The Starch Statolith Hypothesis and the Interaction of Amyloplasts and Endoplasmic Reticulum in Root Geotropism

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Abstract
Roots of cress (Lepidium sativum L, ) seedlings continuously stimulated at an angle of 135°—root tips pointing obliquely upwards—develop a larger final geotropic curvature than roots stimulated at 45° or 90°. This well-known behaviour has previously been interpreted as support for the starch statolith hypothesis. In the present experiments two groups of cress and lettuce (Lactuca sativa L.) seedlings were used: (a) the control group in which the roots were allowed to curve without adjustment of the stimulation angle, and (b) the test group in which the roots were readjusted at different time intervals to the original stimulation angle. They were stimulated continuously at 45°, 90°, or 135° and the development of root curvatures was followed over a period of 5–8 h. Initially (1–2 h) the rate of curvature was approximately the same for 135° and 90° control and tested cress and lettuce roots. Thereafter the test roots stimulated at 135° followed a linear curvature pattern. Seedlings stimulated at 45° and 90° did not show the same linearity in curvature development in the test group. The rates of curvature in the test group were generally higher than in the control group at angles less than 135°. Cress seedlings were examined by light and electron microscopy in order to follow the movement of the cell organelles in the statocytes. In the statocytes of roots of test seedlings the starch statoliths were located in the position attained before the first readjustment of the stimulation angle. In the statocytes of control roots the starch statoliths followed the curvature of the root tip sliding along the cell walls and attaining the rest position as in normally orientated roots. The behaviour of control and readjusted roots is interpreted as a result of interaction between starch statoliths and endoplasmic reticulum membranes.

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