DURHAM, NC – Duke researchers have been studying something that happens too slowly for our eyes to see. Philip Benfey’s biologist’s lab team wanted to see how plant roots dig into the soil. So they set up a camera on rice seeds exploding in clear gel, taking a new photo every 15 minutes for several days after germination.
When they played their films back at 15 frames per second, squeezing 100 hours of growth into less than a minute, they saw that rice roots are using a trick to get their first leg in the soil: their growing tips produce corkscrew-like, waggling and winding movements in a helical path.
Using their time-film, along with a root-like robot to validate ideas, the researchers gained new insights into how and why root tips circulate as they grow.
The first news came from something else the team noticed: some corkscrew dance roots can’t do it. The culprit, they discovered, is a mutation in a gene called HK1 that causes them to grow straight down, instead of twisting and bending as other roots do.
The team also noted that the mutant roots grew twice as deep as normal ones. Which raised the question: “What does the most typical spiral tip growth do for the plant?” said Isaiah Taylor, a graduate fellow in the Duke’s Benfey lab.
Curved movements in plants were a “surprise enjoyed by Charles Darwin,” even 150 years ago, Benfey said. As for shoes, there is an obvious convenience: connection and rotation make it easier to grip while ascending towards the sunlight. But how and why it occurs was rooted in greater mystery.
Sprouting seeds has a challenge, the researchers say. If they are to survive, the first tiny root that emerges needs to anchor the plant and study down to get the water and nutrients the plant needs to grow.
Which made them think: perhaps in basic recommendations this spiraling growth is a strategy of exploration – a way to find the best path, Taylor said.
In experiments conducted in the laboratory of physics professor Daniel Goldman at Georgia Tech, observations of conventional and mutant rice roots grown over a molten plastic plate showed that conventional spinning roots were three times more likely to bore a hole. find and grow through to the other side.
Colleagues at Georgia Tech and the University of California, Santa Barbara built a pliable soft robot that opens from its top as a root and releases it in the course of an obstacle made up of wide-angle pegs. uneven.
To create the robot, the team took two inflatable plastic pipes and nestled them inside each other. By changing the pressure of the air he pushed the soft tube inside from the outside, causing the robot to expand from the tip. Contracting against pairs of artificial “muscles” caused the tip of the robot to bend side by side as it grew.
Even without sensors or solemn controls, the robotic root was able to make its way past obstacles and find a way through the pegs. But when the side-by-side bending stopped, the robot swung against a peg.
Eventually, the team grew conventional and mutant rice seeds in a dirt mixture used for basketball fields, to test them for the barriers that would take root in soil. Certainly, while the mutants had difficulty getting a toe, the normal roots with spiral growth tips were able to get through.
The growth of corkscrew tip root is coordinated by the plant hormone auxin, a growth substance that the researchers believe may move around root crops growing in a wave-like pattern. Auxin buildup on one side of the root causes these cells to swell less than those on the other side, and the top of the root bends to that side.
Plants carrying HK1 mutations cannot dance because of a deficiency in how auxin is transported from cell to cell, the researchers found. Suppress this hormone and roots lose the ability to circulate.
The work helps scientists understand how roots grow in tight hard ground.
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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), the Foundation for Food and Agricultural Research (534683), National Institutes of Health (GM122968) and Dunn Family Professor.
CITATION: “Root Circulation Equipment and Function,” Isaiah Taylor, Kevin Lehner, Erin McCaskey, Niba Nirmal, Yasemin Ozkan-Aydin, Mason Murray-Cooper, Rashmi Jain, Elliot W. Hawkes, Pamela C. Ronald, Daniel I. Goldman, Philip N. Benfey. Proceedings of the National Academy of Sciences, February 19, 2021. DOI: 10.1073 / pnas.2018940118.
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