A device that operates in space to study how plant cells sense and respond to different levels of gravity has achieved a milestone, recording the longest continuous observation of cellular response to gravity.

The project was led by researchers from Purdue, NASA’s Ames Research Center and the University of Texas at Austin.

Understanding how gravity impacts plants is key for determining the conditions necessary to grow plants in space, says Jenna Rickus, associate vice provost for teaching and learning, professor of agricultural and biological engineering, and professor of biomedical engineering.

Three disc-shaped “bioCDs” used sensors to measure the calcium signaling activity of fern spores at varying gravity levels. A satellite carrying the system was launched in 2014 aboard a SpaceX Falcon 9 rocket from Cape Canaveral, Fla. Known as SporeSat, the autonomous spacecraft operated for several days to investigate how variations in gravity affect calcium signaling in germinating spores of the fern Ceratopteris richardii. Calcium signaling — a gravity-directed process — acts as a compass for plants, determining the directions sprouts and roots grow during germination.

“Spores of the fern Ceratopteris richardii are a valuable system for studying cellular gravity response,” Rickus says.

Each bioCD contained four rings of eight fern spores. During the experiment, two of the bioCDs were caused to spin, simulating gravity, while the third remained stationary as a microgravity control. The spore rings were subjected to different levels of gravity depending on how far they were from the center of the disc. The bioCDs were developed by Marshall Porterfield, professor of agricultural and biological engineering, who initiated the project. Microelectrodes measured the spores’ calcium signaling activity and transmitted the data back to Earth.

Research findings were detailed in a paper published in the journal Lab on a Chip. Data from the project demonstrated that the degree of cellular response correlates with the magnitude of the g-force applied via rotation of the bioCDs. The paper’s lead authors were UT Austin postdoctoral research associate Mari Salmi; Joon Park, a research engineer at Purdue’s Birck Nanotechnology Center and the Bindley Bioscience Center; Stanley Roux, a professor at UT Austin; Porterfield; Rickus; Anthony Ricco, a scientist at the NASA Ames Research Center; and Amani Salim, who was a postdoctoral researcher at the Birck Nanotechnology Center.

“The experiments recorded the longest continuous observation of a cellular response to different levels of gravity, and they demonstrate the potential utility of the device for assaying the threshold of cells’ g-force responses in spaceflight conditions,” Rickus says.

Collaborators at UT Austin have created mutant spores to learn more about the mechanism.

“The SporeSat bioCD can now be used as an important tool to evaluate the contribution of individual gene targets to this gravity-induced electrochemical signal,” Rickus says. “It can also be used to determine the minimum gravitational force needed to activate a single cell response.”

SporeSat’s microsensor technology provides a foundation for future studies of cell activity. Previous experiments to study the effects of varying gravity on plant germination were operated on airplanes that fly in parabolic maneuvers, providing minutes of weightlessness. “But, until we built this device, there was no way to do variable gravity for hours,” she says.

There is a critical window during which the direction of the spore’s growing rhizoid — or root tip — is irreversibly set. Previous experiments on the Columbia Space Shuttle and the International Space Station revealed that spores in microgravity will germinate, but in random orientations.

“The Space Station is much further out than SporeSat was, so it’s really low gravity. When you germinate seeds, the first thing that happens is the root tip comes out. And if you do this in super-low gravity like on the Space Station, they’ll germinate but they shoot out in all random directions,” she says.

SporeSat flew in low-Earth orbit around 200 miles above the planet. The “nanosatellite” weighed about 12 pounds and measured 14 inches long by 4 inches wide by 4 inches tall. The spores were loaded onto the bioCDs in the dark and then tightly packed in such a way that allowed them to withstand the jarring vibrations of launch. Special lighting, heating and sensors also were incorporated into the design. Once in orbit, researchers activated LED lighting to initiate germination.

“Once you turn on the light and initiate germination, we know those first, probably 24 to 48, hours are important, but we didn’t have the technology to measure continuously in those conditions,” Rickus says. “So this was an extension of prior works. A lot of custom technology went into this research.”

In response to gravity, the cells undergo “calcium polarization,” or the movement of calcium ions to either pole of the cell. The cells know which direction to grow because of “gravity-directed polar development.” Calcium sensors positioned on either side of the spores made electrical measurements.

This polarized orientation of calcium transport and growth direction of the root tip are directly linked to the force and orientation of gravity.

“Each spore is a single cell, and before the first division of the cell, the orientation of cellular polarity is set in response to the force exerted on the cell by gravity,” Rickus says.

Findings from the research also could yield findings with broad applications. For example, Rickus says, a neuron’s development involves extending the cell’s axon in a similar manner as rhizoid growth.

“It’s actually a really fundamental principle in cellular development. This polarization step is very relevant in human development,” she says. “There are really interesting similarities in the role of calcium and some other signaling molecules in some of those processes. So, it’s a fundamental biological process that gets repeated in different ways and different organisms.”

Because human cells use calcium signaling, the study also is an important step toward understanding the effects of space’s microgravity on the human body.