AUXIN AND CYTOKININ CONTROL OF FEMALE GAMETOPHYTE DEVELOPMENT IN ARABIDOPSIS THALIANA

The seed contains and protects the embryo, and it represents the unit of dispersion of the progeny for the higher plants. The seeds are also essential for human and animal nutrition and therefore, they are the subject of studies and research to implement their number per plant as well as their size and quality. The seeds in Angiosperm, as well as Arabidopsis thaliana, are produced after a process called, double fertilization. This process occurs between two haploid individuals, the male gametophyte or pollen grain and the female gametophyte or embryo sac. The two gametophytes are enclosed within the reproductive organs: the anthers carry the pollen grains, whereas the female gametophyte is wrapped by the sporophytic tissues of the ovule that in turn is enclosed within the pistil. Ovules are the precursors of seeds and for this reason, their development which is a tightly regulated process has been studied in details. Ovule development is divided in two phases: the megasporogenesis and the megagametogenesis. The megasporogenesis starts when a somatic cell differentiates in the megaspore mother cell (MMC) that undergoes meiotic division giving rise to four haploid spores. Three of them degenerate and the one surviving differentiates in the Functional Megaspore (FM) and starts to divide mitotically to give rise to the mature embryo sac. At the maturity the female gametophyte consists of seven cells. Simultaneously with the formation of the female gametophyte, the integuments are formed. These structures protect the embryo sac during its development but also provide the female gametophyte of all the signals and factors required for its correct development. Mutants defective in integument formation exhibited also alteration in female gametophyte formation and specification. Hormones have been shown to play an important role in the control of embryo sac development, including auxin and cytokinins, which appear to play a key role in this organogenesis process. In this thesis, we’ve demonstrated that the phenotype of two mutants defective in ovule development, ant-4 (AINTEGUMENTA) and bel1-1 (BELL1), is caused by the alteration of the dynamic distribution of auxin, which not only determines the failure to form the integuments but also it leads defects in megasporogenesis-magagametogenesis progression and consequently the sterility of the ovule. By studying the potential role of cytokinins during ovule development, we demonstrated that an alteration in the levels of this hormone has an impact on the process of megagametogenesis. In conclusion, auxins and cytokinins are essential for female gametophyte determination and development.