Investigators had the opportunity to confirm and extend the results of Tropi in a follow-up
experiment during Increment 22 on the ISS. Major modifications between part 1 and part 2 of
Tropi took place to ensure optimal experimental conditions. Instead of using analog videos to
capture images, part 2 used direct downlinks from the ISS. This allowed for much quicker
processing of experiment images and results. Due to modifications in the storage time of the
seedlings, the second part of Tropi demonstrated better seed germination (approx. 90%) and
improved seedling growth compared to part 1. The last major modification in part 2 of Tropi
involved a switch to use powered GLACIER freezers for the transfer of the frozen samples from
the ISS to the space shuttle then to Earth. These freezers helped to maintain the low
temperatures required to keep RNA intact (Kiss 2011).
Previous experiments have dealt with the effects normal gravity and microgravity on plant
biology, however Tropi tested the effects fractional gravity has on plant development. More
specifically, the effect fractional gravity has on hypocotyl and root development. Tropi part 2
confirmed red-light-based positive phototropism found in hypocotyls under microgravity. When
gravity levels began to elevate, a decreased response was noted. Under microgravity
conditions, roots displayed a positive phototropic response to red light. This response was only
observed under microgravity because when 0.1g was induced, the red-light response was
attenuated. Tropi was also utilized to determine the relationship between red-light
pretreatments prior to blue-light exposure in hypocotyls and roots. Hypocotyls displayed a
decreased response in phototropism while roots exhibited an increase response in
phototropism when exposed to a red-light pretreatment. In response to these findings,
investigators wanted to determine if there was a direct or indirect relationship between the
red-light pretreatments in the hypocotyls and roots. In regards to the hypocotyls, these space
studies support the concept that red-light is indirectly involved in blue-light-based
phototropism through the attenuation of gravitropism. These results support the hypothesis
that red-light enhancement of blue-light root phototropism is direct. Taking Tropi as a whole,
these observations show that the microgravity conditions found in laboratories aboard the ISS
can provide a unique research tool to gain insights into fundamental mechanisms in plant
biology (Kiss 2012).
PUBLICATION(S)
Sindelar TJ, Millar KD, Kiss JZ. Red light effects on blue light–based phototropism in roots of
Arabidopsis thaliana. International Journal of Plant Sciences. July 2014 ;175:731-740. doi:
10.1086/676303.
Kiss JZ, Millar KD, Edelmann RE. Phototropism of Arabidopsis thaliana in microgravity and
fractional gravity on the International Space Station. Planta. 2012;236(2):635-645. doi:
10.1007/s00425-012-1633-y.
Kiss JZ, Millar KD, Kumar P, Edelmann RE, Correll MJ. Improvements in the re-flight of
spaceflight experiments on plant tropisms. Advances in Space Research. 2011;47(3):545-552.
doi: 10.1016/j.asr.2010.09.024.