Although leaves may look solid, their surface is covered with microscopic pores, termed stomata. These pores play a key role in controlling how much carbon dioxide enters the plant for photosynthesis and, at the same time, how much water escapes. The pores are formed by two cells (guard cells) which swell and shrink via the control of an internal pressure called turgor. This pressure is the result of the pumping of salts into and out of the cells, leading to the flow of water into and out of the cells. The changes in turgor pressure lead to changes in guard cell shape so that as pressure increases the cells swell and bend apart, creating the pore through which carbon dioxide and water can diffuse. This shape change in response to pressure change is thought to reflect special properties of the guard cell walls, in the same way that the shape of a balloon depends on the physical properties of the skin of the balloon. However, this textbook explanation is based purely on observations of how the cell wall looks under the microscope, with no measurements of the actual mechanical properties of the guard cells.
To resolve this question, I will be using a technique termed Atomic Force Microscopy (AFM) to measure the mechanical properties of living guard cells. AFM is widely used in the physical sciences but it’s application to living biological material is much less advanced. Essentially it involves bringing a microscopic tip close to the surface of a material and then tapping the surface. The resistance to this tapping can be measured and by repeating the process over the surface of a cell one can gain a measure of the wall mechanical properties. However, this is not only technically tricky, the interpretation of the data is also not straightforward. AFM was initially designed for investigating uniform materials which do not change significantly in composition over time, whereas plant cell walls (like most biological materials) are definitely not uniform and definitely do possess the potential to change in composition and structure over time! I will be working in the Sainsbury Laboratory in Cambridge where I will be able to collaborate with a leading group in this area, as well as forming important links with mathematical modellers and physicists whose inputs are vital for such an interdisciplinary project at the interphase of biology and physics.
This is an increasingly topical area since although we understand the control of gene activity in intricate detail, at some point gene expression must lead to control of the mechanical properties of cells and tissues which define the size, shape and function of plants and animals. At present our understanding of how this works is very much incomplete. The Leverhulme funding will thus provide a unique opportunity for me to explore this important but challenging area.
An AFM image of a stomata on the surface of a leaf. The pore is formed by two guard cells which swell to open the pore. Image: Ross Carter, University of Sheffield.
Professor Andrew Fleming
University of Sheffield