A role for CHROMOMETHYLASE3 in mediating transposon and euchromatin silencing during egg cell reprogramming in Arabidopsis.

During embryogenesis there is a major switch from dependence upon maternally-deposited products to reliance on products of the zygotic genome. In animals, this so-called maternal-to-zygotic transition occurs following a period of transcriptional quiescence. Recently, we have shown that the early embryo in Arabidopsis is also quiescent, a state inherited from the female gamete and linked to specific patterns of H3K9 dimethylation and TERMINAL FLOWER2 (TFL2) localization. We also demonstrated that CHROMOMETHYLASE 3 (CMT3) is required for H3K9 dimethylation in the egg cell and for normal embryogenesis during the first few divisions of the zygote. Subsequent analysis of CMT3 mutants points to a key role in egg cell reprogramming by controlling silencing in both transposon and euchromatic regions. A speculative model of the CMT3-induced egg cell silencing is presented here, based on these results and current data from the literature suggesting the potential involvement of small RNAs targeted to the egg cell, a process conceptually similar to the division of labor described in the male gametophyte for which we show that H3K9 modifications and TFL2 localization are reminiscent of the female gametophyte.