The Characterizing Arabidopsis Root Attractions (CARA) experiment looks at mechanisms at the molecular and genetic level that influence the growth of a plant’s roots in the absence of gravity, and how those change with or without light. Researchers expose one set of seedlings to light, keep another set in the dark, and then examine how each environment influences the patterns of root growth. Some of the plants are also imaged with the Light Microscopy Module on orbit, and at the end of the experiment, all plants are harvested by the astronaut, and preserved for their return to Earth in order to evaluate genes associated with plant responses on orbit.
Characterizing Arabidopsis Root Attractions (CARA) advances the fundamental understanding of the molecular biological responses to extraterrestrial environments. This understanding further defines the impacts of space flight on biological systems to better enable the United States’ future space exploration goals.
The hypothesis to be tested is that the differences between the WS and Col-0 will reveal key genes involved in the morphology of root growth on orbit. Further, it is hypothesized that phyD contributes to the light-mediated signal transduction that influences the tropic direction of root growth on orbit, and that Col-0 plants deficient in this gene will mimic the negatively phototrophic patterns of WS roots on orbit. Two tools are used for analyses: whole genome transcriptome analyses and morphometric analysis. The results anticipated include the identification of a number of differentially expressed genes that help define gravity-independent responses unique to each ecotype, and insight into the role of the phyD gene in root growth. The fundamental scientific relevance of this experiment is that it provides insight into the signal transduction pathways that control tropism and adaptive physiology in plants. The experiment also showcases how the unique research environment of the ISS provides insight into fundamental and widely applicable biological questions that cannot be answered on earth where gravity would mask many of the underlying phenomena.
Twenty petri plates, ten KFTs and one Harvest Kit are transferred from ambient stowage on the ascent vehicle to ISS. To activate the experiment, blackout cloths wrapped around each petri plate are removed and petri plates are attached to a wall (using Velcro) to be exposed to ambient light. After 1 to 12 hours of light exposure, ten petri plates are re-wrapped in blackout cloth and returned to ambient stowage. The remaining ten petri plates remain attached to a wall for the duration of the experiment run. After an experiment duration of 11 days, all of the petri plates, KFTs and the Harvest Kit are transferred to the Maintenance Work Area for the harvest activity. The ABRS Photogrid, currently on ISS, is the preferred method to secure the petri plate during harvest and to provide a black background for photography. Each petri plate is photographed, one photo with the lid on and one with the lid off, and harvested. The KFTs have a mesh divider so that one dark grown and one light grown petri plate can be harvested into a single KFT. The harvest activity requires the crewmember to use forceps from the Harvest Kit to pull the plants from the agar surface of the petri plate. Once the plants are in a KFT, the KFT are actuated to deliver the RNALater chemical preservative to the plants. The harvest procedure is completed for all twenty petri plates. Upon completion of the harvest, the petri plates and Harvest Kit are trashed. Eight to twenty-four hours following the harvest, the ten KFTs are transferred to MELFI at -80°C. The KFTs return in a Cold Bag at -20°C.
Plants physiologically adapt to a changing environment by regulating patterns of gene expression. This “transcriptome” (the segments of DNA copied into RNA) creates a map of the response, which can then be compared among varieties, and between treatments – like between plants grown in space, and those grown on the ground. Plants did not evolve in an environment without gravity, so they do not have an evolutionary history of how to metabolically deal with such a stimulus. Because plants have not developed adaptive strategies to cope with microgravity, it raises the possibility that signals in this novel environment inappropriately activate non-adaptive responses. This study investigated whether inappropriate, or non-adaptive responses to microgravity could be identified by comparing the spaceflight transcriptomes of genetically similar cultivars of Arabidopsis thaliana (Arabidopsis), and also by comparing the spaceflight transcriptomes of plants differing by only a single gene. RNA sequencing (RNAseq) was used to identify the differentially expressed genes in root tips of plants grown on the ISS in microgravity, and in comparable plants on the ground. RNAseq provided a quantitative measure of how variations in genetic background and lighting impacted the expression of genes in the spaceflight environment. Results showed that all plants grown in microgravity in this study expressed genes differently relative to their ground controls, indicating that changes in gene expression are required for physiological adaptation to spaceflight. The relative impact of the spaceflight environment was inferred by the number of genes that were differentially expressed. In other words, a plant genotype that required the differential expression of a large number of genes was thought to struggle more to adjust to spaceflight than a genotype that expressed fewer genes. The genotype of the cultivar WS required far fewer genes to physiologically adapt to spaceflight than the genotype of cultivar Col-0. This study then asked whether manipulating the genotype of the Col-0 plant could alter the plant’s response to microgravity. It was found that disabling just one gene in Col-0 could reduce the number of genes needed by Col-0 to physiologically adapt to spaceflight. These results suggest that a strategy of genome manipulation could be used to improve the adaptability of plants to spaceflight by reducing the transcriptome cost of physiological adaptation. In addition, the observation that of the hundreds of genes that are differentially expressed in each genotype, only about a dozen are held in common to all three, thus hundreds of differentially expressed genes are likely not to be truly necessary for physiological adaptation to spaceflight. Taken together, these results suggest that responses to truly novel environments may include responses from signals that are incorrectly activated, and further suggests that these incorrect responses can be genetically removed to lighten the metabolic load on the adaptive process. In conclusion, these data suggest that genetic manipulation can produce varieties that are better adapted for growth in microgravity through the elimination of unnecessary environmental responses (Paul, 2017)
Transcriptome datasets available through the NASA GeneLab Data System: GLDS-120
|Mission||Launch/Start Date||Landing/End Date||Duration|