In Control of Transpiration: The Evolutionary Interplay between Cuticle, Stomata, and Air Pores

If you scrape your fingernail lightly across the surface of some plants like the Cactus, you may pick up a smattering of wax, as if you'd run your fingernail down the side of a candle. In fact, almost all the surfaces of plants, the stems, leaves, flowers and fruit are covered in a tiny layer of wax called the cuticle. This layer is too thin to see because it is thousands of times thinner than a human hair. However its effects can be seen. It is this waxy layer that makes leaves look shiny, allows you to polish your apple, and causes water droplets to roll smoothly off the surface of leaves in a rain shower.
It is this waxy cuticle that allows plants to live on the land without drying out. As such, the cuticle is one of the most important evolutionary inventions in the history of our planet because it has allowed life to leave the watery oceans and survive on dry land. Life and land has never been the same since. The cuticle also does a lot of other useful things for the plants. For example, it blocks bacteria and fungi from infecting the plant, much like human skin. Indeed, one of the reasons that fruit can last for days in the fruit bowl without becoming rotten is due to the protective effects of the waxy cuticle.
We have a lot to learn about how plants make waxes, and move these waxes from where they are manufactured inside the plants to the surface of the plant. The waxy components of the cuticle are made, transported and assembled on the surface by proteins, which are encoded for by genes in a plant's DNA. However we have still to identify many of the genes involved in making the cuticle. It is important to identify these genes because it could help us to design better crops to resist diseases and to create fruit that last longer, and have a longer shelf life with less food waste. It may also help us to commercially synthesise waxes by copying these genes into the DNA of other organisms.
Remarkably we also do not know how plants first evolved the wax cuticle. We do not know what the function of the waxy layer was in the first land plants, what steps were involved in the evolution of the waxy layer, and how it affected the biology of these land plants. We don't know which genes were important in its evolution or how the cuticle has changed and evolved over millions of years. However, by studying the cuticle in plants that represent the first lineages to survive on land, we can get a sense of how the cuticle has changed through evolution and with changing climate.
In this project I would look at living relatives of some of the earliest plants to move onto land. I will compare the DNA of plants that never moved onto land and do not have a cuticle, with DNA from land plants that do have a cuticle. This will help detect genes that are involved in making the cuticle and reveal how these genes have changed over time. I will interfere with these genes to stop them working, in order to see how they make the waxy cuticle in these early plants. Together this will help us to better understand to what extent all land plants have the same genes to make cuticle in the same way, and to what extent the cuticle had similar properties and functions in the past and present.
Plants are constantly absorbing water from the soil and transferring it to the atmosphere via tiny pores called stomata - a process called transpiration. Together plants all over the planet release an enormous amount of moisture into the air, which in turn forms clouds and rain. The waterproof cuticle drastically reduces transpiration and consequently affects the global climate. We do not know how the cuticle of plants will respond to man made changes to the climate. This study will lead to better understanding of the cuticle across all land plants and allow us to predict the effect of changing temperature, carbon dioxide, and drought on the cuticle. This in turn will allow us to better understand how plants will respond to the changing climate

Climate Change Science. Our rapidly changing global climate impacts on plant traits such as stomata, which can then feedback on global processes including transpiration. It is an important applied research to evaluate and predict how plants will respond to changing environments and climate. These predictions may indicate when and where plants may mitigate or exacerbate the effects of climate change. Such predictions depend in part on a thorough understanding of plant physiology, and how plant processes will respond to environmental stimulus. In this regard it is far from clear to what extent the cuticle or cuticle-stomatal interactions respond to changing environment and this is a significant gap in our knowledge that may affect predictive power. This proposed research will give some insight into these uncertainties and allow better predictions about plant responses in a changing world.

Agriculture and Food Security. The cuticle performs many important roles in crop plants. These functions include: 1) acting as physical barrier to pathogen attack and a source of signals in plant-pathogen interactions; 2) maintaining the integrity, appearance, and shelf life of fruit, and; 3) determining the degree of water loss from crop plants, in conjunction with stomatal pores. The cuticle is an important but under-utilised target for crop improvement and marker assisted breeding, and offers a important target for crop improvement particularly for arid agricultural land. An example of a crop that has been improved by breeding of a cuticle trait is the naked hulled variety of barley, used for human consumption as it is easily threshed. The easily removed barley hull in this variety is due to an allelic mutant of the SHN gene family that controls cuticle formation and hull adhesion in the barley seed. The research proposed here will reveal the conservation and diversification of genetic pathways regulating cuticle formation, and highlight important targets within genetic 'toolkit' that may be manipulated to improve crop plants. Modifications to cuticle on fruits for example could lead to better fruit preservation, longer shelf life, and less food waste. Better understanding of cuticle-stomatal interactions will help understand potential trade-offs associated with cuticle modification.

Biofuels Industry. Waxes, such as occur in plant cuticle, are high-value, high-energy products that are used in a variety of industrial and commercial applications e.g jojoba wax esters. Most commercial waxes are produced by chemical synthesis, fossil fuel extraction, and from a small number of plant and animal species. The diversity and yield of these waxes is low. Improved understanding of plant wax biosynthesis will allow genetic engineering of wax synthesis in higher yielding oil-rich seed crops, such as oil seed rape, on in lipid-rich algal species. Such approaches would produce higher yields and improved commercialization of waxes and generate a greater range of different waxes with different properties. The value and the range of applications of biofuels would be greatly increased. This proposal would generate abundant data regarding the conservation of was biosynthesis components across land plants and algae and highlight genomic diversity in these genetic components that would be useful in synthesizing a range of wax products in diverse organisms.

Public Interest and Students. This research touches on several areas of research that are of topical interest to the public: agriculture, food security, biofuels and climate change. The data and publications from this research will therefore enhance and inform public debate. Such practical concerns are also engaging points of discussion and provide useful foci for high school and undergraduate teaching.