GENETICS OF DEVELOPMENT.
FLOWER DEVELOPMENT REPORT (ARABIDOPSIS THALIANA)
Arabidopsis thaliana is the scientific name used to identify a tiny weed whose common name is ‘mouse ear cress’. It is a dicotyledonous angiosperm since its mature embryo carries two leaves (dicotyledonous) and its seeds are contained in an ovary within the flower (angiosperm). Like many other plants, Arabidopsis thaliana is an autotrophic plant capable of producing its own food through photosynthesis. Being firmly rooted to the soil this weed is non-motile and continuously produces new organ systems throughout its life cycle (Masson 2004). Arabidopsis thaliana possesses certain characteristics that make it a suitable experimental plant model. Arabidopsis thaliana is a non-fastidious plant, it has a comparatively shorter generation time than other higher plants, it is able to reproduce by both self-fertilization and cross-pollination and produces abundant seeds which can range from 10 000 to 40 000. Furthermore, Arabidopsis thaliana shares similar patterns of growth, development, flowering and seed production to higher plants. The high germination rate of this plant allows researchers to analyze large populations of seedlings for a specific phenotype. Besides all these characteristics, Arabidopsis thaliana is a small plant, requiring relatively little sunlight within temperatures of 22C to 26C. Thus Arabidopsis thaliana can be grown easily in the laboratory and also in abundant quantity (Masson 2004). A homeotic gene is important for controlling the early development and differentiation of embryonic tissues in eukaryotic organisms which leads to determination of a tissue’s identity during development. The homeotic genes in Arabidopsis thaliana are related with organ identity genes. The development of an organ in the incorrect region of the plant is a homeotic mutation (Fosket 1994). The ABC model was introduced to explain how the organs in the whorls of a flower are normally identified and why the organs were misidentified in the mutants (Howell 1998). The model postulates that there are three classes of homeotic genes in the flower that determine the identity of the different organs. These classes of genes are designated as class A, B and C respectively. In Arabidopsis thaliana, class A is compromised of the genes APETELLA1 (AP1) and APETELLA2 (AP2), class B genes are APETELLA3 (AP3) and PISTILLATA (PI), and class gene includes AGAMOUS (AG). The determination of the floral organs is the results of the products of these classes of genes, when expressed alone or in combination. The genes involved in the floral organ identity of the Arabidopsis thaliana are summarized in the Appendix – Table 1. There are three types of interaction between the three classes of genes in the ABC model. These are intraclass, interclass and cadastral interactions. Intraclass interaction is the interaction between the members of the same gene class. For example, the interaction of AP1 and AP2 gives rise to sepal formation. Intraclass interaction id determined to be cooperative or positive for class A and B genes respectively (Howell 1998). Interclass interaction on the other hand, is the interaction between the members of different classes which is expressed in the same regions of the flower (Howell 1998). Interclass interaction can be observed is observed in whorls 2 and 3 where the interaction of class A and B genes in whorl 2 gives rise to petal formation, and the interaction of class B and C in whorl 3 gives rise to stamen formation. Cadastral interaction is a competitive interaction between members of different classes expressed in different parts of the flower (Howell 1998). Cadastral interaction can be observed in the interaction of class A genes with class C gene. They mutually exclude the expression of each other in the same whorl so that whenever class A genes are active, class C genes are repressed or vice versa...
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