Figure 4. DNA methylation changes during plant development.
Diagram of an Arabidopsis (A) flower, and (B) the male and female gametophytes. Male gametogenesis occurs in stamens (St) and generates tricellular pollen grains that contain a vegetative nucleus (VN) and two sperm cells (SC). Female gametogenesis occurs in ovules (Ov) and produces a multicellular gametophyte with three antipodal cells (AC), two synergid cells (SC), one egg cell (EC) and a diploid (2n) central cell nucleus (CCN)78. (upper) Model showing transposon reactivation and siRNAs production specifically in the VN. These siRNAs may travel to the SCs to reinforce transposon silencing. (lower) Model showing siRNAs in the CCN, which may arise as a consequence of global demethylation. These siRNAs may travel to the EC and reinforce silencing. Reinforced silencing in the sperm and egg cells could account for the observed hypermethylation of the embryo. (C) Fertilization of the EC and CCN generate the embryo and endosperm, respectively. The embryo will give rise to the mature Arabidopsis plant while the endosperm is a terminally differentiated tissue. Imprinting is observed the endosperm, which nourishes the embryo, and is thus analogous to the placenta in mammals, where imprinting also occurs. In plants, maternal imprinting results from demethylation in the CCN by the DME glycosylase, which likely accounts for the observed hypomethylation in this tissue. After fertilization, the unmethylated (open circles) maternal alleles (♀) are expressed in the endosperm, while the paternal allele (♂) is methylated (closed circles) and silent.