Molecular analysis of the developmental mechanisms that establish the body plan of petunia

Flowering plants have developed many ways to arrange their flowers. A flowerbearing branch or system of branches is called an inflorescence. The number of flowers that an inflorescence contains ranges from a single flower to endless flower-clusters. Over the last centuries, botanists have classified inflorescences based on their morphology, which has led to an unfortunate maze of complex botanical terminology. With the rise of molecular developmental biology, research has become increasingly focussed on how inflorescences develop, rather than on their morphology. It is the decisions taken by groups of stem cells at the growing tips of shoots, called meristems, on when and where to produce a flower or a shoot that specify the course of inflorescence development. Modelling is a helpful aid to follow the consequences of these decisions for inflorescence development. The socalled transient model can produce the broad inflorescence types cyme, raceme and panicle, into which most inflorescences found in nature can be classified. The analysis of several inflorescence branching mutants has led to a solid understanding of cymose inflorescence development in petunia (Petunia hybrida). The cyme of petunia is a distinct body plan compared with the well-studied racemes of Arabidopsis and Antirrhinum, which provides an excellent opportunity to study evolutionary developmental biology (evo-devo) related questions. However, thus far, limited use has been made of this opportunity, which is, at least in part, due to researchers getting lost in a maze of botanical terminology. We discuss some general issues here, while focussing on inflorescence development in petunia.