Professor and Miller Chair in Plant Developmental Biology
Ph.D., University of Alberta, 1983

Program Affiliation: Molecular Biology & Genetics | Plant Biology

Research Groups Affiliation: Biochemistry | Cell Biology | Development | Genetics |Genomics & Bioinformatics | Plant Biology

Phone: 812-856-1216
Fax: 812-855-6082
Email Mark

Estelle Lab Website

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.

Dharmasiri, N., Dharmasiri, S., and M. Estelle. 2005. The F-box protein TIR1 is an auxin receptor. Nature 435:441-445

N. Dharmasiri, S. Dharmasiri, D. Weijers, E. Lechner, M. Yamada, L. Hobbie, J. S. Ehrismann, G. Jürgens, and M. Estelle. 2005. Plant development is regulated by a family of auxin receptor F-box proteins. Dev Cell, 9:109-119.

L. Navarro, P. Dunoyer, F.Jay, B. Arnold, N. Dharmasiri, M. Estelle, O.Voinnet, J. D.G. Jones. 2006. An Inducible MiRNA Contributes to Arabidopsis Basal Resistance by Repressing Auxin Signaling. Science, 312:436-439

Parry, G., Ward, S., Cernac, A. Dharmasiri, S., and M. Estelle. 2006. The Arabidopsis SUPPRESSOR OF AUXIN RESISTANCE proteins are nucleoporins with important roles in hormone signaling and development. Plant Cell, 18:1590-1603.

Dharmasiri, S., Swarup, R., Mockaitis, K., Dharmasiri, N., Singh, S.K., Kowalchyk, M., Marchant, A., Mills, S., Sandberg, G., Bennett, M., and M. Estelle. 2006 AXR4 is required for localization of the auxin influx facilitator AUX1. Science, 312:1218-1220.

Dharmasiri, S., Dharmasiri, N., Hellmann, H., and M. Estelle. 2003 The RUB/Nedd8 conjugation pathway is required for auxin response and early development in Arabidopsis. EMBO J 22:1762-70.

Hellmann, H., Hobbie, L., Chapman, A., Dharmasiri, S., Dharmasiri, N., del Pozo, C., Reinhardt, D., and M. Estelle 2003. AXR6 encodes CUL1 implicating SCF ubiquitin ligases in auxin regulation of embryogenesis. EMBO J. 22:3314-3325

Dharmasiri, N., Dharmasiri, S., Jones, A.M., and M. Estelle. 2003,  Auxin Action in a Cell-Free System. Current Biology, 13(16):1418-22.

Hellmann, H. and M. Estelle.  2002.  Plant Development:  Regulation by Protein Degradation.  Science 297:793-797.

Gray, W. M., Kepinski, S., Rouse, D., Leyser, O., and M. Estelle.  2001.  Auxin Regulates SCFTIR1-Dependent Degradation of the AUX/IAA Proteins.  Nature 414:271-276.