Effects of hypergravity stimulus on global gene expression during reproductive growth in Arabidopsis.

The life cycle of higher plants consists of successive vegetative and reproductive growth phases. Understanding effects of altered gravity conditions on the reproductive growth is essential, not only to elucidate how higher plants evolved under gravitational condition on Earth but also to approach toward realization of agriculture in space. In the present study, a comprehensive analysis of global gene expression of floral buds under hypergravity was carried out to understand effects of altered gravity on reproductive growth at molecular level. Arabidopsis plants grown for 20-26 days were exposed to hypergravity of 300 g for 24 h. Total RNA was extracted from flower buds and microarray (44 K) analysis performed. As a result, hypergravity up-regulated expression of a gene related to ß-1,3-glucanase involved in pectin modification, and down-regulated ß-galactosidase and amino acid transport, which supports a previous study reporting inhibition of pollen development and germination under hypergravity. With regard to genes related to seed storage accumulation, hypergravity up-regulated expression of genes of aspartate aminotransferase, and down-regulated those related to cell wall invertase and sugar transporter, supporting a previous study reporting promotion of protein body development and inhibition of starch accumulation under hypergravity, respectively. In addition, hypergravity up-regulated expression of G6PDH and GPGDH, which supports a previous study reporting promotion of lipid deposition under hypergravity. In addition, analysis of the metabolic pathway revealed that hypergravity substantially changed expression of genes involved in the biosynthesis of phytohormones such as abscisic acid and auxin.

Growth stimulation in inflorescences of an Arabidopsis tubulin mutant under microgravity conditions in space.

Cortical microtubules are involved in plant resistance to hypergravity, but their roles in resistance to 1 g gravity are still uncertain. To clarify this point, we cultivated an Arabidopsis α-tubulin 6 mutant (tua6) in the Cell Biology Experiment Facility on the Kibo Module of the International Space Station, and analyzed growth and cell wall mechanical properties of inflorescences. Growth of inflorescence stems was stimulated under microgravity conditions, as compared with ground and on-orbit 1 g conditions. The stems were 10-45% longer and their growth rate 15-55% higher under microgravity conditions than those under both 1 g conditions. The degree of growth stimulation tended to be higher in the tua6 mutant than the wild-type Columbia. Under microgravity conditions, the cell wall extensibility in elongating regions of inflorescences was significantly higher than the controls, suggesting that growth stimulation was caused by cell wall modifications. No clear differences were detected in any growth or cell wall property between ground and on-orbit 1 g controls. These results support the hypothesis that cortical microtubules generally play an important role in plant resistance to the gravitational force.

Radial widening of the Casparian strip follows induced radial expansion of endodermal cells.

The Casparian strip, the barrier to apoplastic transport that is located at the endodermis in roots and stems, is formed by individual endodermal cells and is constructed as a highly organized mesh within the primary wall. Since little is known about the mechanism of formation of the strip, we tried to obtain morphological evidence for the existence, prior to suberization and lignification, of some regulatory system at the expected site of the strip. Endodermal cells in etiolated pea stems were induced to expand in the radial direction by piercing the stems through the cortex before formation of the strip. The radial width of the strip increased significantly with the expansion of the radial walls of these endodermal cells. The expansion of the cells occurred before the formation of the strip. However, strips that had already been formed when the stems were pierced did not increase in width despite an induced expansion of the radial walls. These observations suggest that some positional information exists in the radial wall of endodermal cells that defines the future site of formation of the strip and its width.

An agravitropic mutant of Arabidopsis, endodermal-amyloplast less 1, that lacks amyloplasts in hypocotyl endodermal cell layer.

We have isolated a new recessive mutant of Arabidopsis thaliana for gravitropism, endodermal-amyloplast less 1 (eal1). eal1 shows reduced gravitropism in hypocotyl, and completely lacks gravitropism in inflorescence stems; root gravitropism is not affected. Starch staining with I-KI solution reveals almost no amyloplasts in eal1 hypocotyls when grown on a sucrose-free medium, though the root columella cells contain as many amyloplasts as wild type. On a medium containing 1% sucrose, eal1 hypocotyls contain as many starch granules as those of wild type, suggesting that starch synthesis is not affected in eal1. The endodermal cell layer which is thought to function as statocytes in hypocotyls is present in eal1. These results suggest that differentiation or development of gravity-responsive amyloplasts are affected in eal1 hypocotyls.