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. [...]
[...] The precise molecular nature of these MADS-domain protein complexes, however, has still not been defined. Early studies showed that although all of the class B and C MADS-domain proteins could interact with each other; only certain combinations (AP1 homodimers, AP3/PI heterodimers and AG homodimers in Arabidopsis) could bind DNA in vitro. Heterodimers formed between A and B class proteins and those between B and C class proteins did not bind to DNA, suggesting that unique heterodimers between B and C class proteins are not present within each whorl. [...]
[...] The quartet model proposes that distinct complexes of four MADS-domain proteins function in each whorl of the flower. An AP3-PI- AP1-SEP complex is proposed to function in second whorl cells and confer petal identity, an AP3-PI-AG-SEP complex is proposed to function in the third whorl and specify stamen identity and an AG-AG-SEP-SEP complex is proposed to act in the fourth whorl cells to specify carpel identity. These proposed tetramers may correspond to a pair of dimers with each dimer binding to adjacent CArG box sequences in the promoters of target genes. [...]
[...] Mutations in any one or two of these genes do not significantly alter floral organ identity but sep1 sep2 sep3 triple mutants produce indeterminate flowers consisting only of sepals. These flowers resemble those produced in ap3 ag and pi ag double mutants, indicating that the B and C functions are not active. The initial expression patterns of AP3, PI and AG are normal in sep1 sep2 sep3 triple mutants showing that the SEP genes are not required for activation of the B and C class genes. [...]
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