Microgravity research in plants

Science and Society Microgravity research in plants

A range of platforms and options allow research on plants in zero or low gravity that can yield important insights into plant physiology.

"The Brick Moon" by Edward Everett Hale, published in eighteen sixty-nine in the Atlantic Monthly, is the first science fiction story of an artificial satellite. Hale described how the low gravity on board enabled plants to evolve at high speed, providing the inhabitants of the Brick Moon with plenty of food. He already foresaw two aspects of space facilities: their dependency on sustainable food supply and their astronomical price: The Brick Moon required twelve million bricks and cost two hundred fifty thousand dollars, an astronomical sum by the time he wrote his story.

"When Charles Darwin published his first work on plant tropisms in eighteen eighty, he or his fellow researchers did not have the opportunity to perform experiments in real microgravity."

Now, one hundred fifty years later, a human-inhabited satellite, the International Space Station, has become a reality and plant research is one of the numerous scientific disciplines that is being studied there. Recently published review articles have summarized the achievements of decades of plant research in microgravity. In this article, we look at the socioeconomic aspects of this research and the impact it had in recent years and will have in the future, and at the costs equated to other, equally ambitious research projects. To compare space-based microgravity research and to highlight this unique environment, we take a look at some other facilities that are used to provide a low-gravity environment for research and at their costs and constraints.

Facilities to conduct experiments in simulated or real microgravity

When Charles Darwin published his first work on plant tropisms in eighteen eighty, he or his fellow researchers did not have the opportunity to perform experiments in real microgravity. Instead, scientists had to rely for a long time on changing or randomizing the gravity vector by turning plants from a vertical into a horizontal position, or by using centrifuges. Over time, more sophisticated tools, such as clinostats and random positioning machines, were developed. However, these platforms only provide what we call simulated microgravity and not real microgravity, and, depending on the biological system that is studied, the interpretation of results might be challenging.

"Only free fall can achieve real microgravity."

Only free fall can achieve real microgravity. A drop capsule in free fall, a plane during a parabolic flight, a sounding rocket, or a satellite creates a centrifugal force, which compensates the gravitational pull of the Earth with residual acceleration forces in the range of ten to the negative two to ten to the negative six G. These experimental platforms differ in their time of microgravity provided, the mode of operating the hardware, the quality of microgravity, and the price.

[The ISS] is by far the most expensive low-gravity platform to perform experiments, but is a unique experimental platform for microgravity research.

The free fall in the vacuum tube of a drop tower generates up to five seconds of exposure to high-quality microgravity or up to ten seconds when a catapult is used to propel the experiment into the drop tower. With six thousand to ten thousand euros per drop, it is one of the cheapest microgravity platforms and the turnaround time of two to three drops per day widely surpasses that of other platforms. The major constraints are the short time and the up to fifty G of landing acceleration, which does not make drop towers suitable for every experimental hardware. For experiments with biological samples, such as plants, the drop tower has become more attractive in recent years as fast molecular readouts, such as phosphoproteomics and secondary messenger signaling that play a role during the first seconds of a microgravity response, came more into focus.

The other ground-based facility is airplane parabolic flights that provide up to thirty seconds of microgravity. In Europe, the largest aircraft for parabolic flight research is the Airbus A three ten ZERO-G, operated by Novespace by the order of the European Space Agency, and the French space agencies. The plane regularly flies thirty-one parabolas with microgravity of up to twenty-two seconds per parabola.

Three features make parabolic flights unique. One is the ability to produce partial G-levels, as experienced on the Moon or Mars. The second is that, unlike the drop tower, experiments can be manually operated by crew members, allowing to change experimental parameters during the flight. Third, the parabolic flight is the only microgravity platform that allows biomedical experiments with human subjects. The deceleration of one point eight G is also much lower compared to the drop tower. A constraint is the low level of microgravity of ten to the negative two G and the start acceleration of one point eight G before and after each parabola. Thus, all biological samples have already been exposed to both hyper- and microgravity after the first parabola, which makes the results more challenging to interpret.

The ISS

Only space enables long-term microgravity experiments. The first plant biology experiment in space took place in nineteen forty-six on board a repurposed V two rocket launched by NASA. It had maize seeds on board, but no plant growth took place during the experiment. Sounding rockets, such as the European MAXUS rocket, provide up to thirteen minutes of microgravity with a level of ten to the negative four G and recovery of the rockets within one to two hours allows for downstream sample analysis. Onboard centrifuges are often used as an inflight one G control to compensate for vibrations and start acceleration. Indeed, start accelerations of six to twelve G are a significant constraint of these experiments, as plants were shown to quickly respond to hypergravity of five G with changes in gene expression.

Growth and developmental responses to alterations in gravity, as measured on Earth using gravitropism assays, require hours of observation on a phenotypical level. Such long-term developmental changes have therefore to be studied in low Earth orbit on board of satellites or human-tended space laboratories, such as in the ISS, the Salyut and Kosmos space stations, Skylab, Space Shuttles, the Russian space station MIR, and, more recently, the Chinese space lab Tiangong. Among these, the ISS is the largest and longest inhabited space station with a continuous human habitation since November two thousand. It is by far the most expensive low-gravity platform to perform experiments, but since it offers months or even years of experimental time, crew operation of the experimental, and download opportunities, it is a unique experimental platform for microgravity research.

The presence of heavy masses in our solar system, notably the planets, does not allow for a real zero-gravity environment. Even on the ISS, the gravity level is still ninety-two percent of the gravity level on Earth-after all, the ISS orbits the Earth at an altitude of merely three hundred fifty kilometers. It is the continuous free fall of the station toward Earth at a speed of around twenty-eight thousand kilometers per hour that creates low-gravity levels of ten to the negative two to ten to the negative six G.

The facilities for plant growth experimentation on board the ISS have significantly grown over time. BIOLAB, the European Modular Cultivation System, KUBIK (ESA incubator with centrifuge platform), and the Cell Biology Experiment Facility are now available to conduct experiments. During the past decade, it became evident that many of the earlier results were hampered by engineering constraints and suboptimal plant growth conditions. All hardware platforms nowadays therefore include one G reference centrifuges to distinguish between non-gravity and space- and growth-dependent effects. Modern technologies are readily adapted to allow leading-edge experimentation, such as FLUMIAS, a structured illumination microscope for live-cell imaging in space built by German industry on a DLR contract.

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,satellites offer an excellent quality of microgravity that is not affected by docking maneuvers or crew movement."

One of the main advantages of the ISS is the possibility to send seeds to the station and start growing plants on board. This way, artifacts from prior hypergravity stress during the rocket launch are mitigated. However, the hypergravity phase during the rocket flight and the long time between the loading of the rocket and the actual experiment on board are still major constraints. Moreover, limited crew time often requires automation of experiments, which is readily achievable for gene or protein expression studies, but presents a challenge for microscopy. In addition, the movement of the crew and docking maneuvers create microgravity changes on board; as a result, the onboard microgravity is, over time, less constant than on ground-based facilities such as the drop tower or other space-based platforms, such as satellites or sounding rockets.

Indeed, satellites offer an excellent quality of microgravity that is not affected by docking maneuvers or crew movement. Experiments can be conducted from weeks to months, limited only by the lifetime of the biological samples. Then, sizes of satellites used for plant experiments range from the Eu:CROPIS with a weight of two hundred thirty kilograms and outside dimension of one by one meter that can house two greenhouses down to CubeSats with ten by ten by ten centimeter size and one kilogram weight. Eu:CROPIS was recently launched with tomato plants and Euglena algae on board. Standardization, off-the-shelf components, and an emphasis on simplicity and low costs allow affordable missions with project timelines of typically nine to twenty-four months from inception to launch. The downsides of satellites are the limited space and the fact that the hardware is often not reusable and that temperature control is challenging in the smaller satellites.