A combinatorial TIR1/AFB–Aux/IAA co-receptor system for differential sensing of auxin

Abstract

The plant hormone auxin regulates virtually every aspect of plant growth and development. Auxin acts by binding to the F-box protein TIR1 and promotes the degradation of the Aux/IAA transcriptional repressors. Here, we show that efficient auxin binding requires assembly of an auxin co-receptor complex consisting of TIR1 and an Aux/IAA protein. Heterologous experiments in yeast and quantitative IAA binding assays using purified proteins showed that different combinations of TIR1 and Aux/IAA proteins form co-receptor complexes with a wide range of auxin-binding affinities. Auxin affinity appears to be largely determined by the Aux/IAA. As there are 6 TIR1/AFBs and 29 Aux/IAA proteins in Arabidopsis thaliana, combinatorial interactions may result in many co-receptors with distinct auxin sensing properties. We also demonstrate that the AFB5-Aux/IAA co-receptor selectively binds the auxinic herbicide picloram. This co-receptor system broadens the effective concentration range of the hormone and may contribute to the complexity of auxin response.

Auxin metabolism and homeostasis during plant development

“…1) was discovered ~70 years ago, although the concept of plant hormones and speculation over their roles in plant development were conceived much earlier (reviewed by Abel and Theologis, 2010). Auxin research took off in the 1980s following the discovery of a battery of genes involved in auxin responses, and more recently with the discovery of the TIR1 auxin receptor family (reviewed by Calderón-Villalobos et al, 2010) and various auxin transporters (reviewed by Zažímalová et al, 2010). These discoveries boosted a wave of research into auxin signalling and the roles of these newly discovered genes during plant development, mostly using the model plant species Arabidopsis thaliana as an experimental system.…”

“…2). The large number of Aux/IAA response genes (29 in Arabidopsis) and ARFs (23 in Arabidopsis) indicate that the auxin response is very complex, depending both on auxin levels and the specificity and strength of the TIR1-Aux/IAA and Aux/IAA-ARF interactions (Vernoux et al, 2011;Calderón-Villalobos et al, 2012). …”

Auxin 2012: a rich mea ho’oulu

“…Working in collaboration with Jennifer Nemhauser (University of Washington, USA), he demonstrated that these TIR1 variants also accelerate Aux/IAA degradation. Variation in this region among the TIR1-AFB family members might explain some of the observed differences in properties of co-receptor pairs (Calderón Villalobos et al, 2012;Havens et al, 2012). Additional TIR1 variants with mutations in the F-box region were also presented.…”

“…Auxin triggers the degradation of Auxin/INDOLE-3-ACETIC ACID (Aux/IAA) repressor proteins by promoting their interaction with TRANSPORT INHIBITOR RESPONSE1 (TIR1)-AUXIN SIGNALING F-BOX (AFB) F-box subunits of the E3 ubiquitin ligases (SCF TIR1/AFB ) ( Fig. 1) (reviewed by Calderón Villalobos et al, 2010). The ubiquitylation and subsequent degradation of Aux/IAA repressors leads to activation of AUXIN RESPONSE FACTOR (ARF) transcription factors.…”

A proposed role for selective autophagy in regulating auxin-dependent lateral root development under phosphate starvation in Arabidopsis

“…25,26 There are AUX/ IAAs in Arabidopsis that lack this domain II (IAA20, IAA30 and IAA32-34) and IAA 31 has partial domain sequence. 27 It has been suggested that there could be degrons outside domain II, which could also influence proteolysis [27][28][29] and Based on our observations with ark2-1/pub9-1 phenotypes and our current knowledge about auxin signaling, we propose a mechanism of selective autophagy that operates to regulate auxin accumulation and lateral root development under Pi starvation. As per our model, Pi starvation triggers the phosphorylation-mediated activation of PUB9 by ARK2 and subsequently the activated PUB9 selectively targets the AUX/IAA or other repressors of auxin accumulation to the autophagosomes for degradation.…”