Bipartite Promoter Element Required for Auxin Response      

Abstract

Multiple mechanisms have been described for coordination of responses to the plant hormones auxin and brassinosteroids (Zhang et al., 2009). One unexplained phenomenon is the reliance of the auxin transcriptional response on a functional brassinosteroid pathway. In this study, we used luciferase reporters to interrogate the promoter of SMALL AUXIN-UP RNA15 (SAUR15), a well-characterized auxin and brassinosteroid early response gene in Arabidopsis (Arabidopsis thaliana). After identifying a minimal region sufficient for auxin response, we targeted predicted cis-regulatory elements contained within this sequence and found a critical subset required for hormone response. Specifically, reporter sensitivity to auxin treatment required two elements: a Hormone Up at Dawn (HUD)-type E-box and an AuxRE-related TGTCT element. Reporter response to brassinosteroid treatment relied on the same two elements. Consistent with these findings, the transcription factors BRASSINOSTEROID INSENSITIVE1-EMS SUPPESSOR1 and MONOPTEROS (MP)/ AUXIN RESPONSE FACTOR5 (ARF5) showed enhanced binding to the critical promoter region containing these elements. Treatment with auxin or brassinosteroids could enhance binding of either transcription factor, and brassinosteroid enhancement of MP/ARF5 binding required an intact HUD element. Conservation of clustered HUD elements and AuxRE-related sequences in promoters of putative SAUR15 orthologs in a number of flowering plant species, in combination with evidence for statistically significant clustering of these elements across all Arabidopsis promoters, provided further evidence of the functional importance of coordinated transcription factor binding.

An Endogenous Carbon-Sensing Pathway Triggers Increased Auxin Flux and Hypocotyl Elongation

“…2D). Results were similar with a natural auxin-responsive reporter (pS15-5E::GUS; Walcher and Nemhauser, 2012;Supplemental Fig. S1).…”

MtLAX2, a Functional Homologue of the Arabidopsis Auxin Influx Transporter AUX1, Is Required for Nodule Organogenesis

“…5A) that the AUXIN RESPONSIVE FACTOR (ARF) family of transcription factors have been shown to bind (Ulmasov et al, 1999;Walcher and Nemhauser, 2012). To test whether MtLAX2 is auxininducible, we treated M. truncatula seedlings with either 1 mM IAA or its structural analog benzoic acid and monitored MtLAX2 gene expression using quantitative RT (qRT)-PCR.…”

The PB1 Domain in Auxin Response Factor and Aux/IAA Proteins: A Versatile Protein Interaction Module in the Auxin Response

“…How these two protein interaction modules might function together in regulating the expression of auxin response genes will require additional investigation. The ARF DD-FD as well as the PB1 domain might also function in recruiting other transcription factors (Ulmasov et al, 1995;Walcher and Nemhauser, 2012;reviewed in Guilfoyle and Hagen, 2012) that function in a cooperative manner to regulate auxin response genes.…”

“…Do ARF7 and ARF19 interact with Aux/ IAA repressors off of ARF DNA targets, and are these Aux/IAA-ARF complexes inhibited from binding AuxREs? Why does auxin increase DNA binding for ARF7 but decrease DNA binding for ARF2 (Walcher and Nemhauser, 2012), and is this directly related to phosphorylation of the ARFs and ARF PB1 interactions with Aux/IAA proteins? How general are observations made with BIN2 phosphorylation of ARF7, given that one or both of the serine phosphorylation sites are poorly conserved in ARF7 homologs found in other species as well as different in ARFs found in the same species?…”