Auxin Transport: Down and Out and Up Again

Almost with clairvoyance, Charles and Francis Darwin in Power of Movement in Plants described in the last century a growth-promoting “influence” which, by its asymmetric distribution in plant organs, causes asymmetrical growth (tropism) after stimulation by gravity or light (1). Fifty years later, this “influence” was identified as indole-acetic acid (IAA), an endogenous member of the class of growth hormones called auxins (2). Auxins are actually morphogens, not hormones, because they are distributed in spatial gradients within plant organs. This gradient conveys to the developing cells their location within the morphogenic field—and their fate. The different concentrations of auxin along this gradient, in combination with other signals, trigger different responses—elongation at one position and differentiation at another, for example (3).