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Plants in search of water: physiological and molecular interplay between root hydraulics and architecture during drought stress

Periodic Reporting for period 1 - DROUGHTROOT (Plants in search of water: physiological and molecular interplay between root hydraulics and architecture during drought stress)

Reporting period: 2016-03-15 to 2018-03-14

In view of global change and strong water demand from intensive agriculture, water deficit is now recognized as the abiotic stress that affects the most crop productivity and quality. Several Mediterranean regions are already under severe risk of drought, extreme temperatures, and other types of abiotic stress linked to water availability (e.g. flooding or salinity). Thus, understanding how plants use water for optimal biomass production has become a fundamental issue worldwide and, particularly, in Europe.
Plants are sessile organisms that cannot escape from environmental constraints and, as a result, have evolved numerous adaptive responses at molecular, cellular and physiological levels to cope with environmental stresses. Plants first respond to water deficit by stomatal closure together with rapid changes in root water permeability. On the long term, plants adjust their root growth to optimize their capacity to take up soil water.
Water uptake by roots is first determined by their architecture, which results from root growth and branching and underlies root ability to explore the soil. A second crucial component is the hydraulics of the root cells and tissues, that is, their intrinsic permeability to water. The cell hydraulics is determined by water channel proteins named aquaporins which facilitate water transport across cell membranes. Water deficit exerts deep effects on both root cellular hydraulics and root architecture. These effects are central for the plant’s adaptation to its environment allowing optimization of water uptake under developing drought conditions. Even though some molecular mechanisms have been recently described, there is no understanding of their integration at whole root level. This question was central throughout the DROUGHTROOT project.
The main aim of DROUGHTROOT was to understand how water deficit alters the ability of the plant root system to acquire water, by considering effects on both root hydraulics and root growth and development. These effects were addressed from an elementary developmental process, lateral root (LR) formation, up to whole root architecture. The molecular and cellular mechanisms involved were investigated in the frame of two specific questions:
- How do water availability, hormones such auxin and abscisic acid (ABA), and aquaporins interact during LR development?
- How effects of hormones on root growth and hydraulics are integrated in the whole root under water deficit conditions?
Objective 1. Interplay between hormones and aquaporins during LR formation under water deficit
To better understand the response of LR development to water deficit, we investigated the dose-dependent effects of solutes such as polyethylene glycol (PEG) on Arabidopsis thaliana seedlings in vitro. Root architecture was characterized with respect to primary root length, number and length of LRs, and development of LR primordia. Under our experimental conditions, low PEG concentrations stimulated the LR initiation and emergence, whereas higher PEG concentrations induced an osmotic stress triggering an inhibition of the LR development. The roles of auxin and ABA during water deficit were explored by monitoring hormone concentrations in roots by means of reporter constructs encoding fluorescent proteins. The impact of both exogenous ABA and water deficit on LR formation in ABA- and auxin-related mutants was investigated. Overall, it was found that that ABA participates in the LR developmental responses to water deficit by interacting with auxin.
Objective 2. To study the effects of water deficit and ABA on root hydraulic architecture
To study the emerging properties of whole root growth responses to water deficit, we used a combination of high throughput phenotyping techniques for root hydraulics (pressure chamber techniques) and root growth which were applied to fully grown Arabidopsis plants cultivated in hydroponics. These approaches were used to investigate root systems in a large set of water deficit and exogenous ABA treatments and genotypes. In particular, dose-dependent effects of PEG showed a stimulatory effect of mild and moderate water deficit on root hydraulics and architecture, whereas these two traits were drastically inhibited under severe water deficit. To explore the role of ABA and auxin on the developmental and hydraulic responses to water deficit, several ABA- and auxin-related genotypes were investigated. In brief, our results revealed that ABA mediates most effects of water deficit on root water uptake capacity, which were dependent on both root architecture and aquaporin activity. Results obtained from this project will be used to develop a model of root hydraulic architecture responses to water limitation.
Results obtained within DROUGHTROOT have been gathered in two research articles which correspond to the two main project’s objectives and are about to be submitted. Dr Rosales also disseminated the outlines and partial results of the project by oral communications and posters in several international conferences and seminars. Furthermore, DROUGHTROOT allowed funding the 3-month internship of a pre-graduated student from the University of Montpellier. Finally, Dr Rosales participated in teaching activities, within the CultiVar program organized by CIRAD and University of Montpellier, and Science vulgarization during the Science Festival 2017 organized by the host department.
This work uncovered a key mechanism that underlies the dual response of plant roots to water deficit. It showed how water deficit induces changes in both root development and aquaporin activity, to adjust water uptake by roots to unfavourable conditions. Furthermore, the use of exogenous hormone treatments and mutants identified ABA as a central regulator and integrator of the water deficit responses in Arabidopsis.
The DROUGHTROOT project addressed an emerging problem in the European agriculture, due to the threats of global change on both plant growth and production of plant foods. We believe that reducing water needs through plant science technologies is feasible. Besides improved water management techniques, crops with improved water use efficiency may be developed. In particular, root functions offer wide opportunities for improving crop performance under environmental stress but their potentialities have not been fully explored. In this context, DROUGHTROOT developed a unique combination of approaches in the model plant Arabidopsis thaliana to enhance our fundamental knowledge of root growth and water transport at two different levels. Firstly, we worked at the tissue level and analysed in detail the physiological and molecular responses involved in LR development. Secondly, we integrated these responses at the whole plant level in the unifying concept of root hydraulic architecture. Another innovative feature of the project was undoubtedly its multiscale and multi-disciplinary dimension: the proposed approach integrated physiological, biochemical and molecular responses to study hormonal mechanisms regulating root hydraulic architecture and its involvement in water deficit tolerance. In summary, DROUGHTROOT has allowed to enhance scientific knowledge on topics of crucial socio-economic importance. It will help in the search for root traits to improve the yield and quality of crops under adverse environmental conditions.
Scheme of the DROUGHTROOT objectives