Where some people see a humble corn plant, University of Missouri biologist David Braun sees a busy, ambitious interstate highway. Carbohydrates — often in the form of sugars — steer deftly around the vasculature of the plant, exiting and entering this internal thoroughfare as they deliver essential resources to various parts of the plant.
For example, during vegetative growth stages, sugars are routed to the young leaves and stem. During grain fill, those same nutrients hit the HOV lane and fast track their way to the corn ears.
This process, known as carbon partitioning, is integral to the plant’s growth and yield. Yet researchers don’t really know much about the system’s traffic controllers — the genes that direct the carbohydrates and tweak their routes as the plant grows.
In breeding for yield, researchers may have been optimizing this process during the years, but not consciously, said Braun. “We can see lines that have much better performance than other lines. But we don’t know why,” he noted.
Braun is now leading researchers from five universities in a project aimed at answering that question. The team is three years into a five-year, $6.6-million National Science Foundation grant to study the genes that control carbon partitioning.
Essential structures of this highway system are proteins called transporters. Like entrance and exit ramps, transporters allow sugars to move across cell membranes. Braun and his team have identified the gene sequences that encode 30 transporters. They know of at least 50 more genes involved in the process, but are still hunting for them. Braun speculated that there could be hundreds, even thousands, more genes involved.
The goal is to identify and analyze these genes so researchers can breed corn with greater precision. Knowing how to control the pathways of carbon partitioning could eventually help researchers make plants that are more drought-tolerant, higher yielding, better suited for biofuels and more resistant to pests.
Environmental stresses like drought, for example, can cause some dramatic rerouting. When water becomes scarce, plants will slow or even stop leafy growth by redirecting carbohydrates down to the roots instead.
Knowing which genes control this course recalculation could change how researchers breed for drought tolerance. “We’d really like to understand that switch, how plants can modulate where they’re sending the sugar, so we could potentially send more sugar down to the roots when there’s an oncoming drought,” Braun said. “They can build that network early and really be in a position to explore more of the soil and hopefully have a better capacity to take up the limited water that’s there and survive that drought.”
Identifying the genes that move carbon throughout the plant could also help researchers breed for plants that can store more carbon during their lifespan, Braun said.
“You can kind of think of it like charging a battery,” he explained. “We could find ways to increase the storage capacity of that battery, to build basically a bigger battery. Then you would have more ability to reallocate that stored carbon down to the ear or the roots.”
Corn growers won’t be the sole beneficiaries of this research, Braun said. The cellulosic industry could benefit, too. Grasses like switchgrass, miscanthus and setaria are close evolutionary relatives of corn, and the genes that Braun’s team turns up could control the same pathways in those species.
“The knowledge will translate, but how you’ll apply them could be quite different, because if your end goal is cellulose or lignin or starch, you’re going to want to tweak the pathway differently,” Braun said. “You want to build up a lot of stored carbon, regardless of the form, you want to be able to maximize its capture — its assimilation from carbon dioxide in the air — and then its storage. And then, depending on how you want to store it, you might target it to different [locations] for different biofuels.”
There have been some unexpected discoveries along the way. As they zoom in on the genes responsible for carbon partitioning, the team is also looking at plants where mutations have essentially closed the on and off ramps. In these plants, sugars and starches pool up in the leaves, and researchers expected these mutants to be delectable sitting ducks for insects.
To their surprise, the pests weren’t particularly interested. “I don’t understand how, because it’s not been an area of focus, but the plant has found a way to be able to accumulate very, very high levels of sugar, and they’re not especially pest sensitive,” Braun said. “So I think that there are opportunities to understand how plants protect themselves from pests when they accumulate all this high sugar and high starch.”
Ultimately, Braun and his team will not be the ones to translate this foundational research to farmer’s fields. Instead, they’re laying the groundwork for others to do so.
“I hope that the knowledge we generate will translate to collaborations with the large ag biotech companies … because they have the infrastructure to really do large-scale breeding and to bring a product to market,” Braun said. “It’s one of these benefits that’s sort of invisible to publicly funded research that definitely benefits the end user. It’s an invisible transfer of knowledge.”