The growth of maize, sugarcane and sorghum has been greatly boosted by modifying the plants to take advantage of higher carbon dioxide levels now found in the air.
This was done by simply increasing the activity of two genes, says at the University of Illinois. The finding should lead to the creation of new varieties whose yields go up as CO2Â levels continue to rise.
For most of the time that photosynthetic cells that turn sunlight into food have existed, atmospheric levels of CO2Â have been much higher (and the planet has been much hotter). As CO2 levels fell particularly low from about 35 million years ago, it exacerbated a fundamental flaw in photosynthesis that involves probably the most abundant protein on the planet, called rubisco.
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Rubisco, which is involved in grabbing a CO2 molecule and adding it to a sugar molecule, is vital to photosynthesis and hence plant growth. The problem is that rubisco sometimes grabs and adds an oxygen molecule by mistake, which stymies growth.
As the level of CO2 in the air dropped, the likelihood increased of rubisco grabbing oxygen instead of CO2, and more energy was wasted by plants – as much as 40 per cent in some crops grown today.
This is a problem for the 95 per cent or so of plant species known as C3 plants, but others have evolved a solution. C4 plants have a chemical pump that concentrates CO2 in the cells where photosynthesis takes place – effectively recreating the ancient atmosphere.
While there are only a few C4 crops – such as maize, sugarcane, sorghum and millet – their high productivity makes them important, says team member at the University of Illinois. “Sugarcane is the highest yielding crop in terms of biomass in the world,” he says.
Rising CO2 levels in recent centuries have boosted the productivity of many plants, but C4 crops haven’t benefited. These plants can pump more CO2 into the photosynthetic cells as atmospheric levels rise, but it is thought the cells don’t make enough rubisco to take advantage of it.
Salesse-Smith and her colleagues have been testing this idea by adding extra copies of the two main genes for making rubisco to C4 plants. In 2018, her team showed that this .
Now Salesse-Smith has used this approach to boost biomass by 16 per cent on average in sorghum grown outdoors in a field. This didn’t translate into increased seed yield, but that is probably because the team modified an old variety and didn’t use any fertiliser, she says. Demonstrating higher growth in a real-world field test is the crucial thing, says Salesse-Smith.
The method was also tried in sugarcane grown in greenhouses, which got boosts of up to 81 per cent, but increases in greenhouses are usually bigger than those achieved in fields, says Long. “We wouldn’t say that’s what we’re going to get in the field.”
The results show that boosting rubisco levels works with at least three different C4 plants, says Salesse-Smith. “So hopefully, this approach could work with all C4 plants.”
“These are really promising results,” says Johannes Kromdijk at the University of Cambridge. But modified C4 crops now need to be tested in a wide variety of climates, he says.
Even if all goes well, the long timescales involved in producing new plant varieties mean it could be two decades before farmers start growing such crops commercially, says Long.
Researchers are also trying to turn non-C4 crops, such as rice, into C4 plants, which could increase yield by reducing the problem of rubisco grabbing oxygen.
“But there’s just so many changes you need to make,” says Long. “Maybe by the late half of this century, it will be achieved.”
Not nearly enough resources are being put into boosting the yield of food crops to feed a growing population in a warming world, he says. “The world needs to wake up to the food security problem ahead.”
bioRxiv
