Relearning our ABCs: new twists on an old model.

Over the past decade, the ABC model of flower development has been widely promulgated. However, correct flower-organ development requires not only the ABC genes but also the SEPALLATA genes. When the SEPALLATA genes are expressed together with the ABC genes, both vegetative and cauline leaves are converted to floral organs. Most of the ABC genes and all three SEPALLATA genes encode MADS transcription factors, which bind to DNA as dimers. Here, amendments to the ABC model are considered that incorporate both the SEPALLATA genes and the ability of MADS proteins to form higher-order complexes.