Auxin Signaling

Abstract

Auxin triggers diverse responses in plants, and this is reflected in quantitative and qualitative diversity in the auxin signaling machinery.

Figure 1.

The main pathway for regulation of transcription by auxin. Auxin-inducible genes have AREs in their promoters, which are bound by dimers of the ARF protein family. Gene expression is prevented by recruitment of Aux/IAA transcriptional repressors to these promoters via their interaction with the ARFs. Aux/IAAs recruit TPL family corepressors, which in turn recruit chromatin modifying enzymes (not shown) that stabilize the repressed state. The steps in the auxin response pathway are indicated by the numbered arrows. 1, Auxin acts as a molecular glue bringing together Aux/IAAs and F-box proteins of the TIR1/AFB family. 2, These F-box proteins are part of an SCF-type E3 ubiquitin protein ligase complex that transfers activated ubiquitin (Ub) from an E1/E2 enzyme system. 3, Polyubiquitination of the Aux/IAAs results in their degradation. 4, This releases repression at ARE-containing promoters.

Figure 2.

Hypothetical intracellular auxin gradients and root hair position. A, Epidermal cells in trichoblast cell files of the Arabidopsis root produce a root hair toward the rootward end of the cell. This is patterned by a root-tip-high auxin gradient, which is predicted to feed a tip-high intracellular auxin gradient in trichoblasts (blue shading), reinforced by shootward localization of the PIN2 auxin exporter (orange line). B, Triple mutation in aux1 ein2 gn_eb flattens the tip-high auxin gradient and presumably also the intracellular auxin gradient. This is associated with shifts in root hair placement. C, Application of auxin to aux1 ein2 gn_eb mutants shootward of the root hair differentiation zone shifts the root hair position to the shootward end of the cell, presumably associated with an inverted intracellular auxin gradient. D, Application of auxin to aux1 ein2 gn_eb mutants rootward of the root hair differentiation zone restores a more wild-type root hair position, presumably associated with restoration of the tip-high intracellular auxin gradient.