ABC model a model for specification of floral organs especially in Arabidopsis thaliana. It views the floral primordium as comprising four whorls whose developmental fate is determined by the concentric and combinatorial activity of three classes of gene, denoted A, B, and C, which encode transcription factors. Class A determines the fate of whorls 1 and 2 (sepals and petals, respectively) and requires the APETALA2 gene (AP2); class B determines whorls 2 and 3 (petals and stamens, respectively) and requires the PISTILLATA (PI) and APETALA3 (AP3) genes; class C determines whorl 4 (carpels) and requires the AGAMOUS gene (AG). These genes are described as ‘homeotic’ even though they encode transcription factors that contain a MADS box instead of homeobox domains. Homologues of these genes occur in other plants.
