The ABC model of flower development

(a) ABC gene expression precedes floral organ formation APETALA1 (AP1) APETALA3 (AP3) PISTILLATA (Pl) AGAMOUS (AG) A + B + C class A class (whorls 1 + 2) B class (whorls 2 + 3) C class (whorls 3 + 4) (whorls 1, 2, 3, + 4) (b) The ABC model Genes . . . Distribution of the ABC gene products in the four floral B i + C whorls determines the identity 8 g of organs in the whorl. Complex of MADS box proteins in ”SE , air @Eyp @Ep The four SEPALLATA proteins the floral whorls Egg (SE9 provide transcriptional activa- ¢ ‘ ¢ ¢ tor activity. Organ identity Sepals Petals Stamens Carpels Stamens Petals Sepals Floral whorl 1 2 3 4 3 2 1