Just because plants photosynthesize doesn't mean they can't get a form of sunburn—damage caused by overexposure to light. That's why all plants rely on a mechanism that defends against excessively bright sunlight by converting photons into harmless heat. But like someone who is slow to doff their sunglasses indoors, this botanical sun shield is slow to turn off when a shadow passes over a leaf. The result: Photosynthesis stays depressed.
Now, plant biologists seeking improved photosynthesis—and, ultimately, more bountiful crops—have cleverly manipulated plants to adjust more quickly to shade. Genetically engineered into tobacco plants, the faster response yielded up to a 20% increase in biomass. The proof-of-concept study, which appears today in Science, is "ground-breaking" and the first convincing field trial in the hot area of engineering photosynthesis, says Robert Furbank, an integrative plant biologist at the Australian National University in Canberra.
Traditional plant breeding has greatly boosted yields of popular crops. During the green revolution, for example, Norman Borlaug and others nearly doubled wheat yields by creating plants with short, sturdy stems that could hold a greater load of grain. Nowadays, breeders can get crops to put about 50% to 60% of their biomass into seeds. But the gains have stagnated at less than 1% per year because plant growth is now limited by the efficiency of photosynthesis itself.
Research teams are trying to break the bottleneck in multiple ways. One long-held dream is taking a high-power type of photosynthesis found in corn and three other crops, called the C4 pathway, and putting it into rice. Another goal is improving RuBisCO, the notoriously sluggish enzyme that catalyzes early stages in the conversion of carbon dioxide into useful organic molecules.
More recently, a few researchers have contemplated streamlining an aspect of photosynthesis called photoprotection. To guard themselves from bright light—as well as balance their metabolic processes—plants rely on a mechanism called nonphotochemical quenching (NPQ), in which chloroplasts divert photons from their light-harvesting molecules and simply waste them as heat. In dim conditions, plants can turn off NPQ to boost photosynthetic efficiency. But although they can raise the shield in a few minutes, lowering the defenses can take hours, which limits photosynthesis in the shade.
This time lag isn't a problem for wild plants, for which survival and reproduction are paramount, but it's a disadvantage for farmers who want to maximize biomass. In 2004, plant physiologist Stephen Long of the University of Illinois in Urbana and colleagues calculated that NPQ operating under typical conditions for a midlatitude farm can reduce the amount of carbon dioxide turned into sugars by up to 30%.
After reading Long's paper, geneticist Krishna Niyogi of the University of California, Berkeley, had an idea for how to turn off NPQ faster. The strategy was to add extra copies of three genes whose proteins are responsible for relaxing the protection. The higher protein levels should speed the response to shade. Niyogi, Long, and their postdocs took these genes from the widely studied mustard Arabidopsis thaliana and inserted them into tobacco plants, which are relatively easy to modify. After lab and greenhouse testing, they planted them in a test field near the University of Illinois. The modified tobacco bulked up their leaves, stems, and roots, weighing 14% to 20% more than unmodified plants after 22 days.
"To see something like that increase in a field trial was astonishing," Niyogi says. The bonus came without apparent side effects, although the researchers could not test for any loss of disease resistance or stress tolerance in such a small field trial.
The big question is whether similar manipulations in food crops will mean more consumable yield. To find out, Niyogi and Long have already started to put the genes into elite breeding lines of rice and maize, and other crops could follow. Long also predicts that researchers will find ways to turn off NPQ even faster, and perhaps generate even more biomass. "We think this could still be bigger than we have now."
It's also possible that the same effect could be achieved without moving genes between species, which might ease regulatory approval or improve consumer acceptance. Plants normally silence any extra copies of their own genes, but editing the genes or promoters using CRISPR or another technique could get around that barrier, allowing researchers to work with a species's own genetic material.
Regardless of how it's done, boosting photosynthesis could help researchers answer critics of plant biotechnology who complain that genetically modified plants have not boosted harvests, says Dario Leister, a plant molecular biologist at Ludwig-Maximilian University of Munich in Germany. "Making plants that yield more: That is something that everyone should be happy about."