Research Highlights

Finding Traits to Help Soybeans Withstand Drought

By Sarah Hill

When it comes to soybeans, no two varieties look the same. Some phenotypes are referred to as slow canopy wilting, meaning they retain water better and don’t wilt as quickly as traditional soybean varieties. Soybean researchers and geneticists are working to crossbreed soybeans with landraces that are not indigenous to North America to result in soybeans that are more resistant to adverse conditions like drought.

One example of these soybean varieties is called PI 567731, referred to as “731.” Troy Wood, professor of chemistry at the State University of New York at Buffalo, has been collaborating with University of Missouri Plant Sciences Professor Henry Nguyen, one of the world’s foremost soybean genetics experts. The scientists compared 731 to another soybean variety called Pana.

“The soybean plants are grown in Missouri, but we analyze them here in New York,” Wood says.

Compare and Contrast

Wood and Nguyen found a lot of differences when it comes to the two soybean varieties. To start, Pana is less tolerant of dry conditions than typical soybeans.

“When Pana has a lack of access to water, things start happening quickly,” Wood says. “You can’t keep water content in the plant. We look at 731, and it’s good at retaining water.”

Water molecules pass through tunnels called aquaporins to escape plant leaves. By inhibiting that process, water loss is slowed down. Crossbreeding has slowed this aquaporin process down, leading to plants that are heartier and more tolerant of drought.

Another trait where 731 has the advantage over Pana is a higher chlorophyll content. 

“In soybeans, chlorophyll A is the most dominant, but 731 has more chlorophyll B than Pana,” Wood explains. “We wonder if chlorophyll B has more energy transfer molecules that take energy from the sun and shifts them over to chlorophyll A, giving the plant an additional energy supply mechanism.”

Pheophytins are very closely related to chlorophyll, and during drought conditions, 731 had greater levels of pheophytins. 

“They’re an indicator that some chlorophyll has degraded,” Wood says. “At the end of the season, you’d expect there to be a higher pheophytin to chlorophyll ratio, and that’s what we found in our research.”

It’s this ratio that enables the soybean plant to harvest sunlight. As that ratio changes, the plant’s leaves start turning from green to brown, yellow, or orange, as its chlorophyll is converted to pheophytin. Soybean seeds can struggle to fully change color in dry conditions, remaining undesirably green, but a higher pheophytin level means that 731 is better at completing this process.

The soybean variety 731 also has more plastoquinone, which is an effective electron transport molecule. Basic chemistry tells us that when molecules have an odd number of electrons, an undesirable radical is produced. Elevated levels of radicals indicate high oxidation damage to the plant, while the presence of more plastoquinone creates more opportunity for electrons to be transported away, resulting in less oxidative damage. 

Wood notes that it’s interesting that 731 has larger sugar chains—another beneficial quality when faced with drought.

“Sugars are a source of energy, and when that energy is stored up from what the soybean plants harvest from the sun, it’s then available when the plant needs it, such as during a drought—when a soybean plant needs all the energy it can get,” Wood says. “We saw evidence that the 731 soybean plants were creating sugar from the beginning of germination, which had grown to significant levels by two to three weeks. The soybean plants are trying to maximize their survival by building up those sugars.”

Evaluating Lipid Composition

“At a metabolic level, we don’t yet know how drought impacts stomatal closures and hormone signaling,” Wood says. “One adaptation that’s made 731 more resistant to slow canopy wilt seems to be somewhere in the lipid composition.”

Soybean plants have a waxy layer called cutin that helps prevent water evaporation from leaves, according to Wood. The components of the cutin are different for 731 and Pana, likely contributing to 731’s better water retention.

Wood and Nguyen started their analysis by looking at the leaves and then moved on to the seeds, focusing on soybeans’ nutritional value. They used a traditional process to evaluate the soybeans’ metabolic profile.

“We used a mortar and pestle to pulverize the plant tissue, then used a solvent like ethyl acetate to extract the lipid components,” Wood says. “We then analyze the components to find out what’s there by separating the components using liquid chromatography.”

Each lipid component moves down a column at a different rate and time, so they can use that method to determine the type of lipids. If that doesn’t give them enough information, a mass spectrometer can identify different compounds by classifying the molecular formula. The research team then compares the formula to a database to see which compounds it matches. 

The researchers will continue working to understand the compounds that help 731 tolerate drought. These researchers are taking another step forward in developing soybean varieties that can produce higher yields even in challenging weather conditions.

Published: Oct 23, 2023