Abstract
Cellular growth in higher plants is generated (powered) by internal turgor pressure. Basic physics shows that the pressure required to deform a plastic tube by elongation is inversely proportional to the tube's diameter. Accordingly, the turgor required to drive tip growth of very narrow cylindrical plant cells becomes very high, probably too high to be realized in living cells. The non-involvement of turgor in tip growth is demonstrated directly in living diatoms secreting fine tubular spines of silica. In some species, the membrane at the tip of the rigid tube is deformed inwards into its lumen during normal extension, whereas in other species, many cells are partly plasmolysed during normal, active spine ('seta') extension. Evidence from other cells is consistent with the general conclusion that turgor is not significant in tip growth. We support the alternative hypothesis proposed by M. Harold and colleagues that extension in tip cells can be amoeboid, driven by cycling of the actin cytoskeleton. Actively growing setae display an internal, fibrous, collar-like sleeve, probably of actin at the tip; it is visualized as a molecular treadmill ('nanomachine') that uses as its support-base the rigid tube that has just been secreted. This scenario can thereby explain how the perfectly even diameter of very long, fine setae is maintained throughout their extension, even when their tips are far distant from the cell body.