Phototropism of Arabidopsis thaliana in microgravity and fractional gravity on the International Space Station
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While there is a great deal of knowledge regarding plant growth and development in microgravity aboard orbiting spacecraft, there is little information available about these parameters in reduced or fractional gravity conditions (less than the nominal 1g on Earth). Thus, in these experiments using the European Modular Cultivation System on the International Space Station, we studied the interaction between phototropism and gravitropism in the WT and mutants of phytochrome A and B of Arabidopis thaliana. Fractional gravity and the 1g control were provided by centrifuges in the spaceflight hardware, and unidirectional red and blue illumination followed a white light growth period in the time line of the space experiments. The existence of red-light-based positive phototropism in hypocotyls of seedlings that is mediated by phytochrome was confirmed in these microgravity experiments. Fractional gravity studies showed an attenuation of red-light-based phototropism in both roots and hypocotyls of seedlings occurring due to gravitational accelerations ranging from 0.l to 0.3g. In contrast, blue-light negative phototropism in roots, which was enhanced in microgravity compared with the 1g control, showed a significant attenuation at 0.3g. In addition, our studies suggest that the well-known red-light enhancement of blue-light-induced phototropism in hypocotyls is likely due to an indirect effect by the attenuation of gravitropism. However, red-light enhancement of root blue-light-based phototropism may occur via a more direct effect on the phototropism system itself, most likely through the phytochrome photoreceptors. To our knowledge, these experiments represent the first to examine the behavior of flowering plants in fractional or reduced gravity conditions.
KeywordsArabidopsis Gravitropism Microgravity Phototropism Phytochrome Space biology
European Modular Cultivation System
International Space Station
This work was supported by the National Aeronautics and Space Administration [NNX10AF44G to J.Z.K.]. The authors gratefully acknowledge the support of NASA’s Ames Research Center (Mountain View, CA, USA), especially Marianne Steele, Kenny Vassigh, Ken Souza, Sid Sun, Bob Bowman, Kris Vogelsong, and Dave Heathcote. They also thank the Norwegian User Support and Operations Center (especially Carina Helle Berg, Gjert Aanes, and Knut Olav Helleseng), the European Space Agency, and the EADS team (especially Thomas Niedermaier and Anna Grinberg) for their excellent technical support during spaceflight operations. In addition, Caitlin Bregitzer, Maggie Brown, Jessie Hall, and Katie Huntoon aided in the data collection at Miami University. Special thanks to Astronauts Jeffrey Williams and T.J. Creamer and other crew members for performing their experiments on board the ISS.
Supplementary material 2 Suppl. 1 Fig S1 A time-lapse movie of phototropic curvature in hypocotyls of phyB seedlings in response to the unilateral red light from the left of the screen. The movie was compiled from digital still images taken over the 40-h photostimulation period. Note the active nutation of the hypocotyls during the phototropic curvature (MPG 3972 kb)
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