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Plants are an important component of the controlled ecological life-support system (CELSS) for future long-term spaceflight and the International Space Station. Therefore, it is critical to understand the susceptibility of plants to pathogen infection in microgravity. An increase in both hyphal growth and sporangia formation in Phycomyces blakes in microgravity has been described. Plant cell walls, a critical barrier for pathogen invasion, have been reported to undergo changes in microgravity including changes in the wall structure. For example, a decrease in the crystalline cellulose content and an increase in the hemicellulose content in cell walls of plants grown in clinostats and in microgravity have been reported. Based of these previous reports, we hypothesize that susceptibility of plants to pathogen infection in microgravity would be increased relative to the ground control.

The study of pathogenicity of higher plants under conditions of microgravity is of great importance for the future production of food in space. Previous work suggests that microgravity affects both microbes and plants. Bacterial numbers increased after 17 days in an algae-bacterium association on the biosatellite "Kosmos-1887". This was speculated to result from an increase in the multiplication rate of the bacteria. Sporangia of both Actinomices brevis, in the shuttles "Soyuz-19" and "Appolon", and Phycomyces blakes, in biosatellite "Kosmos-936", formed after 10 days in microgravity. Sporangia did not form in the ground controls in the same time suggesting that the rate of fungal development is enhanced in microgravity. Plant responses to pathogens in microgravity have not been studied, however, microgravity profoundly impacts plant cell development, cytology, and physiology. In microgravity, developing cell walls are thinner and contain less lignin than ground-grown plants. The demonstrated effects of microgravity on both plants and microbes lead us to hypothesize that plants may be more susceptible to pathogens under conditions of microgravity. The aim of this study was to determine the influence of microgravity on the susceptibility of soybean to the fungal root rot pathogen, Phytophthora sojae.

Electron micrographs of columella cells from sweet clover seedlings grown and fixed in microgravity revealed longitudinal and cross sectioned cortical microtubules. This is the first report demonstrating the presence and stability of this network in plants in microgravity.

In higher plants, calcium redistribution is believed to be crucial for the root to respond to a change in the direction of the gravity vector. To test the effects of clinorotation and microgravity on calcium localization in higher plant roots, sweet clover (Melilotus alba L.) seedlings were germinated and grown for two days on a slow rotating clinostat or in microgravity on the US Space Shuttle flight STS-60. Subsequently, the tissue was treated with a fixative containing antimonate (a calcium precipitating agent) during clinorotation or in microgravity and processed for electron microscopy. In root columella cells of clinorotated plants, antimonate precipitates were localized adjacent to the cell wall in a unilateral manner. Columella cells exposed to microgravity were characterized by precipitates mostly located adjacent to the proximal and lateral cell wall. In all treatments some punctate precipitates were associated with vacuoles, amyloplasts, mitochondria, and euchromatin of the nucleus. A quantitative study revealed a decreased number of precipitates associated with the nucleus and the amyloplasts in columella cells exposed to microgravity as compared to ground controls. These data suggest that roots perceive a change in the gravitational field, as produced by clinorotation or space flights, and respond respectively differently by a redistribution of free calcium.

Although spaceflight does not appear to significantly affect seed germination, it can influence subsequent plant growth. On STS-3 and SL-2, decreased growth (measured as plant length, fresh weight and dry weight) was noted for pine, oat and mung bean. In the CHROMEX-01 and -02 experiments with Haplopappus and in the CHROMEX-03 experiment with Arabidopsis, enhanced root growth was noted in the space-grown plants. In the CHROMEX-04 experiment with wheat, both leaf fresh weight and leaf area were diminished in the space-grown plants but there was no difference in total plant height (CS Brown, HG Levine, and AD Krikorian, unpublished data). These data suggest that microgravity impacts growth by whole plant partitioning of assimilates. The objective of the present study was to determine the influence of clinorotation on the growth and morphology of soybean seedlings grown in the BRIC (Biological Research In Canister) flight hardware. This experiment provided baseline data for a spaceflight experiment (BRIC-03) flown on STS-63 (Feb. 3-11, 1995).