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. [...]
[...] All elements required for proper spatial and temporal regulation of AG expression are contained within this intron. Although the LFY- and WUS- binding sites are physically close, LFY and WUS appear to bind independently to their respective sites. No physical interaction or cooperation of binding was observed in gel shift experiments Transcriptional repression of AG The lack of AG expression in vegetative tissues and the outer two floral whorls is a consequence of both the absence of AG activators and the activity of AG repressors. [...]
[...] Maintenance of class B gene expression The maintenance of AP3 and PI expression in flowers requires the activity of both AP3 and PI indicating that these two proteins function in a positive autoregulatory loop. The existence of such a loop was first postulated when it was observed that AP3 expression was not maintained in the third whorl of pi mutants. PI expression is not maintained in ap3 or pi mutants and the early fourth whorl expression of PI in wild-type flowers is also not maintained in older flowers. [...]
[...] Translational repression of AP2 The only floral organ-identity gene that does not encode a MADS-domain protein is the class A gene AP2. Although AP2 is expressed at the mRNA level in all four whorls of developing flowers, its activity is restricted to the outer two whorls of the flower. The mechanism behind this spatial restriction of AP2 activity is now becoming clearer. AP2 is regulated post- transcriptionally by the activity of a microRNA (miRNA). miRNAs are non- coding RNAs of 21–22 nucleotides that are processed from longer hairpin transcripts and are thought to play important roles in development. [...]
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