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Root waving and skewing - unexpectedly in micro-g

DOI: 10.1186/1471-2229-12-231

Keywords: Arabidopsis, Auxin, Cytoskeleton, Extracellular ATP, International space station, Roots

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Abstract:

In a recent paper published in BMC Plant Biology, Paul et al. [1] describe novel data that challenge a long-held hypothesis on how gravity affects patterns of root growth. When plants are grown on a solid surface, their roots show growth patterns of waving, which are undulations due to the regular periodic change in the direction root tips grow, and skewing, which is the slanted angle of growth roots exhibit when they are growing along a nearly-vertical surface. The generally accepted explanation for these patterns is that they result in large part from a combination of the touch stimulus arising from contact of the root tip with the surface and gravity, which increases the force of this contact. Using a specialized plant growth facility on the International Space Station, the Advanced Biological Research System (ABRS) with imaging hardware, Paul et al. collected digital photographs of plants every six hours during 15 days of spaceflight. The novel imaging system allowed the authors to observe the growth pattern of roots when their orientation was directed not by gravity but by overhead LED lights. Because the roots are negatively phototropic they grew away from the lights. In the micro-g environment of the ISS, plants grew more slowly than their 1-g controls grown under the same temperature and lighting on earth, but they still showed root waving and skewing. Their experimental design and controls made it unlikely that these growth patterns could be attributed to airflow, μg vectors or other directional environmental factors.Waving and skewing are largely surface-dependent phenomena, and do not appear in roots that grow embedded in agar [2]. Certainly, gravity-driven interactions of roots with surfaces influence waving and skewing, but these novel results of Paul et al. now make it clear that these growth patterns can also occur in micro-g when directional root growth that is driven by negative phototropism interacts with surfaces. Until these observations the only

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