Tweaking how plants manage a crisis boosts photosynthesis

Enhancing just three genes helps plants harvest more light, raising new hopes for developing crops that can keep up with food demands from a crowded planet.

Genetically engineered tobacco plants, chosen to test the concept, managed the unusual feat of growing 14 to 20 percent more mass — meaning more crop yield — than untweaked plants, says Krishna Niyogi of the University of California, Berkeley and Lawrence Berkeley National Laboratory. The gains came from inserting different versions of three genes that control how quickly plants ramp back up to full energy-harvesting capacity after going into a protective mode to protect themselves from too-bright sunlight, researchers report in the Nov. 18 Science.

Among results published so far, “to my knowledge, this is the first example where crop growth has been enhanced by improving photosynthesis,” says plant physiologist John Evans at Australian National University in Canberra, who wasn’t part of the new project.

Photosynthesis, the basic green chemistry for converting the sun’s energy into food, isn’t a perfectly efficient process (SN: 2/20/16, p. 12). And the quest to improve efficiency by manipulating the interlocking steps of more than 100 reactions in living crops has been complex. “We can make things worse, but this is the first time we can make something better,” Evans says.

The underlying idea for the tobacco experiment came from an appreciation of how light and shade dance over leaves throughout the day in a farm field. Sudden blasts of intense sunlight are dangerous stuff; an overload can lead to chemical scorching in a plant’s light-catching chloroplasts. So when the sun’s movement or a toss from a breeze suddenly exposes a chloroplast to more sunlight than it can handle, a protection system kicks in.

Enzymes in the leaf create a surge of a paprika-colored molecule called zeaxanthin, which helps offload the excess energy as heat. This protection turns on within minutes, but turns off more slowly when the crisis is over, Niyogi says.

Restoring full photosynthesis takes a lot more than just enhancing the back-to-normal mechanisms. An enzyme called ZEP dismantles protective zeaxanthin when it’s no longer needed. But making the plant simply build more ZEP keeps the protective system from turning on properly in the first place — which could put a plant at risk. So researchers also enhanced the enzyme called VDE that builds the protective zeaxanthin. With those two enzymes in balance, a chloroplast can still rid itself of excess energy but get back to full operations faster.

Enhancing a third protein, PsbS, also helped, although researchers don’t yet understand the full details of how. Tobacco plants with modified versions of all three proteins grew bigger, as measured by the weight of dried plant material, than others.

The extra growth those genes produced “is a major, economically important gain,” says Maureen Hanson of Cornell University, who is working on a different approach to improving photosynthesis. Now, she says, the new paper’s idea is ready for attempted transfer to plants that people harvest for grains or fruits. Hanson is hopeful that size will increase there, too.

Coaxing plants to calm down faster after a crisis is just one strategy to make photosynthesis more efficient. Evans and Hanson are among those involved in efforts to improve a notoriously slow and distractible photosynthetic enzyme called Rubisco (SN Online: 9/19/14). Other researchers are trying to transfer a naturally more efficient photosynthetic system found in some tropical and subtropical plants, called C4 photosynthesis, into rice, one of the world’s main grains.

Older strategies for wringing more food from farms are not on track to keep up with soaring human population and food demands, Niyogi says. The United Nation’s Food and Agriculture Organization has estimated that feeding the world in 2050 could require boosting food production by an additional 70 percent. But the success of all of this, Niyogi notes, may depend on how people around the world feel about genetically engineered food.