Fig. 1
The classical model of auxin-mediated gravitropism (left) is controlled by generation of an auxin maximum on the bottom root side after amyloplast sedimentation results in polarization of PIN3 in the columella cells, followed by increased levels of apically localized PIN2 on the underside of the root. Cytokinin-mediated hydrotropism is controlled by asymmetric distribution of cytokinins (right), and induces root turning towards an area of higher water potential. Despite turning away from the gravity vector, amyloplast sedimentation does not result in PIN redistribution and thus no auxin asymmetry, making hydrotropism overriding the gravitropic bending.
