The role of amyloplasts during gravity perception in gynophores of the peanut plant (Arachis hypogaea)

Gravitropic perception and response are essential for the completion of the reproductive life cycle of the peanut plant (Arachis hypogaea L.). The dev eloping seeds are buried in the soil by a specialized organ, the gynophore, allowing the fruit to mature underground. Controversy exists about the sit e of graviperception in the gynophore: previous workers suggested that the intercalary meristem was the zone where gravity was perceived. Taking the s tarch statolith hypothesis for graviperception as a framework, we explored the possibility that the starch-grain filled plastids (amyloplasts) in the starch sheath of the gynophore may be acting as gravisensors. We show that these amyloplasts sediment readily with respect to the gravity vector withi n 30 min of reorientation, and before there is a measurable gravitropic res ponse. Gynophore explants were incubated with gibberellic acid and kinetin, in darkness, to remove starch from the amyloplasts. Destarching the gynoph ores did not inhibit overall growth of the organ, but reduced the gravitrop ic response curvature by 82% compared to water-treated controls. In additio n, gynophores placed on a rotating clinostat (without hormone treatment) al so showed a reduced gravitropic response. In conclusion, the evidence prese nted in this work strongly suggests that the amyloplasts of the starch shea th are responsible for gravitropic perception in the peanut gynophore. A mo del for graviperception in the gynophore is presented. (C) 1999 Annals of B otany Company.