Diversity and specificity: auxin perception and signaling through the TIR1/AFB pathway

Abstract

Auxin is a versatile plant hormone that plays an essential role in most aspects of plant growth and development. Auxin regulates various growth processes by modulating gene transcription through a SCF(TIR1/AFB)-Aux/IAA-ARF nuclear signaling module. Recent work has generated clues as to how multiple layers of regulation of the auxin signaling components may result in diverse and specific response outputs. In particular, interaction and structural studies of key auxin signaling proteins have produced novel insights into the molecular basis of auxin-regulated transcription and may lead to a refined auxin signaling model.

Figure 1. A model for the TIR1/AFB-mediated auxin signaling pathway

A. DIII/IV regions of Aux/IAAs and ARFs are homologous and the DIII/IV of most Aux/IAAs and ARFs form a PB1 domain that comprises both an acidic (+) and a basic (−) faces. B. When auxin level is low, Aux/IAA proteins and ARFs form multimers through directional interaction between the acidic and basic interfaces of their DIII/IV regions. Aux/IAAs in the multimers recruit co-repressor complexes, which at least consist of TPLs and HDACs and repress transcription of target promoter through removing acetyls (Ac) from local chromatin. In addition, Aux/IAAs in the multimers may block ARFs from efficient binding to AuxREs in their target promoters. When auxin level is high, auxin promotes ubiquitination and degradation of Aux/IAAs through a SCF^TIR1/AFB-proteosome module, and the released ARFs form dimers and even higher-order complexes that activate expression of target genes.