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The APETALA1 (AP1) gene specifies flower meristem identity and is also required for normal development of sepals and petals. Consistent with its meristem identity role, AP1 RNA begins to accumulate in young flower primordia as they arise on the flanks of the inflorescence meristem (IM). Consistent with its organ identity role, AP1 RNA accumulates in sepals and petals throughout their development.


Gene Sequence

Map position

Although AP1 RNA accumulates uniformly throughout young flower primordia, it later becomes excluded from the third and fourth whorls, cells that will give rise to stamens and carpels. Interestingly, the loss of AP1 RNA in the third and fourth whorls corresponds to the onset of AG expression in these same cells. Indeed, analyses in mutant backgrounds confirmed that AG is a negative spatial regulator of AP1 RNA accumulation, and specifically prevents AP1 RNA from accumulating in stamens and carpels.

 ap1_early_df.gif (31117 bytes)ag_early2_df.gif (31938 bytes)

The first pair of images show the onset of AP1 (green) and AG (red) expression early in flower development. AP1 expression is observed throughout stage 1 and 2 flower primordia on the flanks of the inflorescence meristem. By stage 3, AP1 RNA has been excluded from the center of the young flower primordium. In contrast, AG RNA, which is not detected during stages 1 and 2, first accumulates early during stage 3, when sepal primordia are just beginning to emerge on the flanks of the flower primordium. From this stage forward, AP1 and AG RNAs accumulate in a mutually exclusive and complementary pattern.

The second pair of images show how AP1 (green) and AG (red) are restricted to the outer and inner two whorls of the flower respectively, later in flower development.

ap1_late_df.gif (30223 bytes)ag_late2_df.gif (26377 bytes)

Studies of AP1 expression in single and double mutant combinations demonstrated that AG negatively regulates AP1 RNA accumulation in stamens and carpels. Shown below is the expression of AP1 RNA in agamous mutant flowers. Whereas AP1 RNA is normally excluded from the two inner whorls, beginning at stage 3, AP1 RNA accumulates throughout agamous mutant flowers at all stages of development.

ap1_ag-1_df.gif (27972 bytes)

Selected References:

Drews, G.N., Bowman, J.L., and Meyerowitz, E.M. (1991). Negative regulation of the Arabidopsis homeotic gene AGAMOUS by the APETALA2 product. Cell 65, 991-1002.

Gustafson-Brown, C., Savidge, B., and Yanofsky, M.F. (1994) Regulation of the Arabidopsis floral homeotic gene APETALA1. Cell, 76:131-143.

Mandel, A.M., Gustafson-Brown, C., Savidge, B., and Yanofsky, M.F. (1992). Molecular characterization of the Arabidopsis floral homeotic gene APETALA1. Nature 360, 273-277.