From photons to food, fuels and chemicals: improving nature

Understanding and improving photosynthesis could be the key to providing better crops for food and fuel, and may even affect climate change – Professor Douglas Kell tells us more

Life as we know it depends on the ability of plants of various kinds to store sunlight as chemical energy in the form of organic compounds. The more we can boost the yields from photosynthesis the less land we need to do it in. In a bid to boost biotechnology for sustainable bioenergy, to develop higher yielding or nutritionally enhanced crops, and to produce important chemical products without further depleting oil reserves, the UK Biotechnology and Biological Sciences Research Council (BBSRC) is partnering with the US National Science Foundation (NSF) to give researchers the opportunity to develop proposals in the area of enhanced photosynthesis. Successful applicants will be invited to a five-day intensive “Ideas Lab” where partnerships and plans will form. BBSRC and NSF will create a fund totalling up to $8m from which the proposed research may be funded. This is ‘high risk, high reward’ activity for funding organisations – folk have been trying to do this for a while, and the chances of success are relatively uncertain, even given the best scientific proposals, but where the research does yield breakthroughs in biotechnology it could save the planet.

Plants are comparatively inefficient at fixing carbon dioxide from the atmosphere and storing it as various sugars, starches, fibres and oils. Nevertheless, the fact that they do this sustainably makes plants attractive as the necessary long-term and sustainable alternatives to fossils fuels for energy generation, transport fuels and chemicals. For example, the BBSRC Sustainable Bioenergy Centre (BSBEC) aims to produce liquid transport fuels such as ethanol and butanol from non-edible food waste and non-food perennial crops that can be grown on marginal land. Indeed, willow chips and Miscanthus straw (both from perennial crops) are already used as fuel in power stations and are earmarked as prime carbon sources for liquid fuels produced through fermentation.

However, there are many (albeit unknown) significant limiting factors that prevent plants from reaching the full efficiency of carbon fixation that, on paper, the photosynthetic reactions they carry out could achieve. If it becomes possible to overcome these limitations, it is feasible to produce crops that are essentially carbon neutral – they absorb as much carbon dioxide as they grow as is released into the atmosphere when they burn. The same advances in photosynthetic rate and efficiency can be applied to improving food crops as well as these non-food sources of fuel, of course. Plants with increased carbon fixing abilities will also prove crucial in our efforts to supplement or perhaps completely replace the use of fossil oil in many industrial applications. In addition, soils contain twice as much carbon as does the atmosphere, so if the non-harvested (root and related) soil carbon biomass could be increased in the steady state by 15% we would decrease the CO₂ content of the atmosphere by 30%, thereby mitigating much of the threat of global warming.

Because plants cannot move, they must go to great lengths to deal with a changing environment. The process of converting carbon dioxide to sugars using energy from the sunlight has to be able to provide the plant with the energy it needs throughout the lifecycle of the plant. Sometimes this will mean dealing with very high or very low levels of light. The molecular systems involved in photosynthesis can easily be overwhelmed by very strong sunlight – an effect that is exacerbated by high temperature and/or low availability of water. Plants protect themselves from such onslaughts remarkably well, but often at the cost of the rate of photosynthesis. There are many useful questions to be answered about the molecular mechanisms involved in sunlight damage and the mechanisms plants use to protect themselves. It might then be possible to develop varieties with improved rates of photosynthesis and the potential to produce greater amounts of carbohydrates that can be used for food, fuel and other products.

Most of what limits photosynthesis is down to the ‘dark reactions’, i.e. the metabolic pathways that make the carbohydrate products. For example, the enzyme Rubisco – a catalyst in the Calvin cycle that converts carbon dioxide to sugar – has been known, for many years, to be extremely inefficient, working on perhaps only a couple of carbon dioxide molecules per second. So far a solution to this problem has been elusive but with new approaches it might be possible to look at the properties of Rubisco and examine a natural variation of it or other molecules as a possible route to better crops through engineering, breeding or mutagenesis. Other elements of metabolism may also be important and in particular, the link between metabolic control and responses to challenges such as high or low temperature and a lack of water. The signalling pathways that regulate this link can have a very great effect on the capacity of a plant to photosynthesise, and represent another important area of enquiry if we are to improve the rates and efficiencies of carbon fixation by plants.

What all of this (and our biochemical knowledge) tells us is that improving biochemical processes is a systems property, and that this requires that multiple steps must be modulated simultaneously; increasing the activity of just one gene (enzyme) will not work. If only three genes need to be modified, but the choice is from 1000 genes, the number of ways of finding the 3 in 1000 is about 100 million. Doing this by exhaustive testing of 108 genotypes in the field is infeasible, but trivial for in silico modelling. That statement is generally true, as it follows simply from the arithmetic properties of combinations of things, and so clever modelling is likely to play an important role in this or any other form of metabolic engineering.

In the discussion of improving photosynthetic rate we must also consider what happens to the carbon compounds that are produced. Plants have to store carbon – usually as starch or oil – to provide a source of energy when photosynthesis isn’t possible, e.g. during the night or during germination under the soil. These stores represent food, fuel and raw materials for humans as well so it will be important to look at the overall processes of photosynthesis with a view to examining the balance between immediate use and storage of the sugars made during this process.

With the global population expected to reach nine billion by 2050 we have a challenge to produce enough safe, nutritious and affordable food. We will effectively need to produce more calories per hectare and so improving the yield and nutritional value of crops will be one important element of crop development over the coming years. As well as maximising the efficiency of photosynthesis, we will also need a greater knowledge of how and where plants then store the sugars that are produced. The power of plant genomics will be important here, particularly now that we have the opportunity to delve, for instance, into the wheat genome and understand where wheat matches or differs from the model plants where we have largely studied photosynthesis until now. An important aim of plant biology research is to increase yield of the edible parts of staple crops so that – per stem and without increasing the need for fertiliser, water or pesticides – the plants are converting more carbon dioxide into food for humans and animals. One element of this could be through enhanced photosynthesis coupled with an understanding of how to ensure that carbon is stored in usable parts of the plant – seeds for example.

Oil reserves are not at all infinite, and as well as extensive use of oil and gas for energy production and diesel and petrol in transport vehicles, there is a huge industry of other oil-based and oil-derived (petrochemical) products. Plant oils and starches have the potential to replace some of these raw materials. Plants can offer oil-derived products such as lubricants, hydraulic oils, paints and other coatings, and surfactants. Starches and proteins can be used to make adhesives and plastics. Polylactic acid (PLA), for example, is now fairly widely used as a starch-derived alternative to petrol-based plastics. PLA also has the advantage of being readily biodegradable and so avoids the landfill problem that has arisen with oil-derived plastics. Whilst these are promising prospects, there will always be competition for space to grow such crops and so it is vital that any crops grown as fossil oil supplement or replacement are as efficient as possible at storing energy, e.g. as starches and oils. So, again, an increase in photosynthesis rate and efficiency has the potential to impact greatly on our ability to continue making products that have become central to modern day living.

Climate change and carbon dioxide emissions raise important challenges across the world and not least to crop production. As carbon dioxide in the atmosphere rises, we might expect plants to become more productive. Rubisco, for example, would, in theory, work better if carbon dioxide rises to, say, 500ppm in the next 30-40 years and this might be expected to offset some of the negative effects on productivity such as temperature change and drought. In fact, according to large scale experiments in 500ppm carbon dioxide environments this offsetting is probably not has much as is predicted. It’s not clear why this is the case, but it may be to do with the way that immediate usage and storage of sugars are balanced.

This is not the place to rehearse all the possible areas of biology, informatics and control engineering that might be anticipated to contribute to improved photosynthesis. Certainly there is much discussion of incorporating the more efficient ‘C4’ photosynthetic pathways into the more common ‘C3’ plants, but this is highly non-trivial. Some think that bypassing the dark reactions altogether (biomimetic photovoltaics) may be the way forward, while others pin their hopes on algae rather than flowering plants.  

At all events, if research can bring about a step change in the efficiency of photosynthesis there is a real potential for plants to play an even more central part in meeting some of the grand challenges facing the world right now, specifically in food and energy security and in climate change. To achieve this step change will be a huge triumph, unlocking many more of the benefits of plants for humans and securing our future health and well being. This is an exciting time for bioscience and there is no guarantee of success in enhanced photosynthesis; it must be done in the context of wider plant metabolism and network biology research, but it is well worth going for it with the unrivalled knowledge and expertise we have at our disposal in the UK.