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. 2013 Dec 31;8(12):e85480. doi: 10.1371/journal.pone.0085480

Figure 5. Diagram of the expanded ABCDE model of floral development.

Figure 5

a. In the model, A, B, C, D, and E class MADS-box proteins interact, leading to the formation of homodimers and heterodimers called “floral quartets.” The complexes then activate floral organ-specific expression programs [6-8]. Class A genes (APETALA1, AP1) control sepal development and, class A and class B genes (e.g., PISTILLATA, PI, and APETALA3, AP3) jointly regulate petal formation. Class B and class C genes (e.g., AGAMOUS, AG) jointly mediate stamen development. Class C genes determine the formation of carpel alone. Class D genes (e.g., SEEDSTICK, STK and SHATTERPROOF, SHP) specify the identity of the ovule within the carpel. Class E genes (e.g., SEPALLATA, SEP) are necessary for the proper formation of all floral organs. In orchids, male and female tissues fuse into a gynostemium or column. The orchid code theory suggests that class B AP3/DEF-like genes play a crucial role in lateral petal and lip identity and the class PI/GLO-like genes and the A, C, D and E class genes have unchanged function [8,50-52]. b. Floral organs of C. ensifolium. Se: Sepals (whorl 1); Pe: Petals and Li: Lip (whorl 2); Co: Column including Ac: Anther cap and Ca: Carpel (whorl 3+Whorl4).