The physics of plant morphogenesis: dynamics and mechanics of the plant cell wall in the meristem – MechaStem

We have developed approaches to measure the mechanical properties of the cells at the shoot apical meristem (a group of dividing cells that is controlling the production of all above ground organs) and we have correlated these measurements to gene expression and the biochemical modifications of tissues. In parallel, growth and morphogenesis were analyzed by confocal microscopy, to link gene expression, mechanical properties and growth altogether. To do so, we have adapted atomic force microscopy (AFM) to measure local properties on living tissues (2 application notes in collaboration with Bruker), we have developed new protocols to interpret the AFM output (scripts Matlab, modeling) and we have developed new quantification tools to measure tissue geometry (curvature, strain rate) and the behavior of intracellular actors (gene expression, cytoskeleton behavior). Last, we have generated statistical tools to correlate the data with one another (e.g. curvature, stiffness and gene expression) and to formalize the results in predictive models.

We have generated a map of the mechanical properties of the meristem at the very local scale (subcellular) and more global scale (tissue). A relation between the hormone auxin, the biochemical modification of the cell wall, the mechanical properties of the tissue and organogenesis at the meristem has been established. We have identified the mechanical identity of stem cells within the meristem. We have shown that the dynamics of the cytoskeleton provides the cells with a competence to respond to mechanical properties, to control growth and morphogenesis. Last, we have established a chain of causal events between these local molecular and mechanical features and the global architecture of the plant. In parallel to the publication of research articles in top journals, all the protocols that were developed in the frame of this project have been disseminated in the form of methodological articles and application notes. In the footsteps of this project, three ERC grants have been obtained (5 millions euros in total) to further explore the many prospects of this project.

We expect that the results of this project will mainly result in publications in high ranking
peer reviewed journals given the interdisciplinary approach we propose, and the essential
biological questions and modelling challenges we will address:
- Biology: Linking mechanical properties of tissues to specific morphogenetic
events in a stem cell niche
- Biophysics: Quantification of the mechanical properties leading to complex
morphologies in growing systems
- Complex system analysis: Integration of quantified mechanical properties and
genetics to understand the mechanical basis of morphogenesis.

Tissue mechanics and architecture:
Peaucelle et al., 2011 Development, Landrein et al., 2013 Curr. Biol.

Tissue mechanics and organogenesis:
Peaucelle et al., 2011 Curr. Biol., Braybrook et al., 2013 PlosOne

Cell mechanics and gene expression:
Milani et al., 2014 Plant Physiol., Landrein et al., submitted

Local cell wall mechanics and cell morphogenesis :
Sampathkumar et al., 2014 eLife

Mechanics of cells in culture (comparison between plant and animal cells):
Durand et al., 2014 Biophys. J. ; Digiuni et al., submitted

Cytoskeleton response to mechanical stress
Uyttewaal et al., 2012 Cell, Alim et al., 2013 Front. Plant Sci., Burian et al., 2013 J. Exp. Bot.

Physics/Mathematics of indentation :
Vella et al., 2011 Phys Rev Lett.

Protocols:
Milani et al., 2011 Plant J ; Boudaoud et al., 2014 Nat. Protocols, Peaucelle, 2014 JoVE ; Hamant et al., 2014 Meth. Mol. Biol., Milani et al., 2013 Bruker application notes

Reviews :
Mirabet et al., 2011 Ann. Rev. Plant Biol., Bringmann et al., 2012 Trends in Plant Sci., Asnacios and Hamant, 2012 Trend Cell Biol., Landrein and Hamant, 2013 Plant J., Milani et al., 2013 J. Exp. Bot., Robinson et al., 2013 J. Exp. Bot

The role of physical forces in morphogenesis is widely described as an essential aspect of developmental biology. Nevertheless the contribution of mechanics in shaping organisms is still largely unknown. Building on recent advances from the participants of the project, we will re-examine this issue exploiting new live imaging technologies combined with genetic and micromechanical (atomic force microscopy, AFM) approaches. Hereby we will concentrate on the shoot apical meristem in higher plants, a population of stem cells which continuously initiates aerial organs and, therefore, is a major determinant of plant architecture. Using AFM, we will measure and alter the mechanical properties of the meristematic tissue and associate these properties to the gene network controlling cell wall synthesis and dynamics, thus linking gene and shape via the mechanical properties of the cells.