Three Types of Homeotic Genes Control Floral Organ Identity

Five different genes are known to specify floral organ identity in Arabidopsis: APETALA1 (API), APETALA2 (AP2), APETALA3 (AP3), PISTILLATA (PI), and AGAMOUS (AG) (Bowman et al. 1989; Weigel and Meyerowitz 1994). The organ identity genes initially were identified through mutations that dramatically alter the structure and thus the identity of the floral organs produced in two adjacent whorls (Figure 24.5). For example, plants with the ap2 mutation lack sepals and petals (see Figure 24.5B). Plants bearing ap3 or pi mutations produce sepals instead of petals in the second whorl, and carpels instead of stamens in the third whorl (see Figure 24.5C). And plants homozygous for the ag mutation lack both stamens and carpels (see Figure 24.5D).

Because mutations in these genes change floral organ identity without affecting the initiation of flowers, they are homeotic genes. These homeotic genes fall into three classes—types A, B, and C—defining three different kinds of activities (Figure 24.6):

Floral Homeotic Genes
FIGURE 24.5 Mutations in the floral organ identity genes dramatically alter the structure of the flower. (A) Wild type; (B) apetala2-2 mutants lack sepals and petals; (C) pistillata2 mutants lack petals and stamens; (D) agamousl mutants lack both stamens and carpels. (From Bewley et al. 2000.)

FIGURE 24.6 The ABC model for the acquisition of floral organ identity is based on the interactions of three different types of activities of floral homeotic genes: A, B, and C. In the first whorl, expression of type A (AP2) alone results in the formation of sepals. In the second whorl, expression of both type A (AP2) and type B (AP3/PI) results in the formation of petals. In the third whorl, the expression of B (AP3/PI) and C (AG) causes the formation of stamens. In the fourth whorl, activity C (AG) alone specifies carpels. In addition, activity A (AP2) represses activity C (AG) in whorls 1 and 2, while C represses A in whorls 3 and 4.


Activity type


Activity type



















1. Type A activity, encoded by API and AP2, controls organ identity in the first and second whorls. Loss of type A activity results in the formation of carpels instead of sepals in the first whorl, and of stamens instead of petals in the second whorl.

2. Type B activity, encoded by AP3 and PI, controls organ determination in the second and third whorls. Loss of type B activity results in the formation of sepals instead of petals in the second whorl, and of carpels instead of stamens in the third whorl.

3. Type C activity, encoded by AG, controls events in the third and fourth whorls. Loss of type C activity results in the formation of petals instead of stamens in the third whorl, and replacement of the fourth whorl by a new flower such that the fourth whorl of the ag mutant flower is occupied by sepals.

The control of organ identity by type A, B, and C homeotic genes (the ABC model) is described in more detail in the next section.

Floral Organ
FIGURE 24.7 A quadruple mutant (apil, ap2, ap3/pi, ag) results in the production of leaf-like structures in place of floral organs. (Courtesy of John Bowman.)

The role of the organ identity genes in floral development is dramatically illustrated by experiments in which two or three activities are eliminated by loss-of-function mutations (Figure 24.7). Quadruple-mutant plants (apl, ap2, ap3/pi, and ag) produce floral meristems that develop as pseudoflowers; all the floral organs are replaced with green leaflike structures, although these organs are produced with a whorled phyllotaxy. Evolutionary biologists, beginning with the eighteenth-century German poet, philosopher, and natural scientist Johann Wolfgang von Goethe (1749-1832), have speculated that floral organs are highly modified leaves, and this experiment gives direct support to these ideas.

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