Primary Root

Abstract

The primary root, or the root that is initiated in the embryo within the seed, is the original root of a plant to discover its soil environment. It is essential for the anchoring of the plant and ensures plant growth through the uptake of water and nutrients. In this review, we describe the tissue organisation of the primary root, not only largely focusing on the model species Arabidopsis thaliana but also mentioning some other plant species where relevant. We also flag, non‐comprehensively, some of the molecular factors important for proper root formation and growth, plant growth regulators or hormones that determine primary root growth as well as interactions with the environment that shape the root system and refer to relevant literature for further detailed information.

Key Concepts

  • The primary root is initiated in the embryo within the seed.
  • The primary root is only a (small) part of the complex root system architecture.
  • The primary root is an ideal model system to study cell division, differentiation and identity.
  • There are complex and tightly regulated molecular patterning events to ensure proper primary root tissue organisation and a lot of these regulations involve plant hormones.
  • The primary root is a dynamic interface with the soil environment.

Keywords: primary root; hormones; cell types; Arabidopsis thaliana ; model system; root architecture; transcription factors

Figure 1. Germinating wheat (a) and Arabidopsis thaliana (b) seed with important organs indicated. Wheat seed germinated on ½ MS medium in the dark, 48 h after transfer to germination medium. A. thaliana seedling, 5 days after germination.
Figure 2. Cell types of the Arabidopsis thaliana primary root (5 days after germination). (a) Longitudinal section through the root meristem of a propidium iodide‐stained primary root tip. (b) Tracing of the primary root meristem in (a) with indication of the different cell types and tissues (colour scheme based on Cruz‐Ramirez et al., ). (c) Longitudinal division of the primary root into three functional zones: the meristematic, elongation and differentiation zone.
Figure 3. Genes and pathways involved in the establishment of the Arabidopsis thaliana primary root. (a) Apical meristem and stem cell maintenance. Figure adapted from Drisch and Stahl, 2015, Front Plant Sci 6:505. (b) Specification of endodermal tissue layer and involvement of SHORTROOT (SHR)/SCARECROW(SCR) in cortical/endodermis initials. Adapted from Cruz‐Ramirez et al. 2012 and Sozzani and Iyer‐Pascuzzi 2014 ACD, asymmetric cell division © Elsevier.
Figure 4. Environmental factors shaping the root system. (a) Normal plant growth versus tropic growth and growth affected by salt stress and nutrient availability. (b) Detailed gravitropic responses in apical root meristem. The statoliths/amyloplasts reorient depending on the gravistimulus. As a consequence, an asymmetric auxin gradient originates, which results in more auxin at the lower side of the root to inhibit cell elongation and less auxin at the upper side of the root to increase cell elongation. Adapted from: https://atlasofscience.org/control‐of‐plant‐architecture‐by‐gravity.
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Verstraeten, Inge, and De Smet, Ive(May 2017) Primary Root. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0002059.pub2]