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Potential sites for the perception of gravity in the acellular slime mold Physarum polycephalum
1DFVLR--Institute for Aerospace Medicine, Cologne, FRG.
Abstract:
Recently a gravisensitivity of the acellular slime mold Physarum polycephalum, which possesses no specialized gravireceptor, could be established by conducting experiments under simulated and under real near weightlessness. In these experiments macroplasmodia showed a modulation of their contraction rhythm followed by regulation phenomena. Until now the perception mechanism for the gravistimulus is unknown, but several findings indicate the involvement of mitochondria: A) During the impediment of respiration the 0g-reaction is inhibited and the regulation is reduced. B) The response to a light stimulus and the following regulation phenomena strongly resemble the behavior during exposure to 0g, the only difference is that the two reactions are directed into opposite directions. In the blue-light reaction a flavin of the mitochondrial matrix seems to be involved in the light perception. C) The contraction rhythm as well as its modulations are coupled to rhythmic changes in the levels of ATP and calcium ions, involving the mitochondria as sites of energy production and of Ca(++)-storage. So the mitochondria could be the site of the regulation and they possibly are the receptor sites for the light and gravity stimuli. Also the observation of a morphologic polarity of the slime mold's plasmodial strands has to be considered: Cross-sections reveal that the ectoplasmic wall surrounding the streaming endoplasm is much thinner on the physically lower side than on the upper side of the strand--this applies to strands lying on or hanging on a horizontal surface. So, in addition to the mitochondria, also the morphologic polarity may be involved in the perception mechanism of the observed gravisensitivity and of the recently established geotaxis. The potential role of the nuclei and of the contractile elements in the perception of gravity is also discussed.
Insights
The slime mold Physarum polycephalum exhibits gravity sensitivity, with mitochondria potentially acting as the primary gravireceptors. Morphological polarity may also contribute to this organism's response to gravity.
Area of Science:
- Cell Biology
- Biophysics
- Astrobiology
Background:
- The acellular slime mold Physarum polycephalum lacks specialized gravireceptors but exhibits gravisensitivity.
- Previous experiments under simulated and real near-weightlessness demonstrated a modulation of its contraction rhythm and regulatory phenomena.
Purpose of the Study:
- To investigate the perception mechanism for gravistimuli in Physarum polycephalum.
- To explore the potential roles of mitochondria and morphological polarity in gravisensitivity.
Main Methods:
- Experiments were conducted under simulated and real near-weightlessness conditions.
- Observations included macroplasmodia's contraction rhythm, response to light stimuli, and morphological analysis of plasmodial strands.
Main Results:
- Inhibition of respiration reduced the 0g-reaction and regulation, suggesting mitochondrial involvement.
- The response to light stimuli mirrored gravity responses, with mitochondria implicated in light perception via flavins.
- Contraction rhythm and modulations correlated with ATP and calcium ion level changes, with mitochondria as key sites.
- Morphological polarity, characterized by asymmetric ectoplasmic wall thickness, was observed in plasmodial strands.
Conclusions:
- Mitochondria are strongly suggested as the primary sites for gravity and light perception and regulatory mechanisms.
- Morphological polarity, in addition to mitochondria, may play a role in perceiving gravisensitivity and geotaxis.
- The study highlights the complex interplay of cellular components in sensing environmental stimuli.