Section: New Results

Signaling and transport for tissue patterning

Participants : Romain Azaïs, Guillaume Cerutti, Christophe Godin, Bruno Leggio, Jonathan Legrand, Teva Vernoux [External Collaborator] .

• Research Axes: RA1 (Representations of forms in silico) & RA2 (Data-driven models)

• Key Modelling Challenges: KMC3 (Realistic integrated digital models)

One central mechanism in the shaping of biological forms is the definition of regions with different genetic identities or physiological properties through bio-chemical processes operating at cellular level. Such patterning of the tissue is often controlled by the action of molecular signals for which active or passive transport mechanisms determine patterning spatial precision. The shoot apical meristem (SAM) of flowering plants is a remarkable example of such finely controlled system where the dynamic interplay between the hormone auxin and the polarization of efflux carriers PIN1 during growth governs the rhythmic patterning of organs, and the consequent emergence of phyllotaxis. Using Arabidopsis thaliana as a model system, we developed an integrated view of the meristem as a self-organizing dynamical form by reconstructing the dynamics of physiological processes from living tissues, and by proposing computational models integrating transport and signaling to study tissue patterning in silico.

Automatic quantification of auxin transport polarities. Time-lapse imaging of living SAM tissues marked with various fluorescent proteins allows monitoring the dynamics of cell-level molecular processes. Using a co-visualization of functional fluorescent auxin transporter (PIN1-GFP) with a dye staining of cell walls with propidium iodide (PI), we developed a method to quantify in 3D the polarization of auxin transport for every anticlinal wall of the first layer of cells. The digitally reconstructed network evidenced an overall stable convergence of PIN1 polarities towards the center of the meristem, with local front lines matching dynamic accumulations of auxin [15]. It also showed that the apparent crescent shape often thought to indicate polarities in cells might sometimes be misleading, and opens the way for a new view of how auxin transport is regulated.

Temporal auxin signaling in meristem organ patterning. Morphogenetic signals such as auxin define spatial distributions that are thought to control tissue patterning, but it has been proposed in animals that they also carry temporal information in their dynamics. A recent model developed by our group has postulated the existence of a stochastic mechanism to explain disturbed phyllotaxis patterns. This model assumes that organ initiation results from a temporal integration of a morphogenetic signal that buffers molecular noise [22]. Using a quantitative analysis of the dynamics of auxin distribution and response, we provide evidence that organ initiation in the SAM is indeed dependent on the temporal integration of the auxin signal [15]. The duration of cell exposition to auxin is used to differentiate temporally sites of organ initiation, and provide robustness to the rhythmic organ patterning.

Computational models of integrated transport and signaling. To interpret these new observations of auxin signaling and transport in the meristem, we investigate theoretical and computational models to study dynamic auxin distributions and the consequent organ patterning at the level of the meristem. Building on existing models of auxin transport [23], [25], we investigate different competing hypotheses on the auxin-PIN interplay, through numerical simulations based on rate equations for molecular transport and efflux carrier polarization. Quantitative comparisons with in vivo observations will provide cues on how the system responses are linked to memory effects and information exchanges between auxin and PINs.

These works were part of the BioSensors HFSP project and are carried out in the Phyllo ENS-Lyon project and gave rise to a journal article submitted for publication. These results have been presented at the International Worskhop on Image Analysis Methods for the Plant Sciences in Nottingham in January 2018 and in several invited talks given by Teva Vernoux and Christophe Godin.