Solving the puzzle of gravitropism--has a lost piece been found?

Attempts to devise models to account for the gravitropic behaviour of plant organs have been limited conceptually by the predominance of studies on the gravitropic behaviour of organs of young seedlings. The dramatic growth responses induced by gravitropic stimulation of young shoots or roots, which rapidly restore the elongating axes to vertical, are experimentally convenient but theoretically limiting because gravitropism needs not simply restore an organ to vertical. Evidence is reviewed that suggests that plant organs must possess a mechanism which will allow them to attain a stable gravitropic position at any angle and that each organ has a characteristic gravitropic set-point angle (GSA). The GSA can be changed developmentally and is also regulated in a reversible manner by environmental parameters such as light. It is speculated that gravity may itself influence the GSA of an organ. The recognition that plant organs can grow at angles other than vertically up or down is not new, but previously it has been accepted that angles other than vertical were the consequence of the vectorial resultant effect of two different, opposing mechanisms. The new GSA concept proposes that a single mechanism might be sufficient to account for all forms of gravitropism in roots and shoots. This unifying concept proposes that the ability to change the angle of an organ with respect to the vertical is part of the basic gravitropic mechanism and that models of gravitropism must be able to account for this important feature.