Changes in operational procedures to improve spaceflight experiments in plant biology in the European Modular Cultivation System

The microgravity environment aboard orbiting spacecraft has provided a unique laboratory to explore topics in basic plant biology as well as applied research on the use of plants in bioregenerative life support systems. Our group has utilized the European Modular Cultivation System (EMCS) aboard the International Space Station (ISS) to study plant growth, development, tropisms, and gene expression in a series of spaceflight experiments. The most current project performed on the ISS was termed Seedling Growth-1 (SG-1) which builds on the previous TROPI (for tropisms) experiments performed in 2006 and 2010. Major technical and operational changes in SG-1 (launched in March 2013) compared to the TROPI experiments include: (1) improvements in lighting conditions within the EMCS to optimize the environment for phototropism studies, (2) the use of infrared illumination to provide high-quality images of the seedlings, (3) modifications in procedures used in flight to improve the focus and overall quality of the images, and (4) changes in the atmospheric conditions in the EMCS incubator. In SG-1, a novel red-light-based phototropism in roots and hypocotyls of seedlings that was noted in TROPI was confirmed and now can be more precisely characterized based on the improvements in procedures. The lessons learned from sequential experiments in the TROPI hardware provide insights to other researchers developing space experiments in plant biology.     

The gravitropic and phototropic responses of wheat grown in a space greenhouse prototype with hemispherical planting surface

The time course of gravicurvature of 3-day-old wheat (Triticum aestivum L., cv. Apogee) coleoptiles and 7-day-old wheat stems were studied in darkness and under red and red-blue light illumination after declination from the vertical at various angles. The experiments showed that the shortest gravitropic curvature corresponded to 30° initial angle of gravistimulation (IAG). The time course became longer as the IAG increased and with plant age. The effects of unilateral red (660 nm) and red-blue light (660 nm; 470 nm) at photosynthetic photon flux (PPF) of 30 μmol m⁻² s⁻¹ on the curvature of 3-day-old coleoptiles were evaluated. Red light did not produce phototropic bending of wheat coleoptiles in contrast with red-blue light. The analysis of experimental data showed that the curvature in response to a gravitropic stimulus or to combined gravity-light stimuli were not statistically different. Time course of gravitropic curvature were used to determine the acceptable crop rotation rate around the horizontal axis. Approximation of stem bending to a linear dynamic system described by a first-order aperiodic element with a lag allowed the determination of the dependence of the amplitude of apex oscillations on the rate of horizontal rotation under 1-g conditions. The calculated lowest minimal rotation rate (MRR) minimizing the gravitropic effects on wheat was about 1 revolution per hour (rph). Rotating the plant growth chamber (PGC) at a rate of more than MRR eliminated the effect of gravitropic curvature.     

Improvements in the re-flight of spaceflight experiments on plant tropisms

In order to effectively study phototropism, the directed growth in response to light, we performed a series of experiments in microgravity to better understand light response without the “complications” of a 1-g stimulus. These experiments were named TROPI (for tropisms) and were performed on the European Modular Cultivation System (EMCS), a laboratory facility on the International Space Station (ISS). TROPI-1 was performed in 2006, and while it was a successful experiment, there were a number of technical difficulties. We had the opportunity to perform TROPI-2 in 2010 and were able to optimize experimental conditions as well as to extend the studies of phototropism to fractional gravity created by the EMCS centrifuge. This paper focuses on how the technical improvements in TROPI-2 allowed for a better experiment with increased scientific return. Major modifications in TROPI-2 compared to TROPI-1 included the use of spaceflight hardware that was off-gassed for a longer period and reduced seed storage (less than 2 months) in hardware. These changes resulted in increased seed germination and more vigorous growth of seedlings. While phototropism in response to red illumination was observed in hypocotyls of seedlings grown in microgravity during TROPI-1, there was a greater magnitude of red-light-based phototropic curvature in TROPI-2. Direct downlinking of digital images from the ISS in TROPI-2, rather than the use of analog tapes in TROPI-1, resulted in better quality images and simplified data analyses. In TROPI-2, improved cryo-procedures and the use of the GLACIER freezer during transport of samples back to Earth maintained the low temperature necessary to obtain good-quality RNA required for use in gene profiling studies.