Research team designs efficient bioenergy crops that require less water to grow

Drought stress has long been a limiting factor in agricultural production worldwide, a challenge exacerbated by climate change.

For more than a century, scientists have targeted a key plant trait called water-use efficiency (WUE) to help crops grow with less water and avoid drought stress. Better WUE can help plants avoid drought stress, but for most crops, it’s also associated with lower productivity when water is abundant.

In a pair of studies published in the Journal of experimental botanyResearchers at the Center for Advanced Bioenergy and Bioproducts Innovation (CABBI) have used genetic engineering to advance improved water use in climate-friendly C4 bioenergy crops without sacrificing yield, a significant step forward for the development of a sustainable bioeconomy.

In the first study, the CABBI team was able to reduce the amount of water escaping from sorghum plants by decreasing the number of stomata, or pores, on the leaf surface, thereby improving water evacuation without limiting photosynthesis and biomass production. The researchers inserted a gene into the plants that altered their developmental pattern and reduced stomatal density.

“With C4 species, we think we can get a free lunch. We can improve water-use efficiency without having to compromise the growth of the plant when it doesn’t have enough water. And this is a special case,” said Andrew Leakey, director of CABBI and team leader of both studies.

The researchers in the second study found that the reduction in stomatal density in sugarcane and other C4 crops coincided with wider pore openings. This offset some of the expected improvement in water efficiency.

The mechanism underlying this response is not fully understood, so the discovery represents a valuable new target for designing an even more efficient plant.

Together, these findings will help maximize the production of bioenergy feedstocks, help crops mitigate the effects of inadequate water supplies and open new avenues for plant research, said Leakey, the Michael Aiken Chair and professor in the departments of plant biology and crop sciences and the Carl R. Woese Institute for Genomic Biology (IGB) at the University of Illinois at Urbana-Champaign.

“This provides an exciting opportunity for new scientific discoveries and engineering strategies,” said Daniel Lunn, a CABBI postdoctoral researcher in plant biology, IGB and the Illinois Center for Digital Agriculture, lead author of the sugarcane study.

CABBI’s principal collaborators on this research were Tom Clemente, Eugene W. Price Distinguished Professor of Biotechnology at the University of Nebraska, and Fredy Altpeter, professor of agronomy at the University of Florida. The lead author of the sorghum paper was John Ferguson, a former postdoctoral researcher at IGB.

During photosynthesis in plants, light energy is captured and used to convert water and carbon dioxide (CO₂) into energy-rich organic compounds.

Water-use efficiency refers to how much photosynthetic carbon a plant obtains—or more broadly, how much biomass it produces—relative to how much water it uses. In these studies, the researchers focused on the leaf level, measuring the amount of water and CO₂ entering and exiting through the stomata.

In the vast majority of cases, and in the vast majority of efforts to increase water efficiency in plants, scientists face a tradeoff that holds back crop improvement: Making them more water-efficient reduces their inherent productivity, their photosynthetic carbon gain, and their growth rate. “So plants do better when they don’t have enough water, but they do worse when they have enough. From a broader agricultural perspective, that’s a pretty undesirable tradeoff,” Leakey said.

But C4 crops, including sorghum, sugarcane and miscanthus, the bioenergy crops targeted by CABBI, are built differently. They have a “fuel-injection version of photosynthesis” that concentrates CO₂ inside the leaf before capturing it, “whereas most plants are like a Model T Ford, where they run on a naturally aspirated engine,” Leakey said.

Although C4 crops represent only 5% of all plant species, they are becoming increasingly vital for agricultural production of food, fuel and fibre. They are important examples of emerging biomass crops, including sugarcane and miscanthus, which sequester carbon while providing a basis for the manufacture of bioproducts.

With the new WUE research, “we’re taking plants that already have an advantage as crops and potentially making them even better without slowing down carbon gain,” Leakey said.

The team is studying this engineering approach in other CABBI plant species and refining the design. The groundbreaking work on miscanthus by other researchers in CABBI’s feedstock production team (sequencing the miscanthus genome and developing the first gene editing techniques) “will allow us to pursue this engineering strategy in a very important emerging perennial feedstock crop that can sequester a large amount of carbon,” Leakey said.

“Overcoming water limitations in agricultural production is truly critical to achieving our mission of supporting a profitable, sustainable and resilient bioeconomy,” he added.

Leaves, roots and other plant features have evolved to address the fundamental tradeoff between carbon gain and water loss in photosynthesis, and these processes have a primary impact on where crops can grow without irrigation, Leakey said.

Developing crops that require 10 to 20 percent less water could expand the rain-fed agricultural region of the United States farther west and allow farmers in the current growing area to maintain profitable crops even in years without sufficient rainfall—a more common threat under climate change.

“Part of what we’re trying to do here is maintain increased productivity in times and places where water supplies are scarce,” Leakey said.