Auxin-induced, SCFTIR1-mediated poly-ubiquitination marks AUX/IAA proteins for degradation
Article first published online: 1 APR 2009
© 2009 The Authors. Journal compilation © 2009 Blackwell Publishing Ltd
The Plant Journal
Volume 59, Issue 1, pages 100–109, July 2009
How to Cite
Dos Santos Maraschin, F., Memelink, J. and Offringa, R. (2009), Auxin-induced, SCFTIR1-mediated poly-ubiquitination marks AUX/IAA proteins for degradation. The Plant Journal, 59: 100–109. doi: 10.1111/j.1365-313X.2009.03854.x
- Issue published online: 25 JUN 2009
- Article first published online: 1 APR 2009
- Received 21 November 2008; revised 11 February 2009; accepted 19 February 2009; published online 1 April 2009.
- auxin response;
- transcriptional repressor;
- F-box protein;
- E3 ubiquitin ligase;
- 26S proteasome;
The plant hormone auxin (indole-3-acetic acid or IAA) regulates plant development by inducing rapid cellular responses and changes in gene expression. Auxin promotes the degradation of Aux/IAA transcriptional repressors, thereby allowing auxin response factors (ARFs) to activate the transcription of auxin-responsive genes. Auxin enhances the binding of Aux/IAA proteins to the receptor TIR1, which is an F-box protein that is part of the E3 ubiquitin ligase complex SCFTIR1. Binding of Aux/IAA proteins leads to degradation via the 26S proteasome, but evidence for SCFTIR1-mediated poly-ubiquitination of Aux/IAA proteins is lacking. Here we used an Arabidopsis cell suspension-based protoplast system to find evidence for SCFTIR1-mediated ubiquitination of the Aux/IAA proteins SHY2/IAA3 and BDL/IAA12. Each of these proteins showed a distinct abundance and repressor activity when expressed in this cell system. Moreover, the amount of endogenous TIR1 protein appeared to be rate-limiting for a proper auxin response measured by the co-transfected DR5::GUS reporter construct. Co-transfection with 35S::TIR1 led to auxin-dependent degradation, and excess of 35S::TIR1 even led to degradation of Aux/IAAs in the absence of auxin treatment. Expression of the mutant tir1-1 protein or the related F-box protein COI1, which is involved in jasmonate signaling, had no effect on Aux/IAA degradation. Our results show that SHY2/IAA3 and BDL/IAA12 are poly-ubiquitinated and degraded in response to increased auxin or TIR1 levels. In conclusion, our data provide experimental support for the model that SCFTIR1-dependent poly-ubiquitination of Aux/IAA proteins marks these proteins for degradation by the 26S proteasome, leading to activation of auxin-responsive gene expression.