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  1. Plants have been recognized as key components of bioregenerative life support systems for space exploration, and many experiments have been carried out to evaluate their adaptability to spaceflight. Unfortunately, few of these experiments have involved monocot plants, which constitute most of the crops used on Earth as sources of food, feed, and fiber. To better understand the ability of monocot plants to adapt to spaceflight, we germinated and grew Brachypodium distachyon seedlings of the Bd21, Bd21-3, and Gaz8 accessions in a customized growth unit on the International Space Station, along with 1-g ground controls. At the end of a 4-day growth period, seedling organ’s growth and morphologies were quantified, and root and shoot transcriptomic profiles were investigated using RNA-seq. The roots of all three accessions grew more slowly and displayed longer root hairs under microgravity conditions relative to ground control. On the other hand, the shoots of Bd21-3 and Gaz-8 grew at similar rates between conditions, whereas those of Bd21 grew more slowly under microgravity. The three Brachypodium accessions displayed dramatically different transcriptomic responses to microgravity relative to ground controls, with the largest numbers of differentially expressed genes (DEGs) found in Gaz8 (4527), followed by Bd21 (1353) and Bd21-3 (570). Only 47 and six DEGs were shared between accessions for shoots and roots, respectively, including DEGs encoding wall-associated proteins and photosynthesis-related DEGs. Furthermore, DEGs associated with the “Oxidative Stress Response” GO group were up-regulated in the shoots and down-regulated in the roots of Bd21 and Gaz8, indicating that Brachypodium roots and shoots deploy distinct biological strategies to adapt to the microgravity environment. A comparative analysis of the Brachypodium oxidative-stress response DEGs with the Arabidopsis ROS wheel suggests a connection between retrograde signaling, light response, and decreased expression of photosynthesis-related genes in microgravity-exposed shoots. In Gaz8, DEGs were also found to preferentially associate with the “Plant Hormonal Signaling” and “MAP Kinase Signaling” KEGG pathways. Overall, these data indicate that Brachypodium distachyon seedlings exposed to the microgravity environment of ISS display accession- and organ-specific responses that involve oxidative stress response, wall remodeling, photosynthesis inhibition, expression regulation, ribosome biogenesis, and post-translational modifications. The general characteristics of these responses are similar to those displayed by microgravity-exposed Arabidopsis thaliana seedlings. However, organ- and accession-specific components of the response dramatically differ both within and between species. These results suggest a need to directly evaluate candidate-crop responses to microgravity to better understand their specific adaptability to this novel environment and develop cultivation strategies allowing them to strive during spaceflight. 
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