Plant Roots the Hidden Half Cellular Patterning of the Root Meristem: Genes and Signals

This article was downloaded by: 10.3.98.104 On: 27 Sep 2021 Access details: subscription number Publisher: CRC Press Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: 5 Howick Place, London SW1P 1WG, UK Plant Roots The Hidden Half Amram Eshel, Tom Beeckman Cellular Patterning of the Root Meristem: Genes and Signals Publication details https://www.routledgehandbooks.com/doi/10.1201/b14550-6 Kimberly L. Gallagher Published online on: 17 Apr 2013 How to cite :- Kimberly L. Gallagher. 17 Apr 2013, Cellular Patterning of the Root Meristem: Genes and Signals from: Plant Roots, The Hidden Half CRC Press Accessed on: 27 Sep 2021 https://www.routledgehandbooks.com/doi/10.1201/b14550-6 PLEASE SCROLL DOWN FOR DOCUMENT Full terms and conditions of use: https://www.routledgehandbooks.com/legal-notices/terms This Document PDF may be used for research, teaching and private study purposes. Any substantial or systematic reproductions, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The publisher shall not be liable for an loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. 3 Cellular Patterning of the Root Meristem: Genes and Signals I. Introduction .............................................................................................................................3-1 II. Anatomical Coordinates ........................................................................................................3-1 III. Definition of the Term “Meristem” .......................................................................................3-2 IV. SAM ...........................................................................................................................................3-3 V. Root System ..............................................................................................................................3-3 VI. Structure of the RAM .............................................................................................................3-4 VII. Significance of an Open versus a Closed Meristem ...........................................................3-8 VIII. Control of Meristem Size ........................................................................................................3-8 IX. Changes in the Patterning of the Meristem ......................................................................3-10 X. Importance of the QC in Maintaining Root Growth ......................................................3-11 XI. Reformation and Maintenance of a QC ............................................................................. 3-14 XII. Stem Cell in the Root Meristem ..........................................................................................3-15 XIII. Stem Cell Niche Concept in the Root .................................................................................3-16 XIV. Importance of Cell Signaling in Root Growth ..................................................................3-18 Kimberly L. Gallagher XV. Conclusions.............................................................................................................................3-21 University of Pennsylvania References ...........................................................................................................................................3-21 I. Introduction the regulation and maintenance of the RAMs of angiosperms. Particular attention is paid to Arabidopsis development as much It is easy to see how an infant child relates to the parent. With of the molecular and genetic analysis of RAM formation and respect to overall pattern, it is essentially the adult in minia- maintenance has been done in this system. When possible, ture: 2 arms, 2 legs, 10 fingers, and 10 toes. The same is not true comparisons are drawn to other systems. As there are sepa- when one compares a newly emerged plant seedling to the adult. rate chapters in this book that concentrate entirely on vascular The adult plant body plan is often a highly elaborated structure patterning of the root and hormone signaling, these topics are with multiple lateral appendages and organs. These structures given only modest coverage here, although they clearly play very may be underdeveloped or completely lacking in the newly ger- important roles in the patterning of the root. Prior to discussing minated seedling. This is because much of the adult plant body the RAM, I provide definitions of anatomical terms and a brief is patterned postembryonically through the action of coor- description of the SAM as a basis for comparison to the RAM. dinated groups of stem cells in the shoot and root apices: the shoot and root apical meristems (SAM and RAM, respectively). II. Anatomical Coordinates Postembryonic development of the adult body plan allows the plant the plasticity to adjust its overall structure, to optimize There is inconsistent use of anatomical terminology in the lit- the acquisition of resources like light and water, and to respond erature particularly with respect to defining relative directions appropriately to biotic and abiotic signals. of growth and cell division (Baluska et al. 2005). Therefore, As the SAM and RAM collectively give rise to all of the tis- these terms will be defined here. By convention the tip of the sues of the adult plant, much attention has been given to how shoot and the tip of the root are referred to as the shoot and they are structured, how they function, and how they form. In root “apex,” respectively (Figure 3.1A). Consequently in both previous editions of this book, this chapter discussed the physi- the root and shoot, the term “apical” describes a location that, cal and mechanical properties of the RAM and provided an relative to the structure or feature being described, is toward the excellent comparative analysis of RAM form and function con- tip. For example, the epidermal cell labeled “a” in Figure 3.1B sidering multiple different taxa. In this edition, I will focus on is apical to the cells labeled “b” and “c.” Conversely “basal” is 3-1 Downloaded By: 10.3.98.104 At: 08:24 27 Sep 2021; For: 9781439846490, chapter3, 10.1201/b14550-6 © 2013 by Taylor & Francis Group, LLC 3-2 Root Structure (B) (A) SAM Basal Apical 2 3 proximal Shoot distal root-ward shoot-ward (D) c 1 b 1 Anticlinal a (C) 2 Periclinal Apical Root Basal distal proximal 3 Anticlinal root-ward shoot-ward 1 2 T 3 T RAM FIGURE 3.1 Anatomical coordinates. (A) Cartoon image of a hypothetical seedling. The junction between the root and shoot is indicated by the dashed line. Directional terms are labeled. (B) A transverse slice through an Arabidopsis root. (C) Medial longitudinal cross section through the Arabidopsis root meristem. The lowercase labels “a,” “b,” and “c” on the column of cells in (B) are for orientation and are referenced in the text. In both (B) and (C), oriented divisions are shown as bold black lines in the cells in white. The numbers associated with these divisions refer to the images in (D). “T” cell divisions are indicated in (C). The division on the left contributes to the plant body, whereas the one on the right adds to the root cap. away from the apex and toward the point where the root joins the orientation of cell divisions in the root meristem, they can the shoot (dotted line in Figure 3.1A; referred to by Dolan as the be confusing when referring to divisions when cells are not collet zone in Arabidopsis). The terms “proximal” and “distal” cuboidal or when divisions are not clearly oriented parallel or are also used to describe the position of a feature relative to the perpendicular to the organ surface. For example, the apical cell junction of the root and the shoot (usually the root–hypocotyl in Azolla filiculoides Lam. (discussed in more detail later; see junction). “Proximal” indicates toward the root–shoot junction, Figure 3.3E and F) is a tetrahedral and most of the formative cell whereas away is “distal.” In Figure 3.1B, cell “c” is proximal to divisions are oblique to the root surface. In this case, descriptors “b.” Recently some have adopted the terms “root-ward” and other than periclinal and anticlinal are useful in explaining the “shoot-ward” in favor of proximal and distal (Baskin et al. 2010). orientation of cell division, and the orientation may be described In the root, shoot-ward is proximal, whereas in the shoot, proxi- relative to the axis of the cell. mal is root-ward. With respect to cell division, the terms “anticlinal” and III. Definition of the term “Meristem” “periclinal” (Figure 3.1D) are used to describe the orientation of cell division relative to the surface of the organ, usually the The term meristem was coined in 1858 by Carl Wilhelm von epidermis. Anticlinal cell divisions create a new cell wall that Nägeli to refer to populations of cells in the plant that are able is perpendicular (at a right angle) to the organ surface. In the to give rise to entire organs, for example, the SAM and RAMs root, anticlinal cell divisions can occur in two orientations: (von Nägeli 1858). Part of the motivation for developing this new perpendicular or parallel to the axis of root elongation. The term was to distinguish

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