If you have missed my previous two columns, I’m currently looking at the radical technologies or events that have the potential to divert us from the trajectory we have been creating for the planet in recent decades. Last time I looked at the effect of a global pandemic, this time I'm looking at how we might change the agricultural landscape by hacking plants.
Our human population is increasing beyond anything Earth has experienced before. The rate of global population growth may now be slowing, but we are still expecting to top nine billion people by 2050, all of whom need to eat. It means we will have to produce more food in the next 40 years than we have had to for the past 8,000 years, when agriculture began. We are on track to convert more and more wild lands – including rainforest and savannah – to cropland. Already 40% of the planet's land surface is used for agriculture.
To interrupt the flow of wholesale land conversion, while needing to produce perhaps 60% more food, would require something quite extraordinary. I’ve discussed issues like re-thinking our use of fertilisers before, but what if we could increase food production at its most fundamental level? What if we could engineer crops to make photosynthesis more efficient?
Ultimately all of our food – indeed, all life on Earth – relies on the conversion of carbon dioxide into sugars by photosynthesis, using the Sun's energy. Most plants, including most crops, use a chemical pathway for photosynthesis that binds three carbon atoms from the air. It's called the C3 pathway. But around 5% of plants have evolved a different pathway that binds four carbon atoms. This C4 pathway is not only more efficient at warmer temperatures, it also uses less nitrogen (fertiliser) and less water during photosynthesis and because the plants' pores, or stomata, need to be open for a shorter time compared with C3 plants to receive carbon dioxide. This means there is less opportunity for the leaves to leak water – C3 plants lose 97% of the water they take up through their roots to transpiration. So the C4 pathway is ideal for the hotter drought conditions that are increasingly prevalent owing to climate change.
C4 plants are so successful, especially in tropical savannahs, that they are responsible for as much as 30% of all terrestrial carbon fixing, even though they make up a tiny percentage of plants. Some of the crops that we cultivate use the C4 pathway, including corn (maize), sugarcane, sorghum and millet. But many of the most popular crops, including wheat and rice, are C3 plants. Their yields suffer in hotter drier conditions – just where and when we need to increase them.
Scientists are now trying to genetically manipulate C3 crops to turn them into C4s. The good news is that the pathway has evolved separately as many as 60 times, which scientist say makes this likely to be possible, and there are potentially a range of different genetic mechanisms to choose from. Many of the proteins used in the C4 pathway are taken from C3 plants and have adapted to a new role. So, scientists think it should be possible to artificially create C4 versions of rice and wheat that would be around 50% more productive while using less water and nutrients.
Researchers have begun with rice because it is a genetically simple plant – with two sets of chromosomes like us – unlike, say wheat, which has six sets. They are working on two fronts: trying to change its leaf anatomy to that of a C4 plant, which have closely packed veins of two cell types; and switching the biochemical make-up of enzymes and proteins to the C4 type. Corn offers a good model because although its main leaves are C4, the small husk leaves (which are far less efficient at photosynthesising) are C3. So geneticists are painstakingly comparing the genetic code of the two leaf types to find out what makes a C4 leaf a C4 leaf.