Growth and stress
Given the small size of their lunar regolith samples, the researchers constructed a system in which small sample wells were filled with 900 milligrams of soil and fed water from underneath. For plants, the researchers chose Arabidopsis, a small flowering plant related to mustard that has been used in biology research. Using a well-understood plant allowed the researchers to track which genes were active in different materials.
About a week after seeds were placed in the soil, things sprouted as normal, so the differences in the soil weren't significant enough to interfere with this process. Several days after, the researchers removed all but one of the plants from each sample well, giving them a look at the developing roots, which were stunted compared to those of seedlings grown in JSC-1A.
On average, growth on all lunar soils was slower and more erratic than growth in JSC-1A. The plants took longer to unfold leaves, they spread to a smaller diameter, they didn't grow as tall, and they had altered pigmentation. But these phenotypes were variable; some plants grown in lunar soil were clearly defective, while others looked normal, albeit a bit smaller. Problems generally correlated with the age of the regolith, with plants in the Apollo 17 sample (the youngest) doing the best.
The researchers found that the plants grown in lunar soil activated many of the genes involved with stress responses, including those involved with phosphate starvation, metal toxicity, and reactive oxygen problems (the last potentially due to the differences in iron between the soils). The genes accounted for over 70 percent of the genes that were activated compared to plants grown in JSC-1A. The rest were mostly involved with nutrient metabolism.
The researchers also divided the plants into three categories: defective, small, and near-normal. The latter two groups had only 100–150 altered genes compared to the JSC-1A controls, with most of the genes involved in responses to drought and salt stress. But the dwarfs saw over 1,000 genes with different activity levels.
What this tells us
There are a couple of ways to look at these results. The first is that the study represented a serious stress test. While the samples were watered using a nutrient solution, they were dumped into lunar regolith as-is—no mixing with organic material and no microbial growth that could sequester some of the metallic toxins before the plants encountered them. So the experimental setup made things harder than they needed to be.
On the flip side, everything that might better prepare the lunar soil for plant growth would take time and mass, two things that can be in very short supply on space missions. Viewed from that angle, the decision to use lunar soil doesn't save as much mass as it might, undercutting one of its advantages.
The work suggests that a few chemical treatments might help leach away some of the heavy metals and convert the iron into an oxidation state more similar to that seen in typical Earth soils. But it's clear that while some plants can tolerate the harsh conditions produced by lunar regolith, they're not happy about it. That means we probably can't expect to use this system to grow anything edible, given that plants seem to struggle to grow at all.
The most promising possibility is to use this information as a springboard to study in more detail what is causing the plants to struggle and then start engineering and selecting for strains that can tolerate the regolith better. But if the study tells us nothing else, it's that we don't have a very good analog for lunar soil, and we certainly don't have enough regolith samples to make these sorts of experiments possible. Working on materials that could enable these kinds of studies is the most pressing need going forward.
Communications Biology, 2022. DOI: 10.1038/s42003-022-03334-8 (About DOIs).
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