More than a century of physiological work, as well as recent contributions from molecular genetics, has clearly shown that auxin is the most prominent intercellular signal in plants, and that it regulates a wide variety of developmental processes.
Auxin distributed over long distances largely contributes to the coordination and integration of growth and development at the level of the whole plant (it coordinates, for example, initiation of side roots with growth of aerial tissues). Auxin distributed over short distances by the polar transport system forms local gradients and thus mediates various patterning events. Furthermore, growth responses to external stimuli such as light or gravity also utilize auxin as a signal and appear to use the same mechanism involving auxin gradients. These auxin gradients are established, maintained and modulated by an active transport system, requiring regulators from the PIN family. The activity of these auxin transport regulators can be modulated at the single cell level by changes in their vesicle-trafficking-dependent polar targeting. Thus, the directional throughput of this complex auxin distribution network can be modulated by both endogenous and exogenous signals and provides, by means of mediating auxin fluxes and creating local gradients, a common mechanism for the plasticity and adaptability of plant development. Another level of control in the whole system occurs downstream of the gradients where a large variety of mutually interacting transcription activators and repressors provide the molecular basis for the multitude of developmental programs, which can be initiated and controlled by auxin.
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