The Arabidopsis LEAFY COTYLEDON1 (LEC1) gene is required for the specification of cotyledon identity and the completion of embryo maturation. We isolated the LEC1 gene and showed that it functions at an early developmental stage to maintain embryonic cell fate. The LEC1 gene encodes a transcription factor homolog, the CCAAT box-binding factor HAP3 subunit. LEC1 RNA accumulates only during seed development in embryo cell types and in endosperm tissue. Ectopic postembryonic expression of the LEC1 gene in vegetative cells induces the expression of embryo-specific genes and initiates formation of embryo-like structures. Our results suggest that LEC1 is an important regulator of embryo development that activates the transcription of genes required for both embryo morphogenesis and cellular differentiation.

MEDEA (MEA) is an Arabidopsis Polycomb group gene that is imprinted in the endosperm. The maternal allele is expressed and the paternal allele is silent. MEA is controlled by DEMETER (DME), a DNA glycosylase required to activate MEA expression, and METHYLTRANSFERASE I (MET1), which maintains CG methylation at the MEA locus. Here we show that DME is responsible for endosperm maternal-allele-specific hypomethylation at the MEA gene. DME can excise 5-methylcytosine in vitro and when expressed in E. coli. Abasic sites opposite 5-methylcytosine inhibit DME activity and might prevent DME from generating double-stranded DNA breaks. Unexpectedly, paternal-allele silencing is not controlled by DNA methylation. Rather, Polycomb group proteins that are expressed from the maternal genome, including MEA, control paternal MEA silencing. Thus, DME establishes MEA imprinting by removing 5-methylcytosine to activate the maternal allele. MEA imprinting is subsequently maintained in the endosperm by maternal MEA silencing the paternal allele.

We isolated mutations in Arabidopsis to understand how the female gametophyte controls embryo and endosperm development. For the DEMETER (DME) gene, seed viability depends only on the maternal allele. DME encodes a large protein with DNA glycosylase and nuclear localization domains. DME is expressed primarily in the central cell of the female gametophyte, the progenitor of the endosperm. DME is required for maternal allele expression of the imprinted MEDEA (MEA) Polycomb gene in the central cell and endosperm. Ectopic DME expression in endosperm activates expression of the normally silenced paternal MEA allele. In leaf, ectopic DME expression induces MEA and nicks the MEA promoter. Thus, a DNA glycosylase activates maternal expression of an imprinted gene in the central cell.

The Arabidopsis LEAFY COTYLEDON2 (LEC2) gene is a central embryonic regulator that serves critical roles both early and late during embryo development. LEC2 is required for the maintenance of suspensor morphology, specification of cotyledon identity, progression through the maturation phase, and suppression of premature germination. We cloned the LEC2 gene on the basis of its chromosomal position and showed that the predicted polypeptide contains a B3 domain, a DNA-binding motif unique to plants that is characteristic of several transcription factors. We showed that LEC2 RNA accumulates primarily during seed development, consistent with our finding that LEC2 shares greatest similarity with the B3 domain transcription factors that act primarily in developing seeds, VIVIPAROUS1/ABA INSENSITIVE3 and FUSCA3. Ectopic, postembryonic expression of LEC2 in transgenic plants induces the formation of somatic embryos and other organ-like structures and often confers embryonic characteristics to seedlings. Together, these results suggest that LEC2 is a transcriptional regulator that establishes a cellular environment sufficient to initiate embryo development.