Isolation and Identification of a Gravity-Induced Growth Inhibitor in Etiolated Radish Hypocotyls

Abstract

A growth inhibitor of which content in etiolated radish hypocotyls was increased under 500 g gravity conditions produced by centrifugation, but decreased under simulated microgravity conditions on a 3-dimensional clinostat, comparing to 1 g conditions, was isolated from etiolated radish hypocotyls grown under 1 g conditions. It was determined to be 3,6'-disinapoylsucrose by its 1 H and 13 C NMR and MS spectral data. When etiolated radish seedlings were placed horizontally, the gravitropic curvature took about 30 min to start. The distribution of endogenous 3,6'-disinapoylsucrose between the peripheral cell layers of the upper and lower flank of radish hypocotyls during gravitropic curvature was analyzed using a physicochemical assay. Its content rapidly increased in the peripheral cell layers of the upper halves, peaking at 30 min after the onset of gravitropic stimulation, then gradually decreased. On the other hand, the content in the peripheral cell layers of the lower halves was almost equal to that in the vertical control during gravitropic curvature. Its unilateral application caused the hypocotyls to bend toward the site of application. These results suggest that gravitropic stimulation suppresses the growth rate of the upper side of radish hypocotyls by increasing the content of the growth inhibitor 3,6'-disinapoylsucrose in the peripheral cell layers of the upper halves, causing a gravitropic response.