Extraterrestrial agriculture: plant cultivation in space

Abstract

Researchers are using various techniques and technologies to study how plants grow in extraterrestrial conditions with the hopes of sustaining longer missions for exploring deep space as well as being able to one day cultivate crops on other planets.

Keywords: :

• extraterrestrial agriculture
• microgravity
• plant experiments
• space

Cultivating galactic gardens

On Earth, plants follow a developmental plan that relies on the sun and gravitational forces to direct gas and liquid flow essential to plant function [1]. In space, plants find themselves operating completely outside the gravitational and solar rules of terrestrial life. However, as humans explore space and embark on longer missions, there is an emerging need for astronauts to have access to plants for both nutritional and wellbeing reasons. Additionally, if we wish to one day sustain life on other planets, the cultivation of crops on those planets will be essential to survival [2,3].

To figure out how to grow plants in space, researchers must analyze how microgravity (reduced gravity), light and nutrient availability and cycling affect plant development. Over the last few years, the US's National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) have conducted several plant experiments in space, which are part of distinct research initiatives using technologies such as The Vegetable Production System (Veggie, NASA), The Biological Research in Canisters (BRIC, NASA) and Micro-Ecological Life Support System Alternative (MELiSSA, ESA).

Defying microgravity & confined environments

Plants on Earth develop the way they do because of gravitropism, the process by which roots grow downwards with the force of gravity and shoots grow upward against gravity. This process is thought to be evolutionarily important for directing plant growth for access to both soil nutrients and light, eventually exhibiting gravity resistance as stems continue to grow against gravity [4]. So, how will plants know which way to grow in the absence of sunlight and terrestrial gravity? By learning how plants are cellularly and molecularly affected by microgravity and the confined spaces of space flight, scientists can determine what genetic modifications would allow plants to grow in space as well as the supplementary nutrient solutions required to support growth. For years, NASA has been conducting research on how plants are affected by space at the International Space Station (ISS).

Previous transcriptomic research has revealed that genes encoding cell wall proteins undergo significant expression changes during spaceflight [5] while proteomic analyses have revealed that proteins involved in cell wall synthesis are upregulated [6]. This research served as a basis upon which a more recent study, published in August of 2023, was conducted to further understand plant cell wall development.

What happens to cell wall glycans when plants are grown on the ISS?

As part of NASA's Veggie initiative, a research team led by Elison Blancaflor used glycome profiling and immunohistochemistry to determine how cell wall composition is affected during spaceflight. Glycome profiling is a method that utilizes different monoclonal antibodies, which recognize glycan epitopes, to study non-cellulosic cell wall composition from extracted RNA [4]. Glycans offer structural support to cell walls, and are involved in trafficking proteins, signaling and plant metabolism [7]. To validate the results of this glycome profiling technique, his team used immunohistochemistry, which is a method that utilizes antibody–antigen interactions to visualize changes occurring in specific tissues and cells [8].

Blancaflor's team investigated Arabidopsis thaliana seedling root development, comparing the non-cellulosic cell wall glycan epitopes of roots grown as part of Veggie on the ISS and those grown on Earth. They observed that roots grown in space were skewed to the side while shoot growth appeared the same regardless of where the plants were grown, encouraging the researchers to focus on root differences.

In both the control and space plants, the team found that xylans and xyloglucans were the most abundant carbohydrates based on higher binding intensities of specific monoclonal antibodies. However, they saw that antibody binding generally produced a higher signal intensity in space-grown plant roots, especially those that were older (11 days old as opposed to 6 days old). When conducting replicate analyses, the team found that the 11-day-old plants were generally more reliable specimens, showing greater consistency than the 6-day-old plants. Immunohistochemistry of two regions (Figure 1), the root–hypocotyl junction (Figure 1C) – which represents mature root tissue – and the primary root tip (Figure 1B), in both space-grown plants and controls was conducted to reveal significant differences in glycan epitope labeling intensities and density, indicating differences in root composition [4].

Figure 1. (A) Space-grown Arabidopsis thaliana seedlings compared with Earth-grown seedlings, examining the (B) root tip (C) and root–hypocotyl junction using immunohistochemistry.

Reproduced from [4], © Springer Nature (2023), published under a CC-BY license.

Although further study is required, this study indicates that there are differences in cell wall glycan composition between plants grown in space and on Earth; it also validates the glycome profiling technique as an effective way to determine carbohydrate composition.

What happens to plants' photosynthetic machinery when grown on the ISS?

Another aspect that must be addressed when growing plants in space is the process of photosynthesis. Extraterrestrial environments are not hospitable for plant growth, but controlled environments within a spaceship could foster growth if they meet the necessary temperature, light, carbon dioxide and water specifications.

Recently, researchers utilized NASA's BRIC hardware and combined transcriptomics and proteomics approaches to investigate how A. thaliana seedlings' photosynthetic machinery is affected by spaceflight. Previously kept in cold storage, 36 seeded 60 mm plates (contained within BRICs, each one holding 6 plates) were sent up on 15 December 2017, to germinate and grow on the ISS for 10 days in ambient temperature, under a specific light wave composition (85% red, 15% blue, and 60 μmol m^-2 s^-1 intensity) and for precise light–dark durations (4–2 h). Therefore, the seedlings had light cues, but no gravitational cues. After 10 days, they were incubated in Invitrogen RNAlater, a solution that prevents the degradation of RNA, and frozen [9].

When back on Earth in May 2018, the space-grown plants were compared to plants that had grown under a similar experimental setup on Earth. RNA extraction from plant shoots was conducted using the Qiagen RNeasy Plant Mini Kit (Qiagen, Hilden, Germany), concentration was determined using the Qubit 2.0 Fluorometer (ThermoFisher Scientific, MA, USA), quality was assessed using the Agilent 2100 Bioanalyzer (Agilent Technologies Inc., CA, USA) and they created an RNA-sequencing library. Alongside this transcriptomic analysis, soluble and membrane proteins from the roots and shoots were extracted before undergoing 6-plex TMT labeling and liquid chromatography–tandem mass spectrometry [9].

Although the overall photosynthetic process wasn't affected by spaceflight, deeper analysis indicated that the molecular functions of the photosystem were affected. The team found that the space-grown plants had altered photosynthetic machinery as many key photosystem-regulating proteins had shifted to other organs compared to the Earth-grown plants. There was an increase in NusG-like protein expression – a protein that moderates RNA synthesis – suggesting that the space-grown plants were boosting their chloroplast ribosomes' translational efficiency to try and adapt to the stresses of space. An abundance of other proteins illuminated that the space-grown plants may have also been trying to enhance their water-oxidizing reaction and repair damage to the photosystem [9].

Additionally, space-grown plants' transcripts did not align with their resulting root and shoot proteins, suggesting substantial post-translational modifications occurred. However, this could be attributed to any number of factors, from disparities in collection and analysis between RNA and protein, to compromised translation mechanisms and upregulation of alternative splicing thanks to the environmental stressors of space [9].

What happens when plants are grown in lunar soil?

Many studies are being conducted to grow crops aboard spaceships to prolong missions and explore deep space. However, there is also a line of study that is concerned with investigating how we can grow crops on other planets – a question that forms the premise of the book and film The Martian – to support human life both for potential habitation and during long missions, especially the return to the moon. This would require the symbiotic interaction of lunar materials and terrestrial systems, an interaction that's study has previously been neglected [10].

In May of 2022, NASA scientists published a paper detailing their Earth-conducted study of plant growth in different kinds of lunar regolith – fragmented, unconsolidated rock on the surface of the moon – sampled during Apollo missions 11, 12 and 17. Utilizing A. thaliana, the team found that when grown in lunar soil, the plants exhibited slow development coupled with signs of significant stress.

Seeds were planted on the surfaces of three different samples of lunar regolith (a sample from each mission); they were placed under special light and irrigation systems in ventilated terrariums. These experimental plants were compared with a terrestrial control (JSC-1A). Within 48–60 h, germination occurred in the lunar plants, and the team observed normal growth of stems and cotyledons. However, between day 6 and 8, the plants exhibited root growth inhibition, which wasn't observed in terrestrial soil-grown plants. The lunar plants also showed slower expanded leaf development along with signs of stress, such as smaller rosette diameter and deeper pigmentation (Figure 2) [10].

Figure 2. (A)

Germination occurs. All seedlings, regardless of growth media, show indistinguishable germination. (B) Lunar regolith-grown seedlings show stunted root growth compared to JSC-1A. ( C) Leaf growth is greater for JSC-1A control than regolith-grown seedlings.

Reproduced from [10], © Springer Nature (2022), published under a CC-BY license.

Transcriptomic analyses were conducted on the leaves and stems of the plants, revealing that the transcriptomes differed depending on the Apollo mission during which the regolith was collected. Comparing the three lunar regolith-grown plants, the researchers found that the sample collected from Apollo 11 had the greatest differential expression, followed by the sample from Apollo 12, and then the sample from Apollo 17. However, all lunar samples had differentially expressed genes (DEGs) indicative of stress; as mentioned, these DEGs varied between Apollo samples but were the same for regolith collected from the same collection location. Additionally, morphology was indicative of DEGs as the severe phenotype (those plants that were distorted and tiny) displayed the greatest DEGs compared to normal looking lunar-grown plants that displayed the fewest DEGs [10].

This study demonstrated terrestrial plants' ability to grow in lunar regolith; however, it shows that it isn't the most hospitable media for growth and can result in significant stress signals and stunted growth. Further research is necessary to determine if and how we could mitigate the negative effects of lunar regolith on plant growth.

A symbiotic relationship between astronauts & plants

Lunar regolith has displayed that it may not be the optimum growing material for plants, so ETH Zurich scientists (Lindau, Switzerland) may have had the right idea when they looked into the possibility of extracting nutrients from organic waste to support crop growth in space, which would be a more sustainable and cost-effective alternative to shipping Earth fertilizers into space. The study, published in February 2024, reviewed the ESA's Micro Ecological Life Support System Alternative (MELiSSA) loop, which is a closed, hydroponic, ecosystem-mimicking loop composed of compartments that hold different processes as shown in Figure 3 [11].

Figure 3. MELiSSA loop as it has been developed by the ESA.

Reproduced from [11], © Elsevier (2024), published under a CC-BY license.

The team set out to determine how and which nutrients from organic waste could be harnessed or expelled to benefit plant growth. Although phosphorous and nitrogen can be extracted from urine, it is unclear how we can extract and remove nutrients, such as chloride and sodium, from urine as well as other forms of organic waste so that these don't progress to other compartments within the loop. The system's ability to extract carbon from solid and liquid waste for CO₂ to support photosynthesis is also a consideration. Additionally, elements like nitrogen, both mineral and gas, are essential to plant health; therefore, scientists must assess whether these organic wastes can provide the nutrients necessary for hydroponic systems to thrive. What's more, they must also assess the efficiency of nutrient uptake by plants, optimizing this as much as possible [11].

Upon reviewing the MELiSSA loop, scientists found that there were two nutrient streams supporting Compartment 4b, which is where higher plants were being grown: Compartment 2 provides phosphate, calcium and magnesium extracted from urine and effluent from the liquified waste while Compartment 3 – the nitrifying compartment – provides nitrogen. Mixing these nutrient streams with water condensation would provide a fairly supportive nutrient solution to the average plant; however, some plants can experience nutrient imbalance or over-consumption on this solution because every crop has different nutrient specifications, so supplemental nutritional streams may be necessary. Furthermore, the pH and element concentrations will need to be monitored throughout plant growth and altered depending on growth phase. Because hydroponic systems grow plants without soil, the nutrient solution's pH must be within the values of 5–7, which is easily thrown off in a case of nutrient imbalance [11].

The researchers suggested an amendment to the current MELiSSA loop (Figure 4), highlighting that Compartment 2 could be more efficient at breaking down liquid organic matter in the presence of CO₂ if the compartment held a microbial electrochemical cell (MEC) coupled with chemical oxidation, in place of photoheterotrophic bacteria [11].

Figure 4. Suggested improvements to the MELiSSA loop schematic.

Reproduced from [11], © Elsevier (2024), published under a CC-BY license.

In order for systems like MELiSSA to thrive, novel techniques and sensors will need to be developed and implemented to monitor plant growth, adjusting for microgravitational conditions and nutrient recycling [11], which I think we can safely assume are in progress behind the scenes at space agencies around the world.

Ready for liftoff?

Research in this extraterrestrial landscape is continuing, providing essential information about how terrestrial biological systems are affected by environmental factors in space and how to circumvent these stressors. Whether we're discussing cell wall glycans, photosynthetic machinery or nutrient availability and growth media, there are several factors that interrupt these structures and processes when plants are trying to grow in extraterrestrial gravitational and light conditions as well as in closed systems. However, steady progress is being made to develop techniques and systems that can sustain diverse crop growth aboard the ISS and maybe even on different planets… one day.