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Regulation of floral organ-identity gene expression

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  1. Introduction.
  2. Floral organ-identity gene expression.
    1. A key advancement in the field of flower development.
    2. Expression of each of the floral organ-identity genes.
  3. Regulation of AP1 expression.
  4. Initiation of B class gene expression by LFY, UFO and AP1.
  5. Maintenance of class B gene expression.
  6. Initiation of AG expression by LFY and WUS.
  7. Transcriptional repression of AG.
  8. Post-transcriptional regulation of AG.
  9. Translational repression of AP2.
  10. Conclusion.

A key advance in the field of flower development has been the uncovering of a direct role for the FM-identity gene LEAFY (LFY) in activation of the floral organ-identity genes. LFY encodes a novel plant-specific DNA-binding protein that can bind in vitro to the promoters of several floral organ-identity genes. While LFY possesses DNA-binding activity, it does not appear to have intrinsic transcriptional-activation activity. Thus LFY may need to interact with additional proteins to function in transcriptional regulation. LFY can directly regulate the transcription of at least one representative of each of the class A, B and C genes but uses distinct mechanisms to activate expression of each of these genes in a region-specific manner. LFY alone is sufficient to activate expression of the class A gene AP1 throughout the flower.

[...] Expression of each of the floral organ-identity genes continues throughout most of flower development. Since LFY is not expressed after stage other factors presumably act to maintain their expression. The plant hormone gibberellin appears to be an important regulator of homeotic gene expression in later stages of flower development (Yu et al., 2004). The GAdeficient mutant ga1 produces flowers that have organs with the correct identity but which are growth arrested and immature. Signalling through the GA pathway involves inactivation of a family of DELLA proteins that act as negative regulators of GA responses. [...]

[...] A possible model for AP2 regulation by miRNA172 can be envisioned in which expression of miRNA172 in the inner two floral whorls causes' translational inhibition of AP2 mRNA in these whorls. In this way, AP2 protein (and thus activity) could be restricted to the outer two floral whorls. This model is supported by the higher levels of miRNA172 in the inner two whorls of the flower, however, the spatial pattern of expression of AP2 protein in inflorescences has not yet been determined. [...]

[...] It has been hypothesized that UFO's role in AP3 activation involves the targeted degradation of a negative regulator of AP3 expression. AP1 also participates in activation of class B gene expression in petals. AP1 is not sufficient for class B gene activation as 35S::AP1 plants do not show transformations in organ identity. However, an activated AP1 (AP1-VP16) can turn on AP3 and PI expression in the first whorl. Furthermore, AP3 expression in 35S::LFY 35S::UFO seedlings is modulated by mutations in AP1. [...]

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