Molecular Genetics of Hormone Action in Arabidopsis
 

We are interested in understanding the molecular mechanisms of plant hormone action. Our favorite hormone is a small molecule called auxin, known to regulate many aspects of plant growth and development through effects on cell division and cell elongation. Our favorite organism is the small crucifer Arabidopsis thaliana, a plant that is particular amenable to genetic studies. In addition, the sequence of the Arabidopsis genome has been determined, enabling a variety of genomic and proteomic strategies.

Regulation of Auxin Response
Our studies have shown that auxin response requires the degradation of a large family of transcriptional repressors called the Aux/IAA proteins. This degradation involves a highly conserved protein degradation system called the ubiquitin-proteasome pathway. When auxin levels are low the Aux/IAA proteins prevent the transcription of auxin-regulated genes. When cells are exposed to auxin, a protein complex called SCF^TIR1, a component of the ubiquitin-proteasome pathway, recognizes the Aux/IAA proteins and promotes their degradation. We are now focusing on how SCF^TIR1 is regulated. In addition, we are interested in identifying proteins that act upstream of SCF^TIR1 in auxin signal transduction.

Genomic Approaches to Auxin Biology
As part of our effort to understand how auxin regulates cell growth, we are studying genome-wide changes in gene expression that are regulated by auxin. By characterizing auxin-regulated gene expression in mutants affected in different aspects of hormone action, we expect to identify the genetic networks active during auxin-induced cell growth. In addition, these studies will identify signaling functions required for auxin response and novel auxin-regulated genes that mediate the cellular responses to auxin.

Ubiquitin Protein Ligases in Plant Growth and Development
Many recent studies in plants, animals, and fungi have shown that the ubiquitin-proteasome pathway has a very important role in cellular regulation. In Arabidopsis about 5% of the genes in the genome encode proteins that function in this pathway. Over 600 of these genes encode the specificity component of SCF-type ubiquitin-protein ligases (called F-box proteins). We are performing biochemical as well as forward and reverse-genetic studies to determine the function of a subset of these proteins. This project is part of a larger effort involving 3 other labs around the country.

Regulation of SCFTIR1 assembly and activity through RUB conjugation and deconjugation. The functional SCFTIR1 is represented by stage (1). AUX/IAA proteins interact with TIR1 and are ubiquitinated in the presence of auxin. Removal of RUB by CSN will release E2 (2) and stimulate the association of CAND1. This will dissociate ASK1 and TIR1 from the SCFTIR1 complex (3). Association of ASK-TIR1 with CUL1 will dislodge CAND1 allowing CUL1 to be modified by RUB1. At low auxin levels, degradation of AUX/IAA proteins may occur at a basal rate. At high auxin levels enhanced interaction between AUX/IAAs and TIR1 will result in increased AUX/IAA degradation and increased transcription of auxin regulated genes.