Youssef Chebli and Prof. Anja Geitmann, Université de MontréalIf the long-term goal of humans is, indeed, space exploration and colonization, then there are many survivability questions that need to be answered. Leaving Earth means leaving the friendly confines of a planet on which we have evolved over the eons and to which our bodies have adapted. We take advantage of what Earth offers us—oxygen, atmospheric protection from solar radiation, and the constant, gentle tug of gravity.

It turns out gravity has a role in successful sexual reproduction, at least for plants.

A group of researchers from the Université de Montréal, Quebec, Canada, have reported on the effects of weightlessness and hypergravity on reproduction performance of plants. The work, led by Prof. Anja Geitmann, specifically focuses on plants, but has broader implications for plants and humans, especially if they are space faring.

“We know that animal and plant organisms are affected by weightlessness or higher gravity, but we do not understand in detail how this effect is mediated at the cellular level,” Geitmann said. “This knowledge will be crucial for future space missions. While the health of human space travelers is obviously a great concern, plants will be important for space exploration as well, since they provide fresh food, aid in recycling waste and producing fresh air, and last but not least, they have significant positive psychological effects on the crew. It is therefore important to understand how plants grow and reproduce under altered gravity conditions.”

Geitmann’s team focused their research on the cellular growth processes of the pollen tube, a rapidly growing plant cell that is affected by both simulated microgravity and hyper-gravity. The pollen tube is the means by which plant sperm is delivered to the plant egg cell.

This is a pollen grain with pollen tubes, viewed in the scanning electron microscope.What they found was changes in gravity significantly affect the reproductive processes of plants. Geitmann explained that gravity modulates traffic on the intercellular “highways” that ensure the growth and functionality of the pollen tube—the male reproductive organ in plants.

“The cellular growth process in the pollen tube is affected by both simulated micro-gravity and hyper-gravity,” Geitmann said. “The effects were noticeable in the altered assembly of the cellular envelope, the cell wall, and in an effect on intracellular trafficking, a process that is crucial for the functioning of all eukaryotic cells in plants and animals.”

“Since plant sperm are not motile, this tube delivers the sperm cells directly to the egg cell,” Geitmann added. “If this cellular growth process and sperm discharge are impeded, fertilization cannot take place and both seed set and fruit formation are impossible, with few exceptions.”

Youssef Chebli, a researcher in Geitmann’s lab added that the work has applications to human health as well.

These are pollen grains attached to the stigma, the female organ of the plant.The research “allows us to not only understand the general principles of the reproductive mechanism in plants but, more importantly, how the intracellular transport machinery in eukaryotic cells respond to altered gravity conditions. Our findings have implications for human health as similar effects are likely to occur in human cells such as neurons, where long distance intercellular transport is crucial.”

Geitmann’s team did their work without leaving Earth. They conducted their studies at European Space Agency’s research facilities in Nordwijk, The Netherlands. They used two devices. The first—a Random Positioning Machine—turns a sample slowly and arbitrarily in space. The effect is not a total removal of gravity, but to a certain extent the continuous changing vector of Earth’s gravity (vs. the orientation of the specimen) cancels out many effects of gravity.

The second device is a centrifuge. By spinning the specimen, the researchers simulated hyper-gravity, which might be encountered on a large planet. This can be done with a simple lab centrifuge. However, the lab centrifuge does not allow for simultaneous observation of the specimen.

“To achieve this, we had to place a microscope, we were able to do brightfield and fluorescence imaging, in the payload gondola of a huge centrifuge (8-m diameter) and spin this entire set-up to achieve g-values of up to 20g,” Geitmann explained. “This experimental set-up posed a series of technical challenges, since microscopes are not made to withstand this kind of force and video imaging was compromised by the vibrations generated by the centrifuge.”

To solve this, the team suspended the microscope on elastic bungee cords.

“The personnel at ESA did extensive test runs to make sure that technically this would work,” Geitmann said, adding that this it is the first time fluorescence imaging was done at hyper-gravity conditions.

The finding that altering gravity can have serious developmental effects on plants and animals raises philosophical as well as scientific issues.

“We take 1g gravity as a given, it is a constant force in our lives, all species have evolved in its presence, and all of us have been exposed to it since conception,” Geitmann said. “We only realize that our bodies actually need this force to function properly when it is taken away.”