On the ground, the "gravity" factor plays an major role in the plants' growth orientation (gravitropism). In fact, plants have the capacity to perceive and reorientate their growth depending on the gravity.
The roots penetrate the soil thanks to the gravity in order to draw water and mineral elements. The aerial parts grow vertically because of the gravity to photosynthesize. Studying the gravity perception and its consequences is thus crucial for the understanding of plants' development mechanisms.
The cytoplasm (content of a living cell) of plant roots contains different specialised structures:
- cells (called statocysts) localised at the end of the roots;
- amyloplasts specialised in starch storing;
- endoplasmic reticulum playing a role towards the proteins;
The statocysts have the characteristic to present a structural polarity depending on the gravity, so interactions between amyloplasts and endoplasmic reticulum are possible.
By changing their contact with the endoplasmic reticulum under the influence of a gravitropic stress, the amyloplasts' displacement could be at the origin of the transduction mechanisms which may lead to a gravitropic curvature.
The PolCa experiment was performed on board the international station within the Kubik European instrument.
Its objective was to analyse the effects of the amyloplasts-endoplasmic reticulum interactions changes on the calcium distribution in order to better understand the plants' mechanisms of gravity perception.
CNES designed the caskets (containing culture chambers), a water tank and a reservoirs dedicated to chemical bonding agents.
The rapeseeds were previously arranged on a paper filter. Once on board the ISS, the experiment was switch automatically on by hydrating the seeds.
The young rapeseed germinations were submitted to one of the following situations:
(1) growth in microgravity
(2) growth in a centrifuge (control group)
(3) growth in microgravity followed by a short growth in a centrifuge 1 g
(4) conversely, growth in a centrifuge 1 g followed by a short growth in microgravity
After a 40h-growth, the bonding agent was automatically injected in the culture chambers. Back on the ground, the samples were prepared to be observed with a microscope.
Rapeseed orientation after a 40h-germination in microgravity - credits: V. Legué University of Nancy, France
Rapeseed orientation after a 40h-germination in a centrifuge 1 g (on board the ISS) - credits: V. Legué University of Nancy, France
The amyloplasts' distribution analysis showed changes in the four studied conditions. This result suggested that the absence of gravity affected the statocysts' polarity. In addition, a change from 1 g to microgravity or conversely (conditions 3 and 4) resulted in a very little displacement of the amyloplasts.
The observations performed by transmission electron microscopy showed the presence of precipitates in the roots' statocysts, near the amyloplasts. This distribution was not significantly different from the one observed in the roots cultivated in microgravity. However, a statocysts' polarity change obtained by changing the gravity level resulted in an increased number of calcium precipitates.
Thus, this space experiment was an amazing tool to change the polarity of the statocysts in microgravity. A very little displacement of the amyloplasts resulted in a modification of the calcium homeostasis (capacity of any system to maintain its functional equilibrium despite the exterior constraints) and confirmed the hypothesis that the receptor of gravity signals were localised near the amyloplasts.