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Molecular biology of floral organogenesis

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  1. Introduction.
  2. Floral organ-identity genes and the ABC model.
  3. Ectopic expression studies providing Support for the ABC Model.
  4. SEPALLATA genes work with ABC genes to specify organ identity.
  5. MADS-domain protein complexes.
  6. Conclusion.

In the early 1990s, genetic studies on Arabidopsis thaliana and Antirrhinum majus led to the isolation and characterization of floral organ-identity genes (also called floral homeotic genes) and the establishment of the seminal ABC model for flower development. This model proposed that different organ-identity genes act alone and in various combinations to specify each of the four types of floral organs. Tests using ectopic expression support the basic tenets of the model. More recent work has addressed additional issues regarding floral organogenesis. What is the molecular basis for the combinatorial nature of the ABC model? The availability of important tools and resources for this model plant (together with a completely sequenced genome, large numbers of insertional mutant lines and DNA microarrays) has contributed greatly to the incredible progress made in flower development over the last several years.

[...] The identities of these different organs are specified by the actions of floral organ-identity genes in different regions of a developing flower. The ABC model for flower development proposed that three classes of these organ identity genes function in overlapping domains to specify sepals in whorl one, petals in whorl two, stamens in whorl three and carpels in whorl four. The A class genes APETALA1 (AP1) and APETALA2 (AP2) act to recognize sepal and petal development, the B class genes APETALA3 (AP3) and PISTILLATA act to specify petal and stamen development and the C class gene AGAMOUS acts to specify stamen and carpel development. [...]

[...] Misexpression of the class C gene AG, under the control of the cauliflower mosaic virus 35S promoter, demonstrated that this gene is sufficient to turn off class A function when it is expressed in the outer two floral whorls. 35S::AG plants closely resemble ap2 mutants. Combined misexpression of both of the class B genes has shown that AP3 and PI are sufficient to specify petal and stamen identity. 35S::PI 35S::AP3 plants produce flowers with petals in whorls one and two and stamens in whorls three and four. [...]

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