Introduction to AstroBotany and online data sharing

Edible microgreens on Earth and Low Earth Orbit (LEO)

Future space missions, as well as extended human space habitation, are critically dependent on sustainably and effectively maintaining both the physiological and psychological health of crew members in space. One important factor, paramount to mission success, is the capability of providing optimal nutritional conditions for crew members while reducing the need for Earth-based resupply. Food packaging, storage, and restocking currently pose significant challenges to extended human presence in space. (To introduce students to Martian exploration, we recommend the “Kids2Space” video program).

The cultivation of edible produce through the integration of plant-based bioregenerative life support systems provides viable solutions to challenges associated with crew health as well as minimizing the costs associated with resupplying. Growing plants throughout space missions has demonstrated its potential to become a source of fresh produce for space crews, while also contributing to gas and water recycling. In addition, the cultivation of plants on the ISS has shown to mitigate psychological stresses and depression associated with long-duration space habitation.

Figure 2: Astronauts eat plants on the ISS. They look really happy! Source: Gioia Massa Growing beyond Earth presentation 2020. NASA and Fairchild Botanical Gardens.

The ideal crop or “Ideotype” for the spaceflight environment?

Ideal plants to be grown in space are selected based on the following criteria: crop potential (yield, edible biomass as well as oxygen and water production); ratio of total biomass to edible biomass (harvest index); crop growth efficiency (growth time and space required, nutrient value; and crop horticultural needs (development, energy requirements, water utilization, harvesting, and crop processing). In summary, plants grown in space should require minimal crew attention and preparation time, while producing nutrient-dense yields. Additionally, these plants need to have fast growth cycles while requiring minimal growth space and water intake.

An ideal crop type that fits this criteria are salad crops. Salad crops are suitable for modular chamber cultivation due to their minimal space requirements. The majority of salad crops have relatively short growth cycles and require minimal horticultural care. In addition, most salad crops can be eaten fresh. This program focuses on the effects of a specific salad specialty crop in microgravity: microgreens.

Microgreens are small (2.5cm-8cm in height), colorful, nutrient-dense crops that are often used to enhance flavor in dishes. Generated from vegetable, legume, grass, and herb seeds, microgreens typically have short species-specific production cycles lasting between one to three weeks. They are harvested after their cotyledons are expanded and the first set of true leaves have emerged. Studies from the USDA have shown that at equal weights, most microgreens contain about 5-30 times more nutrients than what is found in the mature leaves of the same plants. Due to their high nutrient concentrations, fast growth cycles, minimal water/space, and horticultural requirements, microgreens could be considered as an integral component of a bioregenerative life support system. This enables them to contribute to both the dietary and psychological needs of space crews.

Source: https://www.genetics.org/content/208/4/1337

Arabidopsis thaliana developmental stages. Essential for understanding plant morphology changes.

Arabidopsis thaliana is a model plant that has been used to investigate how plant morphology changes in response to their environment as they age. Their genomes express themselves during the different developmental stages they go through. Understanding how Arabidopsis responds to environmental changes gives us the opportunity to potentially engineer improved crops with architecture that more closely resemble the "Ideotype". Practicing time-lapse photographyis a good way to learn how fast different species grow.

The cell biology of “gravitropism”

Gravity plays a key role in plant development. Stems grow toward a light source and roots toward a gravity vector. In the absence of light, stems grow in the opposite direction of the gravity vector. These directional growth responses to external stimuli are known as tropisms. Growth in the direction of light is called phototropism. Gravitropism is the growth response related to gravitational force.

Figure 4: (A) Auxin flows in a “reverse fountain” in the root tip. (B) A gravitropic stimulus (90-degree rotation) causes the auxin to accumulate on the new bottom of the root. (C) Asymmetric distribution of the plant hormone auxin causes organs to bend (Source: Strohm et al., (2012) Molecular mechanisms of root gravity sensing and signal transduction. DOI:10.1002/wdev.14).

Plants use hormones, affected by light and gravity, to regulate the growth of stems and roots. The hormone auxin is integral to both gravitropism and phototropism. The upward growth of stems and downward growth of roots is affected by gravity. Root tips have specialized cells that contain bead-like organelles called statoliths. When a root is repositioned on its side, the statoliths accumulate on the lower side of the cells. The accumulation of statoliths triggers feedback mechanisms in the plant that direct auxin to the site where the statoliths are. High concentrations of auxin in root cells inhibit cell elongation. This causes the bottom cells to stop elongating while the top cells continue at a consistent rate, resulting in the root bend and redirecting itself according to the center of gravity.

The major challenges for plant development in microgravity

We often get asked: How does gravity affect plant development and growth in space?

Microgravity affects root growth patterns as well as various elements in plant biology. Not only does gravity affect how roots and stems grow, but it also affects essential biological processes impacting how the plant develops. One of the processes affected is plant gas exchange and mixing. Convection cells, where warmer air rises and cooler air falls, do not occur in microgravity environments. This reduces the amount of air circulation that occurs, which can cause gas accumulation in cells. As plants give off CO2, it stays close to the plant, displacing O2 and causing hypoxic zones near the plant's surface. This restricted O2 supply can particularly affect areas with high metabolic rates. Areas of plants with high metabolic and respiration rates, such as root tips, are then at risk. This could potentially negatively impact crop yields and the nutritional density of plants grown in microgravity.

Figure 5: How plants respond to microgravity is hard to generalize. It depends on the variety since different growth patterns have evolved and been selected for during crop breeding. Plant size and shape are determined by environmental signals that are perceived and translated into specific developmental responses. Source: Oladosu et al. (2020). Submergence Tolerance in Rice: Review of Mechanism, Breeding, and Future Prospects. https://doi.org/10.3390/su12041632

Our turn to ask you a question: Can you conduct some experiments and share data with us to help us help NASA feed astronauts a healthy diet?