Researchers crack twirling rice root mystery

Normal rice roots grow in a process called circumnutation, in which they spiral and circle as they move downward, while mutant plants stretch their roots straight down. Scientists say both characteristics are useful in particular circumstances. | Reuters/Afolabi Sotunde photo

Plants whose roots grow in a winding, corkscrew motion are better equipped to search for small holes through hard soil

You can’t see it, but the root tips of some rice varieties twirl and wiggle as they grow down into the soil.

Scientists at Duke University in North Carolina used video footage to study the corkscrew-like motions and gain insight into why this occurs.

When they played the footage back at 15 frames per second, compressing over 100 hours of growth into less than minute, they watched as rice roots gained their first foothold in the soil, allowing the seed to anchor firmly so that the plant could grow as the roots burrowed down in a winding, helical path.

“This was a classic example of the serendipity of science,” said Philip Benfey, a professor at Duke’s biology department. “Doing the research that we do, you stumble on things. We were looking for different lengths of roots, roots that would grow deeper and change their architecture.”

He said graduate student Kevin Lehner had evidence pointing to where the gene important in making roots grow longer is located.

After studying mutant rice plants, Lehner saw the genes that had been mutated were located in the same region of the chromosome and those plants had longer roots. He set up a video camera on a normal rice plant alongside the mutant plant and saw that the normal rice plant had a root tip that twisted around in circles while the mutant had a root that grew straight down.

Benfey said the mutant plant does not have a longer root, but stretches its length straight down, whereas the normal root grows in a process called circumnutation, in which it spirals and circles as it moves downward.

The question became why rice root tips adopt this strategy and what value it is to the plants.

Sprouting seeds have a challenge. If they are to survive, the first tiny root must anchor the plant and rapidly probe down to suck up water and nutrients for growth.

“The primary role is early anchoring,” said Benfey. “The second role is that the root is going through the primary topsoil and then it gets down to a more compressed area, soil that is compacted naturally or has become hard because of farm equipment. The roots rarely pound their way through that. They must search for pores or small holes.”

The researchers learned that a normal root tip’s corkscrew growth is co-ordinated by the plant hormone called auxin, a substance that promotes stem elongation.

Auxin can move around the tip of a growing root in a wave-like pattern. It builds up on one side of the root, causing those cells to elongate less than the cells on the other side. This causes the root tip to bend in a particular direction.

“Think of how a bulldozer turns with its tracks,” said Benfey. “You have to stop the bulldozer and then use levers to turn toward a particular direction. With a plant, it will turn by stopping the growth of cells on the lower side and allow the cells on the upper side to keep expanding as they normally do. It causes the root tip to circle.”

But what about the roots that can’t do the corkscrew dance? Researchers discovered that a mutation in a gene called HK1 makes the roots tips reach straight down instead of circling. Plants with the HK mutation have a defect in how auxin is carried from cell to cell. With the hormone blocked, the roots lose their ability to twirl and their ability to navigate obstacles.

Research collaborators at the Georgia Institute of Technology conducted experiments in which they observed normal and mutant rice roots growing over a perforated plastic plate.

The plant roots that could circumnutate were three times more likely to find the holes and go through to the other side while the root tips that did not circle became blocked by the plastic sheeting.

Benfey said both root qualities have value.

“There are circumstances where circumnutation is a good thing,” he said. “That would be if you were trying to plant in soil with lots of obstacles, such as stones or rocks. But there are situations where you don’t want the plant roots to circle but grow straight down fast.”

When rice is planted in some regions of California, the seedlings are started and then airdropped by a low flying plane over paddy fields.

“These seedlings have to anchor really fast or they will get picked up by the wind and blown to the end of the field,” he said. “I would think you don’t want circumnutation. You want your roots to grow really fast straight down.”

Research has shown that some cultivars of rice don’t circumnutate. Breeders may look at some rice varieties and alter root growth by using CRISPR technology to knock out the relevant gene, turning the circumnutating variety into a non-circumnutating one.

Scientists at Georgia Tech used robotics to further explore the mechanics of the root tip. According to the news release, they built a soft pliable robot that unfurls from its tip like a root does and set it down on an obstacle course littered with unevenly spaced pegs.

Even without sophisticated sensors or controls, the robotic root found its way past obstacles and through the pegs. But when the side-to-side bending stopped, the robot got stuck.

“We were exploring how root behaviour works and this robot added additional stuff at the tip of it,” said Benfey. “They put it in a maze with pegs and added forwards and backwards motion. We decided to test real plants in gravel. That was one of the nice features. They did the robotics, which inspired the biologists to do similar experiments.”

Next, the teams at Duke and Georgia Tech plan 3D simulations and more sophisticated robots. Translating that into biology, the data is clarifying what happens at all cell levels.

“We are trying to understand what really happens,” said Benfey. “It turns out that on the surface of rice the cells are not a uniform size. Rice is much more heterogenous and we are trying to understand how that may contribute to roots that circumnutate and further understand from the different rice varieties what the additional genes are that are involved in circumnutation.”

The research was published in Proceedings of the National Academy of Sciences.

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