Different gravitropic responsiveness between proximal and distal sides of epicotyls in early growth stage of etiolated pea seedlings: Relevance to polar auxin transport

Abstract

Polar auxin transport activity in the epicotyl region proximal to the cotyledons (proximal side) of 4.5-day-old etiolated pea seedlings has been found to be approximately twice that in the region distal to the cotyledons (distal side). The study aimed to determine whether the responsiveness to gravitropic stimulation differs between the proximal and distal sides, and to investigate the mechanism that induces gravitropic bending. When the seedlings were placed horizontally with the proximal side facing downward, the onset of bending occurred earlier and the bending angle was greater than when the distal side was facing downward. In the seedlings placed horizontally, tissue tension substantially decreased in the lower flank, while that in the upper flank remained the same as that in upright epicotyls. Decreased tissue tension in the lower flank was observed earlier in the seedlings with the proximal side facing downward, indicating that the asymmetrical distribution of tissue tension in the epicotyl provides a net gravitropic moment. Gravitropic stimulation increased cell wall extensibility in the lower flank, but not in the upper flank. These results suggest that differences in polar auxin transport between the proximal and distal sides is closely related to gravitropic responsiveness, and that changes in cell wall extensibility regulate asymmetrical tissue tension to cause gravitropic bending in etiolated pea seedlings.