Light and gravity interact
One of Kiss’ most striking discoveries has implications for growing plants on other worlds. It also sheds light on how evolution has changed the way the plants respond to their environments.
Two of the most important environmental stimuli for plants are light and gravity.
Generally, plants grow toward light sources. Put a potted plant on a window sill and you may find it takes on a distinctive curved profile as it grows toward the light. This tendency is called phototropism.
Likewise, plants respond to the force of gravity. Generally, roots grow down toward the gravitational pull, shoots and stems away from it.
There are subtle interactions between these two tendencies that would be nearly impossible to detect on earth, but which have come into sharp focus in the microgravity environment of low Earth orbit.
Kiss and his collaborators send hundreds of tiny seeds into space, to the International Space Station. Once there, they’re placed in a centrifuge and cultivated. As the seeds sprout and become seedlings, they can be spun at different speeds in the centrifuge to simulate different levels of gravity — such as the levels on the moon and Mars — to assess how they respond. If the centrifuge is off, then the young plants are in a microgravity environment.
On Earth, when flowering plants grow toward light, they’re responding to the higher frequency, blue wavelengths. Shine a red light on a flowering plant, and, although it can use that energy for photosynthesis, it won’t grow toward the light source.
It turns out, this bias toward blue light is switched off in low gravity environments. In lower gravity, Kiss’ plants would also grow toward red light.
“That was really an amazing discovery,” Kiss says.
Ambassador for Space Science. Kiss has been an outspoken advocate for NASA and its research and exploration mission. In 2014, he was awarded NASA’s Medal for Outstanding Public Leadership for both his research accomplishments and his outreach and advocacy on behalf of NASA’s work.
Decoding genetic secrets
The switch is likely based on mechanisms deep in the plant’s genetic code that could go back some 140 million years, when the earliest flowering plants evolved an affinity to blue light. That affinity may relate to the refraction of light through Earth’s ancient atmosphere, which was quite different millions of years ago compared to today.
Understanding the genetic component of these tropisms is one of postdoctoral fellow Josh Vandenbrink’s interests. He helps run Kiss’ lab and is a geneticist by training. He’s focused on “looking at the genes that are turned on and off in space.”
In one of their International Space Station experiments, Vandenbrink and Kiss discovered that it’s not just the phototropism of shoots that’s altered by different levels of gravity, but also how plant roots respond to light.
On Earth, roots usually grow away from blue light. After all, if they’re receiving high energy blue light, chances are they’re not in the ground, where they’re supposed to be. But Kiss’ experiments in the last few years have found that in very low levels of gravity, roots, too, will grow toward blue light.
“As you slowly turn up gravity, that reaction will turn off,” Vandenbrink says. “[Plants are] looking at light and gravity … integrating the signals, then deciding which way to grow.”
When these plants were returned to Earth, Vandenbrink used a technique called RNA sequencing to identify which genes were being turned on and off. That, in turn, sheds light on how different components of a plant’s DNA control phototropic and gravitropic activity, and perhaps other types of tropism, too.
It also presents an interesting genetic mystery. “It’s very puzzling to me that you can see some different responses in these plants when there’s never been a gradient of gravity for them to experience,” Vandenbrink says.
Above, Kiss works with the science and engineering team at NASA Ames Research Center prior to the 2017 launch.
Kiss’s advocacy for NASA includes speaking to the media and giving talks to civic clubs, school groups, and other organizations. He has sat on advisory panels for NASA and the European Space Agency, reviewed proposals for the Italian Space Agency, and spoken at a symposium organized by the Japanese Space Agency.
Growing plants in space
Unlocking these genetic and physiological components that control tropisms will help scientists better understand how and why plants grow the way they do — whether on Earth or on a space station.
The research has implications for how we grow plants in different off-Earth environments.
The phototropic changes Kiss has identified are seen in low-gravity environments, like the near-zero gravity of the space station. But as the speed of the centrifuge — and hence the G-force — increases, these responses switch off. At around 0.3 G, the plants behave like they do on Earth.
Why does that matter? Because the moon’s gravity is about 0.17 G, so these unusual phototropic effects would apply to plants grown there. But Mars’ gravity is stronger — about 0.38 G — above the level at which these phototropic effects switch off.
“The gravity on Mars is probably enough for these plants to grow like they would on Earth,” Kiss says. “But on the moon, it wouldn’t be enough.”
That might suggest different designs for cultivation systems developed for the moon, for Mars, or for a long-range spacecraft, like that which would be needed for the months-long voyage to the red planet.
There’s plenty more work to be done. There are other types of tropisms, such as hydrotropism — growth toward water. And different species can exhibit different tropistic behaviors. Sunflowers, which Vandenbrink studied earlier in his career, turn themselves overnight to face east in the morning when the sun rises.
Understanding the genetic and physiological components underlying tropisms provides a rich vein for research.
“I think there are other potentially important discoveries we can make in microgravity,” Kiss says. “There’s this sense of discovery, that’s part of why we do it.”
And ultimately, he says, “I would argue that space will maybe help us solve some of the longer-term problems on Earth.”
For that to happen, humans will have to continue to explore “the final frontier,” going farther and staying longer. And we’ll probably take plants with us.
Kiss is the former president of the American Society for Gravitational and Space Research. In his spare time, he reviews popular books about space exploration and space history.
