Class A floral organ identity genes

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The Class A Floral Organ Identity genes are necessary for the proper development of the sepals and petals normally found in the first and second whorls of the flower . The Class A and C activities are mutually antagonistic, meaning in Class A mutants, the Class C activity expands to the outer two whorls.
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ap1 flower

Both of APETALA1 and APETALA2 show a class A activity and, in the absence of either gene, sepals and petals fail to develop properly.Although AP1 RNA expression occurs throughout young flower primordia, it later becomes localized to sepals and petals, consistent with its role in determining the identity of these two organs. The absence of AP1 RNA from stamens and carpels is due to negative regulation by the C-function gene AGAMOUS.

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ap2 flower

Although the activity of AP2 in specifying organ identity is restricted to the outer two whorls, AP2 RNA accumulates in all four whorls of the flower. AP1 and AP2 likely encode DNA-binding transcriptional regulators, since AP1 is a MADS-box gene and AP2 is related to a novel class of DNA binding proteins.
Selected References:

Irish and Sussex, Plant Cell 2:741-753, 1990

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

Mandel et al. (1992) Molecular Characterization of the Arabidopsis floral homeotic gene APETALA1. Nature 360:273-277

Gustafson-Brown et al. (1994) Regulation of the Arabidopsis floral homeotic gene APETALA1. Cell 76: 131-43.

Jofuku et al., Plant Cell 6, 1211-1225, 1994

Weigel, Plant Cell 7:388-389, 1995.