How the humble hornwort could supercharge agriculture
You’re here because of one very important enzyme. But don’t look inside to find ribulose-1,5-bisphosphate carboxylase/oxygenase, more happily known among scientists as rubisco. Instead, look at the food you eat and the trees that make oxygen, because that’s the protein that makes photosynthesis possible. Without it, life on Earth as we know it would not exist.
Despite all this hard work, rubisco is remarkably inefficient. The enzyme converts carbon dioxide into sugars that sustain plants. But it easily confuses and reacts with oxygen in a process that creates a toxic byproduct, wastes energy and limits the rate of plant growth. (Which is not to blame for rubisco: it’s a very difficult reaction to pull off, and just look at how many plants have been successful for hundreds of millions of years.) This includes the essential crops that feed humanity—grains, vegetables, fruits—that could theoretically grow better if their rubisco worked more efficiently. This problem becomes more pressing as global temperatures rise, as this essential protein becomes even less effective in the heat.
Enter hornwort. This small plant, related to moss, grows like a green leaf on the ground. It is the only known land plant that has found an (evolutionary) way to supercharge rubisco by concentrating CO2 around the enzyme.
Now an international team of scientists claims to have figured out how hornwort does this and how it could apply this superpower to crops. This could mean significantly improved yields, so farmers wouldn’t need to cultivate as much land to produce the same amount of food. “It’s very impressive,” said Robert Wilson, a biochemist who studies rubisco at the Massachusetts Institute of Technology. (Wilson was not involved in the research but is collaborating with one of the scientists.) “This is interesting, because this is a completely new and novel mechanism by which an important aspect of rubisco biochemistry occurs.”
Scientists have long known that certain species of algae also enhance the effectiveness of rubisco. Inside their chloroplasts – the structures in cells where photosynthesis takes place – they have developed specialized compartments called pyrenoids. These concentrate the CO2 around the enzyme, minimizing its reaction with oxygen. “This prevents rubisco from touching oxygen, because it introduces it into a house and then pumps a lot of CO2 into it,” said Laura Gunn, a synthetic plant biologist at Cornell University and co-author of a new paper describing the work. “The rubisco is therefore completely saturated with CO2 and all the oxygen is outside the house.”
But because algae is so far removed from the foods we eat, it would be difficult to genetically modify crops to mimic these pyrenoids. In contrast, the cornea is more closely related. These researchers discovered that it has a unique way of making these pyrenoid structures, thanks to a protein they call RbcS-STAR. The Rubisco in all plants is made of protein, but the hornwort version has an extra “tail” that helps tie the enzymes together to create compartments into which CO2 is pumped, increasing the efficiency of the process.
To this end, the researchers introduced the RbcS-STAR protein into a closely related hornwort species that lacks pyrenoids, and sure enough, its rubisco rearranged itself to create these compartments. They then did the same with Arabidopsis – a small flowering plant commonly used in laboratory experiments as a model organism – and it also responded. “They form pyrenoid-like structures, and this will be a very important step toward engineering better photosynthesis using this type of CO2 concentration mechanism,” said Fay-Wei Li, a plant biologist at Cornell University and the Boyce Thompson Institute and co-author of the paper. The team is now looking to do the same by genetically modifying crops: Gunn said that by adding pyrenoid structures, researchers could increase growth and yields by up to 60 percent.
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But so far, researchers have only built this pyrenoid-shaped house. “Basically we built the walls, right, we built the roof,” Gunn said. “But we don’t have an HVAC system yet. We don’t have a system yet that’s going to pump the CO2 out and then blow the sugary stuff out at the end.”
Once researchers figure this out, it could mean a field day for farmers around the world. Since Rubisco is very inefficient, plants must produce a large quantity of it. To encourage this, farmers must apply tons of synthetic fertilizers. All of these chemicals are not only expensive, but also terrible for the environment: their production requires an immense amount of energy, then pollutes waterways as they run off the fields.
And speaking of water, this advancement could also mean that crops use less of it. Plants are dotted with small structures called stomata, which open to “inhale” CO2. Because rubisco is very inefficient, plants have to breathe deeply to get enough gas, but this releases more water vapor through the stomata, forcing them to constantly irrigate the crops. “If a rubisco doesn’t react as much with oxygen, plants can close their stomata more often,” Li said. “If you can close your stomata, you won’t lose as much water.”
But if algae and cornea figured out how to improve rubisco, why didn’t all plants do it? Algae live in aquatic environments, where CO2 dissolves poorly, so they had to make the most of the little they had. For hornworts, the reason is not really clear, especially since related species living in the same environment have not evolved pyrenoids. As with all other land plants lacking a better system, one reason could be that rubisco evolved at a time when there wasn’t much oxygen in the atmosphere, meaning plants didn’t have to worry about the enzyme being distracted when it tried to process CO2.
All these years later, scientists have cracked the code and may soon make rubisco an even more essential enzyme. “I think it’s very likely that improvements in crop yields through synthetic plant biology will occur over the next 10 years,” Wilson said. “And that’s very exciting, because that’s where the field has been trying to go for many decades now.”
