ABC model of flower development

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.

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".

The transition from the vegetative phase to a reproductive phase involves a dramatic change in the plant's vital cycle, perhaps the most important one, as the process must be carried out correctly in order to guarantee that the plant produces descendants. This transition is characterised by the induction and development of the meristem of the inflorescence, which will produce a collection of flowers or one flower. This morphogenetic change contains both endogenous and exogenous elements: For example, in order for the change to be initiated the plant must have a certain number of leaves and contain a certain level of total biomass. Certain environmental conditions are also required such as a characteristic photoperiod. Plant hormones play an important part in the process, with the gibberellins having a particularly important role.

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 meristem can be defined as the tissue or group of plant tissues that contain undifferentiated stem cells, which are capable of producing any type of cell tissue. Their maintenance and development, both in the vegetative meristem or the meristem of the inflorescence is controlled by genetic cell fate determination mechanisms. This means that a number of genes will directly regulate, for example, the maintenance of the stem cell's characteristics (gene WUSCHEL or WUS), and others will act via negative feedback mechanisms in order to inhibit a characteristic (gene CLAVATA or CLV). 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. Along with the WUS gene the SHOOTMERISTEMLESS (STM) gene also represses the differentiation of the meristematic dome. This gene acts by inhibiting the possible differentiation of the stem cells but still allows cell division in the daughter cells, which, had they been allowed to differentiate, would have given rise to distinct organs.