My work is largely focused on the evolution of ABC program, a genetic module that controls the establishment of floral organ identity. This model describes how the overlapping domains of three classes of genes function to establish floral organ identity by producing a combinatorial code such that A function alone encodes sepal identity; A and B function, petal identity; B and C function, stamen identity; and C function alone, carpel identity (figure below). Genetic analyses of mutations in A, B and C class genes from Arabidopsis and Antirrhinum indicate that this program is functioning similarly in both model species, despite their differences in floral morphology and phylogenetic position (Carpenter and Coen, 1990; Bowman et al., 1991). These mutants exhibit homeotic phenotypes, each displaying a transformation of floral organ identity in two adjacent whorls. In mutants in the B group genes, for instance, petals are transformed into sepals and stamens, into carpels (Bowman et al., 1989). When the genes corresponding to the three classes of mutants were cloned it was found that Arabidopsis genes from each class are homologous to those in the corresponding classes of Antirrhinum (Coen and Meyerowitz, 1991). Furthermore, all but one of the so-called ABC genes are members of the MADS-box family of transcription factors, specifically MIKC-type MADS box genes (reviewed Theissen et al., 2000). More recently, an additional class of these genes, the SEPALLATAs (SEPs or E class genes) have been shown to be necessary for the proper functioning of the original ABC genes (Pelaz et al., 2000; Honma and Goto, 2001).
In a series of elegant genetic experiments using both mutants and lines exhibiting ectopic expression of the ABC genes, it has been shown that the identities of all the floral organs in an Arabidopsis flower are interchangeable and depend entirely on the ABC code expressed in the developing primordia (Bowman et al., 1991; Mizukami and Ma, 1992; Krizek and Meyerowitz, 1996). On the whole, comparative studies suggest that while the B and C components of the program are generally conserved, they are not static entities and have been impacted significantly by patterns of gene duplication and divergence (reviewed Litt and Kramer 2010; Rijpkema et al. 2010). Moreover, across the angiosperms there are many examples of novel floral organ types that do not fit simply into the ABC model. Even less is known about novel genetic programs that may act in parallel or downstream of organ identity in order to produce complex organ elaborations. This raises a series of questions that our lab is interested in tackling. These include:
Bowman, J.L., Smyth, D.R., and Meyerowitz, E.M. (1989) Genes directing flower development in Arabidopsis. Plant Cell, 1:37-52.
Bowman, J.L., Smyth, D.R., and Meyerowitz, E.M. (1991) Genetic interactions among floral homeotic genes of Arabidopsis. Development, 112:1-20.
Carpenter, R., and Coen, E.S. (1990) Floral homeotic mutations produced by transposon-mutagenesis in Antirrhinum majus. Genes and Development, 4:1483-1493.
Coen, E.S., and Meyerowitz, E.M. (1991) The war of the whorls: genetic interactions controlling flower development. Nature, 353:31-37.
Honma, T., and Goto, K. (2001) Complexes of MADS-box proteins are sufficient to convert leaves into floral organs. Nature, 409:525-529.
Krizek, B.A., and Meyerowitz, E.M. (1996) The Arabidopsis homeotic genes APETALA3 and PISTILLATA are sufficient to provide the B class organ identity function. Development, 122:11-22.
Litt, A. and Kramer, E.M. (2010) The ABC model and the diversification of floral organ identity. Seminars in Cell & Developmental Biology, 21:129-137.
Mizukami, Y., and Ma, H. (1992) Ectopic expression of the floral homeotic gene AGAMOUS in transgenic Arabidopsis plants alters floral organ identity. Cell, 71:119-131.
Pelaz, S., Ditta, G.S., Baumann, E., Wisman, E., and Yanofsky, M. (2000) B and C floral organ identity functions require SEPALLATA MADS-box genes. Nature, 405:200-203.
Rijpkema, A. S., Vandenbussche, M., Koes, R., Heijmans, K. and Gerats, T. (2010) Variations on a theme: Changes in the floral ABCs in angiosperms. Seminars in Cell & Developmental Biology, 21:100-107.
Theissen, G., Becker, A., Di Rosa, A., Kanno, A., Kim, J.T., Munster, T., Winter, K.-U., and
H., Saedler. (2000) A short history of MADS-box genes in plants. Plant Molecular Biology, 42:115-149.