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Mapping genetic information into physical space: shape growth in plants

Final Report Summary - GENEMECHMAP (Mapping genetic information into physical space: shape growth in plants)

The growth of an organism is a physical process. In plants, growth can generate and respond to physical forces owing to the presence of a cell wall. The cell wall links sibling cells together, they do not slide, and as such if one grows the other must either follow or tolerate the stress placed on it by its neighbour. The wall also constrains each individual cell, acting as a physical control barrier for a given cell’s expansion. In the case of most plants, organs and tissues are comprised of many cells, connected together by cell walls. A group of cells growing in one place can exert forces on neighbouring groups and so on, creating a complex and informative field of innate stress that instructs growth even as it results from it. The aim of this project was to investigate the changes to the cell wall as a material that contribute to shape formation in plant organs, specifically the extreme directional growth of the young seedling, and their underlying molecular control mechanisms.
When a seed germinates under the soil, it uses the environmental cues of light and gravity to direct its growth upwards out of the soil. The young seedling achieves this by elongating an organ called the hypocotyl (between root and shoot tip) in a very directional manner, favouring growth in height over girth. The focus of this project was to 1) develop a quantitative description of hypocotyl elongation in time, 2) develop new methods of quantifying cell wall mechanical behaviour, and 3) identify transcription factors underlying wall mechanics and elongation.
With the support provided by this fellowship, we have been able to generate a detailed, quantitative, description of hypocotyl elongation over time at the organ and cellular levels. This work includes changes in height (length) and girth (width) whereas other published accounts focus on height alone. Our work with sugar responses, supplemental to the original aims, revealed that data on height and girth are essential to understanding hypocotyl elongation and its modulation, height alone is misleading when girth also shifts. We have also developed Atomic force microscopy (AFM) based methods for examining cell wall elasticity (a material property) in internal tissues. By cryo-sectioning plant tissue and exposing the cut face of the section to AFM indentation, we have been able to assess cell wall mechanics inside of organs; our previous methods allowed access to the outside cell layer (the epidermis). We have also developed and AFM-based indentation method to examine cell wall viscosity allowing us to determine that, during growth, a cell wall becomes more like a fluid and less like a solid. These insights, and methods, are providing step-changes in the field of plant cell wall mechanics. The experiments performed on elongating hypocotyls have allowed us to identify an epidermal mechanism that enhances the upward direction of elongation: changes in the mechanical properties (elasticity and viscosity) of vertical walls allows for enhanced upward elongation of the organ. Finally, we have begun dissecting the genetic network underlying the changes in cell wall mechanics described above. This essential work indicates a suite of cell wall modifying enzymes are orchestrated together to effect the change in cell wall mechanics.

The project has enabled the training of two EU researchers and two non-EU researchers in the methods and interpretations of biological physics. It has also provided interesting results which we believe will help improve breeding in crops to combat early seedling emergence losses: it can be challenging for young seedlings to break through complex soils. The training of the fellow has been enhanced greatly by this project, enabling them to move on to the next step in their career. Work by the fellow’s research group can be followed on the lab website: https://braybrookgroup.wordpress.com/