Root Apical Meristems


Apical root growth requires iterative processes of cell division, elongation and differentiation. Root apical meristem localised at the root tip harbours stem cells that divide asymmetrically and generate initial cells for all the cell types in the root. Fates of these initial cells are determined by positional signals that integrate both intrinsic and extrinsic cues. The quiescent centre maintains the stem cells and thereby sustains a constant supply of cells for root growth. Another important aspect of root growth regulation is to balance the rate of cell division and differentiation. The crosstalks between plant hormones auxin and cytokinins help to establish this balance. Gene regulatory networks that govern root meristem activities and growth have been discovered over the years in the model plant Arabidopsis thaliana. This introductory article provides an overview of the organisation of the root meristem and the underlying regulatory mechanisms.

Key Concepts:

  • Roots support the areal parts of plants and absorb water and nutrients that are essential for plant growth and survival.

  • Roots, which made their first appearance in vascular plants, have evolved into a multitude of sizes and shapes.

  • A root is composed of the outermost epidermis, ground tissue and the innermost stele as well as the quiescent centre and root cap.

  • Root apical meristem serves as the source of new cells for root growth.

  • In the root apical meristem, the stem cell niche sustains pluripotent stems cells to ensure a constant supply of cells for continuous root growth.

  • Two transcription factor pathways led by the SHORTROOT–SCARECROW and the Auxin–PLETHORA play a central role in maintaining the root stem cell niche.

  • Crosstalks between auxins and cytokinins establish a balance between cell proliferation and cell differentiation in the root meristem.

  • In the root meristem, cell fate decisions are made based on positional information and various intrinsic and extrinsic signals.

  • Cell‐to‐cell communications mediated by mobile transcription factors, peptides and small RNAs play critical roles in the root cell type patterning.

  • Owing to the vital importance of roots in overall plant health and survival, study of roots and root meristem poses utmost significance in plant biology.

Keywords: root; apical meristem; stem cell niche; quiescent centre; plant hormones; positional signals

Figure 1.

The atlas of the root tip of Arabidopsis thaliana as a representative RAM. The organisation of the RAM is enlarged and the major tissues are colour‐coded. The first T‐shape arrangement of divided cells (marked in bold lines) indicates the asymmetric cell division. Major developmental zones along the longitudinal axis of a root are marked in colour bars. EZ, elongation zone; MZ, maturation zone; PM, proximal meristem; SCN, stem cell niche; TZ, transition zone.

Figure 2.

Gene regulatory pathways for formative cell divisions and tissue specification. Gene regulatory pathways in (a) epidermis (b) endodermis and cortex (c) xylem and phloem and (d) QC are shown. ↓, positive regulation; T, negative regulation; …, protein movement. Colours of tissues are denoted as in Figure .

Figure 3.

Regulation of auxin and cytokinin in the RAM activity. Red arrows, gene activation; purple line, gene repression; green arrows, auxin flow.

Figure 4.

Regulation of RAM organisation by plant hormones: (a) The gravitropic response by auxin. Green lines indicate the flow of auxin. The relative amounts are indicated by the line thickness. (b) Crosstalk between auxin and cytokinin in the regulation of vascular tissue boundaries.



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Further Reading

Bishopp A, Help H and Helariutta Y (2009) Cytokinin signaling during root development. International Review of Cell and Molecular Biology 276: 1–48.

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Scheres B (2007) Stem‐cell niches: nursery rhymes across kingdoms. Nature Reviews Molecular Cell Biology 8: 345–354.

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

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How to Cite close
Sebastian, Jose, and Lee, Ji‐Young(May 2013) Root Apical Meristems. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0020121.pub2]