Auxin transport

The Cholodony-Went hypothesis for gravitropism suggests that differential elongation between opposite flanks of graviresponding organs is linked to auxin transport, concentration, and/or sensitivity of the responding tissue. When a root is positioned horizontally, auxin redistributes to the lower side of the root. This redistribution promotes growth on the upper side and inhibits growth on the lower side of roots, resulting in downward curvature.
Auxin transport is clearly involved in root gravitropism since auxin transport inhibitors such as naphthylphthalamic acid (NPA) and triiodobenzoic acid (TIBA) inhibit root curvature and the differential induction of an auxin-responsive promoter. This model has been validated by the discovery of transport (PIN) proteins and the auxin-dependent degradation of inhibitors, changes in auxin sensitivity remain to be characterized.
Current work includes examining the connection of auxin transport with cytoskeletal organization and the interdependency of auxin transport and cellular organization.