Apical Meristems


Apical meristems are regarded generically, as the growing tips of the higher plant. However, neither the shoot apical meristem (SAM) nor the root apical meristem (RAM) is apical, the former is protected by young unexpanded leaves and the RAM by the root cap. The SAM is an exponentially growing morphogenetic centre, it initiates leaves, flowers, nodes and internodes and in culture can regenerate a whole plant. Genes that regulate the SAM include the homeodomain genes, WUSCHEL (WUS), SHOOTMERISTEMLESS (KNOTTED), KNOTTED‐like genes and the WUS‐related WOX9 together with the nonhomeodomain CLAVATA gene family. Leaf initiation, an event characterized by an early change in the plane of cell division on the flanks of the SAM, is characterized by the expression of MGOUN while AINTEGUMENTA expression is restricted to the primordium. The RAM shows steady‐state growth kinetics and roots in culture form more roots. QUIESCENT CENTRE HOMOBOX (QHB) is a gene expressed in the quiescent centre, a population of slowly dividing founder cells that is located at the pole of the RAM. QHB may function in a manner analogous to WUS. However, ROOTMERISTEMLESS is a nonhomeodomain gene that is also central to normal function of the RAM.

Keywords: cell division; growth rate; cell cycle; plant development

Figure 1.

Median longitudinal section of a shoot apical meristem (SAM) of the grass, Dactylis glomerata. A is the apical dome; B is the subapical region containing the new leaf primordium as a bump on the left side adjacent to a central region in which the progenitor cells of the node and internode are located. Bar, 100 μm.

Figure 2.

Schematic of the shoot apical meristem (SAM) and (RAM) with ‘expression locations’ for various genes (see text for details). In the shoot apex model, the SAM is partitioned into central zone (CZ), peripheral zone (PZ), (SAR) and youngest leaf primordium (LP). WUSCHEL (WUS) is expressed in the lower half of the CZ and induces expression of CLAVATA (CLV) 3. The latter encodes a ligand that binds to CLV1 which in turn represses WUS in a feedback control loop. SHOOTMERISTEMLESS (STM) is expressed throughout the SAM except in the region of the PZ where the next primordium would form which is where MGOUN (MGO) genes would be expressed. AINTEGUMENTA (ANT) is expressed in the LP. In the root apex model, the RAM is partitioned into cortical initials (CI), stele initials (SI) and quiescent centre (QC). The CI and SI domains would extend to the pole of the RAM. Subtending the RAM is the root cap initials (RCI). A network of QUIESCENT CENTRE HOMOBOX (QHB), PLETHORA (PLT) and PIN FORMED (PIN) genes is portrayed.



Aida M, Beis D and Hiedstra R (2004) The PLETHORA genes mediate patterning of the Arabidopsis root stem cell niche. Cell 119: 109–120.

Belles‐Boix E, Hamant O, Witiak SM et al. (2006) KNAT6: An Arabidopsis homeobox gene involved in meristem activity and organ separation. Plant Cell 18: 1900–1907.

van den Berg C, Willemsen V, Hendriks G, Weisbeek P and Scheres B (1997) Short‐range control of cell differentiation in the Arabidopsis root meristem. Nature 390: 287–289.

Blilou I, Frugier F, Folmer S et al. (2002) The Arabidopsis HOBBIT gene encodes a CDC27 homolog that links the plant cell cycle to progression of cell differentiation. Genes and Development 16: 2566–2575.

Blilou I, Xu J and Wildwater M (2005) The PIN auxin efflux facilitator network controls growth and patterning in Arabidopsis roots. Nature 433: 39–44.

Brand U, Grunewald M, Hobe M and Simon R (2000) Regulation of CLV3 expression by two homeobox genes in Arabidopsis. Plant Physiology 129: 565–575.

Byrne ME, Simorowski J and Martienssen RA (2002) ASYMMETRIC LEAVES1 reveals knox gene redundancy in Arabidopsis. Development 129: 1957–1965.

Cheng JC, Seeley K and Sung ZR (1995) RML1 and RML2, Arabidopsis genes required for cell proliferation at the root tip. Plant Physiology 107: 365–376.

Clark SE, Jacobsen SE, Levin JZ and Meyerowitz EM (1996) The CLAVATA and SHOOT MERISTEMLESS loci competitively regulate meristem activity in Arabidopsis. Development 122: 1567–1575.

Clowes FAL (1954) The promeristem and the minimal construction centre in grass root apices. New Phytologist 53: 108–116.

Clowes FAL (1956) Localisation of nucleic acid synthesis in root meristems. Journal of Experimental Botany 7: 307–312.

Clowes FAL (1967) The quiescent centre. Phytomorphology 17: 132–140.

Clowes FAL (1970) The immediate response of the quiescent centre to X‐rays. New Phytologist 69: 1–18.

Francis D (2007) The plant cell cycle – 15 years on. New Phytologist 174: 261–278.

Francis D (2008) G2/M transition in eukaryotes. In: Bryant JA and Francis D (eds) The Eukaryote Cell Cycle, pp. 81–99. Abingdon: Taylor & Francis.

Gonthier R, Jacqmard A and Bernier G (1987) Changes in cell cycle duration and growth fraction in the shoot meristem of Sinapis alba during floral transition. Planta 170: 55–59.

Green PB (1994) Connecting gene and hormone action to form, pattern and organogenesis: biophysical transductions. Journal of Experimental Botany 45: 1775–1788.

Gruissem W (2007) Function of the retinoblastoma‐related proteins in plants. In: Inze D (ed.) Cell Cycle Control and Plant Development, pp. 164–186. Oxford: Blackwell Publishing.

Hanstein J (1870) Die Entwicklung des Keimes der Monokotylen und der Dikotylen. Botanisch Abhandlen 1: 1–112.

Jenik PD, Gillmore CS and Lukowitz W (2007) Embryonic patterning in Arabidopsis thaliana. Annual Review of Cell and Developmental Biology 23: 207–236.

Kamiya N, Nagasaki H, Morikama A, Sato Y and Matsuoka M (2003) Isolation and characterisation of a rice WUSCHEL‐type homeobox gene that is specifically expressed in the central cells of the quiescent centre in the root apical meristem. Plant Journal 35: 429–441.

Laufs P, Dockx J, Kronenberger J and Traas J (1998) MGOUN1 and MGOUN2, two genes required for primordium initiation at the shoot apical meristem in Arabidopsis thaliana. Development 125: 1253–1260.

Lyndon RF (1970) Planes of cell division in the shoot apical meristem of Pisum. Annals of Botany 34: 19–28.

Lyndon RF (1977) Interacting processes in development at the shoot apex. Society for Experimental Biology Symposium 31: 221–250.

Lyndon RF (1998) The Shoot Apical Meristem. Cambridge: Cambridge University Press.

Mauseth JD (1991) Botany. Orlando, FL: Holt Rhinehart & Winston.

Mayer KF, Schoof H, Haeker A et al. (1998) Role of the WUCHSEL in regulating stem cell fate in the Arabidopsis shoot meristem. Cell 95: 805–815.

Menges M and Murray JAH (2008) Plant D‐type cyclins: structure, roles and functions. In: Bryant JA and Francis D (eds) The Eukaryote Cell Cycle, pp. 1–28. Abingdon: Taylor & Francis.

Nougarède A (1967) Experimental cytology of the shoot apical cells during vegetative growth and flowering. International Review of Cytology 21: 203–351.

Ramirez‐Parra E, Del Pozo JC, Desvoyes B, De La Paz Sanchez M and Guitierrrez C (2007) E2F‐DP transcription factors. In: Inze D (ed.) Cell Cycle Control and Plant Development, pp. 138–1639. Oxford: Blackwell Publishing.

Schoof H, Lenhard M, Haeker A et al. (2000) The stem cell population of Arabidopsis shoot meristems is maintained by a regulatory loop between the CLAVATA and WUSCHEL genes. Cell 100: 635–644.

Schüepp O (1917) Untersuchungen uber Wachstum und Formwechsel von Vegetationspunkten. Jahrbuche für Wissenschaften Botanisch 57: 17–79.

Sorrell DA, Chrimes D, Dickinson JR, Rogers HJ and Francis D (2005) The Arabidopsis CDC25 induces a short cell length when over expressed in fission yeast: evidence for cell cycle function. New Phytologist 165: 425–428.

Springer PS, McCombie WR, Sundaresan V and Martienssen RA (1995) Gene trap tagging of PROLIFERA, an essential MCM2‐3‐5‐like gene in Arabidopsis. Science 268: 877–880.

Vlieghe K, Inze D and De Veylder L (2007) Physiological relevance and molecular control of the endocycles in plants. In: Inze D (ed.) Cell Cycle Control and Plant Development, pp. 227–248. Oxford: Blackwell Publishing.

Wang H, Zhou Y, Antonio J, Acosta T and Fowke L (2007) CDK inhibitors. In: Inze D (ed.) Cell Cycle Control and Plant Development, pp. 62–86. Oxford: Blackwell Publishing.

Wu X, Dabi T and Weigel D (2005) Requirement for the homeobox gene STIMPY/WOX9 for Arabidopsis meristem growth and maintenance. Current Biology 15: 436u–440u.

Further Reading

Bryant JA and Francis D (2008) The Eukaryote Cell Cycle. Abingdon: Taylor & Francis.

Inze D (2007) Cell Cycle Control and Plant Development. Oxford: Blackwell Publishing.

Lyndon RF (1998) The Shoot Apical Meristem. Cambridge: Cambridge University Press.

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Francis, Dennis(Sep 2008) Apical Meristems. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0002050.pub2]