The polar transport of auxin and vein patterns in plants

Abstract

The hormone auxin is transported through many plant tissues with a definite velocity. It is thought that certain channels, or pumps, located at the basal ends of cells, are responsible for the hormone’s transport. It is also known that auxin will induce veins when applied to suitable tissues. T. Sachs has suggested that it is the flow of the hormone that induces vessels. He suggests that discrete strands form because the transport capacity of a pathway increases with the flux that that pathway carries, leading to a canalization of flow. I cast this in the form of a more specific hypothesis: I suppose the permeability for the transport of auxin through the basal plasmalemma of a cell (by means of whatever kind of pump or channel) to increase with flux. I then show that discrete veins will form provided that the transport permeability increases rapidly enough with flux, and provided that the movement of auxin is not too polar, in the sense that there is a substantial amount of diffusive movement of auxin in addition to polar transport. The same hypothesis offers an explanation for the loops of veins found under certain conditions.