Flower organs form in concentric rings, called whorls, starting with the outer whorl. Sepals form first, followed by petals, and then by stamen, and finally by carpels.

Below is an image of a wild type Arabidopsis thaliana flower, and a diagram of the flower outlining the four whorls. Wild type Arabidopsis flowers have, from outside to inside, 4 sepals (which are not clearly visible in the photograph), 4 petals, 6 stamens, and 2 carpels which fuse to form the central pistil.

How do the proper flower parts form in the proper arrangment?

Members of Elliot Meyerowitz's and Enrico Coen's laboratories have used genetic dissection to identify genes required for proper floral organ identity in Arabidopsis thaliana and in Antirrhinum majus(snapdragon).

These scientists identified homeotic mutations that resulted in the misplacement of floral organs.

Three classes of mutations were identified in Arabidopsis.

Class A: flowers with these mutations have (unfused) carpels instead of sepals in whorl 1, and stamens instead of petals in whorl 2. The pattern of organs (from outside to inside) is carpel, stamen, stamen, carpel. The genes containing these mutations were named APETALA1 (AP1) and APETALA2 (ap2). Below is an image of an apetala2 mutant flower (right) next to a wild type flower (left).

Class B: flowers with these mutations have sepals in whorl 2 instead of petals, and (unfused) carpels in whorl 3 instead of stamens. The pattern of organs (from outside to inside) is sepal, sepal,carpel, carpel. The genes containing these mutations were named  APETALA3 (AP3) and PISTILLATA (PI). Below is an image of a pistillata mutant flower (right) next to a wild type flower (left).

Class C: flowers with this mutation have petals in whorl 3 instead of stamens, and sepals in whorl 4 instead of carpels. In addition, the floral meristem is not determinate - flowers continue to form within the flowers, so the pattern of organs (from outside to inside) is:
sepal, petal, petal; sepal, petal, petal; sepal, petal, petal, etc. The gene containing this mutation was called AGAMOUS (AG). Below is an image of an agamous mutant flower (right) next to a wild type flower (left).

The ABC model:

Each class of genes is required in two adjacent whorls. 

• Class A genes are required in whorls 1 and 2.  Class A gene products repress expresson of class C genes.
• Class B genes are required in whorls 2 and 3.  Both class A andclass B genes are required in whorl 2; both class B  and class C genes are required in whorl 3.
• Class C genes are required in whorls 3 and 4.  Class C gene products repress expression of Class A genes.

What turns on the different transcription factors in different cells?

• Induction and inhibition by one cell signalling to a neighboring cell.
• Repression of expression of A gene by C product, and of C gene by A product.
• Numerous other developmental gene products regulate or respond to A, B, and C.