How do plants grow in microgravity?

Image Credit: Professor Takayuki Hoson, Osaka City University

Aniruddha Maram discusses the impact of microgravity on the growth patterns of plants.

The question ‘How do plants grow in microgravity?’ could be answered in two different ways, one considering the growth and development of plants from earth in space or defining the differences between the microbial flora found in space to flora found on earth. This article explains the effects of microgravity on the development of plants and its future scope.

Plants are complex multicellular organisms that can grow, adapt and evolve in the most unlikely of places. Previous spaceflight studies have shown that seeds can germinate, develop, and mature into plants in outer space. They can blossom and even bear fruit in such an environment—under the influence of microgravity—if grown in-vitro. 

Plants became terrestrial creatures when they moved from the oceans around 450 million years ago. However, they had to endure numerous environmental stresses and other external stimuli such as drought, in order to survive, develop and reproduce. Sessile plants evolved mechanisms to sense light, water, and gravity to alter their growth orientation in response to environmental stress. Gravi Morphogenesis, in which plant development is affected by gravity, is one of these adaptive methods. Gravitropism, in which roots grow downward and stems grow upward, circumnutation, a process where  the stem or root tips display helical or spiral movement (a climbing vine, for example, exhibits remarkable circumnutation); and peg formation, which aids cucurbitaceous seedlings in shedding their seed coats are examples of this phenomenon. The space environment is a perfect setting for studying these gravity-dependent growth processes in plant development.

Gravitropism is a bending reaction that occurs when plant organs develop at various rates in response to gravity. Experiments comparing ground-grown and space-grown Arabidopsis and rice were done aboard the space shuttle voyage STS-95, which featured Astronaut Chiaki Mukai. Plant aerial portions (shoots) grow upward on Earth, whereas roots grow below. However, the studies revealed that growth direction is uncontrolled in a microgravity setting, with some roots even extending in the same direction as the aerial stems.

Scientists were startled to find that plant development is not hampered by the absence of gravity, the force that has formed our biological processes. As plants grow away from their seeds in quest of nourishment on Earth, they generate a filigree-like network of roots. For a long time, scientists thought that the motions were governed in part by gravity. Without gravity to guide them, roots on the International Space Station followed the same path.

“Plants don’t really care about gravity so much if you can get the environment right,” Gioia Massa a NASA scientist says.

A few plants researched on the ISS (International space station) by multiple countries are, Oryza sativa(rice), Arabidopsis thaliana, Cucumis sativus (Cucumber), Ipomoea purpurea(Common morning glory) and  Gossypium hirsutum(Cotton). These species were also used to test the effect of gravitropism on the development of seedlings into fully fledged plants.

Scientists use a system called Veg - 01 which involves two control chambers, one on the ISS and one on earth that shadows the experiments done on the ISS. A seed is planted in the 'plant cushion' in Veggie system which consists of  unique bags containing 'space soil' that regulate fertilizer release and control. A seed is then placed inside the bag with a plant wick inserted and positioned such that the roots grow into the bag and the stem emerges up and out of the bag. The plants are given a sense of direction by the blue and red LED lights, which are tuned for photosynthesis. The Veggie chamber's walls extend to accommodate the growing produce. For years, scientists have flown experiments to the International Space Station (ISS) to grow plants, freeze them, and bring them back to Earth to investigate and identify how growing in microgravity affects the plant's DNA.

Larger plants may now be cultivated on the ISS thanks to a recent addition. The Advanced Plant Habitat (APH) is the biggest growing chamber ever built. It is about the size of a mini-fridge, and is meant to see which growth conditions plants prefer in space while also giving them a bigger root and shoot area. The APH system is essentially self-contained, with an Earth-based computer sending commands and changes to manage and regulate temperature, humidity, oxygen, and carbon dioxide levels, which will be critical for growing various different plants.

From the research conducted on both the ISS and Earth, scientists have started to understand the effects of gravity on plant development. It is thought that gravity can be sensed by root cap cells termed columella, which are located at the root tips in the event of root gravitropism. Starch-filled amyloplasts settle within the columella cells due to gravity, producing a shift in the flow of the plant hormone auxin. It is a hormone that travels in a set route from an aerial shoot, comprising the apical meristem and early leaves, to the roots, via a central cylinder. Auxin begins to flow in the opposite direction, as if doing a U-turn, along the roots after being transported down to the root tips by polar transport. When the roots are inclined and given a gravitational stimulation, U-turning auxin, on the other hand, tends to travel downward rather than upward. As a result, auxin concentration rises in the bottom portion of the elongation zone in inclined roots, producing a growth differential between the lower and higher parts; the lower section's growth rate falls behind the top part's, leading the root to bend downward. This is how plant roots on Earth respond to gravity by growing downward. However, because amyloplasts do not settle into root cap cells in microgravity, gravity is not felt, and asymmetric auxin distribution is not produced. This is probably why the growth direction in space is uncontrollable.

An opinion by Dr. Susanne Schilling, Assistant professor at UCD School of Biology and Environmental Science, on “how much does gravity affect the development of a plant, do you think it plays a major role in determining the cellular development of shoot and root ?” was “gravity certainly has a major impact on plant development - after all plants evolved to deal with 1g of gravity here on earth. Plants can sense gravity and move according to it, a phenomenon called gravitropism. When grown in microgravity, i.e. in space on a space station, it appears that roots are still growing into the medium that the plant is cultivated on. But their root pattern can be different, "skewed" to the side or into certain patterns depending on species and specific conditions. It seems that there are other factors in root patterning, usually masked by gravity, which influence root growth and shape once gravity is omitted. Further, some research found that the plants can form adventitious roots under microgravity, meaning roots that are formed from non-root tissue, which is often a stress response when plants are grown on earth. Looking at gene expression in the whole plant, genes that affect light response, cell wall synthesis, and the cytoskeleton have been found to change their expression during microgravity as compared to ground conditions, which indicates that multiple developmental processes are affected during plant growth in microgravity.”

Considering the new trend of scientific advancement leading to colonization of Mars, the foremost step is to check plant viability for nutrition, and speculate which  plants can be grown on Mars as  most will likely be grown in climate-controlled greenhouses You can't grow a salad in a petri dish, so anyone living on Mars will require a lot of these alien gardens. Astronauts have already had the chance  of eating a salad made of space grown plants. In 2015, astronauts aboard the International Space Station were given the opportunity to sample the leaves of a red romaine lettuce grown in NASA's first fresh-food growing chamber.

To summarize, plants depend on gravity, and auxin transport, which is influenced by gravity, for their development. It's hypothesized that the lack of gravity in space causes aberrant plant growth and development because auxin transport isn't regulated. However, the specific mechanism by which gravity controls auxin transport is unclear. This process will not only boost plant productivity on Earth once it is understood,  but also pave the path to expanding agriculture to outer space.