Regulation of embryogenesis by LEAFY COTYLEDON2

Embryogenesis can be conceptually divided into two phases, morphogenesis and maturation. During morphogenesis the body plan and tissue systems of the embryo are established. During maturation the embryo expands to fill the seed while accumulating storage reserves, desiccating, and entering a quiescent state. The LEAFY COTYLEDON (LEC) genes of Arabidopsis thaliana (L. Heynh.) are central regulators of embryogenesis that are important for both phases of embryo development. For my dissertation I have focused on understanding the role of LEC2 in the regulation of embryogenesis. More specifically, I have explored the role LEC2 plays in regulating maturation and somatic embryogenesis (SE).;LEC2 encodes a B3 domain transcription factor. Ectopic LEC2 activity in seedlings can trigger the accumulation of maturation specific RNAs. LEC2 directly activates many genes important for maturation, and the activity of LEC2 is enhanced by abscisic acid (ABA). As ABA is normally high during maturation this provides insight into LEC2's mechanism of action. LEC2 also induces RNAs encoding gibberellic acid (GA) degrading enzymes and as such may increase the ABA to GA ratio during maturation and facilitate its own activity.;Here I have demonstrated that the maturation program in normally kept off in seedlings by Polycomb Group mediated histone 3-lysine-27-trimethylation (H3K27me3). I have also shown that ectopic LEC2 activity causes the loss of the H3K27me3 histone modification at maturation loci.;Ectopic LEC2 activity can trigger somatic embryo development in seedlings. SE in most systems requires an induction signal and a tissue competent to respond to said signal. I propose that auxin acts as the major induction signal, and that a high ABA to GA ratio may be important for competence of tissues. LEC2 activity directly upregulates RNAs important for auxin biosynthesis and transport, and the biological consequences of this may explain why LEC2 requires no exogenous auxin to form somatic embryos.;In conclusion, LEC2 is a central regulator of embryogenesis as demonstrated within this dissertation. LEC2 plays important roles in regulating somatic embryogenesis and seed maturation genes, as well as the hormones required for these processes.