A Review of Auxin Response Factors (ARFs) in Plants

Abstract

Auxin is a key regulator of virtually every aspect of plant growth and development from embryogenesis to senescence. Previous studies have indicated that auxin regulates these processes by controlling gene expression via a family of functionally distinct DNA-binding auxin response factors (ARFs). ARFs are likely components that confer specificity to auxin response through selection of target genes as transcription factors. They bind to auxin response DNA elements (AuxRE) in the promoters of auxin-regulated genes and either activate or repress transcription of these genes depending on a specific domain in the middle of the protein. Genetic studies have implicated various ARFs in distinct developmental processes through loss-of-function mutant analysis. Recent advances have provided information on the regulation of ARF gene expression, the role of ARFs in growth and developmental processes, protein-protein interactions of ARFs and target genes regulated by ARFs in plants. In particular, protein interaction and structural studies of ARF proteins have yielded novel insights into the molecular basis of auxin-regulated transcription. These results provide the foundation for predicting the contributions of ARF genes to the biology of other plants.

Keywords: ARF; Arabidopsis; DBD domain; a type I/II Phox and Bem1p (PB1); auxin; auxin response DNA elements.

Figure 1

The key components in auxin perception and signaling in Arabidopsis. ARF proteins contain a non-conserved AD or RD flanked by an N-terminal DBD (composed of a B3 domain, a dimerization domain: DD, and a Tudor-like ancillary domain within the C-terminal region of the flanking domain: FD) and a C-terminal PB1 domain (previously referred to as domain III/IV). Parts of the DD and FD are found both N-terminal and C-terminal to the B3 domain. In this pathway, the TRANSPORT INHIBITOR RESPONSE 1/AUXIN SIGNALING F-BOX proteins (TIR1/AFBs) are F-box proteins that, together with other proteins (ASK1, CUL1, RBX), form the ubiquitin protein ligase complex, SCF^TIR1. At low auxin levels (A), the Aux/IAA proteins form multimers with ARFs and recruit TPL to the chromatin. High levels of auxin (B) promote ubiquitination and degradation of Aux/IAAs through SCF^TIR1∕AFB and the 26S proteasome (Kim et al., ; Guilfoyle and Hagen, , ; Szemenyei et al., ; Boer et al., ; Guilfoyle, ; Korasick et al., ; Salehin et al., 2015).