A sweet surprise: Revealing new roles for sugars in plants
The fifth post of our Celebrating Basic Plant Science series comes from Mike Haydon, a lecturer at the University of York. He and his research group work on understanding signalling in plants. Here he explains some of his work on integrating sugar metabolism with light signals. You can see more about Mike and his group on his website. The work he discusses below was published in the journal Nature last year (Haydon et al. Nature 502:689-692).
A life based on sugar
Most of us think about sugar every day, be it consciously as we consider our calorie intake, or unconsciously when our brain tells us it’s mealtime. Sugars are among the simplest of carbohydrates and they are the raw material for cellular respiration, which produces energy for almost all living cells. Glucose, a monosaccharide, is the preferred sugar for cellular respiration. Sucrose, most familiar to us as the granulated sugar in our kitchens, is a disaccharide made of glucose and fructose. These, and other simple carbohydrates, are used to build complex carbohydrates such as starch and cellulose in plants, and glycogen and chitin in animals. Sugars are the foundation of cellular metabolism, and produce the wide array of molecules that sustain our carbon-based existence.
The most important process on the planet
Plants, along with algae and some species of bacteria, use photosynthesis to convert carbon dioxide in the air into glucose using energy from sunlight, while producing oxygen as a by-product. Photosynthetic bacteria were responsible for the Great Oxidation Event, which occurred from about 2.5 billion years ago and led to the life-sustaining atmosphere we now live in. Photosynthetic organisms are called autotrophs, because they produce their own sugars to use in cellular metabolism. All other organisms, called heterotrophs, must somehow get their sugars from their environment. For animals, this is ultimately through the plant-based component of their diet. So essentially all the carbon in 
our bodies was, at some point, converted from carbon dioxide into glucose by photosynthesis. Thus, photosynthesis is probably the most important metabolic process on the planet.
You might think that something so fundamental in biology would be completely understood, and we certainly do know a lot about carbohydrate metabolism. We also know that sugars have functions outside of this basic metabolism. For example in plants they can act as hormones, regulating processes such as cell growth, cell division, flowering time and disease resistance. But there is still a lot we don’t know about how plants regulate carbohydrate metabolism, and sometimes we still find entirely new functions for sugars in biological processes.
How time matters in sugar metabolism
The circadian clock is a biological time-keeping mechanism that allows living things to predict daily rhythms of the environment. This time-keeper regulates many aspects of metabolism and physiology – it is the cause of the jetlag we experience when we travel between time zones. In plants, photosynthesis is regulated by the circadian clock to synchronise cellular metabolism with day and night. This leads to rhythmic production of sugars, which peak around the middle of the day.
My team at the University of York research the impact of sugars in circadian clock function in the model plant, Arabidopsis thaliana. Our recent research revealed that the rhythm of sugar accumulation helps to tell the plant what time of day it is by regulating PSEUDO RESPONSE REGULATOR 7 (PRR7), a gene that is expressed in the middle of the day and functions at the core of the plant circadian clock. By abolishing the rhythmic production of sugar by inhibiting photosynthesis in the light (either by growing plants in carbon dioxide-free air, or adding a chemical inhibitor), or adding sugars to plants growing in the dark, we could separate the effects of light and sugar on the circadian clock.
These experiments show us that, in addition to the previously well-established setting of the circadian clock by light at dawn, photosynthetically-derived sugars provide a second reference point for circadian rhythms in plants. This is a bit like when we check our watch to see if it’s time for morning tea. We have termed this a ‘metabolic dawn’. Thus, our basic research defined a new role for sugars in a fundamental aspect of plant biology.
For me, our findings highlight the potential for exciting new discoveries in fundamental plant science, even in an area as well-researched as sugar metabolism. Continuing work in my lab aims to identify new ways that plant cells sense and signal sugar availability. Our goal is to contribute something novel to our understanding of very basic biological processes, and hopefully find our data in textbooks. Of course by working on plants, this could also lead to improvements in agriculture by enhancing growth efficiency – but the opportunity to learn something completely new or unexpected is the primary motivation for our research.
Images: spoons and sugar cubes by humusak2 via stock.xng; Arabidopsis thaliana flower c/o Ruth Bastow; figure from Haydon et al. Nature 502:689-692.
Leave a Reply