mechanisms that govern pattern formation during plant development
	Vivian Irish, Ph.D. Professor of Molecular, Cellular & Developmental Biology and Ecology & Evolutionary Biology Email: vivian.irish@yale.edu Room: OML 252A Phone: (203) 432-5572/ (203) 432-5571 Web site B.A. Wesleyan University 1980; Ph.D. Harvard University 1986

A wild type Arabidopsis flower.

AP3 expression in the Arabidopsis inflorescence as visualized by an AP3 promoter:GUS reporter gene. AP3 expression is first seen in very young flower buds as a ring encircling the meristem corresponding to the presumptive petal and stamen primordia; later in flower development, AP3 expression is seen in these organs as they mature. Lower levels of expression appear pink, higher levels of expression appear blue. We are also investigating the evolution of molecular genetic mechanisms controlling floral form in other plant species. Normally poppy flowers contain four petals, but in this poppy homeotic mutant, some stamens are converted to petals, resulting in multiple extra petals.

Flowers have a very regular architecture, yet floral form varies immensely between different species. We are interested in understanding how the stereotypical floral pattern arises, as well as how these developmental processes have been modulated in different plant species. We are utilizing Arabidopsis for many of these investigations; Arabidopsis has a short generation time, it can be transformed and genetically manipulated, and the Arabidopsis genome is the first plant genome to be completely sequenced, making it an ideal system in which to study pattern formation.

We are focusing on analyzing Arabidopsis petal development as a paradigm for organogenesis and pattern formation in plants. Petals are composed of only a few cell types, yet a complex series of cell divisions and differentiation processes need to take place to orchestrate the formation of this organ. We are investigating the roles of several genes in regulating these processes. These include two homeotic genes, APETALA3 (AP3) and PISTILLATA (PI), which encode MADS-box containing transcription factors that are required to specify petal identity. Using molecular, biochemical and genetic techniques, we are examining both how homeotic gene expression patterns are initially established in the floral meristem, as well as how AP3, PI and a number of other transcription factors regulate petal growth, development, and differentiation.

In addition to our work on Arabidopsis, we are interested in understanding the underlying basis of the tremendous variability in floral form in the angiosperms. To this end, we are characterizing homologs of the AP3 and PI genes from a variety of other species. In particular, we are interested in understanding whether the functions of homologs of these genes have been conserved, and to that end we are developing genetic tools that can be used in non-model systems. By carrying out functional analyses in non-model angiosperm species, we can determine the degree to which the developmental circuitry specifying floral organ identity is similar among species. These studies are part of a larger effort to characterize the changing roles of MADS domain containing transcription factors in regulating floral development in different angiosperm species.

By combining both genetic and molecular approaches to the study of floral development, we hope to elucidate how dividing floral meristematic cells acquire information about their position and then differentiate accordingly, as well as how these processes may have been modulated during evolution.

Selected Publications

Zik, M., and Irish, V.F. (2003). Global identification of target genes regulated by APETALA3 and PISTILLATA floral homeotic gene action. Plant Cell, 15: 207-222.

Drea, S., Hileman, L.C., de Martino, G., and Irish, V.F. (2007). Functional analyses of genetic pathways controlling petal specification in poppy. Development 134: 4157-4166.

Chae, E., Tan, Q. K-G., Hill, T.A. and Irish, V.F. (2008). An Arabidopsis F-box protein acts as a transcriptional cofactor to regulate floral development. Development 135: 1235-1245.

			The Arabidopsis AP3 and PI proteins are both required for petal and stamen identity and function as a heterodimeric transcription factor. A. wild type Arabidopsis flower. B. apetala3 (AP3) mutant in which petals are converted to sepals and stamens are converted to carpels. C. pistillata (pi) mutant displays the same homeotic muant phenotype as AP3 mutants. D. AP3 and PI proteins must form a heterodimer to bind to DNA. In this gel-shift assay, AP3 or PI proteins cannot bind to DNA independently, but when mixed together, can bind to target DNA sequences, resulting in a band shift. Because both proteins are required for function, mutation in either AP3 or PI results in the same homeotic transformation.

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