and-reproduction/plant-development-ii-primary-and-secondary-growth/ |access-date=2024-04-08 |website=organismalbio.biosci.gatech.edu}}</ref> 

==See also==
* [[Primary growth]]
* [[Secondary growth]]
* [[Stem cell]]
* [[Thallus]]
* [[Tissue (biology)|Tissues]]

==References==
{{reflist|30em}}

==Sources==
* Plant Anatomy Laboratory from [[University of Texas]]; the lab of JD Mauseth. [http://www.sbs.utexas.edu/mauseth/weblab/webchap6apmer/6.3-4.htm Micrographs of plant cells and tissues, with explanatory text.]
* {{cite journal | last1 = Schoof | first1 = Heiko| year = 2000| title = ''Arabidopsis'' shoot meristems is maintained by a regulatory loop between Clavata and Wuschel genes | journal = Cell | volume = 100 | issue = 6| pages = 635–644 | doi = 10.1016/S0092-8674(00)80700-X | pmid = 10761929 | last2 = Lenhard | first2 = M | last3 = Haecker | first3 = A | last4 = Mayer | first4 = KF | last5 = Jürgens | first5 = G | last6 = Laux | first6 = T | s2cid = 8963007| doi-access = free }}
* Scofield and Murray (2006). The evolving concept of the meristem. Plant Molecular Biology 60:v–vii.

==External links==
{{Commons|Méristème}}
* [http://meristemania.org Meristemania.org – Research on meristems]

{{Botany}}

[[Category:Plant anatomy]]
[[Category:Plant physiology]]