Editing Meristem (section)

===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>