Abstract
Auxin/indole-3-acetic acid (Aux/IAA) proteins function as repressors of auxin response gene expression when auxin concentrations in a cell are low. At elevated auxin concentrations, these repressors are destroyed via the ubiquitin-proteasome pathway, resulting in derepression/activation of auxin response genes. Most Aux/IAA repressors contain four conserved domains, with one of these being an active, portable repression domain (domain I) and a second being an auxin-dependent instability domain (domain II). Here, we have analyzed the effects of amino acid substitutions in the repression domain of selected Aux/IAA proteins. We show that stabilized versions of Aux/IAA proteins with amino acid substitutions in domain I display contrasting phenotypes when expressed in transformed Arabidopsis (Arabidopsis thaliana) plants. An alanine-for-leucine substitution in the LxLxL (where L is leucine and x is another amino acid) repression domain of IAA3, IAA6, or IAA19 confers enhanced auxin response gene expression and "high-auxin" phenotypes when expressed from the 35S or IAA19 promoter (as tested with IAA19) in transformed Arabidopsis plants. In marked contrast, a single alanine-for-leucine substitution in domain I of IAA12 or IAA17 confers repression of auxin response genes and "low-auxin" phenotypes. These results point to intrinsic differences in the repression domain(s) of IAA proteins and suggest that some IAA proteins have stronger or more complex repression domains than others.
Highlights
Auxin/indole-3-acetic acid (Aux/IAA) proteins function as repressors of auxin response gene expression when auxin concentrations in a cell are low
We have previously reported that mutations in domain II of Aux/IAA proteins, including IAA3/ SHORT HYPOCOTYL2 (SHY2), IAA17/AUXIN RESISTANT3 (AXR3), and IAA19/MASSUGU2 (MSG2), Plant PhysiologyÒ, March 2011, Vol 155, pp. 1252–1263, www.plantphysiol.org Ó 2011 American Society of Plant Biologists Downloaded from on October 5, 2020 - Published by www.plantphysiol.org
To evaluate the effects of mutations in the repression domain of Arabidopsis IAA proteins, we examined five different IAA proteins (i.e. IAA3, IAA6, IAA12, IAA17, and IAA19) that fall into distinct clades on a phylogenetic tree (Remington et al, 2004)
Summary
Auxin/indole-3-acetic acid (Aux/IAA) proteins function as repressors of auxin response gene expression when auxin concentrations in a cell are low. An alanine-forleucine substitution in the LxLxL (where L is leucine and x is another amino acid) repression domain of IAA3, IAA6, or IAA19 confers enhanced auxin response gene expression and “high-auxin” phenotypes when expressed from the 35S or IAA19 promoter (as tested with IAA19) in transformed Arabidopsis plants. It was further shown that these responses occurred in an auxin-independent manner (i.e. free IAA hormone concentrations were not altered in the transgenic lines compared with the wild type) These results supported earlier observations made with transfected protoplasts where IAA17 functioned as a transcriptional repressor of auxin response gene expression that could be converted to a transcriptional activator by fusing a VP16 activation domain onto an IAA17 protein with a compromised repression domain
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