In many ways, plants are remarkable models of efficiency. They take in carbon atoms by photosynthesizing carbon dioxide naturally from the air and absorb hydrogen and oxygen molecules from the water that helps sustain them.
Nitrogen, however, is the problematic curveball. Even though it also exists in the air, plants can’t absorb it that way. It’s a critical component of key processes like protein synthesis and the creation of nucleic acids, but the only way for plants to get it is through the soil. And when there isn’t enough nitrogen there naturally, it’s added through nitrogen-based fertilizer, which can run off into surrounding lakes and rivers, creating a host of environmental problems.
It’s a problem that Professor of Botany Hiroshi Maeda has been wrestling with for years. Maeda’s lab studies plant metabolism — the ways in which plants process the chemicals that keep them alive and help them grow. In a recent study published in the scientific journal Nature Plants, Maeda was part of an international group of scientists, including researchers from Germany, Japan and Michigan State University, that mapped the nitrogen-specific enzymes of a particular plant, suggesting that it may be possible to supercharge plant metabolism and allow it to more efficiently utilize nitrogen.
In the study, the group focused on a particular type of plant enzyme called aminotransferase that moves nitrogen from one molecule to another within the plant, facilitating key chemical reactions like metabolism and growth. They began by studying nitrogen utilization in Arabidopsis, a flowering plant from the mustard family whose genes are already mapped and easily manipulated in the lab setting.
While others have focused on the transporters that uptake nitrogen from the soil into the plant, Maeda’s group has been charting how the nitrogen is utilized once it’s there.
“If we find a way for the plants to efficiently uptake and utilize nitrogen, we may be able to avoid dumping so much nitrogen into the soil, reducing the cost of making fertilizer and also reducing the environmental cost of releasing it into the environment,” says Maeda.
According to Maeda, the study is an early step in what promises to be a long process. There are hundreds of thousands of plant species, and each may leverage aminotransferase differently. Understanding how the enzymes work within Arabidopsis allows scientists like Maeda to begin looking at other types of plants, including grasses, legumes and certain types of algae.
“We are now characterizing this group of enzymes in multiple plant species and trying to understand how this enzyme works in different plants,” he says. “Going from there, we can find the key target enzyme to improve the efficiency of utilizing nitrogen in plants.”
The scientific precedent for this work already exists. Scientists like Maeda and many others have mapped the ways that plants metabolize carbon and have used that understanding to improve the production of proteins, lipids and sugars in different types of crops.
“In the case of nitrogen, we are way behind,” says Maeda. “We don’t know yet how exactly nitrogen is moved around and utilized in plants, but by understanding it, we’ll be able to devise an approach to improve the process.”
The group’s findings stand to alter the way plant scientists think about plant enzymes. By testing more than 4,000 reactions to see which chemicals each plant enzyme would catalyze, Maeda and his fellow researchers discovered that several aminotransferases within the Arabidopsis plant could carry out many more reactions than previously reported. That ability to “multitask” makes the system messier but could also open a lot of avenues for improvement.
“Our computational model suggests that this messiness may be important for the flexibility or the robustness of plants to grow under different conditions, because plants grow under environments where temperature, humidity and nitrogen levels change constantly,” Maeda explains. “Maybe that flexibility and the messiness of this metabolism are important for plants to adapt and be able to be resilient under changing conditions.”
Maeda believes that the conceptual shift in understanding of this complex metabolism is the key to optimizing the system.
The group’s work has been funded both by the U.S. Department of Energy and the National Science Foundation. Maeda greatly appreciates their generous support. Maeda, however, admits that recent uncertainties regarding scientific funding at the federal level remain a cause of stress and concern in running his laboratory, especially supporting students and staff who are carrying out these exciting works.
“Both of these organizations realized the importance of understanding how nitrogen is utilized within the plants and supported our work,” says Maeda. “It’s important for agricultural crop production, as well as bioenergy chemical production. Hopefully, we can continue to explain and convince society that there is a benefit of supporting this kind of fundamental research.”