Cracking the Floral Code- ABC model - flower formation

Three Types of Genes Regulate Floral Development-Mutations have identified three classes of genes that regulate floral development: floral organ identity genes, cadastral genes, and meristem identity genes Floral meristem identity genes initiate a cascade of gene expression that turns on region specifying (cadastral) genes, which further specify pattern by initiating transcription of floral organ identity genes 1. Floral organ identity genes directly control floral identity. The proteins encoded by these genes are transcription factors that likely control the expression of other genes whose products are involved in the formation and/or function of floral organs. Three classes (A, B, and C) of organ identity genes are necessary to specify the four whorls of floral organs They are homeotic genes and include AP2, AGAMOUS (AG), AP3, and PISTILLATA (PI) in Arabidopsis. Most plant homeotic genes belong to a class of related sequences known as MADS box genes When all of these homeotic genes are not expressed in a developing flower, floral parts become leaflike The D-class proteins interact in larger complex with the E-class proteins to specify ovule identity. In the Arabidopsis genome, four class E genes have been found,  SEPALLATA1 (SEP1), SEP2, SEP3 and SEP4, which show partially redundant functions in identity determination of sepals, petals, stamens and carpels  Class A genes (AP2) alone specify sepal development. Class A genes and class B genes (AP3 and PI) together specify petals Class B and class C (AG) genes are necessary for stamen formation Class C genes alone specify carpel formation.