ROOSTOCK : Plant root development affected by soil mechanical stresses
Description of the PhD project
In the context of climate change mitigation, the ‘4 per 1000 Soils for Food Security and Climate’ initiative launched during the COP21 states that an increase in global soil organic matter stocks in agricultural soils by +0.4% per year could offset 20-35% of anthropogenic greenhouse gas emissions. Thanks to photosynthesis, plants are one of the most efficient system to transfer C from atmosphere to soils and to capture C through biomass increase via the development of plant roots. However reliable estimations of root C inputs to soil are currently lacking, partly because of the difficulty in quantifying and predicting the root system extent in opaque and heterogeneous soils. Responsiveness to change in environmental cues, called developmental plasticity, is markedly large for roots. Root system development is highly modulated by biological (competition and symbiosis with other organisms), chemical (nutrient availability, oxygen supply, pH…) and physical properties of soils. The architecture of the root system, that is its three-dimensional shape, largely derives from the distribution and diversity of individual root apical meristems, which continuously sense and adjust their growth according to their local environment. In particular, root growth is affected by the mechanical strength of the soil and by the presence of obstacles which scale match or exceed that of the root. As a consequence, the presence of zones of high mechanical resistance in the soil is one of the most common physical limitations to soil exploration by roots, with direct consequences on the C sequestration but also on the agricultural yield. In contrast to aerial organs, roots apices must exert a growth pressure to penetrate strong soils and reorient their trajectory to cope with obstacles like stones or to follow the tortuous paths of the soil porosity. When the soil becomes too compact due for instance to droughts, the mechanical strength of the soil increases and the distribution of poral network changes, resulting in growth arrest.
In this interdisciplinary context, we have developped at PMMH experimental model systems to study the biophysical mechanisms underlying root responses to obstacles. The model systems include single or a collection of obstacles mimicking the mechanical heterogeneities in a soil. In particular, we investigate the growth response of a root pushing against a single obstacle such as a force sensor or growing inside a 3D-printed array of stiff obstacles. By coupling force and growth kinematics measurements under infra-red lighting, we probed the force-growth relationship of a primary root contacting a stiff resisting obstacle.
The PhD-project aims at identifying the biophysical laws governing the spatial extent of a root system in an heterogeneous soil. A first approach is to investigate the response of the root to prescribed compression steps with controlled force to study its time-dependent mechanical properties and to decouple the mechanical responses from the biological feedbacks. Another approach is to use microfluidic systems to build arrays of granular substrates of soft hydrogel to vary the stiffness of obstacles encountered by roots as well as controllig the osmolarity of the external medium.
Keywords
Biomechanics, plant roots, C storage, mechanical stresses, morphogenesis
Research Unit, UMR number and acronym
ESPCI – Physique et Mécanique des Milieux Hétérogènes (PMMH), UMR7636
Description of the research Unit/subunit
The PMMH laboratory is organized around three main research areas. The “Root-Soil Interaction” team falls under the “Physical Mechanics and Statistical Physics” research area, which focuses on slender structures, disordered solids such as granular materials, growth phenomena, and the mechanical behavior of heterogeneous media. The overarching question is how materials respond to forces and deformations, combining approaches from hydrodynamics, soft matter, and mechanics. The team is also part of the cross-cutting topic “Physics of Living Systems,” which addresses fundamental questions in biophysics.
The “Root-Soil Interaction” team conducts experiments on real plant roots in specially designed, controlled substrates, but also works on biomimetic systems consisting of fibers in granular media, thereby mimicking the mechanical contribution of roots in a heterogeneous substrate.
Name of the supervisor
Evelyne KOLB (evelyne.kolb@sorbonne-universite.fr)
Name of the co-supervisor
José BICO (jose.bico@espci.fr)
3i Aspects of the proposal
Both of the thesis advisors have proven track records in doctoral supervision, interdisciplinary collaboration, and mentoring of early-career researchers. They are highly qualified experts in their field. E. Kolb has a long term of experience in research on granular materials and biomechanics of plant roots as well as in teaching to graduated and undergraduated students (soft matter, hydrodynamics, elasticity). She developed pioneering research on the growth of plant roots in model soils. .J. Bico is professor at ESPCI and has been rewarded by the 2025 APS fellowship for his « outstanding contributions to elasto-capillary phenomena, elastic instabilities, and shape-morphing materials that combine creative experiments and elegant modeling ». E.Kolb will be fully available for monitoring a new PhD thesis next year and will benefit from the internal collaboration with J. Bico.
Intersectoriality
The project on roots is part of the team’s broader research focus on the interactions between grains and fibers. E. Kolb is leading a work package (WP) as part of an ANR grant between SVI Saint-Gobain (P. Jop, project leader) and J.Y. Delenne (INRAE Montpellier) on the project titled CONFIG (Constrained fibers in granular media). This existing collaboration with Saint-Gobain could open up new opportunities for the future doctoral student.
International
The current project follows a previous PhD project on the effect of the structure of granular media on root growth (Jiaojiao YAO, 2025). It was conducted as part of an international collaboration with L.X. Dupuy’s group (Neiker Institute, Spanish Basque Country), through a joint supervision agreement. L.X. Dupuy is an Ikerbasque Research Professor working on Biophysics of plant-environment interaction and a member of a research consortium, Root2Res (Root phenotyping and genetic improvement for rotational crops resilient to environmental change). He was invited in PMMH through the funding of chaire Joliot from ESPCI. This collaboration will continue through this new PhD.
E. Kolb is also a member of an informal consortium of European researchers in soft and active matter, robotics, and civil engineering that submitted a proposal for a doctoral student network focused on fibers (passive, active or living) and grains. Although the ITN project was not funded, collaborations still continue.
Interdisciplinarity
Understanding root growth in a complex soil environment relies on an interdisciplinary approach. The thesis project involves experimental physicists from PMMH, biologists from INRAE and theoreticians from MSC in Paris. E. Kolb is accustomed to working at the interface between different scientific communities. She has previously published with soil scientists from IRD1 and biologists from the iEES laboratory2. She maintains an ongoing collaboration with ecophysiologists at INRAE Nancy, notably through joint publications and INRAE-funded projects. Together with colleagues from civil engineering, agronomy, and soil science, she coordinated and authored a book chapter on root-soil interactions in a book titled ”Soft Matter in Plants : From Biophysics to Biomimetics”3.
This PhD project fits within this highly interdisciplinary context. It will also benefit from regular meetings organized by the GDR PhyP4, as well as informal bibliographic meetings among groups of physicists and biologists working on roots (INRAE Nancy, Clermont, MSC, PMMH, I2BC Paris-Saclay).
1 IRD (Institut de Recherche Pour le Développement)
2 Institut d’Ecologie et des Sciences de l’Environnement de Paris
3 Chapter 5, Soft Matter in Plants : From Biophysics to Biomimetics, edited by K. Jansen and Y. Forterre and published by the Royal Society of Chemistry.
4 GDR Biophysique et Biomécanique des Plantes
Expected profile of the candidate
The candidate must have a primary background in physics or biophysics. She or he must have completed laboratory internships with a strong experimental component. Experience in soft matter, biophysics or biomechanics and image analysis is a bonus.
Important dates
Call for applications : from September 1st to October 31st 2026
Eligibility check results : November
3i Committee evaluation results : December
Interviews from the shortlisted candidates with the Selection Committee : January 2027
Start of the PhD : March 1st 2027