The global population is set to increase to 10 billion by 2050 and our current food production just won’t cut it. Among the many solutions proposed to fix this potential disaster is to increase the efficiency of photosynthesis itself. But how does one improve upon a process that has been fine-tuned by natural selection for billions of years? Fortunately, nature has done much of the work for us. A modified form of photosynthesis called C4 is found in many plants, conferring greater efficiency of carbon fixation, water and nitrogen use.
In plants, an enzyme called RuBisCO uses carbon dioxide (CO2) extracted from the air to synthesise carbohydrates, termed fixation. However, as enzymes go, RuBisCO is rather promiscuous. As well as fixing CO2 into carbohydrates, it also wastes some of the plant’s hard-earned energy by fixing oxygen in a process called photorespiration—where instead of CO2, RuBisCo adds Oxygen, resulting in a product useless to the plant. This inefficiency increases as the temperature rises, with progressively more energy wasted on photorespiration.
However, some plants have fought back, developing a new kind of photosynthesis called C4. Unlike in the typical C3 photosynthesis, where CO2 is initially fixed by RuBisCO into a three carbon compound, these plants initially fix carbon into a four carbon compound using a much more efficient enzyme, called PEPC, which doesn’t carry the flaw of photorespiration. The carbon this enzyme fixes is then transferred into specialized cells around the plant’s vascular tissue called bundle sheath cells, where it is then converted back to CO2. CO2 is channeled into these cells from all of the surrounding leaf cells, increasing the concentration of CO2 in these cells to around ten times that found in the atmosphere. The result of this is that RuBisCO spends more of its time fixing carbon for the plant, and less time wasting energy on photorespiration.
All of this begs the question of whether C4 could be introduced into C3 plants to increase their efficiency. This is the aim of the C4 rice project, a operation launched in 2008 with funding from the Bill & Melinda Gates Foundation, which hopes to genetically engineer rice (naturally a C3 plant) to photosynthesize via the C4 method.
Such a change could have a huge impact on food security. The current expected production of rice – a crop that provides 19 percent of global dietary energy – falls 394 million tons short of the expected demand in 2050. By engineering C4 rice, proponents claim rice production could increase by 50 per cent, easily covering our estimated shortfall and helping to solve the coming global food shortage.
Manipulation of the biochemistry is in reality fairly straightforward. The genes involved in the C4 pathway have been identified and research is currently underway towards introducing this pathway into the cells around leaf veins.
The second feature of C4 plants that the C4 rice project aims to introduce is called Kranz anatomy, the structural modification involving the enlarged bundle sheath cells in which photosynthesis occurs in C4 plants. Unfortunately, introducing this structure into rice may prove difficult, as it is currently unknown how the development of Kranz anatomy is regulated.
Eventually Kranz anatomy and C4 biochemistry will be combined in a single rice plant. But is such a feat of genetic engineering even possible? The C4 rice project is optimistic that it is, an attitude drawn from the fact the plants have evolved C4 from C3 independently over 60 times throughout their history, suggesting that the transition might not be so complicated after all.
However, C4 rice is likely still a long way off and – given the scale of the expected population increase – must be only one of a number of innovations needed in the battle against the impending food crisis.