Using plants to harvest the Sun’s energy

In the search for fuels of the future, one of the most promising areas for the production of sustainable energy is the plant world. So-called ‘biofuels’ are renewable and home-grown, key advantages in the face of rising oil prices and instability in oil-producing countries. And, as important, biofuels are potentially ‘carbon neutral’ – carbon absorbed by the plant as it grows is balanced by the carbon released when used for energy – and so using biofuels can contribute to environmental goals to reduce greenhouse gas emissions.

Of course, in practice it’s not that simple – from the moment the seed is planted, energy is required to farm and process the crops, reducing the potential carbon savings that could be made. However, if the energy balance could be tipped further into favour, the environmental and energy security advantages are clear. As a step towards this, researchers in the Department of Biochemistry are studying the way plant cell walls are built, in an effort to unlock their energy stores with greater efficiency. Despite these cell wall constituents being some of the most abundant on Earth, and having enormous nutritional, agricultural and industrial importance, until now surprisingly little has been known about the genes and enzymes that make them.

First- and second-generation fuels

So-called ‘first-generation’ biofuels are made from food crops. In Europe and the USA, first-generation fuel is often made either from rapeseed oil, which is converted to biodiesel, or from maize, which is converted to glucose syrup and fermented to ethanol. We now know that there are substantial problems with the longer term use of maize or rapeseed oil to make biofuels: because of the energy required to farm and process these crops, the reduction in carbon emission through their use is relatively low, or may even be non-existent. Moreover, as the world population continues to rise, there are justifiable concerns that the use of food for energy will lead to increases in food prices and food shortages.

Much effort has therefore focused on ‘second-generation’ biofuels, made from any source of woody plant material, which could include straw, wood and food waste. In the UK, miscanthus grass and short rotation coppice willow, both of which will grow on marginal land that cannot be used to farm food crops, are now being bred for increased biomass yield and other improved sustainability qualities. Not only do these perennial species require lower agricultural input of fertilisers, pesticides and herbicides, but they are also much more productive, with a longer growing season than the conventional annual crops.

Breaking down barriers

So why aren’t second-generation biofuels being used already for transport fuels? One of the main reasons is that unlike maize starch, which is easily digested, the tough woody tissues are resistant to enzyme attack. One solution is to use the microbes in the guts of termites and cows that slowly break down wood or grass into sugars, but current industrial techniques for this conversion are inefficient, expensive and not commercially well developed. Another solution is to look at how the sugars are locked up in the plant and develop a way of releasing them more easily: this is the route taken by Dr Paul Dupree’s research group.

Plant sugars make the rigid cell walls that give the plant its strength and shape. Long chains of linked sugars known as polysaccharides are tightly interwoven into a structure that is physically flexible, yet strong and resistant to digestive enzymes. The main cell wall polysaccharide, cellulose, is made of pure glucose just like maize starch, but because the sugars are linked into chains in a different way, the plant cell walls are resistant to enzyme attack.

Plants vary enormously in the types and amounts of sugars used to build their walls, as well as in the way the sugars are linked together. These factors influence how easily the plants can be digested and how much of the wall can be fermented to ethanol. As this variability is under genetic control, Dr Dupree and his colleagues are searching for the plant’s genes that control this variation. It is likely that several hundred genes in plants are involved in making their walls and, until recently, this genetic complexity has hindered investigations of the process.

Each plant cell has hundreds of cell wall polysaccharide factories, known as Golgi bodies, which weave the sugars into the polysaccharides. To find the genes that direct the formation of the cell wall polysaccharides, Dr Dupree’s group has examined the enzyme machinery in these factories, with the help of Dr Kathryn Lilley’s group in the Cambridge Centre for Proteomics in the Cambridge Systems Biology Centre. Now, the function of more than 10 individual components of this machinery has been discovered, and more than 100 further possible enzymes have been identified.

Unlocking the potential

Having found the genes involved in making the cell wall polysaccharides, the question could then be asked: is it possible to find plants that have defective components that are broken down more easily? Biofuel generation from such plants would be faster, more efficient and closer to carbon neutrality. The answer is yes – Dr Dupree’s group has now found plants in which some of the polysaccharides are not linked together in the normal way. Remarkably, the plants seem to grow normally. By applying enzymes from wood-rotting microbes to the plant walls, more of the sugars could be released from the cell walls than usual. And even more encouragingly, a further study has shown that the activity of some genes can be increased, with the effect of increasing the amount of fermentable sugars in the plant cell walls. The expectation is that these studies will allow future bioenergy crops to be bred to provide greater quantities of biomass that can be converted more simply and cheaply to biofuels: a step towards bringing fuels of the future closer to fruition.

For more information, please contact the author Dr Paul Dupree (p.dupree@bioc.cam.ac.uk) at the Department of Biochemistry.