Laboratoire Matière et Systèmes Complexes

Thèse de Mathieu Rivière effectuée sous la direction de Stéphane Douady, Alexis Peaucelle et Julien Derr.

Soutenance le lundi 20 novembre 2017 à 14h00.

Lieu : bâtiment Condorcet, Amphithéâtre Pierre-Gilles de Gennes (sous-sol). La soutenance sera suivie d’un pot au 6e étage.

Shape, rhythms and growth heterogeneities of a leaf

Abstract:

Plant growth, the irreversible volume increase of a cell or an organ, remains an exciting biological puzzle. Growth is essential for shape regulation of the developing organ. It is also the key for every plant, developing or mature, to adapt to its environment.

The cellular mechanisms of plant growth rely on a competition between the inner pressure of the cell and the elastic properties of the cell-wall. Heterogeneities of these mechanical properties across the tissue result in differential growth and eventually lead to growth motions at higher scales. These macroscopic motions can thus be read as an outward signal of the cellular mechanisms underlying shape regulation and growth itself. Here, we propose to take benefit from the intimate link between growth and its related motions to gain a new insight on growth through a multiscale approach, from the organ to the cell-wall level.

The compound leaves of Averrhoa carambola display two marked motions, unfurling and nutation, that are widespread among developing leaves. Our approach consists in assuming that different phases of the motion correspond to different growth status within the tissues of the organ.

First, we show that the shape of the leaf results from an active regulation, especially of its macroscopic mechanical properties. The kinematics of the two latter motions are then compared to the associated growth. Doing so, we put forward the specific patterns of growth and growth heterogeneities underlying the development of the leaf. Based on these findings, we build a kinematic model for nutation which---accordingly with previous findings---suggests the occurrence of local contractions during growth. The mechanics of the cell wall are then investigated. Our results suggest that spatial heterogeneities of cell wall rigidity within the leaf are correlated with the direction of the nutation. Finally, immunohistochemistry reveals heterogeneities in the repartition of de-methylesterified pectins within the leaf, possibly consistent with the direction of nutation.