Gravitropism of hypocotyls of wild-type and starch-deficient Arabidopsis seedlings in spaceflight studies

The major purpose of this spaceflight project was to investigate the starch -statolith hypothesis for gravity perception, and a secondary goal was to s tudy plant growth and development under spaceflight conditions. This resear ch was based on our ground studies of gravity perception in the wild type a nd three starch-deficient (one starchless and two reduced starch) mutants o f Arabidopsis thaliana (L.) Heynh. Dark-grown seedlings that developed in m icrogravity were given one of several (30 min, 60 min, or 90 min) 1-g stimu li by an on-board centrifuge, and additional controls for seedling developm ent also were performed. These latter control experiments included a morpho logical study of plants that developed in space in microgravity (F mu g), i n space on a centrifuge (F 1g), on the ground (G 1g), and on a rotating cli nostat on the ground. Since elevated levels of ethylene were reported in th e spacecraft atmosphere, additional controls for morphology and gravitropis m with added ethylene also were performed. While exogenous ethylene reduced the absolute magnitude of the response in all four strains of Arabidopsis, this gas did not appear to change the relative graviresponsiveness among t he strains. The relative response of hypocotyls of microgravity-grown seedl ings to the stimuli provided by the in-flight centrifuge was: wild type > s tarch-deficient mutants. Although the protoplast pressure model for gravity perception cannot be excluded, these results are consistent with a statoli th-based model for perception in plants.