As plants convert sunlight into sugar, their cells are playing with fire. Photosynthesis generates chemical byproducts that can damage the light-converting machinery itself—and the hotter the weather, the more likely the process is to run amok as some chemical reactions accelerate and others slow. Now, a team of geneticists has engineered plants so they can better repair heat damage, an advance that could help preserve crop yields as global warming makes heat waves more common. And in a surprise, the change made plants more productive at normal temperatures.
“This is exciting news,” says Maria Ermakova of Australian National University, who works on improving photosynthesis. The genetic modification worked in three kinds of plants—a mustard that is the most common plant model, tobacco, and rice, suggesting any crop plant could be helped. The work bucked conventional wisdom among photosynthesis scientists, and some plant biologists wonder exactly how the added gene produces the benefits. Still, Peter Nixon, a plant biochemist at Imperial College London, predicts the study will “attract considerable attention.”
When plants are exposed to light, a complex of proteins called photosystem II (PSII) energizes electrons that then help power photosynthesis. But heat or intense light can lead to damage in a key subunit, known as D1, halting PSII’s work until the plant makes and inserts a new one into the complex. Plants that make extra D1 should help speed those repairs. Chloroplasts, the organelles that host photosynthesis, have their own DNA, including a gene for D1, and most biologists assumed the protein had to be made there. But the chloroplast genome is much harder to tweak than genes in a plant cell’s nucleus.
A team led by plant molecular biologist Fang-Qing Guo of the Chinese Academy of Sciences bet that D1 made by a nuclear gene could work just as well—and be made more efficiently, as its synthesis in the cytoplasm instead of the chloroplast would be protected from the corrosive byproducts of photosynthetic reactions. Guo and colleagues tested the idea in the mustard Arabidopsis thaliana. They took its chloroplast gene for D1, coupled it to a stretch of DNA that turns on during heat stress, and moved it to the nucleus.
The team found that modified Arabidopsis seedlings could survive extreme heat in the lab—8.5 hours at 41°C—that killed most of the control plants. The same Arabidopsis gene also protected tobacco and rice. In all three species, photosynthesis and growth decreased less than in the surviving control plants. And in 2017, when Shanghai exceeded 36°C for 18 days, transgenic rice planted in test plots yielded 8% to 10% more grain than control plants, the team reports this week in Nature Plants.
The shock was what happened at normal temperatures. Engineered plants of all three species had more photosynthesis—tobacco's rate increased by 48%—and grew more than control plants. In the field, the transgenic rice yielded up to 20% more grain. “It truly surprised us,” Guo says. “I felt that we have caught a big fish.”
Veteran photosynthesis researcher Donald Ort of the University of Illinois, Urbana-Champagne, says the group presents credible evidence of plant benefits, but he’s not yet convinced that the D1 made by nuclear genes could have repaired PSII in the chloroplast. “Anything this potentially important is going to be met with some skepticism. There are lots of experiments to do, to figure out why this works,” he says.
Guo plans further tests of the mechanism. He also has a practical goal: heftier yield increases in rice. The productivity boost his team saw in modified Arabidopsis was the largest of the three species—80% more biomass than controls—perhaps because the researchers simply moved Arabidopsis’ own D1 gene. Guo thinks rice yield might also burgeon if it could be modified with its own chloroplast gene rather than one from mustard—further heating up these already hot results.