The effect of spaceflight on the gravity-sensing auxin gradient of roots: GFP reporter gene microscopy on orbit

Our primary aim was to determine whether gravity has a direct role in establishing the auxin-mediated gravity-sensing system in primary roots. Major plant architectures have long been thought to be guided by gravity, including the directional growth of the primary root via auxin gradients that are then disturbed when roots deviate from the vertical as a gravity sensor. However, experiments on the International Space Station (ISS) now allow physical clarity with regard to any assumptions regarding the role of gravity in establishing fundamental root auxin distributions. We examined the spaceflight green fluorescent protein (GFP)-reporter gene expression in roots of transgenic lines of Arabidopsis thaliana: pDR5r::GFP, pTAA1::TAA1–GFP, pSCR::SCR–GFP to monitor auxin and pARR5::GFP to monitor cytokinin. Plants on the ISS were imaged live with the Light Microscopy Module (LMM), and compared with control plants imaged on the ground. Preserved spaceflight and ground control plants were examined post flight with confocal microscopy. Plants on orbit, growing in the absence of any physical reference to the terrestrial gravity vector, displayed typically “vertical” distribution of auxin in the primary root. This confirms that the establishment of the auxin-gradient system, the primary guide for gravity signaling in the root, is gravity independent. The cytokinin distribution in the root tip differs between spaceflight and the ground controls, suggesting spaceflight-induced features of root growth may be cytokinin related. The distribution of auxin in the gravity-sensing portion of the root is not dependent on gravity. Spaceflight appears benign to auxin and its role in the development of the primary root tip, whereas spaceflight may influence cytokinin-associated processes. Experiments on the International Space Station (ISS) have shown how key plant developmental hormones respond to microgravity, or not. Robert Ferl and Anna-Lisa Paul of the University of Florida studied the reporter-gene expression patterns of several genes related to auxin and cytokinin in the roots of plants grown on the ISS and on Earth’s surface. Although they saw a difference in the cytokinin response pattern in the ground control compared to the space-grown plants, there were no changes in the auxin-related genes. While auxin is central to the root’s gravity response on Earth, these experiments demonstrate that the distribution of auxin is established by developmental processes independent of gravity. Similar studies of other genes and other contexts will further clarify the role of gravity in plant development.