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.

Nitrate transporters: an overview in legumes

“…As noted by Harris (2015), it is surprising that 10 lM ABA rescues the wild type phenotype either by increasing (for latd mutants) or decreasing (for nitrate-treated npf6.8 mutants) root length, suggesting contradictory results (Fig. 4).…”

“…Interestingly, root development defect of mutants affected in MtNPF1.7 (noted latd) can be rescued by ABA addition (Liang et al 2007), indicating a positive role of ABA in the establishment, or the maintenance, of root meristem function (Fig. 4) (Harris 2015). Since ABA levels in latd mutants are indistinguishable from those in wild type (Liang et al 2007), the defect in latd mutants is likely to be in ABA transport or signaling.…”

Environmental nitrate signals through abscisic acid in the root tip

“…Because the presence of nitrate is usually patchy within the soil, changing concentrations of nitrate can have potent effects on root architecture, locally promoting root growth within a nitrate patch, and systemically inhibiting lateral root elongation when overall levels of nitrate are high. 8 Although ABA is best known for controlling stomatal closure and seed dormancy, 13 it plays a major role in root development, regulating root elongation by modulating the major control points of root growth: cell division and cell elongation. 14,15 Additionally, ABA controls lateral root development by regulating initiation, emergence and meristem activation, as well as elongation (reviewed in 8 ).…”

“…8 Although ABA is best known for controlling stomatal closure and seed dormancy, 13 it plays a major role in root development, regulating root elongation by modulating the major control points of root growth: cell division and cell elongation. 14,15 Additionally, ABA controls lateral root development by regulating initiation, emergence and meristem activation, as well as elongation (reviewed in 8 ). Because ABA mediates responses to many environmental signals, its ability to regulate lateral root formation provides the plant with the flexibility to modulate root architecture in response to a constantly varying environment.…”

“…8,9 ABA mediates responses to a diverse set of environmental signals: drought, salt, cold, osmotic, nitrate, phosphate and light, as well as pathogens. [8][9][10][11][12] Having one hormone mediating responses to multiple environmental signals makes it possible to coordinate and optimize responses at a whole plant level. Based on the role of ABA in multiple environmentally-regulated plant developmental processes, it is interesting to speculate that ABA may play a key role in signal integration, optimizing plant responses to conflicting environmental inputs -a fruitful area for future research.…”