Genetic and Epigenetic Regulation of Embryogenesis

Embryogenesis is a crucial period in the development of eukaryotes. In most plants, embryogenesis begins with an asymmetric cell division that gives rise to a polar embryo having a larger basal cell and a smaller apical cell. The embryoproper develops from the apical cell, and the basal cell develops into the suspensor, which is attached to the ovule and serves as a conduit for nutrient transfer to the developing embryo. The early stages ofArabidopsis embryo development have stereotyped cell divisions. Cotyledons then develop and serve to store nutrients for the seedling after germination. During late seed development, the seed desiccates and remains dormant until conditions are right for germination. Cell division and differentiation during these events follow highly regulated patterns that are influenced by both genetic and epigenetic mechanisms. Mammalian genomes undergo genomic reprogramming during embryogenesis involving global changes in DNA methylation believed to play an important role in developmental regulation of gene expression. DNA methylation is often associated with transcriptionally inactive portions of the genome (heterochromatin), and during early embryogenesis, large portions of the genome undergo a demethylation and subsequent remethylation process that is thought to contribute to chromatin decondensation and transcriptional activation of genes essential for embryo development (Li, 2002). This might have the effect of providing coarse control over gene expression, allowing hundreds of genes specifically required for embryogenesis to be switched on during this critical period and to remain off during other stages of the life cycle. On perhaps a more fundamental level, RNA polymerase II is at the heart of transcriptional machinery that transcribes protein-coding genes into mRNA. The RNA polymerase II machinery contains .85 polypeptides within 10 subcomplexes (Holstege et al., 1998), and the fine control of transcription involves the activity of these and many other interacting factors. Two articles in this issue of The Plant Cell report on genetic and epigenetic aspects of the regulation of gene expression during embryogenesis. Xiao et al. (pages 805– 814) show that DNAmethylation performed by METHYLTRANSFERASE1 (MET1) influences gene expression during embryogenesis in Arabidopsis and is critical for normal development of the embryo and seed viability. In another article, Ding et al. (pages 815–830) show that the pentatricopeptide repeat (PPR) protein GLUTAMINERICH PROTEIN23 (GRP23) is a nuclear protein that interacts with RNA polymerase II and likely functions in regulating gene expression during early embryogenesis. DNA METHYLATION PLAYS A VITAL ROLE IN PLANT EMBRYOGENESIS