The International Space Station (ISS) and Space Shuttle provide a short period of lasting microgravity conditions during spatial missions, which are rare and costly 8.
In general, the way to encounter microgravity environments is either in space or during free fall 7. Understanding gravitational effects on plants requires changing the magnitude of this force. It is important to study auxin responses under microgravity conditions, which is essential for illustrating how plants perceive environmental gravity alterations and developing plant space biology. As the downstream reflection of spatiotemporal distribution, auxin responses depend on not only metabolism but also directional cell-to-cell transportation 6. As a critical regulator of all aspects of plant growth and development 3, including seed germination 4, auxin has been identified to be responsive to gravitational stimuli through the redistribution of auxin gradients 5. Consequently, an alteration of gravity can profoundly influence plant physiological processes and initiate adaptive responses 2. Plants are sessile and well adjusted to this 1 g level and evolved gravitropism 500 million years ago 1. The gravity of Earth is a unique and constant factor throughout the entire life cycle. The presented microfluidic platform provides simulated microgravity conditions in an easy-to-implement manner, helping to study and elucidate how plants correspond to diverse gravity conditions in the future, this may be developed into a versatile tool for biological study on a variety of samples.
Furthermore, pretreatment with an auxin transportation inhibitor (N-1-naphthylphthalamic acid) enables a decrease in the auxin response, which is no longer affected by simulated microgravity, demonstrating that polar auxin transportation plays a vital role in gravity-regulated auxin responses. Simulated microgravity statistically interrupts auxin responses in embryos, even after chemical-mediated auxin alterations, illustrating that auxin is a critical factor that mediates the plant response to gravity alteration. With the benefit of the microfluidic platform to simulate a microgravity environment on-chip, it is found that the auxin response is significantly repressed in levitated seeds. In this paper, a microfluidic negative magnetophoretic platform is developed to levitate Arabidopsis seeds in an equilibrium plane where the applied magnetic force compensates for gravitational acceleration. However, little is known about auxin responses under microgravity conditions due to the lack of a tool that can provide an alteration of gravity. For plants on Earth, the phytohormone auxin is essential for gravitropism-regulated seedling establishment and plant growth.
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