Class B floral organ identity genes

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The Class B Floral Organ Identity genes are necessary for the proper development of the petals and stamens normally found in the second and third whorls of the flower. The APETALA3 (AP3) and PISTILLATA (PI) MADS-box gene products likely interact as a heterodimer to specify the B function of the organ identity model.
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Mutations in either AP3 or PI result in alterations in the 2nd and 3rd whorls, such that petals are replaced by sepals, and stamens are replaced by carpels. These 3rd whorl carpels often fuse together with the 4th whorl carpels to form an enlarged gynoecium.

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ap3 mutant

Consistent with their mutant phenotypes, AP3 and PI RNAs accumulate in the  2nd and 3rd  whorls, beginning at around stage 3 of flower development. In addition, constitutive expression of both AP3 and PI from the CaMV35S promoter leads to 1st whorl sepals developing as petals, and to stamens developing at the expense of the 4th whorl carpels. Extensive molecular and genetic analyses have also been described for the AP3 and PI orthologs from Antirrhinum majus, DEFICIENS and GLOBOSA.
Selected References:

Bowman et al., Development 112, 1-20, 1991.

Goto and Meyerowitz, E.M. Genes Dev. 8:1548-15601994.

Jack et al., Cell 68, 683-688, 1992.

Jack et al., Cell, 76:703-716, 1994.

Krizek and Meyerowitz, E.M., Development 122:11-22,1996.

Schwarz-Sommer et al., Science 250, 931-936, 1990.

Schwarz-Sommer et al., EMBO J. 11, 251-263, 1992.