Plants are proficient in survival and can regulate the direction of their roots to enhance the use of resources around them. Employing specialized cells, they can identify gravity and redistribute hormones, known as auxins, to boost growth and enable vital features of the plant to develop. But, a big puzzle is how this transport procedure occurs at a cellular level. To know more, scientists in Japan analysed cucumber seedlings germinated under the very small gravity – or microgravity – conditions of the International Space Station.
Cucumbers were selected for the study as they prefer other ‘cucurbitaceous’ seedlings like pumpkins, melons and squash feature specialized pegs, or protuberances, whose formation is controlled by the gravity. Such pegs form during the plant’s early growth stage to aid the seedlings to emerge from their hard seed coat and anchor the introducing plant in the soil white its roots form.
When seeds are put before germination either in a vertical state with their embryonic roots directing down, or subjected to microgravity conditions, a peg develops on each of the sides, explains Hideyuki Takahashi, a member of the Adaption and Space Biology Laboratory at Tohoku University’s Graduate School of Life Sciences. But when they are put horizontally on the ground, peg creation on the upper side is suppressed in response to the gravity.
In the current round of work, using samples extend on board the International Space Station, Takahashi, and his group highlight the valuable contribution of the gravity sensitive CsP1N1 protein to this procedure. The function of the protein in facilitating the transport of the growth hormones had been primarily suggested in previous experiments conducted on Earth.
To gain further insights, the scientists loaded cucumber seeds into specifically designed canisters that were sent up to the space station. Here, the water-absorbent plastic foam in the container was irrigated and the new – germinating seedlings were grown in the microgravity section of the cell biology study facility for 24 hours. Then the cucumber seedlings were either maintained in microgravity or stimulated by gravity applying a 1g centrifugal force for two hours more.
One of the major challenges experiencing the group was finding a suitable fixative to ‘freeze’ the status of seedlings germinated on the space station so that the samples could be analysed in detail back on the Earth. Particularly, the change in protein position was identified to occur in the transition zone of the cucumber seedling – the space between the roots and stem, where the pegs develop. It appears that such behaviour boosts the formation of a cellular canal proficient of transporting growth hormones from one area to the other.
“Such result helps to explain the gravity controlled the decrease in auxin level and hence suppression of peg formation on the upper side of horizontally growing seedlings of cucumber seedlings,” confirms Takahashi. The place simply, such findings point towards the mechanism by which the seeds are able to transform on and off the growth of their anchoring pegs in relation to their orientation with respect to gravity.
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