Influence of Microgravity on Ultrastructure and Storage Reserves in Seeds of Brassica rapa L.

Abstract

Successful plant reproduction under spaceflight conditions has been problematic in the past. During a 122 d opportunity on the Mir space station, full life cycles of Brassica rapa L. were completed in microgravity in a series of three experiments in the Svet greenhouse. Ultrastructural and cytochemical analyses of storage reserves in mature dry seeds produced in these experiments were compared with those of seeds produced during a high-fidelity ground control. Additional analyses were performed on developing Brassica embryos, 15 d post pollination, which were produced during a separate experiment on the Shuttle (STS-87). Seeds produced on Mir had less than 20% of the cotyledon cell number found in seeds harvested from the ground control. Cytochemical localization of storage reserves in mature cotyledons showed that starch was retained in the spaceflight material, whereas protein and lipid were the primary storage reserves in ground control seeds. Protein bodies in mature cotyledons produced in space were 44% smaller than those in the ground control seeds. Fifteen days after pollination, cotyledon cells from mature embryos formed in space had large numbers of starch grains, and protein bodies were absent, while in developing ground control seeds at the same stage, protein bodies had already formed and fewer starch grains were evident. These data suggest that both the late stage of seed development and maturation are changed in Brassica by growth in a microgravity environment. While gravity is not absolutely required for any step in the plant life cycle, seed quality in Brassica is compromised by development in microgravity.

Seed-to-Seed-to-Seed Growth and Development of Arabidopsis in Microgravity

“…The high forced airflow rates (2-3 m/s) and accompanying ethylene removal provided by ADVASC improved growing conditions for the aerial part of the plants when compared to the previous studies by Kuang et al (2000) and Merkys and Laurinavicius (1983).…”

“…Other symptoms may include aberrant deposition of starch grains and thickened cell walls (Ramonell et al, 2001). In a full life-cycle experiment with Brassica, Kuang et al (2000) found protein bodies that were 44% smaller (cross-sectional area). There was an accompanying 80% reduction in the cotyledon cell number, and starch grains were aberrantly deposited in the seed.…”

“…Elevated ethylene levels (1100-1600 ppb on a shuttle) caused anomalous seedling growth of Arabidopsis in spaceflight studies, although they had no effect on relative graviresponsiveness . Furthermore, ethylene levels on the Mir space station were very high (800-1200 ppb) during a Brassica study reported by Kuang et al (2000). It is notable that Brassica plants produced seed at this ethylene level, since the same environment stopped a wheat crop from producing seed on board Mir (Campbell et al, 2001).…”

“…Seeds were examined by using light microscopy and transmission electron microscopy to check for alterations in embryo development, deposition of starch grains, reduction in cell numbers, or reduced seed protein content, since these phenotypes were reported for seed produced on previous long-duration spaceflights (Merkys and Laurinavicius, 1983;Kuang et al, 2000). We also looked for cell wall thickening, since this was seen in Arabidopsis leaves grown under reduced oxygen atmospheres and may be indicative of hypoxia (Ramonell et al, 2001).…”

“…They observed some viable seed, but most seed had nonviable embryos. The second successful experiment was performed with Brassica rapa and was reported by Musgrave et al (2000) and Kuang et al (2000). Though the Brassica seed was healthy and viable, the seed was observed to have less protein, fewer cotyledon cells, and aberrant deposition of starch grains.…”