LEAFY COTYLEDON2 (LEC2) is a central regulator of embryogenesis sufficient to induce somatic cells to form embryos when expressedectopically. Here, we analyze the cellular processes induced by LEC2, a B3 domain transcription factor, that may underlie its ability to promote somatic embryogenesis. We show auxin-responsive genes are induced after LEC2 activation in seedlings. Genes encoding enzymes involved in auxin biosynthesis, YUC2 and YUC4, are activated within 1 h after induction of LEC2 activity, and YUC4 appears to be a direct transcriptional target of LEC2. We also show ectopic LEC2 expression induces accumulation of seed storage protein and oil bodies in vegetative and reproductive organs, events that normally occur during the maturation phase of embryogenesis. Furthermore, LEC2 activates seed protein genes before an increase in RNAs encoding LEC1 or FUS3 is observed. Thus, LEC2 causes rapid changes in auxin responses and induces cellular differentiation characteristic of the maturation phase. The relevance of these changes to the ability of LEC2 to promote somatic embryogenesis is discussed.seed development ͉ totipotency A n outstanding characteristic of plants is their totipotency. That is, a variety of cells can be induced to regenerate the adult organism, and several cell types do so by undergoing embryogenesis. For example, the fertilized egg cell undergoes zygotic embryogenesis, a number of differentiated cells of the sporophyte can be induced to undergo somatic embryogenesis, microspores can be diverted from their development into pollen grains to enter microspore embryogenesis, and a number of ovule cell types undergo asexual embryo development in a suite of processes known collectively as apomixis (1-4). Evidence suggests the morphological pathways of development used by these different types of embryos are similar to those of zygotic embryos (5, 6). However, the processes that induce a cell to change its fate and enter an embryonic program of development are not known, nor is it known whether a common pathway is used to initiate embryonic development in all of these diverse cell types.To gain insight into these questions, we focused on Arabidopsis LEAFY COTYLEDON2 (LEC2). LEC2 regulates many distinct aspects of embryogenesis (7,8). For example, during the early morphogenesis phase of embryogenesis in which the basic body plan of the embryo is established, loss-of-function mutations in LEC2 affect the maintenance of embryonic cell fate and specification of cotyledon identity. Later in embryogenesis, lec2 mutants have cotyledon tips that do not accumulate storage reserves nor acquire desiccation tolerance, indicating defects in the initiation and/or maintenance of the maturation phase. Consistent with the pleiotropic effects of the lec2 mutation, LEC2 encodes a transcription factor with a B3 domain, a DNA binding region found thus far only in plant proteins (8-10). Two transcription factors most closely related to LEC2, ABA INSENSITIVE3 (ABI3) and another LEC protein, FUSCA3 (FUS3), also play criti...