Auxin in transcription factor complex controls polarity in plant organogenesis

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Auxin comprises the most important intercellular signal in plant development and functions in organogenesis and patterning through biosynthesis, transport and signalling. Auxin signalling occurs through binding of the auxin molecule to a TIR1/AFB F-box protein allowing interaction with Aux/IAA transcriptional repressor proteins. These are subsequently degraded via the 26S proteasome leading to derepression of auxin response factors (ARFs).
Different combinations between members of the ARF, Aux/IAA and AFB families are believed to provide cell and tissue specificity to the auxin response. Moreover, auxin signalling also occurs through the auxin-binding protein 1 (ABP1), which – in the presence of auxin – inhibits endocytosis of PIN auxin efflux carriers to regulate directional transport.
From experiments carried out in yeast and plants, we have identified a protein-protein interaction between two key regulators of polarity establishment during Arabidopsis gynoecium development. These are the auxin response factor ETTIN (ETT) and the bHLH protein IND. Interestingly, this protein complex can bind the natural auxin, indole-3-acetic acid (IAA) and our preliminary data suggest that binding of IAA changes the ETT/IND target gene set. This suggests the existence of an alternative signalling pathway for auxin, and since ETTIN is involved in the initiation and patterning of other organs, this pathway may be widespread throughout plant development.
Here we will carry out studies to reveal the significance, mechanism and conservation of this novel IAA-controlled module, which is distinctly different from previously established modes of auxin action.

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