biologyScience

The interval reveals the rootkit dance

Scientists see a plant root, then a root-like robot, swinging its way around obstacles to find out how seedlings get their first foothold

Credit: Screenshots from Benfey Labs / Goldman. Produced by Veronique Koch.

Durham, North Carolina – Duke researchers have been studying something that happens so slowly that our eyes cannot see it. A team in the lab of biologist Philip Penvey wanted to see how plant roots dig into soil. So they set up a camera on the rice seeds sprouting in a transparent gel, and took a new photo every 15 minutes for several days after germination.

When they played their shots again at 15 frames per second, and squeezed 100 hours of growth to less than a minute, they saw that the rice roots use a trick to gain a first foothold in the soil: their growing limbs make key-like movements, wiggling and winding in a spiral path.

Using their time-lapse shots, along with a root-like robot to test ideas, the researchers gained new insights into how and why the tips of the plant’s roots rotate as they grow.

The first clue came from another thing the team noticed: Some roots cannot perform the key dance. They found that the culprit is a mutation in a gene called HK1 that causes them to grow straight, rather than spinning and zigzag as other roots do.

The team also noted that the mutant roots grew as deep as twice the natural roots. Which raised the question: “What does a typical spike-tip growth do?” Said Isaiah Taylor, postdoctoral fellow at Banffee Lab in Duke.

Banffe said that the sinuous motions in plants were “a phenomenon that fascinated Charles Darwin” even 150 years ago. In the case of buds, there is a clear benefit: the twist and rotation make them easier to control as they ascend toward the sunlight. But how and why it occurs in the roots was an even more mystery.

Researchers say seed germination is a challenge. If they want to survive, the first young rootstock that appears must anchor the plant and drill down to absorb the water and nutrients the plant needs to grow.

Which got them thinking: Maybe this spiral of root advice is a search strategy – a way to find the best way forward, Taylor said.

In experiments conducted in the laboratory of physics professor Daniel Goldman at Georgia Tech, observations of natural and metamorphic roots of rice growing over a perforated plastic plate revealed that natural helical roots were three times more likely to find a hole and grow to the other side.

Collaborators at Georgia Tech and the University of California, Santa Barbara have built a soft, flexible robot that opens at its end like a root and places it on an obstacle course consisting of unevenly spaced pegs.

To make the robot, the team took two inflatable plastic tubes and placed them inside each other. The change in air pressure forces the soft inner tube from the inside out, causing the robot to lengthen on its end. Contracting with opposing pairs of artificial “muscles” causes the robot’s tip to bend side to side as it grows.

Even without sophisticated sensors or controls, the robotic root was still able to weave its way through obstacles and find a path through pegs. But when the bending stopped from side to side, the robot quickly got stuck in the wedge.

Finally, the team planted natural, transformed rice seeds in an earthen mix used on baseball fields, to test them on obstacles the rootstock might actually encounter in the soil. Sure enough, while mutants had trouble gaining a foothold, natural roots with spiral growing tips were able to bore.

Root tip spiral growth is coordinated by the plant hormone auxin, a growth material that researchers believe may move around a growing root tip in a wave-like pattern. The accumulation of auxin on one side of the root causes those cells to elongate less than those on the other side, and the tip of the root bends in that direction.

The researchers found that plants carrying the HK1 mutation could not dance due to a defect in how auxin was transported from cell to cell. Blocking this hormone, the roots lose their ability to circulate.

The work helps scientists understand how roots grow in hard, compact soils.

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This work was supported by a grant from the National Science Foundation (PHY-1915445, 1237975, GRFP-2015184268), Howard Hughes Medical Institute, Gordon and Betty Moore Foundation (GBMF3405), Food and Agricultural Research Foundation (534683), National Institutes of Health (GM122968) and the Dunn Family Professorship .

Quote: “The Mechanism and Function of Root Rotation”, Isaiah Taylor, Kevin Lehner, Irene McCasky, Nipa Nirmal, Jasmine Ozcan Aydin, Mason Murray Cooper, Rashmi Jane, Elliot W. Hawks, Pamela C. Ronald, Daniel E. Goldman, Philip N. Deny. Proceedings of the National Academy of Sciences19 February 2021. DOI: 10.1073 / pnas.2018940118.

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