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ABC model of flower development

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**1. Floral Transition and Induction:**
– Transition from vegetative to reproductive phase involves induction and development of inflorescence meristem.
– Morphogenetic change requires a specific number of leaves, total biomass, and environmental conditions.
Plant hormones, especially gibberellins, play a crucial role in the process.
– Genes like FLOWERING LOCUS T, LEAFY, and SOC1 regulate floral transition in Arabidopsis thaliana.
– SOC1 integrates responses to photoperiod, vernalization, and gibberellins.

**2. Formation of Floral Meristem and Genetic Regulation:**
Meristem contains undifferentiated stem cells capable of producing any type of cell tissue.
– Genetic cell fate determination mechanisms control maintenance and development of meristems.
– Genes like WUSCHEL and CLAVATA regulate stem cell characteristics and inhibit characteristics, respectively.
– Feedback loops and genetic elements contribute to the robustness of the system.

**3. Floral Architecture and Genetic Regulation:**
Flower anatomy consists of sepals, petals, stamens, and carpels positioned according to a specific pattern.
– Three classes of genes regulate floral development: Meristem identity genes, Organ identity genes, and Cadastral genes.
– The ABC model describes genetic mechanisms establishing floral organ identity in Rosids and Asterids.
– Homeotic genes determine organ identity, and loss of specific gene expression results in altered floral composition.

**4. Genetic Analysis and Techniques:**
Flower development methodology involves gene identification and genetic analysis.
– Mutations in genes can lead to abnormal floral morphology.
– Techniques for detecting differential expression include cloning studies and serial analysis of gene expression.
– MADS factors, transcription factors, are present in various plant species and have structures similar to factors in yeasts and animal cells.

**5. Specific Gene Functions in Floral Development:**
– Genes exhibiting type-A function include AP1 and AP2 in Arabidopsis.
– Genes with type-B function are AP3 and PISTILLATA in Arabidopsis.
– Functions D and E have been proposed in addition to A, B, and C functions.
– Genes exhibiting type-C function in Arabidopsis thaliana include AGAMOUS, PLENA, and FARINELLI.

The ABC model of flower development is a scientific model of the process by which flowering plants produce a pattern of gene expression in meristems that leads to the appearance of an organ oriented towards sexual reproduction, a flower. There are three physiological 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's 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.

ABC model of flower development guided by three groups of homeotic genes.

An external stimulus is required in order to trigger the differentiation of the meristem into a flower meristem. This stimulus will activate mitotic cell division in the apical meristem, particularly on its sides where new primordia 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 stem elongation among the successive whorls or verticils of the primordium. These verticils follow an acropetal development, giving rise to sepals, petals, stamens and carpels. Another difference from vegetative axillary meristems is that the floral meristem is "determined", which means that, once differentiated, its cells will no longer divide.

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:

"...we may equally well say that a stamen is a contracted petal, as that a petal is a stamen in a state of expansion; or that a sepal is a contracted stem leaf approaching a certain stage of refinement, as that a stem leaf is a sepal expanded by the influx of cruder saps".

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