Reprogramming of DNA Methylation in the Pollen Vegetative Nucleus Guides Epigenetic Inheritance via Small RNA

Genome reprogramming of DNA methylation is important for transposon silencing and epigenetic inheritance, and might ultimately lead to the formation of epialleles and phenotypic variation. We are currently trying to elucidate the mechanisms underlying epiallele formation and epigenome reprogramming in the Arabidopsis male gametophyte. Dynamic changes of DNA methylation occur during pollen development, where two sperm cells and a vegetative cell differentiate from a precursor microspore after post-meiotic cell divisions. Interestingly, previously identified variable epialleles and imprinted genes are actively demethylated in the vegetative nucleus, which seems to be required for their stable repression in the germline and subsequent generations. In contrast, CHH methylation is lost from retrotransposons in microspores and sperm cells, and restored by de novo DNA methylation activity guided by 24 nt small interfering RNA both in the vegetative nucleus and in the embryo after fertilization. I will discuss the possible implications of epigenetic reprogramming in the pollen vegetative nucleus, guiding heritable silencing of transposable elements via RNAi.