Analysis of a Novel Sensory Mechanism in Root Phototropism (Tropi) - 10.11.17

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ISS Science for Everyone

Science Objectives for Everyone
Analysis of a Novel Sensory Mechanism in Root Phototropism (Tropi) studies Arabidopsis thaliana plants sprouting from seeds to gain insights into sustainable agriculture for future long-duration space missions.
Science Results for Everyone
Tropi looks at the combined effect of light and gravity on plant growth. Results suggest that red light promotes growth in space, whereas it does not on the ground . A greater growth response is also observed to blue light, particularly following brief exposure to red light. Red light appears to indirectly affect blue-light-based phototropism (growth in response to light) by reducing the effects of gravity, and to directly enhance blue-light root phototropism. These observations show that microgravity aboard the ISS provides a way to study plant biology. The work also produced important modifications to image and sample processing and analysis.

The following content was provided by John Z. Kiss, Ph.D., and is maintained in a database by the ISS Program Science Office.
Experiment Details


Principal Investigator(s)
John Z. Kiss, Ph.D., University of North Carolina-Greensboro, Greensboro, NC, United States

Melanie J. Correll, Ph.D., University of Florida, Gainesville, FL, United States
Roger P. Hangarter, Ph.D., Indiana University, Bloomington, IN, United States
Richard E. Edelmann, Ph.D., Miami University, Oxford, OH, United States

NASA Ames Research Center, Moffett Field, CA, United States

Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)

Sponsoring Organization
Human Exploration and Operations Mission Directorate (HEOMD)

Research Benefits
Information Pending

ISS Expedition Duration
October 2009 - September 2010; March 2011 - September 2011

Expeditions Assigned

Previous Missions
While Tropi is a new experiment, other studies of plant growth and development have been conducted on ISS.

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Experiment Description

Research Overview

  • Analysis of a Novel Sensory Mechanism in Root Phototropism (Tropi) is a plant growth experiment that investigates how plant roots from Arabidopsis thaliana (thale cress) respond to varying levels of light and gravity.

  • Plant growth under various gravity conditions (0g to 1.0g) is achieved using a rotating centrifuge.

  • Plants grown are analyzed to determine which genes are responsible for successful plant growth in microgravity.

  • This experiment helps gain insight into plant growth in space in order to help create sustainable life support systems for long term space travel.


Analysis of a Novel Sensory Mechanism in Root Phototropism (Tropi) consists of dry Arabidopsis thaliana (thale cress) seeds stored in small seed cassettes. A. thaliana is a rapidly growing, flowering plant in the mustard family. The seed cassettes are contained inside the European Modular Cultivation System (EMCS). The seeds are installed into the EMCS dry and at ambient temperature until hydrated by an automated system of the EMCS. At specified times during the experiment; the plants are stimulated by different light spectrums and by different gravity gradients. The only operations required by the crew are to replace video tapes and harvest the plants following growth. Once the plants are harvested, they are stored in the Minus Eighty Degree Laboratory Freezer for ISS (MELFI) until they are returned to Earth (STS-116 and STS-117). Investigators gather data from video-taped images of the plants, observing their roots as they undergo development in the EMCS, as well as conducting biochemical analyses of the returned plants.

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Space Applications
During long-term space exploration, it will be necessary to provide crewmembers with regenerative sources of food, as well as supplemental methods to recycle carbon dioxide into breathable oxygen. As new information about how plants grow in microgravity emerges, sustainable plant-based life support systems may be developed.

Earth Applications
Further understanding of how plants grow and develop at a molecular level can lead to significant advancements in agricultural production on Earth.

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Operational Requirements and Protocols

During its ascent, Tropi requires no resources. The dry seeds are contained in five seed cassettes located in the EMCS. Once in orbit, the seeds are hydrated automatically. Video is taken throughout the experiment, allowing the ground team to evaluate and study the development of the plants. Ninety percent of the science return is based upon video analysis. The crew harvests the plants and stores the specimens in MELFI. They are also required to change out the video tapes.

Seed cassettes containing dried Arabidopsis thaliana seeds are initiated in the EMCS automatically. The EMCS introduces water to the seeds to activate growth. Video cameras record the growth and development of the plants. Once the plants are grown, the crewmembers harvest the plants and place them in the cold storage in MELFI. The plants are returned to the ground team for further study.

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Decadal Survey Recommendations

Information Pending

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Results/More Information

Tropi was a multi-part experiment aimed to gain a better understanding of how microgravity affects phototropism, directed growth of a plant in response to a light stimulus, and gravitropism, directed growth in response to gravity, in Arabidopsis thaliana. An in space experiment was necessary because the microgravity conditions of space provide the opportunity to study phototropism in the absence of gravity.

Tropi was initially performed during Increment 14 on the ISS. These experiments were returned to Earth on three space shuttle missions. Several important lessons were gained from this initial run. The first issue arose while using analog video tapes. Although quality images were taken, there was an inefficient use of resources in the time it took for image processing and analysis procedures to be performed. Another issue arose during the seedling hydration procedure. The initial command did not take place and without video downlinks, the entire experiment would not have been completed. An additional issue arose in seed germination. The seeds were stored from six to eight months prior to hydration and resulted in only 58% germination in the first run with progressive deterioration in the following two runs. The last major issue to arise was found in the transfer of the frozen materials from the shuttle to NASA cooler bags. Because the samples were allowed time to "warm up" during transfer, the RNA from the STS-116 samples degraded. This issue was mediated by implementing a "3 minute rule" in the following runs.

Although several issues did arise during the first part of Tropi, invaluable information was gained from this experiment. A novel positive phototrophic response to red light was observed in hypocotyls of seedlings that developed in microgravity. It is hypothesized that A. thaliana exhibits red-light phototropism in microgravity, while on Earth it is suppressed by normal gravity. A greater phototrophic response was also observed in blue-light-based trials. A more prominent curvature was detected when shoots were briefly exposed to red light followed by blue light.

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.

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.

Arabidopsis roots and shoots clearly show different curvatures in different gravity conditions from microgravity, to partial (reduced) gravity, to 1g conditions upon exposure to red or blue light. Plants were grown for 96 h with white light on one side and simulated 1g in an attempt to standardize the orientation of all seedlings . During red or blue light exposure, gravity levels of 0g, 0.1g, 0.3g, 0.5g, 0.8g, or 1g were generated via centrifugation. The phototropic response in roots was measured as positive growth (towards light) or negative growth (away from light) from the initial vertical state. In the better controlled light environment in the European Modular Cultivation System (EMCS), the positive phototropism of roots in response to blue light in microgravity was revealed. In addition, the magnitude of red-light positive phototropic curvature is correlated to the magnitude of gravity. Increasing levels of gravity causes a gradual reduction in the magnitude of curvature with red light as opposed to the drastic cessation observed under blue-light exposure. It is clear that spaceflight experiments, by effectively removing the force of gravity, allow researchers to better understand phototropic responses in plants.

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Results Publications

    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.

    Millar KD, Kumar P, Correll MJ, Mullen JL, Hangarter RP, Edelmann RE, Kiss JZ.  A novel phototropic response to red light is revealed in microgravity. New Phytologist. 2010; 186(3): 648-656. DOI: 10.1111/j.1469-8137.2010.03211.x.

    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. PMID: 22481136.

    Kiss JZ, Kumar P, Millar KD, Edelmann RE, Correll MJ.  Operations of a spaceflight experiment to investigate plant tropisms. Advances in Space Research. 2009; 44(8): 879-886. DOI: 10.1016/l.asr.2009.06.007.

    Vandenbrink JP, Herranz R, Medina F, Edelmann RE, Kiss JZ.  A novel blue-light phototropic response is revealed in roots of Arabidopsis thaliana in microgravity. Planta. 2016 December; 244(6): 1201-1215. DOI: 10.1007/s00425-016-2581-8. PMID: 27507239.

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Ground Based Results Publications

    Correll MJ, Edelmann RE, Hangarter RP, Mullen JL, Kiss JZ.  Ground-based studies of tropisms in hardware developed for the European Modular Cultivation System (EMCS). Advances in Space Research. 2005; 36: 1203-1210. DOI: 10.1016/j.asr.2004.11.003.

    Sindelar TJ, Millar KD, Kiss JZ.  Red light effects on blue light–based phototropism in roots of Arabidopsis thaliana. International Journal of Plant Sciences. 2014 July; 175(6): 731-740. DOI: 10.1086/676303.

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ISS Patents

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Related Publications

    Correll MJ, Kiss JZ.  The roles of phytochromes in elongation and gravitropism of roots. Plant and Cell Physiology. 2005; 46: 317-323.

    Mullen JL, Wolverton C, Hangarter RP.  Apical control, gravitropic signaling, and the growth of lateral roots in Arabidopsis. Advances in Space Research. 2005; 36: 1211-1217.

    Millar KD, Kiss JZ.  Analyses of tropistic responses using metabolomics. American Journal of Botany. 2013 January 1; 100(1): 79-90. DOI: 10.3732/ajb.1200316. PMID: 23196394.

    Molas ML, Kiss JZ, Correll MJ.  Gene profiling of the red light signalling pathways in roots. Journal of Experimental Biology. 2006; 57(12): 3217-3229.

    Matia I, Gonzalez-Camacho F, Herranz R, Kiss JZ, Gasset G, van Loon JJ, Marco R, Medina F.  Plant Cell Proliferation and Growth Are Altered by Microgravity Conditions in Spaceflight. Journal of Plant Physiology. 2010; 167(3): 184-193. DOI: 10.1016/j.jplph.2009.08.012. PMID: 19864040.

    Kiss JZ, Kumar P, Bowman RN, Steele MK, Eodice MT, Correll MJ, Edelmann RE.  Biocompatibility studies in preparation for a spaceflight experiment on plant tropisms (TROPI). Advances in Space Research. 2007; 39(7): 1154-1160. DOI: 10.1016/k.asr.2006.12.017.

    Stimpson AJ, Pereira RS, Kiss JZ, Correll MJ.  Extraction and labeling methods for microarrays using small amounts of plant tissue. Physiologia Plantarum. 2009 March; 135(3): 229-236. DOI: 10.1111/j.1399-3054.2008.01191.x.

    Vandenbrink JP, Kiss JZ, Herranz R, Medina F.  Light and gravity signals synergize in modulating plant development. Frontiers in Plant Science. 2014 October 28; 5: 563. DOI: 10.3389/fpls.2014.00563.

    Kumar P, Montgomery CE, Kiss JZ.  The role of pytochrome C in gravitropism and phototropism in Arabidopsis thaliana. Functional Plant Biology. 2008; 35: 298-305.

    Wyatt SE, Kiss JZ.  Plant Tropisms: From Darwin to the International Space Station. American Journal of Botany. 2013; 100(1): 1-3. DOI: 10.3732/ajb.1200591. PMID: 23281390.

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Related Websites
The Arabidopsis Information Resource
Miami University, Department of Botany
NASA to Study Plants to Help Astronauts Grow Food in Space

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image EMCS sample cartridge that will be spun in the EMCS centrifuge. Inset image is of Arabidopsis thaliana (thale cress) seedlings like the ones that will be used for Tropi. Tropi will compare the growth of root tips toward light in the absence of gravity, and allow the distinction between two redundant internal plant growth systems that respond to different colors of light. Image courtesy of NASA, Johnson Space Center.
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image Screenshot of Tropi during Expedition 14 shows several cassettes in an Experiment Containers (EC) and the growth of young Arabidopsis thaliana seedlings during the "growth phase". Image courtesy of NASA, Johnson Space Center.
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image Screenshot of Tropi at the beginning of "stimulation phase". This image will be used in analysis of tropisms. Image courtesy of NASA, Johnson Space Center.
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image NASA Image: ISS014E10652 - View of the TROPI seedling cassette for the European Modular Cultivation System (EMCS) - Experiment Container (EC) in the Destiny laboratory module during Expedition 14.
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image RNA gels of frozen plant samples from the Tropi investigation. Gel A shows samples returned in Decmeber 2006 did not yield any RNA. Gel B shows RNA in the samples returned in June 2007 following the establishment of cold transfer procedures. Image courtesy of Dr. John Kiss.
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image Close-up of Arabidopsis seedlings with seed cassette cover removed. Image courtesy of Ames Research Center, Moffett Field, CA.
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image Dwarf Arabidopsis plant (left). Dwarf Arabidopsis plant (right). Image courtesy of Ames Research Center, Moffett Field, CA.
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image European Modular Cultivation System (EMCS) Tropi experiment unique equipment (EUE) shown stimulating Arabidopsis seedlings with blue light (left). ENCS Tropi EUE shown stimulating Arabidopsis seedlings with red light (right). Image courtesy of Ames Research Center, Moffett Field, CA.
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image NASA Image: ISS022E087465 - NASA astronaut Jeffrey Williams, Expedition 22 commander, services the Tropism in Plants (TROPI2) experiment in the Columbus laboratory of the International Space Station.
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image Tropi still image. Image courtesy of Prinicipal Investigator.
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image Tropi cassette. Image courtesy of Principal Investigator.
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