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Unravelling the physical basis of morphogenesis in plants

Final Report Summary - PHYMORPH (Unravelling the physical basis of morphogenesis in plants)

Morphogenesis is the remarkable process by which a developing organism acquires its shape. While molecular and genetic studies have been highly successful in explaining the cellular basis of development and the role of biochemical gradients in coordinating cell fate, understanding morphogenesis remains a central challenge for both biophysics and developmental biology. Indeed, shape is imposed by structural elements, so that an investigation of morphogenesis must address how these elements are controlled at the cell level, and how the mechanical properties of these elements lead to specific growth patterns. Using plants as model systems, we tackled the following questions:

i. Does the genetic identity of a cell correspond to a mechanical identity?
ii. Do the mechanical properties of the different cell domains predict shape changes?
iii. How does the intrinsic stochasticity of cell mechanics and cell growth lead to reproducible shapes?

In plants, shape is entirely determined by the extracellular matrix (cell walls) and inner hydrostatic pressure. From that perspective, plants cells involve fewer mechanical parameters than animal cells and are thus perfectly suited to study the physical basis of morphogenesis. Therefore we mostly worked on the shoot apical meristem of Arabidopsis thaliana, a small population of stem cells that orchestrates the aerial architecture of the plant.

We developed a unique combination of physical and biological approaches. For instance, we measured physical properties and growth of specific cell types, using a method that we developed to couple atomic force microscopy and confocal microscopy; we inferred cell wall mechanics and hydrostatic pressure from such measurements. Our main results are that plant stem cells are stiffer than neighbouring cells, which may contribute to quiescence, and that they have heterogeneous hydrostatic pressure, which contributes to generating heterogeneity in growth.

On the long term, our work will help unraveling the physical basis of morphogenesis and shed light on how stochastic cell behaviour can lead to robust shapes.