While the master regulators of floral organ identity have been identified in multiple plantspecies, it remains poorly understood how the downstream transcriptional programs finally lead to the development of the different floral organs, and how evolutionary variations in these programs have yielded the astonishing floral architectural diversity existing in nature. The main objective of my PhD work was to start to address these fundamental questions by analysing floral development in Petunia x hybrida, chosen as model for its elaborate petal architecture combined with the availability of a powerful genetics toolkit. My research started with the identification of the petal transcriptome composition acting downstream of the homeotic genefunctions (Chapter 1). To achieve this, we used an RNAseq strategy on young flowers from a unique collection of floral homeotic mutants, complemented with wild-type samples. We finallyobtained a list of more than 400 potentially interesting genes involved in petal development. Toprovide a detailed analysis for Petunia petal development we used a reverse genetics approachand selected 95 genes expressed during petal development for functional analysis by transposonmutagenesis. I also introduced the CRISPR-Cas9 technology in the team (Chapter 3), targeting3 petal candidate genes for which no transposon insertions in their coding sequence were found. Unfortunately, we did not manage to find eye-catching defects in petal development linked tothe selected mutations. However, in the population generated for the petal reverse genetics screen we encountered a small family in which a mutation segregated causing a novel floral developmental defect strongly affecting petal and stamen development. We confirmed that this mutation was unrelated to the petal candidate gene initially targeted, and by a forward genetic approach we demonstrated that it was instead caused by a different transposon insertion in anR2R3-MYB transcription factor (Chapter 2). With the CRISPR-Cas9 technology I also targetedsome interesting genes involved in flower development like the C-class gene PMADS3. Iobtained KO mutants, and this result was part of a paper (Morel et al., 2018) and allowed a detailed description of the C-class genes function in petunia (Chapter 3). In the last part, we investigated how tube and limb development of Petunia petals depend on the cell-layer specificaction of a MADS-box transcription factor. This allowed to define the contribution of the differentcell-layers in petal development (Chapter 4). Put together, my PhD work should provide a better understanding of floral organ development and architectural diversity.