From morning glories spiraling up fence posts to grape vines corkscrewing through arbors, twisted growth is a problem-solving tool found throughout the plant kingdom. Roots "do the twist" all the time, skewing hard right or left to avoid rocks and other debris.
Scientists have long known that mutations in certain genes affecting microtubules in plants can cause plants to grow in a twisting manner. In most cases, these are "null mutations," meaning the twisting is often a consequence of the absence of a particular gene.
This still left a mystery for plant scientists such as Ram Dixit, the George and Charmaine Mallinckrodt Professor of Biology at Washington University in St. Louis. The absence of a gene should cause all sorts of other problems for plants, yet twisted growth is an incredibly common evolutionary adaptation.
Dixit, with the help of his former PhD student Natasha Nolan and the WashU McKelvey School of Engineering's Guy Genin, has found a possible answer, now published in Nature Communications.
As it turns out, there's no need for a full null mutation for the twist, only a change in gene expression in a particular location - the plant epidermis.
"That might explain why this is so widespread: you don't need null mutations for this growth habit, you just need ways to tweak certain genes in the epidermis alone," said Dixit, who is also chair of biology in Arts & Sciences.
The research emerged from the National Science Foundation Science and Technology Center for Engineering Mechanobiology (CEMB), a nationwide consortium co-led by WashU that brings together biologists, engineers and physicists to understand how physical forces shape living systems.
"This discovery is a perfect example of what our center was designed to do," said Genin, the Harold and Kathleen Faught Professor of Mechanical Engineering and co-director of CEMB. "By combining biological experiments with mechanical modeling, we uncovered a fundamental principle: the outermost layer of the root dominates its twisting behavior through the same torsion physics (twisting from applied torque) that explains why hollow tubes can be almost as strong as solid rods. Geometry matters enormously."
Feeding a changing world
Beyond an evolutionary curiosity, understanding how roots navigate soil is more urgent than ever. As climate change intensifies droughts and pushes agriculture onto marginal lands with rocky, compacted soils, crops with root systems that can thrive in challenging conditions are becoming a critical need.
"Roots are the hidden half of agriculture," said Charles Anderson, a professor of biology and CEMB leader at Pennsylvania State University and a co-senior author on the paper.
"A plant's ability to find water and nutrients depends entirely on how its roots explore the soil. If we can understand how roots twist and turn past obstacles, we could help crops survive in places they currently cannot."
Twisted growth also plays roles in how vines climb, how stems resist wind and how plants anchor themselves against erosion - factors that are critical for both food security and ecosystem resilience.
Solving the mystery
Using a model plant system where roots can skew right or left, Nolan set about trying to see what plant cell layers regulate the twisting behavior.
Plant cells are rigidly locked in place, almost glued together and surrounded by a tough cell wall. The team hypothesized that the twists emerge from the inner cortical layer where mutation causes the cells to be short and wide instead of long and skinny. The thinking was that the twisting phenotypes emerge because the epidermal layer must "lean over" to maintain its structural integrity and reach its squat cortical-layer neighbors.
Nolan, who now works at Pivot Bio, wanted to see if they could restore the straight roots by expressing the wild-type gene in a cell layer-specific manner instead of throughout the root as previously had been done.
The striking finding that emerged was that if they expressed this wild-type gene (that keeps the root straight) in any of the inner cell layers, then those plants still looked exactly like the null twisty mutant. "It didn't matter that you now had that protein being made in some of the inner cell layers, it was as if it didn't exist," Dixit said.
In contrast, when the wild-type gene was expressed only in the epidermis, the roots went straight. That told researchers that "the dominating cell layer, that's really dictating this behavior, is the epidermis," Dixit said.
Mystery solved, the epidermis is calling the shots on the twist. But how? That's where mechanobiologists came in, including co-authors Genin and Anderson.
Anderson's lab measured the orientation of the cellulose microfibrils in mutant and wild-type roots. The twisty defects seem to alter the cellulose deposition, and Genin took that data and created a computer model explaining why the epidermis dominates.
"When you have concentric layers of cells, like rings in a tree trunk, the outer ring has far more leverage over the whole structure than the inner rings," Genin explained. "Our model showed that if only the epidermis has skewed cell files, it can drive about one-third of the total twisting you'd see if every layer were skewed. But if you fix just the epidermis, the whole root straightens out. The math was unambiguous: the outer layer rules."
The model confirmed what Nolan found in her experiments. When she expressed the wild-type (straight root) gene in only the epidermis, it affected even the cortical cells that still carried the mutation. Instead of being short and wide, those inner cells became longer and skinnier, almost like the wild type.
"Somehow the epidermal cell layer is able to entrain inner cell layers," Dixit said. "The epidermis is not a passive skin, but instead a mechanical coordinator of the growth of the entire organ."
Now that scientists understand how plants "do the twist," they can apply those findings to addressing challenges of agricultural science.
"Imagine being able to design plants that dial up or dial down a root's tendency to twist," Anderson said. "In rocky, inhospitable conditions, you might want roots that corkscrew past obstacles. This research gives us a target and a mechanical framework for thinking about root architecture as an engineering problem."
It's the kind of problem that requires multiple perspectives, Genin added.
"A biologist alone might have found that the epidermis matters, but wouldn't have had the tools to explain why. An engineer alone couldn't have done the genetics and phenotyping," he said. "Together, as a center, we got the full picture."
Nolan, N., Jaafar, L., Fan, Y. et al. The epidermis coordinates multi-scale symmetry breaking in chiral root growth. Nat Commun 16, 11022 (2025). https://doi.org/10.1038/s41467-025-66029-8
This work was supported by the Center for Engineering Mechanobiology, a National Science Foundation Science and Technology Center, under grant agreement CMMI: 15-48571 (C.T.A., G.M.G., and R.D.) and the National Institute of General Medical Sciences of the National Institutes of Health under award number R35GM139552 (R.D.). N.N. was supported by a William H. Danforth fellowship in plant sciences. Work by J.G.O. was supported by the Center for Lignocellulose Structure and Formation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0001090.
