The plant hormone auxin regulates virtually every aspect of plant growth and development. Much of this control occurs by auxin regulating the fundamental processes of cell division, cell expansion, and cell differentiation. Given the prominent role of auxin in these basic cellular events, it is hardly surprising that plant biologists have long been intrigued by this hormone and have compiled an enormous amount of physiological data concerning the responses of plants to applied auxin. Over the past decade, impressive molecular insight into the molecular mechanisms underlying auxin signaling has been obtained, with the SCFTIR1 ubiquitin-ligase complex emerging as a central regulator. In response to auxin, SCFTIR1 catalyzes the ubiquitin-mediated degradation of members of the Aux/IAA protein family. The degradation of these negative regulators of auxin response derepresses the pathway, resulting in auxin-mediated changes in gene expression and plant growth and development.

The large family of Small Auxin Up RNA (SAUR) genes are among the most rapidly and strongly auxin-induced genes.  A primary focus of our current research is the elucidation of SAUR protein function. Utilizing multiple reverse genetic strategies, we have found that many SAUR proteins play important roles in auxin-mediated cell expansion.  Mechanistically, this involves SAUR inhibition of members of the D-subfamily of type 2C protein phosphatases (PP2C.D).  These PP2C.D phosphatases control the phosphorylation status and hence activity of plasma membrane H⁺-ATPases, which for over 40 years have been hypothesized to play a central role in the ‘acid-growth’ theory of auxin-mediated cell expansion. However, molecular/biochemical evidence linking auxin to changes in PM H⁺-ATPase activity has been scant.  Our findings that auxin-induced SAURs inhibit PP2C.D activity to trap PM H⁺-ATPases in the phosphorylated, active state has provided a molecular framework for the acid-growth theory of cell expansion.  Ongoing projects in the lab include the elucidation of SAUR-PP2C.D physical interactions; the identification and characterization of distinct SAUR-PP2C.D regulatory modules and their roles in cell expansion, stomatal regulation, and other processes; phosphoproteomic studies aimed at identifying additional phosphoproteins subject to regulation by SAUR-PP2C.D regulatory modules; and the engineering of plants to modulate SAUR/PP2C.D expression in a tissue-specific manner to alter organ size. 

We are grateful to the NIH and NSF for supporting our work.

Spartz A.K., V.S. Lor^*, H. Ren, N.E. Olszewski, N.D. Miller, G. Wu, E.P. Spalding, and W.M. Gray. 2017. Constitutive expression of Arabidopsis Small Auxin Up RNA19 (SAUR19) in tomato confers auxin-independent hypocotyl elongation. Plant Physiol. 173:1453-62.

Haruta, M., W.M. Gray, and M.R. Sussman. 2015. Regulation of the plasma membrane proton pump (H⁺-ATPase) by phosphorylation. Curr. Opin. Plant Biol. 28: 68-75.

Ren, H., and W.M. Gray.  2015. SAUR proteins as effectors of hormonal and environmental signals in plant growth. Mol Plant 8:1153-64.

Spartz, A.K., H. Ren, M.Y. Park, K.N. Grandt^*, S.H. Lee, A.S. Murphy, M.R. Sussman, P.J. Overvoorde, and W.M. Gray. 2014. SAUR inhibition of PP2C-D phosphatases activates plasma membrane H⁺-ATPases to promote cell expansion in Arabidopsis. Plant Cell 26: 2129-42.

Vi, S.L., G. Trost, P. Lange, H. Czesnick, N. Rao, D. Lieber, T. Laux, W.M. Gray, J.L. Manley, D. Groth, C. Kappel, and M. Lenhard.  2013. Target specificity amongst canonical nuclear poly(A) polymerases in plants modulates organ growth and pathogen response. Proc. Natl. Acad. Sci. USA. 110: 13994-9. 

Spartz, A.K., S.H. Lee, J.P. Wenger, N. Gonzalez, H. Itoh, D. Inze, W.A. Peer, A.S. Murphy, P. Overvoorde, and W.M. Gray. 2012.  The SAUR19 subfamily of SMALL AUXIN UP RNA genes promote cell expansion. Plant J. 70:978-90. 

Franklin, K.A., S.H. Lee, D. Patel, V. Kumar, A.K. Spartz, C. Gu, S. Ye*, P. Yu*, G. Breen, J. D. Cohen, P.A. Wigge, and W.M. Gray. 2011. PHYTOCHROME INTERACTING FACTOR 4 regulates auxin biosynthesis at high temperature. Proc. Natl. Acad. Sci. USA. 108:21231-5.

Parry, G., L.I. Calderon-Villalobos, M. Prigge, B. Peret, S. Dharmasiri, H. Itoh, E. Lechner, W.M. Gray, M. Bennet, and M. Estelle. 2009. Complex regulation of the TIR1/AFB family of auxin receptors. Proc. Natl. Acad. Sci. USA. 106: 22540-22545.

Quint, M., L.S. Barkawi, K.-T. Fan, J.D. Cohen, and W. M. Gray. 2009. Arabidopsis IAR4 modulates auxin response by regulating auxin homeostasis. Plant Physiol. 150: 748-58.

Zhang, W., H. Ito, M. Quint, H. Huang, L.D. Noel, and W.M. Gray. 2008. Genetic Analysis of CAND1-CULl Interactions in Arabidopsis Supports a Role for CAND1-Mediated Cycling of the SCFTIR1 Complex. Proc. Natl. Acad. Sci. USA. 105: 8470-8475.

Harmon, F., T. Imaizumi, and W.M. Gray. 2008. CUL1 regulates TOC1 protein stability in the Arabidopsis circadian clock. Plant J. 55: 568-579.

Ito, H, and W.M. Gray. 2006. A gain-of-function mutation in the Arabidopsis pleiotropic drug resistance transporter PDR9 confers resistance to auxinic herbicides. Plant Physiol. 142: 63-74.

Quint, M., Ito, H., Zhang,W., and W.M. Gray. 2005. Characterization of a novel temperature-sensitive allele of the CUL1/AXR6 subunit of SCF ubiquitin-ligases. Plant J. 43:371-383.

Chuang, H.-w., Zhang, W., and W.M. Gray. 2004 Arabidopsis ETA2, an Apparent Ortholog of the Human Cullin-Interacting Protein CAND1, Is Required for Auxin Responses Mediated by the SCF TIR1 Ubiquitin Ligase. Plant Cell, 16: 1883-1897.

Gray, W.M., P.R. Muskett, H.-w. Chuang, and J. E. Parker. 2003. Arabidopsis SGT1b is required for SCFTIR1 -mediated auxin response. Plant Cell, 15:1310-1319.

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

Gray, W.M., J.C. del Pozo, L. Walker, L. Hobbie, E. Risseeuw, T. Banks, W.L. Crosby, M. Yang, H. Ma, and M. Estelle. 1999. Identification of an SCF ubiquitin-ligase complex required for auxin response in Arabidopsis thaliana. Genes Dev. 13: 1678-1691.

Gray, W.M., A. Ostin, G. Sandberg, C.P. Romano, and M. Estelle. 1998. High temperature promotes auxin-mediated hypocotyl elongation in Arabidopsis. Proc. Natl. Acad. Sci. USA. 95: 7197-7202.