Head: Luz Irina Calderón Villalobos 

Indole-3-acetic acid (IAA or auxin) profoundly controls growth and development. Auxin responses depend on hierarchical control of gene expression and are executed by interplay of two classes of transcription factors, known as Aux/IAA (auxin-inducible) and ARF (auxin response factors) proteins.

Aux/IAA genes are rapidly induced by auxin and encode short-lived repressors containing four conserved domains. C-terminal domains III and IV mediate heterodimerization between Aux/IAAs and ARFs, with domain III being predicted to adopt a ßαα-fold similar to the DNA-binding domain of prokaryotic transcription factors such as Arc or MetJ. Domain II contains a degron peptide and confers protein instability, whereas domain I binds to co-repressors. Elevation of auxin concentration rapidly accelerates proteasome-dependent Aux/IAA degradation by recruiting domain II to the TIR1 SCF-type E3 ubiquitin protein ligase. Auxin facilitates the interaction between TIR1 and the degron peptide of Aux/IAAs, indicating that TIR1 and Aux/IAA proteins constitute co-receptors for auxin. Removal of Aux/IAA proteins de-represses ARF function, leading to the activation of target genes, including early Aux/IAA genes as part of a negative feedback regulation of auxin-regulated transcription.The characterization of gain- and loss-of-function mutations of several Aux/IAA genes suggests integration of auxin and light signaling pathways by the Aux/IAA-ARF transcription factor module.

We previously showed that select Aux/IAA proteins are phosphorylated by phytochrome in vitro and are phosphoproteins in vivo. We hypothesize that phytochrome-dependent phosphorylation of select Aux/IAA proteins increases their nuclear concentration or specific biochemical activities and thus provides a mechanism for integrating auxin and light signaling in plant development.