Date : 18/11/2011

Internship proposal for : Master 2

Laboratory 1
PMMH, ESPCI UMR 7636
CNRS, University Paris 6, University Paris 7
10 rue Vauquelin, 75231 Paris Cedex 5

Laboratory 2
BIOEMCO, ENS
46 rue d'Ulm, 75230 Paris cedex 05, France
Website http://rootmovies.jimdo.com/
Main discipline : physics
Lab director : Philippe Petitjeans

Mentor
Evelyne Kolb
email : This e-mail address is being protected from spam bots, you need JavaScript enabled to view it
phone : +33-1-40-79-58-04

Subjects
1.: morphogenesis
2.: root growth
3.: granular medium

Tools and methodologies
1.: photoelasticity
2.: image analysis
3.: Particle Image Velocimetry and Cell staining

Summary of lab's interests

The mechanical and topological properties of a soil like the global porosity and the distribution of void sizes greatly affect the development of a plant root, which in turn affects the shoot development. In particular, plant roots growing in heterogeneous medium like sandy soils or cracked substrates have to adapt their morphology and exert radial forces depending on the pore size in which they penetrate. Our team is interested in the coupling and retroaction between the root growth and the reorganisation of the soil, i.e. what are the forces a root is able to develop on its environment and how the mechanical stress affects the root growth (morphogenesis) ? This subject emerges from a multidisciplinary collaboration between Evelyne Kolb (PMMH, ESPCI), a physicist specialised in granular materials, Christian Hartmann (IRD), a soil scientist working on soil compaction and rehabilitation, and Patricia Genêt (BIOEMCO,ENS), a biologist specialised in mycorrhizae.

Summary of project

We propose a model experiment in which a pivot root (chick-pea seeds, Cicer arietinum L.) of millimetric diameter has to grow in the constriction between two grains in a two-dimensional cell. The gap between the grains has a well-controlled size and is varied from 0.5 to 2.5 mm. The grains delimiting the gap are photoelastic disks : when they are placed between circular polarizers, optical fringes appear which number is proportional to the compression force acting on the disks. By time-lapse imaging, we continuously monitor the root growth and the development of optical fringes in the photoelastic neighbouring grains when the root enters the gap. Thus we measure simultaneously and in situ the root morphological changes (like length and diameter growth rates, change of curvature, possible circumnutation) as well as the radial forces the root exerts at the level of the constriction. Preliminary measurements have shown that radial forces are increasing in relation with gap constriction and experiment duration but a levelling of the force was not observed, even after 5 days and for narrow gaps. The inferred mechanical stress (force divided by the contact area of the root with the grain) was consistent with the turgor pressure of compressed cells. Therefore our set-up could be a basis for testing mechanical models of cellular growth. During this lab training, we especially want to measure the forces exerted by the root for different locations of the constriction relatively to the root zones (elongation zone or differentiation zone) and for different ages of the root. We want to relate these macroscopic forces to microscopic observations of cell morphology. For this purpose we will use cell staining and post mortem observations to compute deformations of cell walls and change in the number of cell files. We will also develop fluorescent staining of cells for in-vivo observations.