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Developmental plasticity is a hallmark of plant growth, and gravity provides a pervasive cue that influences plant form. Plants perceive gravity in part through the sedimentation of starch-filled plastids found in gravity sensing cells. Upon displacement from vertical, primary roots respond to gravity stimulation at a rate that varies depending on the angle of stimulation. Roots that lack sedimenting plastids show reduced gravitropic response and rates of differential growth that do not depend on the angle of stimulation. These results suggest an alternate mechanism of gravity perception not involving plastid sedimentation. We are planning to characterize this alternate mechanism by applying fractional g treatments to Arabidopsis thaliana wild type and starchless (pgm-1) mutants using centrifugation in the EMCS facility aboard the ISS.

As part of Flight Definition, we are investigating a number of parameters in order to optimize seedling growth in flight hardware. All of our previous work on root gravitropism has been carried out with primary roots between 4 and 5 d old growing on agar-based nutrient media containing 1% sucrose. Agar-based substrates differ significantly from the growth environment in the Seed Cassettes designed for the EMCS, in which seedlings grow along the surface of a membrane. One objective here is to compare development over the period 72 h to 120 h after hydration to determine an optimal developmental stage at which to begin variable g treatments. Results thus far suggest that roots elongate at a sufficient rate as early as 72 h following hydration on the membrane. A second objective is to determine whether seedlings of the pgm-1 mutant require sucrose in the media in order to achieve optimal growth rates, given that the nature of this mutation affects multiple processes involved in carbon metabolism. Our preliminary data indicate that root elongation is minimally impacted by varying sucrose concentration. Finally, we will present data on the growth of pgm-1 and wild type roots at elevated carbon dioxide concentrations, like those on the ISS, to test for negative impacts on the growth of the mutant.

Supported by NASA grant NNX15AG55G.

Faculty Mentor

Chris Wolverton