Supercharged Photosynthesis

In December, geneticists announced that they’d made a major advance in engineering rice plants to carry out photosynthesis in a more efficient way—much as corn and many fast-growing weeds do. The advance, by a consortium of 12 laboratories in eight countries, removes a big obstacle from scientists’ efforts to dramatically increase the production of rice and, potentially, wheat. It comes at a time when yields of those two crops, which together feed nearly 40 percent of the world, are dangerously leveling off, making it increasingly difficult to meet rapidly growing food demand.

The supercharged process, called C4 photosynthesis, boosts plants’ growth by capturing carbon dioxide and concentrating it in specialized cells in the leaves. That allows the photosynthetic process to operate much more efficiently. It’s the reason corn and sugarcane grow so productively; if C4 rice ever comes about, it will tower over conventional rice within a few weeks of planting. Researchers calculate that engineering C4 photosynthesis into rice and wheat could increase yields per hectare by roughly 50 percent; alternatively, it would be possible to use far less water and fertilizer to produce the same amount of food.

The December results, achieved by the C4 consortium and led by Paul Quick at the International Rice Research Institute (IRRl) in the Philippines, introduced key C4 photosynthesis genes into a rice plant and showed that it carried out a rudimentary version of the supercharged photosynthesis process. “It’s the first time we’ve seen evidence of the C4 cycle in rice, so it’s very exciting,” says Thomas Brutnell, a researcher at the Danforth Plant Science Center in St. Louis. Brutnell is part of the C4 Rice Consortium headed by IRRI, which has funding from the Bill & Melinda Gates Foundation, but was not directly involved in the most recent breakthrough.

Despite the genetic changes, the altered rice plants still rely primarily on their usual form of photosynthesis. To get them to switch over completely, researchers need to engineer the plants to produce specialized cells in a precise arrangement: one set of cells to capture the carbon dioxide, surrounding another set of cells that concentrate it. That’s the distinctive wreath anatomy found in the leaves of C4 plants. However, scientists still don’t know all the genes involved in producing these cells and suspect that they could number in the dozens.

New genome editing methods that allow scientists to precisely modify parts of plant genomes could help solve the problem. Using conventional breeding to manipulate more than one or two genes is a “nightmare,” Brutnell says, let alone trying to engineer a plant with dozens of gene changes. Genome editing could make it possible to change a large number of genes easily. Says Brutnell: “Now we have the toolbox to go after this.”

It can be a decade or more before even simple crop modifications reach farmers, let alone changes as complex as reëngineering how plants carry out photosynthesis. But once scientists solve the C4 puzzle in a plant such as rice, they hope, the method can be extended to dramatically increase production of many other crops, including wheat, potatoes, tomatoes, apples, and soybeans.

—Kevin Bullis