Extreme survival of seeds on Earth and in space
Anne Visscher from Kew’s Comparative Plant and Fungal Biology department discusses extreme survival of seeds on Earth and a research proposal to send seeds to the International Space Station to test their survival in outer space.
Extreme survival of dry seeds
Dry seeds can be exposed to extreme environmental conditions on Earth and in space, and their survival is known to vary between species. For example, dry seeds may survive an extreme range of temperatures (from ‑196°C during cryopreservation up to ≤+1000°C in wildfires); ultra-drying conditions associated with the vacuum of space or imposed during seed storage on Earth; and, ultraviolet radiation reaching the surface of the Earth (UV-A, UV-B) or present in space environments (UV-A, UV-B and UV-C) (1).
It is known, for instance, that seeds from many Aizoaceae species, including Mesembryanthemum crystallinum (common iceplant), are able to germinate successfully following extended heat treatments of 103°C, despite the absence of a thick seed coat (2). In addition, research on seven Brassicaceae (cabbage family) species, which were stored for five years under ultra-dry conditions, showed that seeds from Sisymbrium runcinatum benefitted from such storing conditions whilst the other species did not (3).
Little is known about the variation in dry seed survival between different species following exposure to ultraviolet radiation. When seeds from Arabidopsis thaliana (thale cress) and Nicotianum tabacum (cultivated tobacco) were exposed to UV on the outside of the International Space Station for 1.5 years, 23% produced viable plants upon return to Earth (4).
Below I will explain why we are interested in discovering more about the variation in resilience between dry seeds from different species to extreme conditions present in outer space.
Plants and human spaceflight
In the near future, astronauts may be able to spend longer periods of time on the Moon or Mars (for example in a lunar outpost or a Mars base) to improve our understanding of the geology and biology of these different environments.
For human missions that are longer than one or two years, it is expensive and challenging to launch and transport food, water, and atmospheric gases required for survival (5).
Instead of transporting all the necessary food from Earth, photosynthetic plants could be grown in closed life support systems to provide a healthy and varied diet during missions. Plants and microorganisms could also help to recycle waste, water and atmospheric gases, which would further save materials and associated launch costs. For example, photosynthetic higher plants would be able to provide a team of astronauts with a healthy and varied diet in the form of cereals, legumes, oilseeds, fruit and vegetables. In addition, plants could help to revitalise the atmosphere (by liberating oxygen and fixing carbon dioxide) and purify water (via transpiration) (6).
For even larger and longer-term habitats on the Moon or Mars, other benefits from plants could include construction materials, fabrics, medicines, dyes, lubricants, biofuels, and aesthetics.
Storing seeds for growth in space
Since plants have the potential to play several important roles in long-term life support systems on other planets, it is crucial to know how seeds should be stored and transported across space before being germinated and grown in such systems.
On Earth, seeds of many species can be stored for decades under standard seed bank conditions (drying to around 5% moisture content and storage at sub-zero temperatures) (7). But, over time, all seeds lose quality and will eventually die.
In space, seeds could be stored under similar conditions inside a space station or spacecraft, but this may not be necessary for all species. For example, some seeds are known to benefit from ultra-drying and anoxia, both of which are extreme conditions associated with the vacuum of outer space. Seeds that benefit from, or are tolerant of extreme space conditions (for example ultraviolet and ionising radiation, as well as temperature fluctuations) could be transported on the outside of a spacecraft in order to save space for more sensitive materials on the inside.
Comparative seed biology on the ISS
The aim of our proposed project with the European Space Agency is to research how and why seeds from a diverse set of 24 plant species differ in their responses to the outer space environment. As we cannot exactly reproduce the combination of extreme conditions found in outer space in our laboratory on Earth, we are planning to use a seed exposure facility on the outside of the International Space Station (ISS) for our experiments.
For the first time in the history of research on seeds exposed to outer space, we are also planning to monitor changes that are happening to seeds during their stay outside the ISS: for example, we would like to identify the molecules that are outgassed from the seeds in the vacuum of space by using miniature mass spectrometers.
Impact for seed storage on Earth and in space
Results from our research could lead to recommendations for seed transport through space, and improvements to protocols for the long-term storage of seeds on Earth. In addition, our findings may influence the potential design of orbiting seed banks, or even deep space seed banks, with the goal to preserve human life and ecosystems in the event of disaster.
Visscher, A.M., Seal, C.E., Newton, R.J., Latorre Frances, A. & Pritchard, H.W. (2016). Dry seeds and environmental extremes: consequences for seed lifespan and germination. Functional Plant Biology DOI: 10.1071/FP15275. Available online
Daws M.I., Kabadajic A., Manger K. & Kranner I. (2007). Extreme thermo-tolerance in seeds of desert succulents is related to maximum annual temperature. South African Journal of Botany 73, 262–265. Available online.
Mira, S., Estrelles, E. & Gonzalez-Benito, M.E. (2015). Effect of water content and temperature on seed longevity of seven Brassicaceae species after 5 years of storage. Plant Biology 17, 153–162. Available online.
Tepfer D., Zalar A. & Leach S. (2012). Survival of plant seeds, their UV screens, and nptII DNA for 18 months outside the International Space Station. Astrobiology 12, 517–528. Available online.
Barta, D.J. & Henninger, D.L. (1994). Regenerative life support systems - why do we need them. Advances in Space Research 14, 403–410. Available online.
Mitchell, C.A. (1994). Bioregenerative life support systems. American Journal of Clinical Nutrition 60, 820s–824s.
FAO (2014) ‘Genebank standards for plant genetic resources for food and agriculture.’ (FAO: Rome). Available online.