Regulation of Plant Growth by Gravity

Abstract

Gravity is one of the most important environmental signals in regulating plant growth and development. Microgravity conditions in space are thought to be useful to clarify the role of gravity in regulating plant growth. However, the opportunity for experiments in space, which provide us with true microgravity conditions, is limited. True microgravity conditions are produced by a free fall or parabolic flight even on the ground, but the duration of microgravity obtained by these methods is generally too short for plants to exhibit obvious changes in their growth and development. A clinostat is useful for analyzing plant morphogenesis in gravity-free environment. However, the clinostat can eliminate only the dynamic gravistimulation, and does not affect the static stimulation. Hypergravity conditions, a gravitational force of more than 1 G, can be produced easily on earth by centrifugation. Hypergravity stimuli have been shown to inhibit elongation growth of shoots and roots in various plants. Our study has revealed that the metabolism of certain cell wall polysaccharides is modified under hypergravity conditions, which causes a decrease in the cell wall extensibility, thereby inhibiting elongation growth of plants. Under microgravity conditions in space, elongation growth of Arabidopsis hypocotyls and rice coleoptiles was stimulated (Space Shuttle STS-95 experiment). In addition, all cell wall parameters measured in the space experiment were opposite to those induced by hypergravity. These results suggest that hypergravity experiments are effective to clarify mechanisms regulating plant growth under microgravity conditions in space.