Controlling root growth could help crops, combat climate change

A highly conserved ethylene signalling pathway can be targeted to control the direction of root growth, in turn creating deeper root systems that hold on to carbon and remove carbon dioxide from the atmosphere, according to scientists.

Root systems are central to plant survival and productivity, determining the plant’s access to nutrients and water and, therefore, the plant’s ability to withstand nutrient depletion and extreme weather like drought. Now, a research team led by the Salk Institute for Biological Studies has determined how a well-known plant hormone is crucial in controlling the angle at which roots grow. Their study, published in the journal Cell Reports, is understood to be the first time the hormone, called ethylene, has been shown to be involved in regulating lateral root angles that shape root systems — making the findings a revelation for plant scientists optimising root systems.

“Deep roots lead to more durable carbon storage in the soil and can make plants more resistant to drought, so the ability to control how deep roots grow is really exciting for scientists looking to engineer better root systems,” said senior author Professor Wolfgang Busch, Executive Director of Salk’s Harnessing Plants Initiative.

“We’re especially excited that the pathway we found is conserved across many types of plants, meaning our findings can be widely applied to optimise root architecture in all land plants, including food, feed and fuel crops.”

Environmental factors — like average rainfall or abundance of certain nutrients — can influence the shape of a plant’s root system. The angle at which roots grow produces different results in overall root architecture, with horizontal root angles creating a shallower root system and vertical root angles creating a deeper root system. But scientists did not previously understand clearly how these root angles were being determined on a molecular level.

Plant hormones like auxin and cytokinin have been connected to the angle of root growth in the past, but the mechanisms of that connection have remained poorly understood. In searching for molecules and pathways that were involved in setting the angle of root growth, the research team genetically screened Arabidopsis thaliana — a small flowering weed in the mustard family — for root system changes in response to thousands of molecules.

“We noticed this molecule called mebendazole was causing the roots to grow more horizontally,” said first author Wenrong He, a former postdoctoral researcher in Busch’s lab. “When we looked for what target proteins or pathways mebendazole was interacting with to have this effect, we discovered it was ethylene signalling — and ethylene playing such an essential role in root system architecture was really intriguing.”

The team observed that genes throughout the ethylene signalling pathway were activated in response to mebendazole, and, in turn, the pathway was carrying out the resulting changes in root growth. Biochemical investigation of this relationship revealed that mebendazole inhibits the activity of a protein kinase called CTR1. This enzyme negatively regulates ethylene signalling, in turn promoting a shallow root system.

The researchers now plan to target the ethylene signalling pathway in their efforts to engineer plants and crops that can withstand the environmental stresses of climate change and drought, as well as remove carbon dioxide from the atmosphere and store it deep underground. The presence of ethylene in root system architecture also inspires new questions, including whether another molecule exists that makes root systems deeper, or if there are specific genes in the already well-catalogued ethylene signalling pathway that can be targeted most effectively to promote deeper roots.

“Since ethylene signalling is a widely conserved process in land plants, targeting the ethylene pathway is a very promising technique for root system engineering,” Busch said. “Hopefully, now we’ll be able to use this tool to make crop species more resilient.”

Image credit: iStock.com/ThomasVogel