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In recent years, the mechanisms of the origin and development of a flower, which is a characteristic structure of angiosperms, became one of the most intensively studied problems in modern evolutionary studies. Two decades ago, the research of two independent scientific groups, led by E. Coen and E. Meyerowitz on Anthirrhinum majus and Arabidopsis thaliana flowers respectively, resulted in a model of the genetic regulation of the successive whorls (sepals, petals, stamens and carpels) development within the flower, called ABC model. This model assumes that the characteristic whorl phenotype depends on the interaction of three classes of homeotic genes. The expression of the class A genes alone in the outermost whorl is responsible for the sepal formation. Interaction of the genes from the A and B classes in the second whorl and from B and C classes in the third whorl results in formation of the petal crown and stamens, respectively. The carpels develop as a result of the C class genes activity which are expressed exclusively in the inner whorl. Further research revealed that the additional classes of genes, D and E, are necessary for the proper ovary formation (D class genes) and the full functionality of other ABC genes (E class genes). Almost all the genes of the ABC model belong to the MADS-box family. Their classification is based on a gene structure, namely on the presence of four domains: the conservative MADS domain, intervening (I), the typical of plant keratin-like (K) and the C-terminal domain. Initially, the ABC model was proposed for eudicots only, and was widely studied for them. However, it appeared to be a very good tool for the analysis of the monocot flower architecture, too. Research on rice, wheat and corn confirmed the presence and functioning of the ABC genes in parts homologous to the eudicot flower. Furthermore, the analysis of orchid flowers provided the evidence that the combinations of different gene classes together with the presence of paralogous genes within the class are responsible for the identity of the elements and can significantly alter the phenotype of the structure. The ABC model is the basis on which the appearance of many unique structures within the flowers, especially in basal angiosperms, has been explained. One of the theories, called sliding boundary, suggests that the shift of the expression borders of the class B genes is responsible for undifferentiated perianth and explains the origin of e.g. petalody the observed phenomenon, in Magnolia stellata. MADS-box genes have been discovered in different organisms, including seed plants, ferns, mosses and algae. This suggests an early evolutionary emergence of genes of the ABC model. Multiple duplications of the genome in the course of plant evolution coupled with the changes in the flower morphology, required specialization of the new orthologs which gained in turn new roles/functions. The successive gene duplications can be traced directly in the plant phylogenetic tree: for instance, the first known B class gene duplication occurred at the time of separation of magnoliids, the basal angiosperms, from a common tree. The ABC model describes one of the mechanisms in plants, which interacting with other developmental programs results in the wide morphological diversity of flowers. The ABC model provides a new tool to test phylogenetic relationships between plants and generates new insight into plant evolution.

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The Editorial Board
Andrzej Łukaszyk - przewodniczący, Zofia Bielańska-Osuchowska, Szczepan Biliński, Mieczysław Chorąży, Aleksander Koj, Włodzimierz Korochoda, Leszek Kuźnicki, Aleksandra Stojałowska, Lech Wojtczak

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