Arabidopsis basic leucine Zipper transcription factors function as quantitative modulators of auxin mediated transcription

Abstract

The essential plant growth hormone auxin orchestrates a wide range of developmental and environmental processes in the course of plant life. In general, these responses are predominately implemented by the encoded activity of auxin responsive genes, which are corporately regulated by the family of Auxin Response Factors (ARFs) and the class of AUX/IAA proteins. Whereas ARF transcription factors (TFs) exert their trans-activating properties upon direct binding to their cognate Auxin Response Elements (AuxREs), the AUX/IAA transcriptional repressors contact the ARF proteins to modulate their activity. In order to sustain optimal plant growth, auxin mediated responses have to be adjusted according to the prevailing endogenous and environmental conditions. Thus an integration of the corresponding stimuli into auxin-related transcriptional patterns is required. Bioinformatic promoter cis-element analyses revealed that promoters of auxin responsive genes are not only significantlyenriched for AuxREs, but also for the G-BOX RELATED ELEMENTS (GREs) and MYB RESPONSIVE ELEMENTS (MREs). Using the Arabidopsis AtGH3.3 promoter as an auxin responsive model system, a combinatorial control of auxin-mediated transcription by a complex arrangement of these, in part redundantly acting, cis-elements has been demonstrated. Whereas AuxREs function as auxin-dependent switches, GREs and MREs act as quantitative modulators. Applying a trans-activation screening approach in protoplasts, members of the C/S1 network of basic leucine zipper (bZIP) TFs have been identified, which enhance and sensitize auxin-mediated transcription via binding the GRE cis-element. Complementary, gain- and loss-of-function approaches in transgenic plants confirm that the closely related group S1 AtbZIP2, -11 and -44 TFs modulate auxin-induced transcription and are capable to alter typical auxin-related growth-responses, such as primary root growth, lateral root formation, root hair density and gravitropism. Histochemical expression analysis of the auxin respon sive DR5:GFP reporter suggests bZIP-dependent alterations in auxin distribution and/or signalling. Ensuing studies on the mechanistical action of the group S1 bZIPs on auxin mediated transcription revealed that particularly AtbZIP11-related TFs are able to recruit the SAGA-like acetylation machinery via their N-terminal activation domain. Pharmacological and reverse genetic approaches clearly define the impact of histone acetylation in auxin-induced transcription. In fact, Chromatin-Immunoprecipitation (ChIP) analyses confirm bZIP-dependent recruitment of the histone acetylation machinery and RNA-Polymerase II. Altogether, these data suggest a novel bZIP-mediated mechanism to fine-tune chromatin accessibility during auxin-induced gene activation. As C/S1 bZIP-TFs are reprogramming the primary metabolism in response to energy stress, the GRE/bZIP module might function as a “rheostat