Duke researchers have investigated something that is too slow for our eyes to see. A team in the laboratory of biologist Philip Benfey wanted to see how plant roots dig into the ground. So they set up a camera on rice seeds that were sprouting in clear gel and took a new picture every 15 minutes for several days after germination.
When they played back their footage at 15 frames per second and compressed 100 hours of growth in less than a minute, they found that rice roots use a trick to get their first hold in the ground: their growing tips make corkscrew-like movements and wobble and snake in a spiral path.
Using their time-lapse footage together with a root-like robot to test ideas, researchers gained new insights into how and why plant root tips rotate as they grow.
The first hint came from something else the team noticed: some roots can't dance the corkscrew. They found that the culprit was a mutation in a gene called HK1 that made them grow straight down, instead of circling and snaking like other roots.
The team also found that the mutated roots grew twice as deep as normal ones. Which begs a question, "What does the more typical spiral tip growth do to the plant?" said Isaiah Taylor, a postdoctoral fellow in Benfey's lab at Duke.
Winding movements in plants are "a phenomenon that fascinated Charles Darwin 150 years ago," said Benfey. With shoots, there is an obvious benefit: twisting and circling makes it easier to get a grip when climbing towards sunlight. But how and why it happened in roots was more of a mystery.
Germinating seeds have a challenge, say the researchers. To survive, the first tiny root to emerge must anchor the plant and probe down to soak up the water and nutrients the plant needs to grow.
What got her thinking: Maybe that spiral growth in root tips 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 normal and mutant rice roots growing over a perforated plastic sheet showed that normal spiral roots are three times more likely to find a hole and grow through to the other side.
Santa Barbara, an employee of Georgia Tech and the University of California, built a soft, pliable robot that unfolds from its tip like a root and loosens it from unevenly spaced pins on an obstacle course.
To create the robot, the team took two inflatable plastic tubes and nested them together. Changing the air pressure pushed the soft inner tube inside out, causing the robot to expand from the tip. By contracting opposing pairs of artificial "muscles", the tip of the robot flexed back and forth as it grew.
Even without sophisticated sensors or controls, the robot root could overcome obstacles and find a way through the pins. But when the side-to-side bending stopped, the robot quickly got stuck on a pen.
Eventually, the team grew normal and mutated rice seeds in a dirt mix used for baseball fields to test on obstacles that a root would actually encounter in the ground. Sure enough, while the mutants struggled to get a household, the normal roots were able to pierce their way through with spirally growing tips.
Corkscrew growth from a root tip is coordinated by the plant hormone auxin, a growth substance that researchers believe can move in a wave-like pattern around the tip of a growing root. The buildup of auxin on one side of the root causes these cells to elongate less than those on the other side, and the tip of the root bends in that direction.
Plants that carry the HK1 mutation cannot dance because of a defect in the way auxin is transported from cell to cell, the researchers found. If you block this hormone, roots will lose their ability to rotate.
The work helps scientists understand how roots grow in hard, compacted soil.
This work was supported by a grant from the National Science Foundation (PHY-1915445, 1237975, GRFP-2015184268), the Howard Hughes Medical Institute, the Gordon and Betty Moore Foundation (GBMF3405), the Food and Agricultural Research Foundation (534683). , the National Institutes of Health (GM122968), and the Dunn Family Professorship.