February 29, 2024

If you’re ever on your lunch break in space, you might actually want to stay away from the salad. A team of scientists grew lettuce in a simulated microgravity environment on Earth and found that plants have a hard time protecting themselves against harmful bacteria in space.

Space lettuce has been a hit on board the International Space Station (ISS), with astronauts tending to the leafy greens inside a space garden known as Veggie and even chewing on a patch of red leaf lettuce that they’ve grown themselves. It provides a healthy, fresh alternative to the dehydrated, pre-packaged foods that astronauts are made to eat on the space station. New research, however, warns that plants grown in the microgravity environment are more prone to infections like E. coli or Salmonella.

In a paper recently published in Scientific Reports, a group of researchers grew plants in a device called a clinostat, which rotated them like a rotisserie chicken so that they would lose their sense of directionality. “In effect, the plant would not know which way was up or down,” Noah Totsline from the University of Delaware’s Department of Plant and Soil Sciences, and lead author of the new paper, said in a statement. “We were kind of confusing their response to gravity.”

Plants have gravity sensing cells which make their roots grow downwards towards gravity while the plant itself shoots upwards in the opposite direction of gravity. In a microgravity environment, the researchers found that the plant’s natural defense to stressors is rendered less effective.

Stomata are tiny pores in plants’ leaves and stems that help them breathe, and are also used as a defense mechanism by closing up when they sense a stressor like bacteria nearby. But during their carnival ride, the plants became disoriented and opened up their pores in the presence of bacteria instead of closing them, according to the research. “The fact that they were remaining open when we were presenting them with what would appear to be a stress was really unexpected,” Totsline said.

From the experiment, the researchers concluded that bacteria like Salmonella can invade leaf tissue more easily in simulated microgravity conditions, like the one on the ISS.

A previous study published in 2020 analyzed the first crop of lettuce grown on the ISS between 2014 and 2016, and found that while the lettuce contained the same amount of nutrients as the variety grown on Earth, it did seem to have higher levels of bacteria.

The Veggie system on the ISS uses vacuum-sealed seeds that are pre-planted in a pillow filled with ceramic soil and fertilizer; the pillow is then placed on a root mat designed with wicks that deliver the water to the plant, and the whole thing is held down by bungee cords. Despite all these efforts to counteract the effects of the microgravity environment, the ISS is a closed-air system with groups of astronauts living in its headquarters. Wherever there are humans, there are pathogens with the potential of infecting nearby plants.

The results could put a damper on a great source of nutrition that’s also easy to grow in space for future astronauts on longer missions to the Moon or Mars.

In order to reduce the risk of bacterial infections for astronauts who want to munch on space lettuce, the researchers suggest tweaking the genetics of the plants to prevent them from opening their stomata wider in space. The researchers have already started testing different lettuce varieties with different genetics to see how they react in the simulated microgravity environment.

“If, for example, we find one that closes their stomata compared to another we have already tested that opens their stomata, then we can try to compare the genetics of these two different cultivars,” Harsh Bais, plant biology professor at University of Deleware, and co-author of the new study, said in a statement.

“You don’t want the whole mission to fail just because of a food safety outbreak,” he added.

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