Auxin Response Factors and Auxin Regulated Transcription

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AbstractMCB-0080096Thomas Guilfoyle and Gretchen HagenGuilfoyle Auxin is a plant hormone known to play important roles in differential growth, root formation, apical dominance, and vascular patterning. These growth and developmental processes may be regulated, at least in part, by the expression patterns of primary (or early) and secondary (or late) auxin response genes. Early auxin response genes contain one or more Auxin Response Elements [AuxREs] containing the sequence TGTCTC, which confers auxin responsiveness to the promoters. Auxin Response Factors [ARFs] are transcription factors that bind with specificity to TGTCTC AuxREs. ARFs contain an amino-terminal DNA binding domain [DBD], and most ARFs contain a conserved carboxy-terminal domain [CTD] that facilitates their dimerization to themselves or Aux/IAA proteins. Amino acids located between ARF DBDs and CTDs are nonconserved and represent activation or repression domains. Specific ARFs can activate or repress auxin-responsive transcription by binding directly to TGTCTC AuxREs and may further potentiate transcriptional activation or repression by dimerizing with ARF(s) that are bound to AuxREs. While Aux/IAA proteins do not bind directly to TGTCTC AuxREs, they might also potentiate repression [or possibly activation] on AuxREs by forming heterodimers with ARF(s) that are bound to AuxREs. Major questions arise from the identification of many ARF and Aux/IAA proteins in Arabidopsis. How do specific ARFs with similar DNA binding domains target particular TGTCTC AuxREs? What governs the formation of specific pairs of ARFs, Aux/IAA, and ARF-Aux/IAA proteins? How do ARF and Aux/IAA homodimers and heterodimers determine whether an auxin response gene is activated or repressed? Does auxin play a role in targeting specific ARFs to TGTCTC AuxREs, in selecting the types of dimers that form, and/or in determining whether an activation or repression domain is functional when residing at an AuxRE? What other transcription and signalling factors participate along with ARFs and Aux/IAA proteins in regulating primary auxin response genes? The long term goal is to answer these questions and to begin to untangle the seemingly complex network of ARF and Aux/IAA interactions that must occur within cells to regulate auxin-responsive gene expression. The project has the following specific aims: (1) to determine if ARFs and Aux/IAA proteins show selectivity in interactions with one another; (2) to determine if auxin has an effect on ARF activation or repression domains directly or if an ARF CTD is required for auxin-responsive activation or repression; (3) to determine if Q-rich ARF activators play redundant roles in auxin-responsive gene expression; (4) to determine if the gene expression patterns of ARFs might restrict the types of ARF and Aux/IAA dimers that form in planta; and (5) to identify proteins besides Aux/IAAs that interact with ARFs and play roles in auxin signaling and auxin responses.