Laboratoire Matière et Systèmes Complexes

Soutenance d’HDR d’Alexis Peaucelle le jeudi 17 septembre 2015 à 14h.

Attention : la présentation aura lieu à Orsay, rue Noetzlin, Bâtiment 630, partie enseignement.

Plants morphogenesis : the pectin methylation pathway from gene to biomechanics.

Plants are strikingly good at maths, especially geometry. One could find parts or whole plants shaped as spheres, circles, straight lines, and flat surfaces, golden and right angles and all sorts of exotic and pretty combination of shapes. These shapes are generated throw complex tissue growth. We want to understand this puzzling beauty by understanding how a biological tissue is capable of growing. The cell walls block cell movement and cell rearrangement, thus morphogenesis in plants is based on oriented local tissue growth. This process is driven by cell wall expansion, witch is a biophysical prosess per nature. In the past 7 years we focused on describing the chemical and mechanical modification of the cell wall that is associated with growth and its hormonal and genetic regulation. First we developed an Atomic Force Microscopy (AFM) based technique to measure mechanical properties of cell wall at the tissue and cell level in developing tissue. To our amazement, changes in cell wall elasticity measured using compressive forces turn out to be not only predictive of all tissue growth but also of sub-cell expansion in tissue. In parallel, we stumbled on a change in cell wall chemistry (the demethylation of pectin) that is correlated with the change in cell elasticity associated with growth. We demonstrate that both at the tissue level in the meristem but also at the cellular level in the hypocotyl, this change in cell wall chemistry is necessary and sufficient for morphogenesis and for the change in elasticity it is associated with. Finally, we unravelled a very small part of the genetic and hormonal regulation of this process. Focusing on the gene coding for the pectin methyl esterase 5 : PME5, we manage to describe the dual regulation by the transcription factor BELL RINGER. We also demonstrate that auxin promotion of organ growth requires that it increases cell wall elasticity by demethyl esterification of pectin. In a nutshell, we revealed that pectin methyl esterification status changes is necessary for cell and tissue differentiation, growth and its related changes in cell wall elasticity. This work confirms the strong mechanical control of tissue growth. The next step is to pinpoint in time and space how the biological (cell wall synthesis) and the mechanical control of growth interact. In the future, I hope to investigate this process and help to solve paradoxes within the actual paradigms such as cell wall elasticity correlation to growth, microtubule partial correlation with oriented growth, and sound-induced plant growth.

La présentation sera précédée, à 11h, d’une présentation de Elliot Meyerowitz (Meyerowitz Laboratory Howard Hughes Medical Institute and California Institute of Technology), grand spécialiste de la génétique à la fois animale et végétale pionnier dans l’intégration des signaux mécaniques dans la morphogenèse des plantes.

Il nous présentera le travail de son équipe :

Pattern Formation at the Arabidopsis Shoot Apex

The shoot apical meristem uses multiple modes of intercellular signaling to control its size, maintenance, and the patterning of the organs and tissues that derive from it. A peptide signaling system involving the secreted peptide CLAVATA3 and its receptors, the transmembrane kinases CLAVATA1 and BARELY ANY MERISTEM 1, 2 and 3 control meristem size, and maintenance of the stem cell populations. The CLAVATA system is in turn regulated by diffusible hormones of the cytokinin class, which are regulated by synthesis, degradation, diffusion, and through the activities of multiple receptors whose levels are themselves controlled to some degree by cytokinins. Auxins control the pattern in which leaves and flowers are made in the meristem periphery, by modulation of its transporter, and therefore of the directions of auxin transport, as well as by synthesis and degradation. Altogether, therefore, the activities of the meristem are regulated by interlocking feedback ! systems involving at least three classes of hormones. To understand such a system, and to test hypotheses regarding it, requires detailed computational models – as intuition fails in systems with so many feedbacks.