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Cell signalling at the shoot meristem

Nature Reviews Molecular Cell Biology volume 2, pages 276–284 (2001)Cite this article

Key Points

  • Plant development is fundamentally different from developmental patterning in most animals in that most of the body plan is established after embryogenesis. This is possible because plants retain two populations of stem cell — the shoot meristem and the root meristem.

  • Shoot meristems regulate organ formation be carefully balancing the maintenance of indifferentiated stem cells and the commitment of appropriately positioned cells towards differentiation.

  • Genetics screens are now revealing the signalling pathways that regulate this balance and the mechanisms that subsequently mediate organ development.

  • Screens in Arabidopsis thaliana for mutants that lack stem cells or have ectopic stem cells have uncovered a signal-transduction pathway that controls stem cell behaviour. The CLAVATA (CLV) signalling pathway components so far identified consist of a putative receptor kinase (CLV1), its dimerization partner (CLV2) and its proposed ligand (CLV3).

  • Other interacting proteins include the kinase associated protein phosphatase KAPP, which is proposed to negatively regulate CLV1, and ROP, a member of the small GTPase Ras superfamily, which might regulate downstream signalling.

  • WUSCHEL, a putative homeodomain-containing transcription factor that signals the stem-cell fate to overlying cells at the shoot meristem, is regulated, at least indirectly, by the CLV proteins. A feedback loop has been shown between WUS and CLV3, which allows a equilibrium to be achieved, resulting in a stable population of stem cells.

  • After the onset of differentiation, organs are initiated in distinct spatial patterns. Further analysis of PERIANTHIA, which is required for the right number of organs in the flower meristem, might shed light on how this occurs.

  • Other mutants have been identified that mediate organ separation during the early stages of organ intiation. Loss of STM, CUC1 or CUC2 results in fusion of the earliest organs initiated.

  • Organ polarity is initiated by asymmetric growth during the earliest stages of organ development. The molecules that direct this are being uncovered in snapdragon, maize and Arabidopsis, revealing conserved mechanisms.

Abstract

The regulation of cell differentiation at meristems is crucial to developmental patterning in plants. Rapid progress has been made in identifying the genes that regulate differentiation and the receptor-mediated signalling events that have a key role in this process. In particular, we are now learning how the CLAVATA receptor kinase signalling pathway promotes stem cell differentiation in balance with the initiation of stem cells by the transcription factor WUSCHEL.

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Figure 1: An adult Arabidopsis plant.
Figure 2: Cells layers and cell divisions.
Figure 3: The Arabidopsis thaliana shoot meristem.
Figure 4: The CLAVATA1 signalling pathway.
Figure 5: Stem-cell regulators.
Figure 6: Establishment of organ polarities within organ primordia.

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Acknowledgements

Work in the author's lab on meristem development is supported by grants from the NSF Developmental Mechanisms programme and the DOE Energy Biosciences programme. The author thanks John Bowman for providing the photograph used in FIG. 6.

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  1. Department of Biology, University of Michigan, Ann Arbor, 48109-1048, Michigan, USA

    Steven E. Clark

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  1. Steven E. Clark
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DATABASE LINKS

CLV1

CLV2

CLV3

thyroid-stimulating hormone receptor

luteinizing hormone receptor

gonadotropin hormone receptor

RLK5

Rho/Rac

POL

WUS

PAN

STM

AS1

CRC

FIL

REV

ZLL

AGO1

PHB

LEAFY

APETALAI 1

TFL

ERECTA

FURTHER INFORMATION

Clark lab home page

ENCYCLOPEDIA OF LIFE SCIENCES

Arabidopsis thaliana as an experimental organism

Arabidopsis: Flower development and patterning

Positional information in plant development

Glossary

MERISTEMS

Locations on a plant where stem cells are maintained and organogenesis occurs. Root meristems, shoot meristems and flower meristems fit this description.

ORGAN PRIMORDIA

An organ (for example, a leaf, flower or petal) at an early stage of development, immediately after its initiation.

ANGIOSPERMS

Most extant plants are angiosperms, or flowering plants. Non-angiosperms include gymnosperms (for example, pine and cycads), ferns and mosses.

ANTICLINAL

During anticlinal cell divisions, the new cell wall forms perpendicular to the layer of cells. This maintains cells in a single layer.

PERICLINAL

In periclinal cell divisions, the new cell wall forms parallel to the cell layer, effectively thickening that cell layer.

PHYLLOTAXY

The pattern of organ initiation by a shoot or flower meristem.

EPISTATIC

Mutations that are epistatic mask the phenotype of other mutations.

MOSAIC ORGANS

Organs that have cell types characterisitic of two or more organs.

FUSED ORGANS

Organs fused together at early stages of development, usually at the lateral edges.

ABSCISSION

The process by which dead parts of a plant break off naturally (for example, leaves).

MYB TRANSCRIPTION FACTOR

A type of transcription factor first identified in animals. The MYB gene family is greatly expanded in plants, and the proteins that these encode have been shown to control many developmental processes.

ZYGOMORPHIC FLOWERS

Flowers that are asymmetric, and in which the development of specific organs varies depending on the polarities of the flowers. Snapdragon flowers are zygomorphic, whereas roses are not.

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Clark, S. Cell signalling at the shoot meristem. Nat Rev Mol Cell Biol 2, 276–284 (2001). https://doi.org/10.1038/35067079

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