Doctorate thesis of Montpellier University

Monday may 16 at 2h30pm, Amphi Philippe Lamour

Adaptive responses of maize (Zea mays) roots to water deficit

Doctoral school : GAIA – Biodiversité, Agriculture, Alimentation, Environnement, Terre, Eau
Spéciality : BIDAP – Biologie, Interactions, Diversité Adaptative des Plantes
University : Université Montpellier
Reasearch unit : IPSiM –  Institut des Sciences des Plantes de Montpellier

Team: AQUA

Jury :

M. Xavier Draye, Professeur, Université catholique de Louvain, Louvain-la-Neuve (Belgique)  Rapporteur
M.Gregory Gambetta, Professeur, Bordeaux-Sciences Agro, Bordeaux                                    Rapporteur
Mme. Marie-Beatrice Bogeat-Triboulot,Chargée de recherche INRAE, Nancy                          Examinatrice
Mme. Marion Prudent, Chargée de recherche INRAE, Dijon                                                     Examinatrice
M. François Tardieu, Directeur de recherche INRAE, Montpellier                                              Examinateur
M. Christophe Maurel, Directeur de recherche CNRS, Montpellier                                            Directeur de thèse

Abstract :

Water availability is the most limiting factor affecting plant growth and crop productivity worldwide. Understanding and controlling the mechanisms that underlie plant responses to drought has become essential for agriculture in the context of climate change. The root water uptake capacity is governed by the root system architecture (RSA), which is determined by root growth and branching, and the root hydraulic conductivity (Lpr). These two parameters shape the root hydraulic architecture.
Maize (Zea mays) develops a fibrous root system composed of embryonic roots (primary -PR- and seminal -SR- roots) and shoot-borne roots, all of which bear lateral roots (LR). Here, we investigated maize root responses to water deficit (WD) and focused on three major aspects: hydraulics, growth, and signaling. We deliberately targeted embryonic roots and their LRs in 11 to 14-d-old plants of a hybrid genotype (B73H) grown in hydroponics. To extensively study the RSA and Lpr responses of individual roots, WD was induced by adding polyethylene glycol (PEG-8000) to the root solution. To mimic heterogeneous WD, we used a split-root system (Sp.) which allows one-half of the root system to be in well-watered (WW) conditions whereas the other half is under WD.
Morphological and functional analyses revealed precise differences between PR and SR responses. Under homogeneous WD, different behaviors were also observed according to stress intensity. After 1h of moderate WD (150 g L-1 PEG), Lpr was inhibited in both PR and SR. Lpr reduction was markedly reversible with a recovery within 1h in WW conditions, totally in the case of SR and partially for PR. PR may not able to completely reverse their Lpr due to an increased exodermis suberization of their LR. In control conditions, the PIP aquaporin genes were more expressed in the LR of PR than in those of SR. A long-term WD treatment resulted in a reduction of PIP expression in the former but not the latter. By contrast, the WD induced a reduced PIP expression in the axial tips of both PR and SR. In addition, the WD induced in PR and SR a general RSA reduction. Under a light WD (50 g L-1 PEG), PR maintained their Lpr and axial growth at the expense of LR elongation, whereas SR reduced their Lpr and axial growth without affecting LR growth. Thus, our data reveal a higher Lpr plasticity in SR which explore the most variable soil portions regarding water content. In contrast, the PR seem to be more specialized to exploring deeper soil layers that dry more gradually and retain water reserves for longer time.
The Lpr and RSA were integrated in a new mathematical model, which describes water and solute transport in roots under WW and WD conditions. Inversion of this model was used to estimate the axial conductance and radial conductivity of PR and SR in these two conditions.
Heterogeneous WD was found to trigger a compensatory LR growth mechanism in the WW root half. Conversely, an over inhibition of both RSA and Lpr was observed in the water deprived root half. These responses depend, however, on the severity and, possibly, the duration of the WD. We propose that these compensatory mechanisms take place thanks to the contribution of local and long distance signalling, which remain essentially unknown. Indeed, we quantified the hormones present in the shoot and root halves of plants grown for 5 d under split-root conditions but did not detect any systemic effect of water availability on their root accumulation.
In conclusion, our work shows that the plastic response of maize roots to homogeneous and heterogeneous WD involves both RSA and Lpr. The gradual water availability along soil depth shapes the specialization of different embryonic root types in order to optimize water uptake in WW and WD conditions. Finally, it is expected that the results provided by this research are not restrained to maize, but can be extended to other cereals.

Key words: water deficit, maize, root, hydraulic, architecture, local and long-distance signaling