Auxin signaling involves regulated protein degradation by the ubiquitin-proteasome pathway

Ubiquitin-mediated protein degradation plays a critical role in the regulation of diverse biological processes in eukaryotes (Hershko and Ciechanover 1998). Recent studies in Arabidopsis also implicate the ubiquitin proteolytic system in auxin response. A combination of genetic and molecular approaches has resulted in the identification of components of a common auxin-response pathway. Mutations in either AXR1 or TIR1 result in decreased auxin response and a variety of auxin-related growth defects. The AXR1 gene encodes a subunit of a RUB-activating enzyme analogous to the El ubiquitin-activating enzyme (del Pozo and others 1998). AXR1 functions as a heterodimer with ECR1 to activate RUB, a ubiquitin-related protein. RUB is conjugated to the cullin CUL1 in an AXR1-dependent manner. CUL1 is a component of an E3-ubiquitin ligase SCF complex along with a Skp1-like gene (ASK1), RBX1, and an F-box protein (Gray and others 1999). The TIR1 gene encodes an F-box protein, and recent data have demonstrated that the role of SCFTIR1 is to degrade one or more negative regulators of auxin response. Further, RUB modification of CUL1 is required for normal SCFTIR1 function. The Aux/IAA genes encode short-lived nuclear proteins that repress auxin-regulated gene expression, possibly through interaction with members of the ARF family of transcription factors. Genetic and biochemical studies have revealed that members of the Aux/IAA family are substrates for SCFTIR1 and that auxin regulates transcription of downstream genes by promoting degradation of the Aux/IAA proteins.