Plant organ morphogenesis : geometry calls for biochemistry and mechanics
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 including 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 genetic, biochemical, and mechanical factors are integrated and coordinated across multiple spatio-temporal scales to robustly produce organ shapes. Our team aims to understand this question in the context of three research axes: the role of cell edges in development, multiscale microtubule response to stress, and establishment of mechanoidentity.
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. 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. 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, 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.
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.
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.
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.
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. 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 investigating 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.
Publications
Trinh, Duy-Chi, et al. “Increased gene expression variability hinders the formation of regional mechanical conflicts leading to reduced organ shape robustness.” Proceedings of the National Academy of Sciences 120.30 (2023): e2302441120.
Colin, Leia, et al. “Cortical tension overrides geometrical cues to orient microtubules in confined protoplasts.” Proceedings of the National Academy of Sciences 117.51 (2020): 32731-32738.
Takatani, Shogo, et al. “Microtubule response to tensile stress is curbed by NEK6 to buffer growth variation in the Arabidopsis hypocotyl.” Current Biology 30.8 (2020): 1491-1503.
Kirchhelle, Charlotte, et al. “Two mechanisms regulate directional cell growth in Arabidopsis lateral roots.” Elife 8 (2019): e47988.
Kirchhelle, Charlotte, et al. “The specification of geometric edges by a plant Rab GTPase is an essential cell-patterning principle during organogenesis in Arabidopsis.” Developmental cell 36.4 (2016): 386-400.
Hamant, Olivier, et al. “Developmental patterning by mechanical signals in Arabidopsis.” science 322.5908 (2008): 1650-1655.
