FRONTIERS Harnessing plants for the future

Plants are excellent at scrubbing carbon from the atmosphere. Every year, the world’s plants breathe in around 64 gigatons of CO2 from the air, storing the carbon in leaves, shoots and roots. To get a sense of the staggering amount of carbon plants absorb, consider that an adult African elephant at the San Diego Zoo can weigh up to 6.8 metric tons. One gigaton is equivalent to more than 100 million of those elephants. Now multiply by 64.

While plants store this carbon in the form of numerous biomolecules, almost all of these materials are degraded by animals, fungi and bacteria and the stored CO2 thereby released. The Salk team has identified one particular plant-made molecule, called suberin, that is highly resistant to this degradation and can thereby remain in the soil. Suberin, better known to wine aficionados as cork, is a waxy, water-repellant and carbon-rich substance, and is at the heart of the Salk team’s strategy to address the problem of meeting human needs while reducing carbon in the atmosphere. Each growing season, annual crops—plants that live only one season, such as corn or wheat—breathe in CO2 to build plump leaves, tall stems and thick roots.

At the end of the season, the plants die and rot, releasing much of that carbon back into the atmosphere.

“What if you could tweak this carbon cycle in the plant so that it absorbed more carbon and released less?” says Joseph Noel, a member of Salk’s plant biology team and Howard Hughes Medical Institute investigator.

“We realized that a crop with a larger, suberin-dense root would capture more carbon in the ground,” he says. “Roots last longer than other parts of plants, particularly in perennial plants that live multiple years. Even in dead roots, suberin decays very slowly.”

Carbon stored in suberin could potentially stay in the soil for hundreds or even thousands of years. So you have two possible benefits: store more carbon and store it for a long time. An expert on the structure and chemistry of compounds produced by plants and on how plants have evolved unique ways to make their own specialized products to adapt to nearly every ecosystem on Earth, Noel is exploring how plant cells can be coaxed to produce more suberin—and thus capture more carbon.

Increasing the amount of suberin each cell produces is one strategy; another is to increase the number of cells—in other words, to grow larger roots. In both cases, the newest addition to Salk’s plant biology team, Associate Professor Wolfgang Busch, is providing critical expertise. Busch combines techniques from genetics, genomics and other science fields to understand how root growth in given environments is determined by a plant’s genes.

“There is still a great deal we don’t know about how genes and molecular mechanisms determine how a plant root decides to grow to a certain size or in a certain direction in the soil,” Busch says. “To breed crops with large roots that produce a lot of suberin, we’ll need a better understanding of how to influence both aspects at the genetic and molecular level.”

In addition to his expertise, Busch brought a plant with him to Salk: Lotus japonicus. A native plant of Japan, known commonly in English as birdsfoot trefoil, Lotus is a wild legume that has become the laboratory stand-in—a “model organism” in scientific parlance—for studying the biology of all legumes, a group of plants that includes chickpeas, beans, peanuts and lentils. In their quest to develop the ideal plant, the Salk team will experiment with both Lotus and Arabidopsis thaliana, the common mustard weed that is the predominant plant research model. Much of what scientists know about the fundamentals of plant molecular and cellular biology has come from studying Arabidopsis. Adding Lotus to the research mix will allow the Salk team to expand its research to focus on a plant more closely related to crops. This is important to the Harnessing Plants Initiative, as the carbon-capturing plants the Salk team is designing would need to be food-producing crops deployed at a scale sufficient to put a significant dent in global warming.

“Most of the world’s arable land is already being used for agriculture, and we are already straining our natural ecosystems and our capacity to generate food and other plant products,” says Busch. “A legume crop bred with a large root could be planted on farms around the world and could potentially have multiple advantages over current crops.”

Deploying an “ideal” legume crop on a relatively small portion of the world’s farmland could capture a significant portion of global CO2 emissions. Among the other advantages of legumes is that crops could be perennials, surviving and producing year after year, which avoids annual replanting. The roots would remain undisturbed, keeping the plant’s carbon haul locked underground. Legumes also tend to make friends. Most grow nodules on their roots as homes for bacteria called rhizobia, which convert nitrogen from the air into usable forms the plants need to thrive. This is why farmers include legumes in crop rotations with other plants to help replenish soil nitrogen. The crops envisioned by the Salk team thus require less fertilizer than normal crops, reducing the harmful farm run-off that damages aquatic ecosystems.