The effect of intracellular geometry on auxin transport - II. Geotropism in shoots

Abstract

When a shoot is tilted, the transport of auxin along its axis acquires a downward component. It has been conjectured that the sinking of dense starch granules, called statoliths, to the lowest part of cells somehow causes this downward diversion of auxin flow, but it is not known how thîs is brought about. I propose a mechanism by which statolith movement could influence auxin transport. Suppose that cytoplasm accumulates in the lower part of the cell around the statoliths, displacing the vacuole upwards and leaving a comparatively thin layer of cytoplasm elsewhere. If auxin moves through cells by diffusion, and if the tonoplast is a substantial permeability barrier, then more auxin will move through the cytoplasm in the lower part of the cell, and there will be a downward diversion of auxin, as required. I give calculations to support this argument, and show that a large enough downward diversion of auxin could be achieved if the permeability of the tonoplast is not too large.