| Embryogenesis in higher plants is a critical period during which the basic body plan of plants is established. A single-celled zygote undergoes a series of proliferation and differentiation events to produce a mature embryo that exhibits shoot-root body pattern and all of the primary tissue types. This process is highly regulated by the orchestrated actions of many genes, although the whole regulatory network has not been elucidated. Arabidopsis LEAFY COTYLEDON1 (LEC1) is an important regulator of embryogenesis that controls various aspects of embryo development from the early morphogenesis phase to the late maturation phase and that has the ability to induce the embryonic pathway in vegetative cells. LEC1 encodes an Arabidopsis homolog of the HAP3 subunit of CCAAT binding factor (CBF). CBFs are evolutionarily conserved transcription factor complexes consisting of at least three subunits. Although a functional plant CBF has not yet been demonstrated, Arabidopsis possesses gene families for three subunits, called AHAP2 (Arabidopsis HAP2), AHAP3, and AHAP5. AHAP3 subunits can be divided into functionally distinct two classes, the LEC1-type and the non-LEC1-type, and LEC1 represents the LEC1-type AHAP3 subunits that specifically function during embryogenesis. In this dissertation, I studied mechanisms underlying the functional specificity of LEC1. By analyzing the ability of mutated LEC1 proteins to suppress genetically the lec1 mutation, I showed that the B domain of LEC1 is important for its specific function, and identified one specific amino acid residue within the LEC1 B domain, Asp-55, that is critical for LEC1's activity during embryo development. To obtain insight about how LEC1 functions specifically as a subunit of the CBF complex, I showed that LEC1 forms a ternary protein complex with AHAP2 and AHAP5 subunits in yeast. I also showed that the LEC1-CBF complex constructed in yeast does not possess intrinsic transcriptional activity and does not bind to a yeast CCAAT element. Based on the results, possible mechanisms by which a LEC1-CBF complex specifically regulates the genes required for embryogenesis and the roles of the Asp-55 residue are discussed. |
