Auxin response factors

Auxin signalling involves the activation or repression of gene expression by a class of auxin response factor (ARF) proteins that bind to auxin response elements in auxin-responsive gene promoters. The release of ARF repression in the presence of auxin by the degradation of their cognate auxin/indole-3-acetic acid repressors forms a paradigm of transcriptional response to auxin. However, this mechanism only applies to activating ARFs, and further layers of complexity of ARF function and regulation are being revealed, which partly reflect their highly modular domain structure. This review summarizes our knowledge concerning ARF binding site specificity, homodimer and heterodimer multimeric ARF association and cooperative function and how activator ARFs activate target genes via chromatin remodelling and evolutionary information derived from phylogenetic comparisons from ARFs from diverse species. ARFs are regulated in diverse ways, and their importance in non-auxin-regulated pathways is becoming evident. They are also embedded within higher-order transcription factor complexes that integrate signalling pathways from other hormones and in response to the environment. The ways in which new information concerning ARFs on many levels is causing a revision of existing paradigms of auxin response are discussed. The importance of auxin in a myriad of plant developmental processes, and the recent elucidation of many aspects of auxin signalling biology has focused attention on auxin response factors (ARFs) as central transcriptional mediators of the auxin response pathway. Recent novel facets and mechanisms of ARF function have been identified, in terms of cooperativity, binding site specificity, chromatin remodelling, translational and post-translational ARF regulation and their function independent of auxin, which have warranted review. The increasing perception that ARFs are components of higher-order protein complexes and are subject to complex regulation to integrate environmental, developmental and other hormone pathways is broadening existing paradigms of ARF function and contributing to the debate of whether auxin acts as facilitator of cell fate rather than as a determiner.