Pericycle

Abstract

Pericycle is a primary tissue of plant roots and is the site for the initiation of lateral roots and two secondary meristems, the vascular cambium and cork cambium (phellogen). In this article the characteristics of the pericycle related to each of its functions are reviewed. During lateral root development, three important and coordinated events take place in the pericycle: priming, founder cells specification and patterned cell division leading to primordium formation. The vascular cambium is initiated from the pericycle, just outside the primary xylem, and from residual procambium that originates from vascular parenchyma cells located between the primary xylem and primary phloem resulting in a continuous cylinder of vascular cambium cells which form the secondary xylem and secondary phloem of the root. The pericycle tends to be ‘pushed’ outwards after the vascular cambium forms and it eventually divides again to become the cork cambium. The cork cambium divides to form phellem cells to the outside and phelloderm to the inside. Aspects of the molecular mechanisms involved in the development and function of the pericycle and its derivatives are also discussed.

Key Concepts:

  • Pericycle is a primary tissue located at the periphery (most outward) of the root vascular cylinder.

  • In roots, the pericycle is a unique tissue in that its cells continue to cycle for a long period and it has three functions – initiation of lateral roots, contribution to the initiation of the vascular cambium and initiation of the cork cambium.

  • Lateral root initiation involves the following steps: pericycle cell priming that occurs in a so‐called oscillation zone, founder cell specification that is dependent on local increase in auxin concentration and first divisions of founder cells leading to primordium formation.

  • Auxin is a key hormone involved in the regulation of pericycle activity related to lateral root formation but it interacts and acts in a complex orchestration with other plant hormones.

  • Auxin homeostasis‐ and signalling‐related genes are involved in pericycle cell development, lateral root initiation and patterned division of founder cells during lateral root formation; some auxin‐independent genes are also important for pericycle development.

  • Vascular cambium forms by division of the pericycle cells just outside the primary xylem and also by division of residual procambium cells located between the primary xylem and primary phloem.

  • Vascular cambium divides toward the inside of the root to form secondary xylem and towards the outside to form the secondary phloem; auxins and cytokinins are essential to initiate the vascular cambium.

  • The cork cambium, or phellogen, forms from the pericycle outside the secondary phloem. It divides to the outside to form a layer of cells called phellem and to the inside to form the phelloderm. The cell walls of these cell layers are impregnated with a waxy substance called suberin to retard water loss out of the root and to prevent the entry of pathogens.

Keywords: Arabidopsis thaliana; lateral root development; lateral root initiation; plant development; primordium; secondary root growth; vascular cambium; cork cambium

Figure 1.

Pericycle in Arabidopsis root. (a) Transverse section of the root 6 mm from the tip. Pericycle cells in direct contact with protoxylem are marked with asterisks and pericycle cells adjacent to protophloem are marked with black dots. (b) Longitudinal optical section at the level of the differentiation zone of the root of J0121 enhancer‐trap line. (c) Longitudinal optical section at the level of the root apical meristem of Rm1007 enhancer‐trap line. Images in (b) and (c) were observed under a confocal laser‐scanning microscope and show a root in the protoxylem plane. Propidium iodide was used for visualisation of the cell walls. Note the GFP‐positive protoxylem‐associated pericycle cells. Arabidopsis Biological Research Center at the Ohio State University is gratefully acknowledged for providing the J0121 seeds. (c) Kindly donated by B. Parizot and T. Beeckman, Gent University and the donation is gratefully acknowledged (see also Parizot et al., ).

Figure 2.

Auxin response at the transcriptional level is the earliest marker of founder cell specification. Shown there are time‐lapse single images from an analysis of DR5rev::GFP live roots observed under a confocal microscope. In the young differentiation zone, pericycle cells showing GFP expression acquire founder cell identity and 15 h later derivatives of these cells become part of a lateral root primordium. At the left is a green channel, at the right is merged image of a bright field and green channel. Red arrowheads indicate end walls of founder cells. Asterisks show reference points used to verify that images are taken at the same focal plane at the beginning and the end of observations. Bar=50 μm. e, epidermis; c, cortex; en, endodermis; P, pericycle. Figure reproduced with small modifications from Dubrovsky et al., with permission of National Academy of Sciences, USA, copyright © 2008.

Figure 3.

Two types of lateral root primordium initiation in Arabidopsis. (a) Unicellular longitudinal; LRP cells are enclosed in the cell wall of the founder cell 6–8 mm from the root tip. (b) Longitudinal bicellular. Shown is the earliest stage of LRP initiation on a histological section of the root portion 2–4 mm from the root tip. Note the first asymmetrical division in two pericycle cells leading to LRP formation. Anticlinal cell walls of the founder cells are marked with asterisks. The location of the cell wall resulting from the first division of founder cells is marked by arrowheads. Black bar, 20 μm. Reproduced from Dubrovsky et al. with the permission of Springer‐Verlag.

Figure 4.

Secondary growth in a root. (a) At the completion of primary growth, an arc of residual procambium cells remains between the primary xylem and primary phloem. The pericycle is a complete cylinder. (b) The residual procambium starts dividing and joins the pericycle cells outside the xylem arms to form a continuous cylinder of vascular cambium. The pericycle just outside the protoxylem points divides to form at least two cell layers. The inner layer joins with the residual procambium to form the vascular cambium; the outer layer remains a part of the pericycle cylinder. (c) The vascular cambium forms secondary xylem internally and secondary phloem externally. The primary phloem is being pushed outward. (d) A cylinder of vascular cambium produces secondary xylem to the inside and secondary phloem to the outside. The primary xylem remains in the centre of the root, the primary phloem has been crushed and the pericycle that remains will form the cork cambium. The cork cambium forms the periderm after the epidermis and cortex die. The term bark refers to everything outside the vascular cambium. From Plant Biology (Non‐Info Trac Version) 1st edn, by Rost/Barbour/Murphy/Stocking. Copyright © 1998. Reprinted with the permission of Brooks/Cole, a division of Thomson Learning: www.thomsonrights.

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Further Reading

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Dubrovsky, Joseph G, and Rost, Thomas L(Jul 2012) Pericycle. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0002085.pub2]