A new look at drought-stressed switchgrass reveals why breeding and biofuel science must work together to fuel the future.
In the wake of increasingly unpredictable weather patterns, researchers are zeroing in on an underappreciated hurdle in the biofuel pipeline — the chemistry of drought-stressed plants.
At the University of Wisconsin-Madison’s Great Lakes Bioenergy Research Center (GLBRC), senior scientist Trey Sato leads efforts to understand why switchgrass grown in dry conditions produces less ethanol. The culprit? A natural compound called saponin.
“In collaboration with others, we found that high concentrations of plant molecules called saponins inhibit the fermentation of switchgrass hydrolysates to biofuels by our yeast strain,” Sato says.
Saponins are part of the plant’s innate defense system, offering protection against fungal pathogens like rust. But their increase during drought becomes a problem for downstream fermentation — the critical step where sugars are converted into usable fuel.
“One answer could be to engineer switchgrass and other bioenergy crops to make fewer saponins,” he says.
“However, saponins are important plant defense molecules… so, reduction in saponin production will likely affect the plant’s fitness in the field and potentially reduce biomass yield.”
Sato sees hope in the middle ground. Breeders might not need to choose between plant health and fermentation potential. The yeast itself could evolve to do the heavy lifting.
“There is hopefully a middle ground, where the plant produces enough saponins to resist most fungal pathogens, while genetic engineering can make the biofuel-producing yeast more tolerant to those saponins,” he says.
The Hidden Chemistry of a Hotter Future
Switchgrass has long been a darling of the biofuel world — a hardy perennial that grows on marginal land and generates six times more energy than it takes to grow. But climate extremes like the 2012 drought in Wisconsin revealed just how much chemistry can change when plants are stressed.
GLBRC researchers compared switchgrass harvested in both drought and non-drought years. The difference in fermentation was striking. When broken down by conventional pretreatment and hydrolysis methods, the drought-year biomass yielded significantly less ethanol.
The solution isn’t to reject drought-grown biomass outright, but to adjust how it’s processed.