Chapter 20. Genes and Development

Floral differentiation is a model for plant development.

The plant Arabidopsis thaliana, a weed commonly called mouse-ear cress, is a model organism for developmental studies and presents a clear case of combinatorial control. As in all plants, Arabidopsis has regions of undifferentiated cells, called meristems, where growth can take place (Chapter 31). Meristem cells are similar to stem cells in animals. They are the growing points where shoots, roots, and flowers are formed. Floral meristems consist of multipotent cells that can differentiate into different structures in the flower. In floral meristems of Arabidopsis, the flowers develop from a pattern of four concentric circles of cells, or whorls, each of which differentiates into a distinct type of floral structure.

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From the outermost whorl (whorl 1) of cells to the innermost whorl (whorl 4), the floral organs are formed as shown in Fig. 20.19:

Cells in whorl 1 form the green sepals, which are modified leaves forming a protective sheath around the petals in the flower bud that unfold like petals when the flower opens.
Cells in whorl 2 form the petals.
Cells in whorl 3 form the stamens, the male sexual structures in which pollen is produced. (The small granules on the stamens in Fig. 20.19b are pollen grains.)
Cells in whorl 4 form the carpels, the female reproductive structures that receive pollen and contain the ovaries.

FIG. 20.19 Development of the Arabidopsis floral organs from concentric whorls of cells in the floral meristem.