The ABC model of flower development is a scientific model of the process by which produce a pattern of gene expression in that leads to the appearance of an organ oriented towards sexual reproduction, a flower. There are three physiology developments that must occur in order for this to take place: firstly, the plant must pass from sexual immaturity into a sexually mature state (i.e. a transition towards flowering); secondly, the transformation of the apical meristem function from a vegetative meristem into a floral meristem or inflorescence; and finally the growth of the flower's individual organs. The latter phase has been modelled using the ABC model, which aims to describe the biological basis of the process from the perspective of molecular and developmental genetics.
An external stimulus is required in order to trigger the differentiation of the meristem into a flower meristem. This stimulus will activate mitosis cell division in the apical meristem, particularly on its sides where new primordium are formed. This same stimulus will also cause the meristem to follow a developmental pattern that will lead to the growth of floral meristems as opposed to vegetative meristems. The main difference between these two types of meristem, apart from the obvious disparity between the objective organ, is the verticillate (or whorled) phyllotaxis, that is, the absence of plant stem elongation among the successive whorls or verticils of the primordium. These verticils follow an acropetal development, giving rise to , , and . Another difference from vegetative axillary meristems is that the floral meristem is "determined", which means that, once differentiated, its cells will no longer cell cycle.
The identity of the organs present in the four floral verticils is a consequence of the interaction of at least three types of gene products, each with distinct functions. According to the ABC model, functions A and C are required in order to determine the identity of the verticils of the perianth and the reproductive verticils, respectively. These functions are exclusive and the absence of one of them means that the other will determine the identity of all the floral verticils. The B function allows the differentiation of petals from sepals in the secondary verticil, as well as the differentiation of the stamen from the carpel on the tertiary verticil.
Goethe's foliar theory was formulated in the 18th century and it suggests that the constituent parts of a flower are structurally modified leaves, which are functionally specialized for reproduction or protection. The theory was first published in 1790 in the essay "Metamorphosis of Plants" (" Versuch die Metamorphose der Pflanzen zu erklären"). where Goethe wrote:
There are many signals that regulate the molecular biology of the process. The following three genes in Arabidopsis thaliana possess both common and independent functions in floral transition: FLOWERING LOCUS T ( FT), LEAFY ( LFY), SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 ( SOC1, also called AGAMOUS-LIKE20). SOC1 is a MADS-box-type gene, which integrates responses to photoperiod, vernalization and gibberellins.
The fact that these homeotic genes determine an organ's identity becomes evident when a gene that represents a particular function, for example the A gene, is not expressed. In Arabidopsis this loss results in a flower which is composed of one verticil of carpels, another containing stamens and another of carpels. This method for studying gene function uses reverse genetics techniques to produce transgenic plants that contain a mechanism for gene silencing through RNA interference. In other studies, using forward genetics techniques such as genetic mapping, it is the analysis of the of flowers with structural anomalies that leads to the cloning of the gene of interest. The flowers may possess a non-functional or gene expression allele for the gene being studied.
The existence of two supplementary functions, D and E, have also been proposed in addition to the A, B and C functions already discussed. Function D specifies the identity of the ovule, as a separate reproductive function from the development of the carpels, which occurs after their determination. Function E relates to a physiological requirement that is a characteristic of all floral verticils, although, it was initially described as necessary for the development of the three innermost verticils (Function E sensu stricto). However, its broader definition ( sensu lato) suggests that it is required in the four verticils. Therefore, when Function D is lost the structure of the ovules becomes similar to that of leaves and when Function E is lost sensu stricto, the floral organs of the three outer most verticils are transformed into sepals, while on losing Function E sensu lato, all the verticils are similar to leaves. The of genes with D and E functions are also MADS-box genes.
This allows the characterization of three classes of mutation, according to which verticils are affected:
The nature of these genes corresponds to that of transcription factors, which, as expected, have analogous structures to a group of factors contained in and animal cells. This group is called MADS, which is an acronym for the different factors contained in the group. These MADS factors have been detected in all the vegetable species studied, although the involvement of other elements involved in the regulation of gene expression cannot be discounted.
In Antirrhinum, the orthologous gene to AP1 is SQUAMOSA ( SQUA), which also has a particular impact on the floral meristem. The homologs for AP2 are LIPLESS1 ( LIP1) and LIPLESS2 ( LIP2), which have a redundant function and are of special interest in the development of sepals, petals and ovules.
A total of three genes have been isolated from Petunia hybrida that are similar to AP2: P. hybrida APETALA2A ( PhAP2A), PhAP2B and PhAP2C. PhAP2A is, to a large degree, homologous with the AP2 gene of Arabidopsis, both in its sequence and in its expression pattern, which suggests that the two genes are orthologs. The proteins PhAP2B and PhAP2C, on the other hand, are slightly different, even though they belong to the family of transcription factors that are similar to AP2. In addition they are expressed in different ways, although they are very similar in comparison with PhAP2A. In fact, the mutants for these genes do not show the usual phenotype, that of the null alleles of A genes. A true A-function gene has not been found in Petunia; though a part of the A-function (the inhibition of the C in the outer two whorls) has been largely attributed to miRNA169 (colloquially called BLIND)
The GLO/ PI lines that have been duplicated in Petunia contain P. hybrida GLOBOSA1 ( PhGLO1, also called FBP1) and also PhGLO2 (also called PMADS2 or FBP3). For the functional elements equivalent to AP3/ DEF in Petunia there is both a gene that possesses a relatively similar sequence, called PhDEF and there is also an atypical B function gene called PhTM6. Phylogenetic studies have placed the first three within the «euAP3» lineage, while PhTM6 belongs to that of «paleoAP3». It is worth pointing out that, in terms of evolutionary history, the appearance of the euAP3 line seems to be related with the emergence of , as representatives of euAP3-type B function genes are present in dicotyledons while paleoAP3 genes are present in monocotyledons and basal angiosperms, among others.
As discussed above, the floral organs of eudicotyledonous angiosperms are arranged in 4 different verticils, containing the sepals, petals, stamen and carpels. The ABC model states that the identity of these organs is determined by the homeotic genes A, A+B, B+C and C, respectively. In contrast with the sepal and petal verticils of the eudicots, the perigone of many plants of the family Liliaceae have two nearly identical external petaloid verticils (the ). In order to explain the floral morphology of the Liliaceae, van Tunen et al. proposed a modified ABC model in 1993. This model suggests that class B genes are not only expressed in verticils 2 and 3, but also in 1. It therefore follows that the organs of verticils 1 and 2 express class A and B genes and this is how they have a petaloid structure. This theoretical model has been experimentally proven through the cloning and characterization of homologs of the Antirrhinum genes GLOBOSA and DEFICIENS in a Liliaceae, the tulip Tulipa gesneriana. These genes are expressed in verticils 1,2 and 3. The homologs GLOBOSA and DEFICIENS have also been isolated and characterized in Agapanthus praecox ssp. orientalis (Agapanthaceae), which is phylogenetically distant from the model organisms. In this study the genes were called ApGLO and ApDEF, respectively. Both contain open reading frames that code for proteins with 210 to 214 . Phylogenetic analysis of these sequences indicated that they belong to B gene family of the . In situ hybridization studies revealed that both sequences are expressed in verticil 1 as well as in 2 and 3. When taken together, these observations show that the floral development mechanism of Agapanthus also follows the modified ABC model.
The PLENA ( PLE) gene is present in A. majus, in place of the AG gene, although it is not an ortholog. However, the FARINELLI ( FAR) gene is an ortholog, which is specific to the development of the and the maturation of pollen.
In Petunia, Antirrhinum and in maize the C function is controlled by a number of genes that act in the same manner. The genes that are closer homologs of AG in Petunia are pMADS3 and floral-binding protein 6 ( FBP6).
The appearance of interesting phenotypes in RNA interference studies in Petunia and tomato led, in 1994, to the definition of a new type of function in the floral development model. The E function was initially thought to be only involved in the development of the three innermost verticils, however, subsequent work found that its expression was required in all the floral verticils.
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