How does the ABC Model of Plant Development work? What are A genes, B genes, and C genes?

Before I start answering the question I would like to mention ABC model is not for plant development rather it is a model for flower development in plants.

A class include APETALA1 (AP1) and APETALA2 (AP2) genes
B class include APETALA3 (AP3) and PISTILLATA (PI) genes
C class include AGAMOUS (AG) gene

Now to the model explanation.
![https://useruploads.socratic.org/ocjcabzbSTGc3kt151ii_abc-model.png)

Class A genes seal the fate of the apical meristem (region from where flower develops) to develop into sepals and when the B class genes are also expressed in the same cells those cells are destined to become petals.

Class C genes when expressed with class B genes they stimulate the formation of stamens the male reproductive parts of the flower, but if they are expressed in the set of cells, those cells are developed into carpel the female reproductive parts in the flower.

as shown in figure the fate of the cells is not random but they are tightly controlled in the formation called whorls. Generally, flower has four whorls as described in the figure.

the model which i have just explained has been updated to include Class D and Class E also. whose role is to provide redundancy to the main ABC class genes but are known to maintain the formation of whorls.