Unlike animals, which make organs only during embryogenesis, plants make organs throughout their lifetime through the action of specialized groups of cells called meristems. Meristems function by setting aside groups of cells to become organ primordia while maintaining a population of undifferentiated cells to support further growth. My lab focuses on axillary meristems, which give rise to branches and flowers and therefore play a fundamental role in plant architecture and reproduction. The goal of our research is to understand how the hormonal signal for axillary meristem initiation is perceived and transmitted to cause the changes in gene expression, cell division and expansion required for the formation of an axillary meristem.

Using a genetic approach, we have identified many mutants with defects in the development of axillary meristems in the maize inflorescence. Characterization of each mutant tells us the biological function of the gene that has been disrupted. Identification of the corresponding gene by positional cloning enables us to determine its mechanism of action using molecular, biochemical and cell biology approaches. Many of the genes defective in these mutants regulate the transport, biosynthesis or signaling of the plant growth hormone auxin, illustrating the importance of this hormone in the process of axillary meristem and organ initiation.

One of the projects we are working on involves the barren inflorescence2 (bif2) mutant, which makes fewer branches and flowers due to defects in the initiation and growth of axillary meristems. bif2 encodes a serine threonine protein kinase expressed in axillary meristems and organ primordia. BIF2 regulates auxin transport by phosphorylating ZmPIN1a, which encodes an auxin efflux transporter. BIF2 also interacts with and phosphorylates BARREN STALK1 (BA1), a bHLH transcription factor that plays an important role in axillary meristem formation. Another project involves the sparse inflorescence1 (spi1) mutant which has a similar phenotype to bif2 in the inflorescence. spi1 encodes an enzyme involved in localized auxin biosynthesis showing that both local synthesis and transport of auxin are required for axillary meristem initiation in the inflorescence. spi1; bif2 double mutants have very severe effects on vegetative development indicating that auxin transport and biosynthesis also play an important role in vegetative development.

Current research uses genetic, genomic, molecular, biochemical, cell biology and physiological approaches to understand the gene regulatory network controlling auxin mediated development in the maize inflorescence. We are currently characterizing and cloning many new mutants with defects similar to spi1 and bif2 as part of an NSF Plant Genome project. See project website www.AuxinEvodevo.org. Through comparison of the role of these genes in maize versus other plant species we are developing an understanding of the evolution of the role of auxin in plant development.

• Researchers find an essential gene for forming ears of corn (Sep 2008)
• The evolution of food plants: genetic control of grass flower architecture (Jan 2006)

Phillips KA, Skirpan AL, Liu X, Christensen A, Slewinski TL, Hudson C, Barazesh S, Cohen JD, Malcomber S, and McSteen P. Vanishing tassel2 Encodes a Grass-Specific tryptophan aminotransferase required for vegetative and reproductive development in maize. Plant Cell 2011;23 (2):550-566.

Woodward JB, Abeydeera ND, Paul D, Phillips K, Rapala-Kozik M, Freeling M, Begley TP, Ealick SE, McSteen P, and Scanlona MJ. A maize thiamine auxotroph is defective in shoot meristem maintenance. Plant Cell 2010;22(10):3305-3317.

McSteen P. Auxin and monocot development. Cold Spring Harbor Perspectives in Biology 2010;2(3):a001479.

Wu X, Skirpan A and McSteen P. Suppressor of sessile spikelets1 functions in the ramosa pathway controlling meristem determinacy in maize. Plant Physiology 2009;149(1):205-219.

Skirpan A, Culler AH, Gallavotti A, Jackson D, Cohen JD and McSteen P. BARREN INFLORESCENCE2 interaction with zmPIN1a suggests a role in auxin transport during maize inflorescence development. Plant and Cell Physiology 2009;50(3):652-657.

Phillips KA, Skirpan AL, Kaplinsky NJ and McSteen P. Developmental disaster1: A novel mutation causing defects during vegetative and inflorescence development in maize (Zea mays, Poaceae). American Journal of Botany 2009;96(2):420-430.

McSteen P. Hormonal regulation of branching in grasses. Plant Physiology 2009;149(1):46-55.

Barazesh S, Nowbakht C and McSteen P. sparse inflorescence1, barren inflorescence1 and barren stalk1 promote cell elongation in maize inflorescence development. Genetics 2009;182(1):403-406.

Skirpan A, Wu X and McSteen P. Genetic and physical interaction suggest that BARREN STALK1 is a target of BARREN INFLORESCENCE2 in maize inflorescence development. Plant Journal 2008;55(5):787-797.

McSteen P and Zhao Y. Plant Hormones and signaling: Common themes and new developments. Developmental Cell 2008;14(4):467-473.

Gallavotti A, Barazesh S, Malcomber S, Hall D, Jackson D, Schmidt RJ and McSteen P. sparse inflorescence1 encodes a monocot-specific YUCCA-like gene required for vegetative and reproductive development in maize. Proceedings of the National Academy of Sciences of the United States of America 2008;105(39):15196-15201.

Barazesh S and McSteen P. Barren inflorescence1 functions in organogenesis during vegetative and inflorescence development in maize. Genetics 2008;179(1):389-401.

Barazesh S and McSteen P. Hormonal control of grass inflorescence development. Trends in Plant Science 2008;13(12):656-662.

Wu X and McSteen P. The role of auxin transport during inflorescence development in maize (Zea mays, Poaceae). American Journal of Botany 2007;94(11):1745-1755.

McSteen P, Malcomber S, Skirpan A, Lunde C, Wu X, Kellogg E and Hake S. barren inflorescence2 encodes a co-ortholog of the Pinoid serine/threonine kinase and is required for organogenesis during inflorescence and vegetative development in maize. Plant Physiology 2007;144(2):1000-1011.

McSteen P. Branching out: The ramosa pathway and the evolution of grass inflorescence morphology. Plant Cell 2006;18(3):518-522.

Keck E, McSteen P, Carpenter R and Coen E. Separation of genetic functions controlling organ identity in flowers. EMBO Journal 2003;22(5):1058-1066.

Hubbard L, McSteen P, Doebley J and Hake S. Expression patterns and mutant phenotype of teosinte branched1 correlate with growth suppression in maize and teosinte. Genetics 2002;162(4):1927-1935.

McSteen P and Hake S. barren inflorescence2 regulates axillary meristem development in the maize inflorescence. Development 2001;128(15):2881-2891.

McSteen P, Laudencia-Chingcuanco D and Colasanti J. A floret by any other name: Control of meristem identity in maize. Trends in Plant Science 2000;5(2):61-66.

McSteen PCM, Vincent CA, Doyle S, Carpenter R and Coen ES. Control of floral homeotic gene expression and organ morphogenesis in Antirrhinum. Development 1998;125(13):2359-2369.

McSteen P and Hake S. Genetic control of plant development. Current Opinion in Biotechnology 1998;9(2):189-195.