Spaceflight experiments with Arabidopsis starch-deficient mutants support a statolith-based model for graviperception

Area of Science:

• Plant Biology
• Gravitational Biology
• Spaceflight Research

Background:

• Ground-based research on plant gravity perception is controversial.
• The starch-statolith theory suggests starch is crucial for plants to sense gravity.
• Previous studies relied on ground-based experiments, necessitating spaceflight validation.

Purpose of the Study:

• To resolve controversy surrounding the starch-statolith theory of plant gravitropism.
• To investigate the role of starch in plant gravity perception using microgravity.
• To compare gravitropic responses of wild-type, reduced-starch, and starchless Arabidopsis mutants in space.

Main Methods:

• Arabidopsis seedlings (wild-type, reduced-starch, starchless) were grown in microgravity on the Space Shuttle.
• Seedlings were subjected to brief gravitational stimuli (30, 60, 90 minutes) on a centrifuge.
• Gravitropic curvature and structural features of space-grown plants were analyzed.

Main Results:

• Wild-type Arabidopsis showed the greatest curvature, while starchless mutants showed the least, after a 90-minute 1-g stimulus.
• Reduced-starch mutants exhibited intermediate gravitropic responses.
• Space-grown plants displayed more root hairs and an anomalous hypocotyl hook structure, potentially due to spacecraft ethylene.

Conclusions:

• Spaceflight results support the starch-statolith theory, showing a positive correlation between starch content and gravity sensitivity.
• The experiment design, providing brief stimuli, allowed for accurate assessment of gravitropism.
• Ethylene in the spacecraft may have influenced some observed morphological changes, but not the core gravitropism findings.