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The  INDOLE-3-BUTYRIC ACID RESPONSE5  ( IBR5 ) gene encodes a dual specificity phosphatase that regulates plant auxin responses.  IBR5  has been predicted to generate two transcripts through alternative splicing, but alternative splicing of  IBR5  has not been confirmed experimentally. The previously characterized  ibr5-1  null mutant exhibits many auxin related defects such as auxin insensitive primary root growth, defective vascular development, short stature and reduced lateral root development. However, whether all these defects are caused by the lack of phosphatase activity is not clear. Here we describe two new auxin insensitive  IBR5  alleles,  ibr5-4 , a catalytic site mutant, and  ibr5-5 , a splice site mutant. Characterization of these new mutants indicates that  IBR5  is post-transcriptionally regulated to generate two transcripts,  AT2G04550.1  and  AT2G04550.3 , and consequently two IBR5 isoforms, IBR5.1 and IBR5.3. The IBR5.1 isoform exhibits phosphatase catalytic activity that is required for both proper degradation of Aux/IAA proteins and auxin-induced gene expression. These two processes are independently regulated by IBR5.1. Comparison of new mutant alleles with  ibr5-1  indicates that all three mutant alleles share many phenotypes. However, each allele also confers distinct defects implicating IBR5 isoform specific functions. Some of these functions are independent of IBR5.1 catalytic activity. Additionally, analysis of these new mutant alleles suggests that  IBR5  may link ABP1 and SCF TIR1/AFBs  auxin signaling pathways.

Alternative Splicing of Arabidopsis IBR5 Pre-mRNA Generates Two IBR5 Isoforms with Distinct and Overlapping Functions