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Genetic analyses of auxin signaling in Arabidopsis
Bonnie Bartel, Professor
Department of Biochemistry and Cell Biology
bartel@rice.edu

The phytohormone auxin promotes cell expansion, division, and differentiation, thereby regulating a plethora of developmental events and environmental responses.  We use Arabidopsis mutants to identify genes providing inputs to the auxin pool, to distinguish precursors from active auxin, and to disentangle contributions of different auxin biosynthetic pathways to development. 

We have identified the enzymes that release auxin from precursors, discovered that the ER and peroxisomes compartmentalize auxin production pathways, and revealed the importance of different auxin sources for lateral root development, cotyledon expansion, root hair lengthening, and hypocotyl elongation.  Moreover, we discovered new classes of regulators of auxin precursor utilization, including components that modulate subcellular metal environments and a transporter family that displays a unique polar localization to the outer face of epidermal cells and effluxes an auxin precursor from roots. 

We also isolated novel auxin-signaling mutants (ecr1, defective in a RUB-activating enzyme; iaa28, a dominant allele of an Aux/IAA transcriptional repressor; and ibr5, defective in a MAP-kinase phosphatase) and exploited these mutants to illuminate the interactions of auxin signaling with other phytohormone pathways and novel facets of auxin signaling.  For example, we found that the ibr5 mutant displays multiple phenotypes suggesting that it responds less than wild type to both exogenous and endogenous auxin.  ibr5 also is less responsive than wild type to abscisic acid and ethylene. Unlike other auxin-response mutants, ibr5 impedes auxin responsiveness without stabilizing Aux/IAA repressors, like IAA28, indicating that IBR5 defines a new regulatory step acting downstream of previously identified auxin signaling modulators. We are taking multiple approaches to identify IBR5 substrates and other IBR5-interacting components, which also may modulate auxin responsiveness.

We continue to isolate and characterize mutants specifically defective in responses to particular auxins, auxin-like molecules, and auxin precursors to distinguish between auxin precursors and active auxins. The phenotypes of these mutants are helping us to assign specific roles to various auxin precursors in plant development.

Lab members with auxin signaling projects:

Post Docs:
Lucia Strader

Graduate students:
Mauro Rinaldi

Undergraduates:
James Liu
Savina Venkova

Former Graduate Students:
Melanie Monroe-Augustus (Ph.D., 2004)
Sarah Ratzel (Ph.D., 2011)
Luise Rogg (Ph.D., 2001)
Andrew Woodward (Ph.D., 2005)

We gratefully acknowledge support for this research from the USDA and the NSF, the NIH (NRSA fellowship and K99 grant to LCS), and Houston Livestock Show and Rodeo Scholarships (AW).

Publications on auxin signaling and auxin precursors:

Other Bartel lab projects:
Auxin conjugates, IBA, peroxisomes

Multiple facets of Arabidopsis seedling development require indole-3-butyric acid-derived auxin.
Strader, L.C., Wheeler, D.L., Christensen, S.E., Berens, J., Cohen, J.D., Rampey, R.A., and Bartel, B. (2011) The Plant Cell 23, 984-999.
Abstract; full text; PDF

Transport and metabolism of the endogenous auxin precursor indole-3-butyric acid.
Strader, L.C. and Bartel, B. (2011) Molecular Plant 4, 477-498. doi: 10.1093/mp/ssr006 (Review Article)
Abstract; full text; PDF

Ethylene directs auxin to control root cell expansion.
Strader, L.C., Chen, G.L., and Bartel, B. (2010) The Plant Journal 64, 874-884.
Abstract; full text; PDF

Conversion of endogenous indole-3-butyric acid to indole-3-acetic acid drives cell expansion in Arabidopsis thaliana seedlings.
Strader, L.C., Culler, A.H., Cohen, J.D., and Bartel, B. (2010) Plant Physiology 153, 1577-1586.
Abstract; full text; PDF

Arabidopsis PIS1 encodes the ABCG37 transporter of auxinic compounds including the auxin precursor indole-3-butyric acid.
Ruzicka, K., Strader, L.C., Bailly, A., Yang, H., Blakeslee, J., Langowski, L., Nejedla, E., Fujita, H., Itoh, H., Syono, K., Hejatko, J., Gray, W.M., Martinoia, E., Geisler, M., Bartel, B., Murphy, A.S., and Friml, J. (2010) Proc. Natl. Acad. Sci. USA 107, 10749-10753.
Abstract; full text; PDF

Silver ions increase auxin efflux independently of effects on ethylene response.
Strader, L.C., Beisner, E.R., and Bartel, B. (2009) The Plant Cell 21, 3585-3590.
Abstract; full text; PDF

The Arabidopsis PLEIOTROPIC DRUG RESISTANCE8/ABCG36 ATP binding cassette transporter modulates sensitivity to the auxin precursor indole-3-butyric acid.  
Strader, L.C. and Bartel, B. (2009) The Plant Cell 21, 1992-2007.
Abstract; full text; PDF

Arabidopsis iba response5 (ibr5) suppressors separate responses to various hormones.
Strader, L.C., Monroe-Augustus, M., Rogers, K.C., Lin, G.L., and Bartel, B. (2008) Genetics180, 2019-2031.
Abstract; full text; PDF

The IBR5 phosphatase promotes Arabidopsis auxin responses through a novel mechanism distinct from TIR1-mediated repressor degradation. 
Strader, L.C., Monroe-Augustus, M., Bartel, B. (2008) BMC Plant Biology 8, 41.
Abstract; full text; PDF

A new path to auxin. 
Strader, L.C, Bartel, B. (2008) Nature Chemical Biology 4, 337-339.
full text

Mutation of E1-CONJUGATING ENZYME-RELATED1 decreases RELATED TO UBIQUITIN conjugation and alters auxin response and development. 
Woodward, A.W., Ratzel, S.E., Woodward, E.E., Shamoo, Y., and Bartel, B.  (2007) Plant Physiology 144, 976-987.
Abstract; full text; PDF

A receptor for auxin.
Woodward, A.W. and Bartel, B. (2005) Plant Cell 17, 2425-2429.
full text

MicroRNA-directed regulation of Arabidopsis AUXIN RESPONSE FACTOR17 is essential for proper development and modulates expression of early auxin response genes.
Mallory, A.C., Bartel, D.P., and Bartel, B. (2005) Plant Cell 17, 1360-1375. (On the cover)
Abstract; full text

Auxin: regulation, action, and interaction. 
Woodward, A.W. and Bartel, B. (2005) Annals of Botany,95, 707-735.
Abstract; full text

An auxin transport independent pathway is involved in phosphate stress-induced root architectural alterations in Arabidopsis. Identification of BIG as a mediator of auxin in pericycle cell activation.
Lopez-Bucio, J., Hernandez-Abreu, E., Sanchez-Calderon,  L., Perez-Torres, A., Rampey, R.A., Bartel, B., and Herrera-Estrella, L. (2005) Plant Physiology 137,681-691.
Abstract; full text

IBR5, a dual-specificity phosphatase-like protein modulating auxin and abscisic acid responsiveness in Arabidopsis. 
Monroe-Augustus, M., Zolman, B.K., and Bartel, B. (2003) Plant Cell 15, 2979-2991.
Abstract; full text; PDF

A gain-of-function mutation in IAA28 suppresses lateral root development. 
Rogg, L.E., Lasswell, J. and Bartel, B. (2001) Plant Cell 13, 465-480.
Abstract; full text; PDF

Auxin signaling: Derepression through regulated proteolysis
Rogg, L.E. and Bartel, B. (2001) Developmental Cell 1, 595-604.  (Review Article)
Abstract; full text; PDF

 
Biochemistry