Auxin Transport

When a plant organ detects, through starch statolith sedimentation or another means, that it is no longer oriented in its preferred direction relative to gravity, a series of cellular and molecular events is initiated that results in a change in its direction of growth. This response, known as differential growth, leads to the eventual curvature of the organ back to its preferred orientation.

Some of the earliest studies of differential growth regulation focused not on gravitropic stimulation, but rather on phototropic stimulation. Charles Darwin was among the first plant scientists to study this phenomenon carefully:

…when seedlings are freely exposed to unilateral light, some influence is transmitted from the upper to the lower part, causing the latter to bend…
Charles Darwin, The Power of Movement in Plants, 1881

This summary came after careful experiments in which he shaded (or painted!) the tips of the growing seedlings and concluded that, while light seemed to be mostly perceived near the growing tip, the response occurred farther down the stem. Thus was set off a search for the nature of the “influence” that was transmitted, eventually leading to the discovery of the plant hormone auxin.

In a series of follow-up experiments to Darwin’s, it was shown that the factor causing differential growth could be extracted from rapidly growing tissues by placing tissue segments on a block of gelatin. The gelatin could then be used as a source of growth factor, and differential growth could be induced by applying the gelatin to one side of intact seedlings. These experiments suggested that differential growth begins with an unequal distribution of auxin, a concept known as the Cholodny-Went theory, first proposed in 1927.

A significant collection of physiological and biochemical evidence accumulated in support of the Cholodny-Went theory, including the discovery that auxin is transported in a polar manner through many organs of the plant. These data pointed to the need for a cell to have a mechanism for controlling the direction of auxin transport. This mechanism has now been identified to consist of several different kinds of auxin transporters, including the efflux carriers (PINs and ABCBs) and influx carriers (AUX1 and LAX3).