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Flower development is the process by which angiosperms 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 endeavours 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. This stimulus will activate mitotic cell division in the 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 erklaren''"). where Goethe wrote: ==Floral transition== 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 descendents. 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, where only one is produced. 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. However, it is worth noting the role of the following three genes in ''Arabidopsis thaliana'': ''FLOWERING LOCUS T'' (''FT''), ''LEAFY'' (''LFY''), ''SUPPRESOR OF OVEREXPRESSION OF CONSTANS1'' (''SOC1'', also called ''AGAMOUS-LIKE20''). These genes possess both common and independent functions in floral transition. ''SOC1'' is a MADS-box-type gene, which integrates responses to photoperiod, vernalization and gibberellins.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「ABC model of flower development」の詳細全文を読む スポンサード リンク
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