Molecular network connected to flower type differentiation in Gerbera hybrida

Evolutionary developmental biology aims at explaining the morphological diversity observed in nature. This study focuses on exploring the regulatory networks connected with flower type differentiation in Asteraceae using Gerbera hybrida as a model. The inflorescence of gerbera is a complex structure composed of morphologically different types of flowers. This is a unique trait that cannot be studied using the classical model species with only single flower forms in their inflorescences. Our aim is to understand how this floral complexity originated during evolution of flowering plants and to identify key regulators that determine the development of various flower types. Plant specific TCP domain transcription factors function as key developmental regulators controlling e.g. shoot branching, flower symmetry and leaf development. We recently identified a novel function for them and showed the first molecular evidence that TCP factors have been ecruited to regulate the complex inflorescence structure in Asteraceae (Broholm et al., PNAS 2008). Interestingly, the CYC/TB1-clade of TCP genes has expanded in Asteraceae dating back to whole genome duplication events. In contrast to only three, partly redundant genes in Arabidopsis, both gerbera and sunflower contain ten gene family members. We suspect that these genes have experienced sub- and/or neofunctionalization contributing to the unique characteristics of the Asteraceae inflorescence.

In this project we explore the evolution and functional roles of gerbera CYC/TB1-like factors by combining expression analyses, phylogenetic approaches and functional studies using transgenic plants. Most importantly, we will focus on the largely unknown molecular network (coregulators, target genes and upstream regulatory elements) connected with them. Yeast two hybrid screens will be completed to characterize the protein complexes involving gerbera CYC-like proteins and to study their role in flower type differentiation. This includes characterization of their target genes using oligonucleotide microarrays and transgenic plants expressing inducible transgene constructs. Currently, nothing is known of the upstream signals that lead to the radial organization of the inflorescence, and that define the differential expression domains for CYC-like genes. Using bioinformatics analysis including several asteraceous species, we will characterize the candidate regulatory domains in CYC-like promoters and directly screen for proteins binding to them. As a first step to understand the role of plant hormones in specifying positional information along the inflorescence radius, we develop tools to visualize and modify hormonal signalling during early stages of development. With these approaches, our aim is to extend the current knowledge on regulation of flower development and on regulatory networks connected to TCP genes that have been shown to be involved in evolution of developmental novelties in plants.