Shape Shifting Stomata: The Role of Geometry in Plant Cell Function

Plants need to draw water up from the soil to the shoots. They do this by losing water vapour via small, controllable pores on the leaf surface, termed stomata. Open stomata allow plants to pull water up to the top of the plant and, at the same time, they allow carbon dioxide into the leaf where it is used for photosynthesis, the process by which all our food is made. However, if stomata were always open this would lead to catastrophic water loss, wilting, and eventual death of the plant. Therefore plants continually adjust their stomata, making sure that they are open enough to allow the plant to grow when conditions are good, but closed when there is the danger of losing too much water.
Evolution has led to two main types of stomata: a simple form composed of just two cells (found in the majority of plants) and a more complex form composed of four cells. These more complex stomata are found in plants such as maize, rice, wheat and barley- the most important crops for feeding the world. One of the reasons why these plants are so successful is thought to be because their stomata function better than those found in other plants, leading to less water loss. However, exactly how the four-celled stomata are "better" than the two-celled type is unclear. Our hypothesis is that it is the structure of the stomata (both the special shape of the cells and/or the mechanical properties of the cell walls in the stomata) that make them a more efficient system for controlling water loss. This project will investigate and test this idea.

To resolve the question of how grass stomata can perform better will involve understanding the mechanical properties of stomata to identify which elements of the structure are most important for stomatal function. Biologists and computational scientists will work together to create a model of the four-celled stomata, using a model grass system, brachypodium, in which significant advances in stomatal biology have recently been made, providing important tools and resources for this project.
By creating a computer model we will be able to rapidly explore ideas on how the stomata work. We will then test these ideas experimentally, creating new types of stomata in the laboratory and evaluating their performance. During the project we will apply new software tools to generate these models. This will allow us to additionally test the idea that the specific shape of a cell can have a major outcome on what a cell does. This will both advance our fundamental understanding of biology and provide a new insight into how stomata work: do apparently minor changes in shape between different stomata on a leaf actually have a large influence on how well the stomata control water loss? As a result of this work we will determine what makes four-cell stomata better than two-cell stomata, answering a long-held question in plant biology and providing information that will be of potential use to crop breeders looking to improve how well crops survive under drought- a major challenge in UK and world agriculture.

Stomata play a central role in controlling plant water use efficiency. Although it has long been accepted that the mechanical properties of the guard cell wall play an important role in the turgor-driven shape changes required for stomatal pore opening to occur, characterisation of the material and geometric properties that allow these cells to undergo repeated and extensive changes in stress/strain as they swell and deflate has remained surprisingly limited. We have recently made significant progress in this area, creating and testing a finite element (FE) model of 2-celled Arabidopsis stomata which has challenged our text-book understanding of stomatal mechanics.
Grasses contain distinct 4-celled stomatal which display improved parameters of performance and it has been suggested this reflects the distinct cell geometry and/or cell wall composition. We will test this hypothesis by creating a model of grass (brachypodium) stomata and investigating (via genetic ablation of cells and enzymatic and genetic modification of cell wall composition) the role of geometry and/or wall structure in stomatal function. Our initial models will be based on idealized 3D structures guided by existing knowledge of the 4-cell configuration. In a second phase of the project we will extract meshes from confocal stacks imaged in individual stomata, then derive FE models for individual stomata. These models will be used to test the hypothesis that each stomate within a leaf might display different performance characteristics depending on slight changes in their geometry. This will provide a novel insight into stomatal biology and address the fundamental question of the extent to which cell behavior is influenced by precise cell geometry.
The results of the project will advance our understanding of a basic aspect of plant biology (stomatal function in grasses) and provide potential leads on how stomatal performance might be improved to limit crop water use whilst protecting grain yield.

Translational Impact
Water is a finite resource which is coming under increasing pressure due to a combination of increased population needing water to live, a requirement for more food to feed this population (dependent on water), industrialisation requiring more water, and a changing climate leading to altered water supply. Increasing the efficiency of crop water use while maintaining or improving yield will help ameliorate these problems.
Our proposal is timely since it has recently been demonstrated in a landmark study, that (optigenetically) enhancing stomatal dynamics in Arabidopsis leads to improved biomass in a fluctuating environment (Papanatsiou et al., Science 2019). Our study will generate an understanding of how evolution has achieved highly dynamic stomata in the grasses. Our research will open the door to commercial organisations interested in exploiting our findings to explore opportunities to modulate the mechanical properties of stomata and improve the water use efficiency of crops. Speed of response to rapid changes in the local environment, and absolute maximal values of transpiration will be heavily influenced by the mechanical properties of stomata. The mechanical properties help define inertia in and the limits of the system. By selecting or genetically improving stomatal mechanics, relatively minor changes might improve water use and photosynthetic efficiency significantly. We will liaise with our established partners in the plant breeding/agritech sector to raise this avenue of translational research. In addition, during the project we will perform preliminary work to provide proof of concept data which can be used as the basis for such future collaborations. These target plants will include UK crops (wheat, barley, potato) and also rice, providing an important line of impact to food security in Asia.

Educational Impact
As part of our impact strategy we will utilise the approaches and results of our research as the basis for outreach to local schools in both South Yorkshire and Norwich. Our research involves a combination of experimental and computational approaches which involve a strong visual element (live imaging and 3D models). These will provide the basis for novel educational tools on a topic (plant water use) which is already embedded in GCSE/A-level courses. This will provide a basis for enthusing young students, both in a fundamental aspect of plant biology and by demonstrating that modern biology requires students comfortable working with physicists and computational scientists. By raising awareness of how successful this combination can be, and how it can be implemented to a real-life problem, we aim to counter-act the silo approach to individual science disciplines which often holds students back at university and research level.