Your Ad Here	Genetics of flower morphology The evolution of flower morphology is beginning to be understood through research on the genetic mechanisms of reproductive development in angiosperms (flowering plants). The model organism Arabidopsis thaliana (thale cress) has figured prominently in these studies. Although Arabidopsis was the first plant completely sequenced (in 2000), earlier genetic studies of Arabidopsis (beginning in the late 1980s) paved the way for evolutionary interpretations of the molecular processes underlying floral diversity
Genes Directing Flower Development Many of the most central genes controlling flower development were discovered in the earliest stages of Arabidopsis mutant analysis MADS-box genes In 1990, the genes representing the deficiens and agamous mutants were  cloned from Antirrhinum and Arabidopsis, respectively. The protein products of DEFICIENS (DEF) and AGA-MOUS (AG) were found to be similar to each other as well as to protein products of the serum response factor (SRF; from humans) and mini-chromosome maintenance-1 (MCMl; from yeast) genes. All four genes encode transcription factors (regulators of the expression of other genes) and contain the same deoxyribonucleic acid (DNA)­binding, 56-amino-acid domain known as the MADS box (named after the first letter of each gene: MCM1, AG, DEF and SRF); these are known together as the MADS-box genes
Non-MADS-box regulators Other principal, non-MADS-box regulators of flower development w re discovered early as Arabidopsis mutants. The APETALA2 (AP2) gene was shown to confer A function along with APETALA1 (AP1). AP1 was cloned first and shown to be a MADS-box gene, but AP2 was found to encode a putative transcription factor from a hitherto unknown protein family. Likewise, the LEAFY (LFY) gene, which was shown to control the entire floral developmental program, was also found to code for a previously unknown type of transcriptional regulator.	Your Ad Here
Regulation of floral organ fate based on the ABC model. (a) Cross section of a mature flower, indicating the four types of organs forming the central gynoecium. (b) Half section of an early flower primordium with the domains of ABC homeotic gene activities, which generate four unique combinations that specify organ fate. (a) a. Carpels b. Stamens c. Petals d. Sepals (b) 1. & 7. Sepals 2. & 6. Petals 3. & 5. Stamens 4. Carpels ABC model It had already been hypothesized from mutant phenotypes that the DEF and AG genes controlled floral organ identity in a combinatorial, whorl-specific fashion; that is, floral organs arise in four concentric rings (whorls), with sepals in the outermost whorl, followed by petals and stamens and finally carpels in the central whorl. An A function was envisioned to direct sepal identity, a B function together with A to specify petals, B plus a C function to designate stamens, and C alone to promote carpel development. This is the so-called ABC model for floral organ specification . The DEF and AG gene products were assigned to the Band C functions, respectively
Your Ad Here	Limitations of ABC model Researchers are discovering that the ABC model may need to be revised, as the model alone cannot explain certain observed complexities of floral developmel1-tal genetics, such as digressions in the conservation of A function, redundant control over the ABC system, and partitioning of genetic function.

Incomplete A function conservation
Through molecular evolutionary and gene exchange studies, sequence and functional homologies were found between AG and LFY and genes present in Antirrhinum. However, even though an AP1-like gene has been cloned from Antirrhinum based on mutant phenotype (SQUAMOSA), the product of this gene appears to operate in a manner distinct from the A function. Indeed, no A-function gene has so far been reported from Antirrhinum. Although AP1- and AP2-like genes are known from a diversity of plants, including Gerbera (Asteraceae), Petunia (Solanacea)    d maize (Poaceae), none of these appear to code for A- function proteins.

Therefore, the existence of a conserved A function in angiosperm flower development is questionable. Instead , it is possible that different genes take on A function in different groups of flowering plants, or that the ABC model i t 0 simple to accommodate all cases of sepal and petal determination.

Based on the presence of B- and C- function MADS-box genes in flowerless gymnosperms, it has been hypothesized that determination of flower organ identity is secondary to a more basic role of these genes in sex determination. It has also been hypothesized that "true" sepals of the kind expressed by Arabidopsis and Antirrhinum are a relatively recent evolutionary innovation, since lower dicots and monocots characteristically lack discrete sepals and petals and bear only one type of organ, tepals ( usually petallike).

Redundant control
A more recently recognized class of MADS-box genes, the SEPALLATA (SEP) genes of Arabidopsis, provide redundant control over the ABC system. These genes were identified through their sequence similarity to AG rather than through individual mutant phenotypes. Triple- null mutants of SEPl­3 produce "flowers" consisting only of sepallike organs, which has indicated to some invesators that these leaflike structures represent the "ground state" for floral parts. However, research on other organisms such as Gerbera has shown that SEP-class homologs play divergent roles in development of the condensed, dislike asteraceous inflorescence as well as in the different floral forms that are borne on it.

Partitioning of genetic function
Speciallly, one SEP-like gene controls the C function only in the staminal whorl of flower borne the inflorescence periphery, whereas another SEP homolog (the probable duplication partner of the first )appears to control the C function only in carpels of inner flowers. This partitioning of genetic function has probably had morphological evolutionary consequences since the outer flowers of Gerbera inflorescences are male-sterile and highly asymmetrical, with fused elongate petals, whereas the inner flowers are bisexual and nearly symmetrical with nonelongate petals. In its totality, the Gerbera inflorescence looks much like a single flower and probably attracts pollinating insects in the same capacity.