Tokyo: Examining cucumber seedlings germinated under the very weak gravity — or microgravity — conditions of the International Space Station, researchers from Japan’s Tohoku University have discovered how a protein helps plants sense gravity to boost their chances of survival. Plants are experts in survival and can control the direction of their roots to maximise the use of resources around them.

Using specialised cells, they can sense gravity and redistribute hormones, called auxins, to stimulate growth and allow vital features of the plant to develop.  However, a big puzzle is how this transport process occurs at a cellular level. In this study, reported in the journal Nature Microgravity and using samples grown on board the International Space Station, the research team highlighted the valuable contribution of the gravity-sensitive CsPIN1 protein to this process.

The role of the protein in facilitating the transport of the growth hormones had first been suggested in previous experiments conducted on Earth. To gain further insight, the researchers loaded cucumber seeds into specially designed canisters, which were sent up to the space station.  Cucumbers were chosen for the study as they — like other “cucurbitaceous” seedlings such as melons, pumpkins and squash — feature specialised protuberances, or pegs, whose formation is regulated by gravity.

These pegs form during the plant’s early growth stage to help the seedlings emerge from their hard seed coat and anchor the developing plant in the soil while its roots form. The experiment showed that CsPIN1 protein can relocalise under the influence of gravity. Specifically, this change in the position of protein was found to occur in the so-called transition zone of the cucumber seedling where the pegs develop.

This behaviour stimulates the formation of a cellular canal capable of transporting growth hormones from one side of this zone to the other, the study said. Put simply, these findings point towards the mechanism by which the seeds are able to turn on and off the growth of their anchoring pegs in relation to their orientation with respect to gravity. And, as result, boost their chances of survival.