Patterns of Arabidopsis gene expression in the face of hypobaric stress

Extreme hypobaria represents an environment that is mostly outside of the evolutionary experience of terrestrial plants, and yet hypobaria may be a part of plant habitats in support of human space exploration. A greenhouse on the surface of Mars, as well as in transit vehicles and space habitats, can be envisioned to operate below the evolutionarily accustomed terrestrial pressures. This work shows how plants use new patterns of gene expression to physiologically adapt to wide changes in atmospheric pressure over time, and provides insight into how the effects of hypobaria can be mitigated or utilized for plant growth in support of exploration. Abstract Extreme hypobaria is a novel abiotic stress that is outside the evolutionary experience of terrestrial plants. In natural environments, the practical limit of atmospheric pressure experienced by higher plants is about 50 kPa or about 0.5 atmospheres; a limit that is primarily imposed by the combined stresses inherent to high altitude conditions of terrestrial mountains. However, in highly controlled chambers, and within projected extra-terrestrial greenhouses, the atmospheric pressure component can be isolated from the associated high altitude stresses such as temperature, desiccation and even hypoxia. Such chambers allow the exploration of hypobaria as a single variable that can be carried to extremes beyond what is possible in terrestrial biomes. Here, we examine the organ-specific progression of transcriptional strategies for the physiological adaptation to various degrees of hypobaric stress, as well as the response to severe hypobaria over time. An abrupt transition from a near-sea level pressure of 97 kPa to a mere 5 kPa is accompanied by the differential expression of hundreds of genes, primarily those associated with drought, hypoxia and cell wall metabolism. However, pressure transitions between these two extremes reveals that plants respond with complex, organ-specific transcriptomic responses, which also vary over time. These responses are not linear; neither with respect to the gradient of hypobaric severity from 75, 50, 25 to 10 kPa, nor with the duration of exposure of up to 3 days at 10 kPa. In the first few hours of hypobaria, plants engage changes in basic metabolism and hormonally mediated growth and development. After 12 or more hours of hypobaria, the gene expression patterns are more indicative of hypoxia and drought environmental responses. The hypobaria transcription patterns were hig