Chemotropic vs Hydrotropic Stimuli for Root Growth Orientation in Microgravity

Area of Science:

• Plant biology
• Space biology
• Astrobiology

Background:

• Gravity significantly influences plant root development and orientation, often masking other tropic responses.
• Understanding plant tropisms in microgravity is essential for developing effective life-support systems for long-term space exploration.

Purpose of the Study:

• To investigate the interaction of hydrotropism and chemotropism in plant root orientation under microgravity conditions.
• To disentangle hydrotropism from chemotropism for root orientation in the absence of gravity stimulus.

Main Methods:

• The MULTITROP experiment was conducted on the International Space Station (ISS) using *Daucus carota* (carrot) seeds.
• Carrot seeds were grown between two substrates, one with water and the other with disodium phosphate solution, in microgravity.
• Root development and orientation were analyzed using 3D-image analysis and X-ray microtomography, with a ground reference experiment for comparison.

Main Results:

• In microgravity, carrot roots demonstrated positive chemotropism towards the disodium phosphate solution.
• On Earth, this chemotropic response was masked by the dominant gravitropic response, with roots growing downwards regardless of nutrient presence.
• Asymmetric embryo position influenced radicle protrusion, a factor to consider in future microgravity root growth studies.

Conclusions:

• Plant roots can exhibit chemotropism in the absence of gravity, indicating a significant interaction between tropic stimuli.
• This research provides crucial insights for optimizing plant cultivation in space for future human missions.
• Further studies are recommended to explore root chemotropic responses with different chemical compounds and under varying gravity conditions.