MOLECULAR CONTROL OF SEED SIZE

Seed derive from fertilized ovules and they constitute complex structures formed by three different parts: the seed coat, and the fertilization products, the endosperm and the embryo. During my Ph.D. I analyzed the role of MADS-domain transcription factor SEEDSTICK (STK) during the development of the seed coat in the model species Arabidopsis thaliana. Part II of this thesis contains the description of the role of STK in the regulation of several metabolic processes in the seed coat, providing evidences for a connection between cell fate determination, development and metabolism. In particular, the focus of the work is on the characterization of the differences, between the wild type and the stk mutant, in the synthesis of the proanthocyanidins (PAs) stored in the seed coat. PAs are a large group of compounds that have been previously reported for their role in different aspects of plant physiology, including defence against pathogens, protection of the embryo against UV-radiation and regulation of seed size. Part III of this thesis is dedicated to the analysis of seed size in Arabidopsis thaliana. We analysed the role of STK and of AUXIN RESPONSE FACTOR 2 (ARF2) in the control of seed coat development and in response to the hormonal cascade of brassinosteroids, known regulators of seed size. We provide evidences that both STK and ARF2 are regulators of cell proliferation and of cell expansion; we show that these two genes act antagonistically to determine seed size and data obtained from the detailed characterization of stk, arf2 and stk arf2 mutants are used to discuss a novel genetic network, pivotal in the maternal control of seed yield.