Scientists have examined cucumber seedlings germinated under microgravity conditions of the International Space Station (ISS), to understand how plants sense gravity and stimulate their own growth.
Cucumbers feature specialized protuberances, or pegs, whose formation is regulated by gravity.
These pegs develop 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.
When seeds are put before germination either in a vertical position with their embryonic roots pointing down or subjected to microgravity conditions, a peg develops on each side.
However, when they are placed horizontally on the ground, peg formation on the upper side is suppressed in response to gravity.
Scientists also unearthed 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.
Examining cross sections of the cucumber seedlings abroad the space station under a microscope, the scientists found that the CsPIN1 protein can relocalize under the influence of gravity.
The change in the position of protein was found to occur in the transition zone of the cucumber seedling, the area between the stem and the roots, where the pegs develop.
It appears that this behaviour stimulates the formation of a cellular canal capable of transporting growth hormones from one side of this zone to the other, researchers said.
The 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
