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CHANGES IN STATOCYTE STRUCTURE AND AMYLOPLAST STARCH IN ARABIDOPSIS THALIANA COLUMELLA CELLS AFTER GROWTH UNDER HYPERGRAVITY CONDITIONS. S.D. Hopkins¹ and J.D. Smith². ¹Lockheed Martin Space Operations, Moffett Field CA, ²NASA Ames Research Center, Moffett Field CA. 

The purpose of this morphological study was to determine if amyloplast starch content and size were reduced in Arabidopsis root caps after growth in a hypergravity environment. During a past space flight experiment it was shown that, in space and on a clinostat, statolith mass in white clover (Trifolium repens) seedlings was dependent on the gravity environment; in an under-stimulated environment (i. e. space flight) it was shown that the volume of statocyte amyloplasts was increased [Smith et. al., (1997) Plant J. 12:1361-1373]. With that in mind we propose the hypothesis that in an over-stimulated environment (i. e. hypergravity on a centrifuge) the statocyte amyloplast volume would decrease. Arabidopsis seedlings were grown on vertical agar plates in an incubator at 25°C for two days under constant illumination from fluorescent lights. After 48 hours the fluorescent lights in the incubator were turned to the off position and the experimental groups were taken to the NASA Ames 1 ft. diameter centrifuge where they were subjected to either 2 g, 4 g or 8 g chronic centrifugation at 25°C in total darkness. After 48 hours of exposure to hypergravity the seedlings were chemically fixed, and the root caps were excised and processed further for transmission electron microscopy. Negatives of longitudinal root tip images were scanned and quantitative morphometrics were completed using NIH Image. Initial observations have shown that the overall root cap and the gross plant morphology remained unchanged by chronic exposure to hypergravity. Preliminary morphometric analysis of columella cell stories 2 and 3 for the 1 g and 8 g samples showed no significant difference in amyloplast starch content; however, the area fraction of amyloplast per cell was changed. Further analysis is underway to substantiate these findings. Continuing investi-gations will focus on vacuole, mitochondria and nucleus area fractions as well as comparisons between cell story and statocyte development. 

(This research was supported by NASA Ames Research Center.)