[Does differential pressure of amyloplasts on a complex endomembrane system cause geoperception in roots?].

In the root cap of Lepidium sativum a complex of multiple rough endoplasmic reticulum develops above the morphological lower transverse cell walls during ontogenesis of the columella cells ("statocysts"). The cisternae of the ER-complex are preferentially oriented parallel to the transverse walls. In normal vertical exposure of the roots the amyloplasts ("statoliths") lie above the ER-complex. They do not touch the plasma membrane, but possibly they press against the ER-complex and thereby bring about geotropic equilibrium.In each storey of the root cap the transverse walls together with their ER-complexes have a parabolic shape. Therefore the surface areas of the central ER-complexes form a right angle and those of the peripheral ER-complexes an acute angle with the organ axis.Owing to the shape of the whole ER-complex within each storey, the amyloplasts in the physically upper peripheral columella cells do not press against the membranes of the ER in the case of horizontal exposition. On the other hand in the physically lower part the amyloplasts are still situated above the ER-complex and can press on the ER.Geoperception in roots may be a function of pressure exerted differentially by amyloplasts on the ER-complex.