Characterization of the PRE protein family in Arabidopsis and their interactions with Aux/IAAs /

Auxin perception and signaling requires the rapid degradation of the Aux/IAA proteins that negatively regulate ARF (Auxin response factors) transcription factors. The SCFTIR1 complex in the ubiquitin proteasome system (UPS) is required for Aux/IAA degradation through its role as the auxin hormone receptor and as the E3 ligase in the UPS. Any proteins that may play a role in this pathway are important for the study of cell elongation and development. KIDARI (KDR), inducible by auxin in the hypocotyl of the model plant A. thaliana, is a member of the PRE (paclobutrazol resistant) family. The PRE family of atypical bHLHs do not bind to DNA directly and instead interrupt the homo-/heterodimerization of other atypical bHLHs or canonical bHLHs. Using a yeast-2- hybrid (Y2H) and bimolecular fluorescence complementation (BiFC), KDR, PRE1, PRE2, and PRE5 are shown to interact with various Aux/IAAs. A mutation in KDR, made by a change in the conserved leucine residue in position 29 (KDRL29E), results in a loss of interaction with Aux/IAAs. Additionally, complementation of the kdr-2 mutant phenotype shows that the short hypocotyls observed in the mutant are indeed due to a loss of KDR. The GUS reporter gene was used to characterize the expression patterns of the KDR and PRE5 promoters and proteins. The GUS reporters revealed expression in the hypocotyls, consistent with a role for these genes in promoting growth. However, it is possible that regulatory elements that facilitate spatial patterning may be absent in the transcriptional fusions of KDR and PRE5