■ Biomechanics of the plant roots in granular soils

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Description of the PhD project

Background: In the context of global warming, the possibility of long-term C (carbon) sequestration in soils is a very promising issue that leads to the so-called 4 per 1000 Initiative during the COP21 conference: Even small increase in soil organic carbon can have large-scale effects both on agricultural productivity and on greenhouse gas balance. Thus, a deep understanding of the inputs to soil carbon stock and fluxes is needed and in this context, roots play a major contribution. However the growth and architecture of roots in soils at different depths are affected by many environmental and anthropogenic factors. One of these factors is the mechanical stress exerted by the soil at the root scale. Under non-stressful biological and chemical conditions, the root growth trajectory highly depends on the mechanical strength of the soil and on the presence of obstacles at the root scale. Then root apices must exert a growth pressure to overcome the resistance to deformation of the surrounding soil or reorient their growth to skirt around obstacles by mechanisms like buckling or active differential growth. In homogeneous soils, increase in soil strength is known to reduce root elongation and alter root diameters as well as the average number of lateral roots that stem from primary axes. However in heterogeneous soils like granular soils, the detailed mechanisms of root growth interacting with an assembly of aggregates and pores are not known and are related to very lively fundamental research areas like mechano-sensing and morphogenesis under mechanical stresses.
Objective: During this PhD, we propose to study the response in growth and morphology of a plant root apex interacting with its mechanical environment: i) with an individual obstacle, ii) with a collection of fixed posts or iii) with an assembly of mobile grains.
Research program: The complexity of the root-soil interaction will be first reduced to the elementary event (i) of a root apex encountering a single mechanical obstacle of adjustable stiffness to mimic the interaction of the root apex with a deformable aggregate in the soil. In a second approach (ii), posts of different shapes in random or regular networks with various spacing will be used to trigger the reorientation of root growth if not stopped. The rigidities of the posts as well as the geometry of the network will be controlled by techniques of 3D printing and microfluidics. By time-lapse photography and image analysis, the pattern of growth will provide information about the competing mechanisms of root growth reorientation due to the presence of obstacle (like thigmotropism or buckling) or due to gravitropism. In a third approach (iii), the root will grow inside a granular medium with various packing fractions. The root trajectory and changes in the growth process will be coupled to the amplitudes and extent of grain reorganizations for characterizing and modeling the root-soil feedback.

Keywords

Plant roots, biomechanics, morphogenesis, fluid-structure interaction, granular soil, agro-ecology

Research unit

UMR7636 Physics & Materials

Description of the research Unit/subunit

The theme “Bio-mechanics and plant root growth” is a part of the Research group “Mechanics and Statistical Physics” inside the laboratory PMMH (Physics and Mechanics of Heterogeneous Media) of the ESPCI. The PMMH laboratory is composed of many experimental teams working in close and active connection on subjects such as hydrodynamics, soft matter, biomechanics, biomimetism, granular materials and mechanical physics of beams and plates. The PMMH laboratory frequently interacts with colleagues from the SIMM laboratory of the ESPCI who are specialists of the mechanics of contacts between soft objects. The ESPCI also benefits from the neighborhood of the Institut Pierre Gilles de Gennes (IPGG), where all the facilities exist for developing micro-fluidic devices.

Name of the supervisor
Evelyne Kolb (evelyne.kolb (arobase) espci.fr)

3i Aspects of the proposal

The potential outcome of our research project is linked to many applied and ecological domains. The reduced development of the root system has repercussions on the accessibility of the plant to water and nutrients, which affects the shoot development and therefore the crop yields in agriculture. In ecology, the lack of deep roots limits the possibility of underground carbon pools. It has also direct consequences on the anchorage of the plant, limiting its resistance to lodging. In turn, soil mechanical properties are highly dependent on the root architecture, as the arming structure formed by roots traps the soil and increases its resistance to shear, reinforcing the stability of slopes or limiting the erosion at river banks. Understanding the root growth pattern from the individual physical interaction of the root apex with obstacles or soil patches of large resistance is a great challenge for modeling and poses a number of fundamental questions regarding sensing, transduction and adaptive response to mechanical stress. It is clearly an interdisciplinary project involving concepts from physics and biology. Concerning the root, this subject addresses questions regarding morphogenesis under mechanical stresses, mechano-transduction and acclimation. Concerning the soil, it involves problems of fluid-structure interaction, reorganization of granular media close to jamming and modifications of the porous networks induced by the root architecture. The project will be performed with the help of an international collaboration with Lionel Dupuy from the Ecological Sciences Group at The James Hutton Institute in Dundee, UK, Scotland. It will be carried out in close collaboration with biologists from the French Institute for Agronomic Research (INRA): M.B. Bogeat Triboulot and V. Legué with whom E. Kolb wrote a recent review [Kolb et al, 2017, Physical Biology]as well as other researchers from INRA, CIRAD and IRD, who developed scientific interaction on soil management with southern countries.

Expected Profile of the candidate

The project is suitable for a candidate with training in physics or related discipline. The candidate should have a strong interest in interdisciplinary research and application of physics to biological systems. The project is particularly well suited to a candidate with experimental skills in soft matter physics. The successful candidate will develop new experimental setups to study the mechanical interactions between plant roots and its environment. Experience with imaging and automation for time-lapse photography, image analysis, programming and computer control of instruments is desirable.

Important dates

Call for applications : from July 16th to September 17th 2018
Eligibility check results : Late September
3i Committee evaluation results : Late October
Interviews from the shortlisted candidates with the Selection Committee : Mid-December (week of December 10th)
Final results : Late December





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