Gene Networks Involved in Hormonal Control of Root Development in Arabidopsis thaliana: A Framework for Studying Its Disturbance by Metal Stress

Abstract

Plant survival under abiotic stress conditions requires morphological and physiological adaptations. Adverse soil conditions directly affect root development, although the underlying mechanisms remain largely to be discovered. Plant hormones regulate normal root growth and mediate root morphological responses to abiotic stress. Hormone synthesis, signal transduction, perception and cross-talk create a complex network in which metal stress can interfere, resulting in root growth alterations. We focus on Arabidopsis thaliana, for which gene networks in root development have been intensively studied, and supply essential terminology of anatomy and growth of roots. Knowledge of gene networks, mechanisms and interactions related to the role of plant hormones is reviewed. Most knowledge has been generated for auxin, the best-studied hormone with a pronounced primary role in root development. Furthermore, cytokinins, gibberellins, abscisic acid, ethylene, jasmonic acid, strigolactones, brassinosteroids and salicylic acid are discussed. Interactions between hormones that are of potential importance for root growth are described. This creates a framework that can be used for investigating the impact of abiotic stress factors on molecular mechanisms related to plant hormones, with the limited knowledge of the effects of the metals cadmium, copper and zinc on plant hormones and root development included as case example.

Keywords: aluminium; cadmium; copper; lateral root; phytohormones; primary root; zinc.

Figure 1

Organization of the Arabidopsis root. (Left): Longitudinal section through the root showing apical-basal polarity. Different cell types (each differently coloured) are arranged in cell files, forming concentric single-celled layers surrounding the central vascular tissue. Distinct developmental zones are formed along the growing root. Cell division occurs in the meristematic zone, especially the apical meristem. Cell division rate slows down in the basal meristem and cells start to elongate in the elongation zone. The boundary between meristematic and elongation zone is indicated as the transition zone. Cell differentiation occurs in the differentiation zone; (Right): Radial polarity in on a cross section of the differentiated root zone showing the formation of root hairs and Casparian strips (Based on Petricka et al., 2012 [1] and Péret et al., 2009 [2]).

Figure 2

Lateral root development. (I) Lateral root initiation—Anticlinal division of lateral root founder cells in the pericycle; (II) Outer and inner cell layers are formed by periclinal divisions; (III) Periclinal divisions of the outer layer makes dome shape of the LRP is apparent (three-layered); (IV) As a result of periclinal divisions the primordium becomes four-layered; (V) After anticlinal divisions, the primordium begins to push through the cortex of the primary root; (VI) Different cell types are being formed; (VII) Lateral root meristem is established and primordium enlarges; and (VIII) Primordium is about to emerge after which the lateral root meristem will be activated (Based on Malamy and Benfey, 1997 [7]).

Figure 3

Schematic representation of the complex cross-talk network of hormones in root development. Individual effects of phytohormone on root development are described in the green boxes. Black lines with a black box describe a cross-talk between two phytohormones. Orange lines indicate that a certain phytohormone partakes in a cross-talk between two hormones. All information in this scheme is described more detailed in the text and can easily be found using the referral letters A–Y. Abbreviations: COI1 (CORONATINE INSENSITIVE 1), EZ (elongation zone), JAZ (JASMONATE ZIM-DOMAIN PROTEINS), LR (lateral root), LRI (lateral root initiation), LRP (lateral root primordium), MZ (meristematic zone), PIN (PIN FORMED), PR (primary root), QC (quiescent centre), RAM (root apical meristem), TZ (transition zone).