During their development, plants must constantly adjust the level of water they contain in response to changing environmental conditions. Roots play a vital role in this process by exploring the soil environment and taking up water. Drought can majorly impact root function by altering cell water permeability (hydraulics) and influencing the growth and architecture of the root system. Water channel proteins named aquaporins adjust root hydraulics in response to many stimuli, including drought stress. Plant hormones (also known as phytohormones) like auxin and abscisic acid (ABA), are important in root growth and development by regulating aquaporins during lateral root formation (LRF). The EU-funded Horizon 2020 DROUGHTROOT project integrated these plant adaptive responses to drought by exploring functional links between root architecture, aquaporins and hydraulics, and phytohormones. The goal was to reach a comprehensive understanding of how plant roots optimise water uptake under drought conditions. Use of plant model Researchers used the model plant Arabidopsis thaliana to study plant responses to water stress from the elementary level of LRF up to the whole root level to identify complex interactions and signaling pathways. “The study of Arabidopsis roots involves a unique combination of developmental biology, genomics, biophysics and mathematical modelling, with the aim of transferring this knowledge to crops,” says project coordinator Christophe Maurel. Scientists applied a series of water stress conditions ranging from mild to severe to investigate the relationship between the architecture of the root system and its water transport activity. Results revealed a dual response of roots to water deficit. “The first involved a stimulatory effect on lateral root development, root architecture and hydraulics, which can be observed under mild water stress. In contrast, higher levels of water stress resulted in general inhibitory effects,” claims Maurel. ABA demonstrated a positive and repressive effect on both root growth and hydraulics depending on its concentration. Therefore, they showed that ABA tightly coordinates root responses to water deficit, to optimise water uptake. “Specifically, we observed that roots exhibit distinct sensitivities to water deficit according to their age and rank,” Maurel explains. Data applied to model DROUGHTROOT provided plant physiologists and mathematical modellers with huge amounts of data that will be applied to a previously developed functional structural model of root water uptake. “The model will help to predict the behaviour of plants affected either in their root architecture or in hydraulic parameters and highlight key elements or parameters for biological testing,” Maurel observes. In the long term, this knowledge will create opportunities for plant breeders by providing tools and proof-of-concept for targeting ABA-dependent signaling pathways that coordinate root responses to water deficit. A better understanding of plant water acquisition strategies will also help optimise irrigation procedures. Overall, these advances will contribute to food safety and sustainable agricultural practices under reduced water availability due to a changing climate.
DROUGHTROOT, water, plant, drought, roots, abscisic acid (ABA), aquaporin, lateral root formation (LRF), auxin