Can plant seeds survive space radiation?

WWill we one day colonize space? Will our children visit other planets? To achieve goals like these, we will face a crucial challenge: how to feed ourselves for long periods away from Earth.

A trip to Mars would take months, and exploring the depths of the galaxy would take even longer. The provision of nutritious food for travelers is a significant barrier. While hoarding food is an option, storing enough of it to last many months strains the weight and space limits in spacecraft, and missions could easily exceed the shelf life of the food. Growing food in space will be essential.

Essential and not necessarily easy. Conditions in the vacuum of space are quite harsh compared to Earth. The seeds in space must be able to withstand large doses of ultraviolet and cosmic radiation, low pressure and microgravity.

Believe it or not, the first space travelers were seeds. In 1946, NASA launched a V-2 rocket carrying corn seeds to observe how they would be affected by the radiation. Since then, the scientific community has learned a great deal about the effects of the space environment on seed germination, metabolism, genetics, biochemistry, and even seed production.

Astrobiologists David Tepfer and Sydney Leach have studied how the seeds would return to Earth after spending long periods on the International Space Station. The experiments they conducted in EXPOSE missions were much longer than many other ISS seed experiments and placed the seeds outside the station, in the heart of space, rather than inside. The goal was to understand not only the effects of long-term radiation exposure, but also a little about the molecular mechanisms of those effects.

The seeds have some defenses

The suits possess a couple of notable traits that Tepfer and Leach speculated would give these “model space travelers” a fighting chance.

First, they contain multiple copies of important genes – what scientists call redundancy. Genetic redundancy is common in flowering plants, especially food products such as seedless watermelon and strawberries. If one genetic copy is damaged, there is still another available to do the job.

Second, seed coatings contain chemicals called flavonoids that act as sunscreens, protecting the seed’s DNA from damage caused by ultraviolet (UV) light. On Earth, our planet’s atmosphere filters out some harmful UV rays before it can reach us. But there is no protective atmosphere in space.

Would these special features be enough for the seeds to survive or even thrive? To find out, Tepfer and Leach conducted a series of experiments – both off the International Space Station and on Earth – with tobacco, Arabidopsis (a flowering plant commonly used in research) and morning glory seeds.

Bombarded with energy

Their EXPOSE-E experiment flew to the International Space Station (ISS) in 2008 and lasted 558 days, so just under two years.

They stored the seeds in a single layer on the outside of the ISS behind a special type of glass that only lets in ultraviolet radiation at wavelengths between 110 and 400 nanometers. DNA readily absorbs UV radiation in this wavelength range. A second identical set of seeds was on the ISS, but fully protected from UV rays. The aim of this experimental project was to observe the effects of UV radiation separately from other types of radiation such as cosmic rays which are everywhere in space.

Tepfer and Leach chose tobacco e Arabidopsis seeds for EXPOSE-E because both have a redundant genome and therefore a good chance of survival. They also included a genetically modified tobacco variety with the addition of an antibiotic resistance gene; the plan was to subsequently test this gene in bacteria and determine if there was any damage. In addition to normal Arapidopsis, they sent two genetically modified strains of the plant that contained low and absent UV protection chemicals in their coat. They also sent purified DNA and purified flavonoids. This provided researchers with a wide range of scenarios for understanding the effects of space on seeds.

A second ISS mission called EXPOSE-R included only the three types of Arabidopsis seeds. These received just over double the dose of ultraviolet light due to the longer experiment time, 682 days. Finally, the researchers performed an experiment on the ground in the laboratory that exhibited Arabidopsis, tobacco and seeds of Morning Glory in very high doses of UV light for only one month.

After all these various exposure conditions, it was time to see how well the seeds could grow.

What would the researchers collect?

When the seeds returned to Earth, the researchers measured their germination rates, which is how quickly the root emerged from the seed coat.

Seeds that had been screened in the lab performed at their best, with over 90 percent germination. Then came the seeds that had been exposed to UV rays for a month in the laboratory, with 80% better germination.

For seeds that travel through space, more than 60 percent of the shielded seeds have germinated. Only 3% of space seeds exposed to UV rays did this.

The 11 Arabidopsis plants that grew from both wild type and genetically modified seeds did not survive once planted in the ground. The tobacco plants, however, showed reduced growth, but that growth rate was recovered in subsequent generations. Tobacco has a much thicker seed coat and a more redundant genome, which may explain its apparent survival advantage.

When the researchers inserted the antibiotic resistance gene into the bacteria, they found it was still functional after its journey into space. This finding suggests that it is not genetic damage that makes these seeds less viable. Tepfer and Leach attributed the reduced germination rate to damage to other molecules in the seed besides DNA, such as proteins. A redundant genome or integrated DNA repair mechanisms would not have overcome that damage, further explaining why the Arabidopsis the plants did not survive the transplant.

In field experiments, the researchers found that radiation damage is dose-dependent: the more radiation the seeds receive, the worse their germination rate.

These findings could provide future indications for research in space agriculture. Scientists may consider that genetically engineered seeds have additional protection for cellular machinery critical to protein synthesis, such as ribosomes. Future research will also need to further explore how seeds stored in space germinate in microgravity conditions, rather than on Earth.

As researchers increase knowledge of how space affects plants and their seeds, we can continue to take the necessary steps towards food production in space. It will be a crucial step towards sustainable colonies capable of surviving beyond the comfortable confines of the Earth’s biosphere.

This article was republished by The Conversation under a Creative Commons license.