Abscisic Acid: Hidden Architect of Root System Structure

Abstract

Plants modulate root growth in response to changes in the local environment, guided by intrinsic developmental genetic programs. The hormone Abscisic Acid (ABA) mediates responses to different environmental factors, such as the presence of nitrate in the soil, water stress and salt, shaping the structure of the root system by regulating the production of lateral roots as well as controlling root elongation by modulating cell division and elongation. Curiously, ABA controls different aspects of root architecture in different plant species, perhaps providing some insight into the great diversity of root architecture in different plants, both from different taxa and from different environments. ABA is an ancient signaling pathway, acquired well before the diversification of land plants. Nonetheless, how this ancient signaling module is implemented or interacts within a larger signaling network appears to vary in different species. This review will examine the role of ABA in the control of root architecture, focusing on the regulation of lateral root formation in three plant species, Arabidopsis thaliana, Medicago truncatula and Oryza sativa. We will consider how the implementation of the ABA signaling module might be a target of natural selection, to help contribute to the diversity of root architecture in nature.

Keywords: abscisic acid (ABA); development; developmental plasticity; lateral root; meristem; root; root architecture.

Figure 1

Regulation of lateral root development by ABA in Arabidopsis and Medicago. Key control points common to both species are initiation and meristem activation. Emergence from the primary root is an additional control point in Medicago. ABA stimulates initiation in both Arabidopsis and Medicago [55,56], but high concentrations of ABA inhibit it [56]. Salt stress inhibits lateral root emergence by stimulating ABA signaling, which induces expression of HB1, which in turn represses LBD1, required for emergence [56]. Lower levels of ABA stimulate lateral root emergence, but it is not known what functions downstream of ABA in this process [56]. Meristem activation is a key control point in both species, with systemic nitrate signaling via the classic ABA biosynthetic pathway in Arabidopsis and requiring ABI4 and ABI5, but not other ABA signaling genes [54]. ABA treatment in the absence of an environmental nitrate signal also regulates meristem activation, but does not require activity of any of the known ABA signaling genes [57]. In M. truncatula, this step requires activity of the MtLATD/NIP (MtNPF1.7) gene [17,58]. ABA treatment can bypass the requirement for MtLATD/NIP, inducing meristem activation in the absence of gene function [17]. Elongation of the lateral root subsequent to meristem activation is regulated by localized nitrate in Arabidopsis and can be repressed by ABA signaling [54].

Figure 2

Interactions between ABA and auxin signaling pathways during elaboration of the Arabidopsis root system. This diagram is restricted to genes mentioned in the text and does not include all ABA or auxin signaling genes. Downward arrows do not imply a simple linear pathway, but are rather used to group genes according to function. Crosstalk between the ABA and auxin signaling pathways are indicated by arrows, signifying a positive interaction, and blocked lines, signifying a negative interaction. When evidence for crosstalk is based on treatment with exogenous ABA or IAA, and a specific gene mediating this activity is unknown, the arrows originate from the hormone name, rather than a gene name. Orange arrows indicate interactions originating from the ABA pathway. Blue arrows indicate connections originating from the Auxin pathway. The red arrow indicates a connection between ABA and Auxin signaling components that may be independent of their function in ABA or Auxin signaling. ABI2 regulates the nitrate transport function of AtNRT1.1/AtNPF6.3, but its effect on auxin transport is unknown [97]. AtRAC7/ROP9 is not included in this diagram, although it is clearly involved in ABA/Auxin crosstalk, because its role connecting these pathways may be context dependent [109,110].