Abstract
Transcription factors play a central role in the regulation and integration of several developmental pathways in all organisms. MADS box proteins are, among transcription factors, key players in the regulation of flower induction, flower architecture and vegetative development and have been isolated and studied in many different plant species. Chapter 2 offers an overview of floral development in several species from which MADS box genes have been analyzed, their function in determining the diversity of floral organs and their role in the evolution of floral morphologies. The rest of the thesis focuses on MADS box transcription factors in Petunia hybrida. The isolation of new genes and their preliminary characterization is described in Chapter 3. Phylogenetic analysis, expression analysis and interaction patterns of all the family members are included in this study. Strategies such as overexpression, cosuppression and knockout via transposon insertion have been adopted for a more detailed functional analysis of a few MADS box genes, as described in Chapter 4. These genes have been selected on the basis of their sequences, expression and interaction patterns which altogether suggested possible roles in floral organ determination, flower induction and vegetative development. A thorough analysis of the genes belonging to the FBP2 subfamily follows in Chapter 5, where it is demonstrated that FBP2 is functionally equivalent to the Arabidopsis SEPALLATA (SEP) proteins on the basis of similarities in sequence, expression, interaction patterns, mutant phenotypes and the functional complementation of the Arabidopsis sep triple mutant with FBP2. In Chapter 6 the function of UNS is studied in detail, by sequence and expression analysis, and overexpression of a full-length and a truncated protein in both petunia and Arabidopsis plants. The function of UNS in petunia is similar to SUPPRESSOR OF OVEREXEPRESSION OF CONSTANS1 (SOC1), a promoter of flowering in Arabidopsis. A possible function of a MADS box complex including C-, D- and E-type proteins in bringing the flower meristem to an end, is proposed in Chapter 7. The hypothesis is supported by the phenotype of transgenic plants simultaneously overexpressing FBP2 and FBP11, which show an early arrest in development and a downregulation of TERMINATOR (TER), the petunia WUS homolog. An upregulation of the petunia C-type gene FBP6 and the D-type FBP7 and FBP11 is also observed. A positive feedback loop among C-, D-, and E- type genes is most likely occurring also within the flower. General considerations about the strategies adopted in this study, the relevance of such a research in the understanding of diversity and conservation in plant development, conclude this thesis.
