Meristems

Abstract

A central feature of plant development is the continuous generation of organs throughout the lifespan of the plant. In the region designated the meristem, undifferentiated cells are maintained throughout the life cycle of the plant, providing a source of cells from which plant organs are derived. Cells that are differentiating as they are channelled toward a particular developmental fate also reside in the meristem, as do the resultant organ primordia. The meristem is a self‐renewing structure and its stem cell population is maintained at a near constant number despite the perpetual mobilisation of differentiating cells into organogenesis. The meristem's capacity to balance continuous differentiation of cells while replenishing the pool of undifferentiated, pluripotent cells is tightly controlled by a very complex and overlapping network of regulatory pathways.

Key Concepts:

  • Plants, unlike animals, have the capacity to generate new organs post‐embryonically, throughout the lifespan of the plant.

  • Shoot and root apical meristems have the capacity to balance perpetual differentiation of cells while maintaining a population of undifferentiated stem cells.

  • Because fates of meristematic cells are determined by their relative positions, they must be in continuous communication with their neighbouring cells.

  • Signals can be transmitted from more mature cells to initial cells to specify the pattern of meristem differentiation.

  • The number of stem cells in meristems is remarkably constant despite the continuous recruitment of differentiating cells into organs.

  • In Arabidopsis shoot meristems, a feedback loop between WUSCHEL and the CLAVATA signalling pathway is crucial for specifying and maintaining the stem cell niche in the shoot apex.

  • Within their niche boundaries, stem cells remain in an undifferentiated state in response to positional cues from neighbouring cells while cells displaced from the niche begin the process of differentiation.

Keywords: development; differentiation; primordium; shoot apex; stem cells; meristem

Figure 1.

The dynamics of shoot meristem development. (a) A single undifferentiated cell (box) in the central zone (CZ). All the cells in (b) and (c) are derived from this cell. (b) The progeny cells (circles) in the peripheral zone (PZ) are channelled toward a particular developmental fate. (c) Cells are increasingly displaced from the centre of the meristem and incorporated into organ primordia (closed circles). RM, rib meristem.

Figure 2.

Apical meristems differ across the plant kingdom. (a) The meristem of red algae consisting of a single apical cell (AC). (b) The meristem of a gymnosperm, pine. The central dome‐like structure is the meristem and it is flanked by organ primordia (OP), which in turn are flanked by organs in the outermost regions. (c) The meristem of maize. The dark tissue indicates the region of KNOTTED expression. KNOTTED in maize, and its counterpart, SHOOTMERISTEMLESS, in Arabidopsis, are essential for maintenance of a population of undifferentiated cells in the meristem. (d) The meristem of an angiosperm, Arabidopsis. The central dome‐like structure is the meristem, and it is flanked by organ primordia.

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Further Reading

Bennett T and Scheres B (2010) Root development‐two meristems for the price of one? Current Topics in Developmental Biology 91: 67–102.

Barton MK (2010) Twenty years on: the inner workings of the shoot apical meristem, a deveopmental dynamo. Developmental Biology 341(1): 95–113.

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Ha CM, Jun JH and Fletcher JC (2010) Shoot apical meristem form and function. Current Topics in Developmental Biology 91: 103–140.

Rieu I and Laux T (2009) Signaling pathways maintaining stem cells at the plant shoot apex. Seminars in Cell and Developmental Biology 20: 1083–1088.

Stahl Y and Simon R (2010) Plant primary meristems: shared functions and regulatory mechanisms. Current Opinion in Plant Biology 13: 53–58.

Tucker MR and Laux T (2007) Connecting the paths in plant stem cell regulation. Trends in Cell Biology 17(8): 403–410.

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Green, Kirsten A(Jun 2012) Meristems. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0002049.pub2]