ABC model of flower development
This stimulus will activate mitotic cell division in the apical meristem, particularly on its sides where new primordia are formed.
These verticils follow an acropetal development, giving rise to sepals, petals, stamens and carpels.
[1] 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.
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").
[2] 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".
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).
In this way both mechanisms give rise to a feedback loop, which along with other elements lend a great deal of robustness to the system.
[9] It was first used as a model to describe the collection of genetic mechanisms that establish floral organ identity in the Rosids, as exemplified by Arabidopsis thaliana, and the Asterids, as demonstrated by Antirrhinum majus.
Secondly, genetic analysis is carried out on the aberrant phenotypes for the relative characteristics of the flowers, which allows the characterization of the homeotic genes implicated in the process.
Supporting evidence for the existence of these mutations comes from the fact that a large number affect the identity of floral organs.
[17] AP2 has also been shown to complex with the co-repressor TOPLESS (TPL) in developing floral buds to repress the C-class gene AGAMOUS (AG).
[18]AP1 functions as a type A gene, both in controlling the identity of sepals and petals, and it also acts in the floral meristem.
It is also likely that post-transcriptional regulation exists, which controls its A function, or even that it has other purposes in the determination of organ identity independent of that mentioned here.
[22] For both species the active form of binding with DNA is that derived from the heterodimer: AP3 and PI, or DEF and GLO, dimerize.
[25] As discussed above, the floral organs of eudicotyledonous angiosperms are arranged in 4 different verticils, containing the sepals, petals, stamen and carpels.
In order to explain the floral morphology of the Liliaceae, van Tunen et al. proposed a modified ABC model in 1993.
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.
When taken together, these observations show that the floral development mechanism of Agapanthus also follows the modified ABC model.
[27] In A. thaliana, the C function is derived from one MADS-box type gene called AGAMOUS (AG), which intervenes both in the establishment of stamen and carpel identity as well as in the determination of the floral meristem.
In addition, the growth in the centre of the flower is undifferentiated, therefore the petals and sepals grow in repetitive verticils.
Equivalent genes were later found in Arabidopsis,[29] where they are also involved in controlling the development of carpels and the ovule and even with structures related to seed dispersal.
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.
