Plant organ morphogenesis : geometry calls for biochemistry and mechanics

Changing shape is changing structure. By definition, this involves the laws of mechanics. Not only do plant generate diverse shapes, but shape itself can serve as an instructive cue to channel morphogenesis, through biochemical and mechanical signaling.

New scientific questions : robustness, multiscale hierarchy, emergent properties

How do organs know when to stop growing ? How do multicellular organisms produce their diverse shapes ? The underlying process, morphogenesis, is complex and characterised by three remarkable features : robustness, hierarchy, and emergence. Morphogenesis robustly produces highly similar forms across a wide range of conditions, despite and sometimes thanks to high levels of local variability. Morphogenesis involves events on a hierarchy of different scales in time and space (from the molecular to the organ scale). Individual factors contributing to morphogenetic events have been studied extensively, but are insufficient to fully predict morphogenesis – morphogenesis has emergent properties. Therefore, a central and challenging question in biology is how different factors are integrated and coordinated across multiple spatio-temporal scales to robustly produce organ shapes.

1. Cell Edges and Morphogenesis

Plants face a particular challenge when forming new organs : their cells share a rigid cell wall, which requires adjacent cells to coordinate their growth.

Growth patterns in Arabidopsis lateral roots

Directional growth depends on differences in cell wall mechanical properties in different regions of the cell. To establish and modulate these, cells precisely coordinate the transport of different cell wall components and their associated biosynthetic machinery to different regions of the cell surface.

Cell edges are a biochemically distinct domain in plant cells. Cell wall biosynthesis involves multiple trafficking pathways to the cell surface, which are regulated by Rab GTPases. RAB-A5c mediates a trafficking pathway specifically to cell edges in Arabidopsis lateral roots.

Our team is particularly interested in cell edges, the geometric domain at the intersection of two cell faces. Our previous work has shown that plants specify a transport route to cell edges which required for directional growth, but does not act through oriented cellulose deposition, the leading paradigm for directional growth control.

In our current work funded by ERC Starting Grant EDGE-CAM, we try to understand why cell edges are important during morphogenesis, and how plant cells specify their edges as distinct domains.

Why cell edges are important during morphogenesis ? Cell edges are notable domains from a topological, geometric, and mechanical perspective. With respect to mechanics, they accumulate stresses arising at the cell level through turgor pressure, and are also exposed to shear stresses arising through differential growth at the tissue scale. We hypothesise that plants specify a cell wall sensing module at cell edges through which they can monitor the mechanical status at the cell wall and adapt growth through fine-tuning trafficking pathways. We are combining forward genetics, proteomics, quantitative imaging, and computational modelling to test our hypothesis and identify new components of edge-based growth control.

A multi-scale model for robust morphogenesis. [1] An RLP-based cell wall sensing module at cell edges senses cell wall changes associated with tissue-level mechanics. [2] Downstream targets of the cell wall sensing module include endomembrane trafficking pathways involved in cell wall assembly. [3] Activity of these pathways determines cellular growth patterns and tissue-level mechanical constraints.

How plant cells specify their edges as distinct domains ? Protein localisation to cell edges has been linked to cytoskeleton organisation and the status of the cell wall, however it is an open question how plant cells identify their edges or differentiate between different edges in the same cell. We are exploring mechanical, geometric, and biochemical factors that may contribute to the recruitment of proteins to cell edges using quantitative imaging and genetics in planta and in an in vitro single-cell system.

Associated team members :
Charlotte Kirchhelle
Antoine Chevallier
Liam Elliott
Nathan German
Zoe Nemec Venza
Claire Lionnet
Marjolaine Martin

2. Multiscale microtubule response to stress

In past work, we showed that cortical microtubules align along maximal tensile stress directions, thereby reinforcing cell walls through the guidance of cellulose deposition.

Cortical microtubules also guide the orientation of the next division plane (through the preprophase band). Combining modeling and experiments, we formally showed that tensile stress prescribes cell division plane orientation. We also unraveled a role of microtubule dynamics and response to stress in organ initiation in organ shape robustness, in organ growth arrest and in organ flatness.

Confined protoplasts in microwells

We are now investigating the possibility that microtubules may be their own mechanosensors, notably using in vitro and microfabrication techniques, such as confined protoplasts in microwells.

In planta, this response may have important implications, relating mechanical conflicts due to differential growth to morphogenesis.

CreLox lines inducing growth mosaics and artificial mechanical conflicts at the shoot apex

Associated team members :
Olivier Hamant
Annalisa Bellandi
Antoine Chevallier
Charlotte Kirchhelle
Claire Lionnet
Marjolaine Martin
Isaty Melogno
Mariana Romeiro Motta

3. Mechanical identity

Beyond the cell cortex, we started to analyze the impact of mechanical signals on gene expression. We found that the expression of key transcription factors (CUC3 and STM) is in part under mechanical control.

STM expression in meristem boundaries, stress hotspots (Landrein et al., 2015)

The nexus between mechanical signals at the cell cortex and gene expression may involve different pathways. Recently, we started to analyze the role of nucleus deformation in gene expression.

We are also investighating the role of the RNA Polymerase-associated factor 1 complex (Paf1c), a central regulator of transcription, in development, at the nexus between transcriptional noise and developmental robustness. We found that Paf1c-dependent transcription contributes to the robustness of phyllotaxis and flower termination. Although at this stage it remains difficult to link mechanosensing with these phenotypes, these results open the possibility for crosstalks between transcriptional noise, mechanotransduction and development.

Severe indetermincay defects in the Paf1c mutant

Associated team members :
Olivier Hamant & Christophe Tréhin
Johanna Dickmann
Denise Arico
Marianne Lang
Claire Lionnet
Marjolaine Martin

4. Science and society

We are also involved in science-society projects, notably on the environmental question sustainability and in several art-science projects. This notably includes the question of robustness.

The case of pollard trees or trunks, of which all the upper and lateral branches are pruned, illustrates the remarkable plant plasticity. The trunks indeed develop a characteristic phenotype with an increased trunk thickness and massive repetitions of shoots. Since the Neolithic, this rural practice has been an essential source of renewable wood and fodder, before declining with the advent of the modern era. Conventional agriculture is increasingly under the spotlight for its negative externalities (greenhouse gas emissions,
pollution, dependence on fossil fuels, loss of biodiversity, desertification ...) and for its low resilience. From this perspective, the pollards find a new agronomic relevance.

Pollard tree drawing

Last, we are also investigating the potential use of plant cell walls as metamaterials, notably their phononic properties. There are numerous examples of animals that manipulate visible light for functional purposes, such as insect wings made of hexagon arrays of cone-shaped nanopillars that provide a graded refractive index for camouflage. Such photonic structures can be mimicked to find innovative solutions for light control in nano- to microstructures. In contrast, there has been no observation of phonon-based biological functions at the supramolecular scale, making phononic bioinspired strategies inoperative. Thomas Dehoux team has recently demonstrated that biological composites in the form of decellularized plant cell scaffolds can behave as phononic materials, including forbidding the propagation of elastic waves in select frequency ranges (i.e. band gaps). This opens the possibility for biobased phononic materials (BPM).

Propagation of an acoustic wave in an onion epidermal peel (T. Dehoux team)

Associated team members :
Olivier Hamant
Sana Dieudonné
Marianne Lang

To explore more science & society questions, visit the Michel Serres Institute website : https://institutmichelserres.fr/