Stimulation of ethylene production in rice seedlings grown under hypergravity conditions

Ethylene is a gaseous plant hormone that influences diverse phases of plant growth and development. The influence of gravitational conditions on ethylene production in plants is a crucial issue for the cultivation of plants under altered gravity conditions, such as in space. The primary objective of this study was to ascertain the impact of gravitational conditions on ethylene production in plants. To this end, rice seedlings were cultivated under hypergravity conditions at 300 g. The ethylene content released in hypergravity-treated samples was three times higher than that in 1 g-grown samples. This is the first evidence that the hypergravity conditions affect the ethylene production in plants. The expression level of the OsACS1 gene for 1-aminocyclopropane-1-carboxylic acid (ACC) synthase, which is a pivotal enzyme in ethylene biosynthesis in plants, was twofold higher in hypergravity-treated shoots compared to 1 g-grown shoots. These results suggest that the upregulation of OsACS1 gene may be associated with the stimulation of ethylene production in rice seedlings grown under hypergravity conditions. In contrast, our previous space experiments have demonstrated that the expression level of OsACS1 in rice shoots substantially decreased under microgravity conditions. The contrasting change in the expression level under hypergravity and microgravity conditions suggests that OsACS1 may be a gravity-responsive gene in rice shoots with a broad spectrum of responsiveness to gravitational forces. Moreover, the present results support the hypothesis that the ethylene production in rice seedlings may be reduced under microgravity conditions.