Roots stop growing in densely packed hard soils not because they’re physically constrained but because of a chemical signal from the plant hormone ethylene, new research suggests, so turning the signal off might improve stunted crop yields.

The study, published January 15 in Science by a diverse group of researchers from around the world, challenges a decades-long theory that plant roots don’t penetrate as deep in compacted soils because they lack the necessary muscle.

“Our work really turns this whole thing on its head, and it suggests that plants stop before they really need to,” said Malcolm Bennett, senior author of the study and a professor of plant sciences at the University of Nottingham School of Biosciences. 

Soil compacts naturally through its own weight and gravity, particularly in deeper areas. But the process has been exacerbated through modern agriculture, as intensified farm operations led to heavier farm equipment and tillage practices. Short crop rotations, intensive grazing and inappropriate soil management can also cause compaction.

The result is the degradation of about 65 million hectares of land worldwide, according to a 2005 literature review, or approximately 160.6 million acres. Compacted soil is more dense and less porous, blocking transportation routes for water and nutrients.

It also limits gas diffusion between roots and their surrounding soils. In the new study, by covering root tips with a barrier that gases couldn’t permeate, the researchers discovered how, when confronted with this conundrum, the plant hormone ethylene becomes trapped in the area around the root tip, ultimately building up in the root tissues. That serves as a stop sign to growing roots, even when they are otherwise able to break through the soil, according to the study.

This reduction in root growth can reduce crop yields by up to one-quarter, the study’s authors note. In the U.K., where Bennett is based, soil compaction costs an estimated 400 million to 650 million euros annually, or about $487 million to $791 million.

“When combined with, for example, drought, when the roots are not able to penetrate the soil and obviously the soil is even harder because of the lack of moisture, you can lose up to 75% of yields,” Bennett said. “So it can be really quite catastrophic.”

In addition to using sand as well as clay soils, the experiment involved both rice, a model cereal that’s a close relative of many important crops such as wheat and maize, and Arabidopsis, which is similar to canola.

“They’re separated by over 100 million years of evolution, and yet they use the same signal and the same mechanism,” Bennett said. “We’re pretty confident that this is a really conserved mechanism amongst plants that grow in soil. And so we’re pretty confident it should work across most cultivated plants or food crops.”

Subject to funding, in another phase of the research, the team would like to conduct field trials to look at how crops with genetically altered ethylene sensing perform under compacted soil or drought conditions. 

“Ethylene as a signal plays several important roles within the plant,” including pathogen response, Bennett explained. “We want to just switch down the ethylene sensing just to where it’s important for soil compaction resistance.” 

Another potential way for farmers to put the finding to use would be to select crops that are already ethylene-resistant. In the field, the team is also interested in screening thousands of crops like wheat and maize for natural variants that may be resistant in the roots but sensitive in the shoots. 

Bennett credited the diverse specialties of his team, which included different types of scientists and mathematicians, with making the findings possible.

“In contrast to many plant scientists, we actually talked to soil scientists,” he said. “Rather than just study the behavior of the plant, we also studied the behavior of the soil. And by having this kind of integrated viewpoint, we were able to get a more informed understanding of the process.”

The study “Plant roots sense soil compaction through restricted ethylene diffusion,” published Jan. 15 in Science, was authored by Bipin K. Pandey, University of Nottingham; Guoqiang Huang, Shanghai Jiao Tong University; Rahul Bhosale, University of Nottingham; Sjon Hartman, Utrecht University and University of Birmingham; Craig J. Sturrock, University of Nottingham; Lottie Jose, University of Nottingham; Olivier C. Martin, Institute of Plant Sciences Paris-Saclay (IPS2); Michal Karady, Laboratory of Growth Regulators; Laurentius A. C. J. Voesenek, Utrecht University; Karin Ljung, Swedish University of Agricultural Sciences; Jonathan P. Lynch and Kathleen M. Brown, Pennsylvania State University; William R. Whalley, Rothamsted Research; Sacha J. Mooney, University of Nottingham; Dabing Zhang, Shanghai Jiao Tong University and University of Adelaide; and Malcolm J. Bennett, University of Nottingham.