Gravitropism and development of wild‐type and starch‐deficient mutants of Arabidopsis during spaceflight

Abstract

The "starch-statolith" hypothesis has been used by plant physiologists to explain the gravity perception mechanism in higher plants. In order to help resolve some of the controversy associated with ground-based research that has supported this theory, we performed a spaceflight experiment during the January 1997 mission of the Space Shuttle STS-81. Seedlings of wild-type (WT) Arabidopsis, two reduced-starch strains, and a starchless mutant were grown in microgravity and then given a gravity stimulus on a centrifuge. In terms of development in space, germination was greater than 90% for seeds in microgravity, and flight seedlings were smaller (60% in total length) compared to control plants grown on the ground and to control plants on a rotating clinostat. Seedlings grown in space had two structural features that distinguished them from the controls: a greater density of root hairs and an anomalous hypocotyl hook structure. However, the slower growth and morphological changes observed in the flight seedlings may be due to the effects of ethylene present in the spacecraft. Nevertheless, during the flight hypocotyls of WT seedlings responded to a unilateral 60 min stimulus provided by a 1-g centrifuge while those of the starch-deficient strains did not. Thus the strain with the greatest amount of starch responded to the stimulus given in flight and therefore, these data support the starch-statolith model for gravity sensing.

Stimulation of elongation growth and xyloglucan breakdown in Arabidopsis hypocotyls under microgravity conditions in space

“…However, in some experiments, space flight did not affect the growth of plants (Thimann 1968;Halstead and Dutcher 1987;Levine et al 2001). In addition, it was reported that growth of cucumber hypocotyls (Halstead and Dutcher 1987) and Arabidopsis hypocotyls (Kiss et al 1998) was inhibited by space flight. These contradictions seem to be caused by differences in the conditions of the space experiments, such as temperature and light.…”

“…Also, the imbibition of seeds in orbit may result in delayed germination and therefore in suppression of growth. In addition, ethylene, which accumulates in the cultural environment in the cabin of spacecrafts, suppresses plant growth (Kiss et al 1998). To avoid such problems, in the present space experiment, Arabidopsis seedlings were grown in complete darkness and independent culture dishes were used to obtain seedlings with different ages.…”

Plastid position in Arabidopsis columella cells is similar in microgravity and on a random-positioning machine

“…This hardware included type-I containers, each with two minicontainers, as described in Katembe et al (1998). The inside of the minicontainers had two surfaces for plant growth and consisted of a``sandwich'' of ®lter paper and cellulose ester membranes held together by o-rings Kiss et al 1998). In some cases, for comparative purposes, seedlings were grown on a nutrient agar in Petri dishes (Kiss et al 1996).…”

“…Dark-grown seedlings of Arabidopsis thaliana (L.) Heynh. (strain Wassilewskija, Ws) were used in these experiments approximately 4 d after hydration of seeds with a growth medium (Kiss et al 1996). Seedlings were grown in space¯ight hardware ( Fig.…”

Auxin Polar Transport and Automorphosis in Plants

“…Studies using Arabidopsis mutants such as hookless, eto1 (ethylene overproducer), ctr1 (constitutive activation of the ethylene response pathway) supported the idea that hook formation is regulated by ethylene (Lehman et al, 1996;Guzman and Ecker, 1990;Roman et al, 1995). Although in STS-81 space experiment an anomalous hook structure in Arabidopsis seedlings similar to that exposed to relatively high ethylene levels was observed, it is considered to be due to particular gaseous environment in the spacecraft, but not microgravityinduced ethylene, since hook structures appeared under simulated microgravity conditions on a clinostat were not significant from those on 1 G conditions (Kiss et al, 1998). Thus, ethylene production may be positively regulated by gravity, regulating in hook formation.…”