So we need to better understand how the lack of gravity affects plant growth. One possibility is that it all comes down to the plant growth hormone known as indole-3-acetic acid or “auxin”. This remarkable molecule is about a thousand times smaller than a dust particle but influences almost every stage of a plant’s life cycle, from germination to death. In my view, auxin is one of the most important small molecules that feeds the world.
The effects of auxin have been studied for over 100 years by scientists including Charles Darwin and his son Francis Darwin. Plants regulate their growth by moving more auxin to the tip of the root and stem compared the rest of the plant. This uneven auxin level inside roots and stems is known as “auxin polarity”.
Whenever plants detect a change in light, water or gravity they can transport auxin to adjust auxin polarity. This enables the shoot to grow towards light and the roots towards gravity and water. Scientists have shown that, in space, water can be used to replace some of the effects of gravity as the signal that influences auxin polarity.
My own research has looked at a small molecule called “TENin1” that interferes with auxin transport. TENin1 causes changes in auxin polarity and removes a plant’s ability to detect gravity. Other studies in space have shown that gravity-related characteristics rely more on auxin than a plant’s ability to sense gravity. These results suggest that auxin transport and signalling are more important than gravity in influencing growth in plants.
So to find the optimum conditions for plant growth in space, scientists are now focusing on how auxin is transported in microgravity. For example, one group is investigating how microgravity affects the cellular distribution of auxin transporters in cucumber plants.
We still need to investigate how plants deal with cosmic radiation and the soils of other planets if we want to take them into space. But knowing how to grow plants in microgravity will make us more equipped for longer space expeditions and future space colonisation. And by improving our understanding of the way plants grow in general, it could even help us to boost agricultural production here on Earth.