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 root apical meristem (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), subapical region (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.



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Further Reading

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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]