Light has a principal role in the Arabidopsis transcriptomic response to the spaceflight environment.

The Characterizing Arabidopsis Root Attractions (CARA) spaceflight experiment provides comparative transcriptome analyses of plants grown in both light and dark conditions within the same spaceflight. CARA compared three genotypes of Arabidopsis grown in ambient light and in the dark on board the International Space Station (ISS); Col-0, Ws, and phyD, a phytochrome D mutant in the Col-0 background. In all genotypes, leaves responded to spaceflight with a higher number of differentially expressed genes (DEGs) than root tips, and each genotype displayed distinct light / dark transcriptomic patterns that were unique to the spaceflight environment. The Col-0 leaves exhibited a substantial dichotomy, with ten-times as many spaceflight DEGs exhibited in light-grown plants versus dark-grown plants. Although the total number of DEGs in phyD leaves is not very different from Col-0, phyD altered the manner in which light-grown leaves respond to spaceflight, and many genes associated with the physiological adaptation of Col-0 to spaceflight were not represented. This result is in contrast to root tips, where a previous CARA study showed that phyD substantially reduced the number of DEGs. There were few DEGs, but a series of space-altered gene categories, common to genotypes and lighting conditions. This commonality indicates that key spaceflight genes are associated with signal transduction for light, defense, and oxidative stress responses. However, these key signaling pathways enriched from DEGs showed opposite regulatory direction in response to spaceflight under light and dark conditions, suggesting a complex interaction between light as a signal, and light-signaling genes in acclimation to spaceflight.