Epigenetic reprogramming and the plant germline

Epigenetic inheritance is more widespread in plants than in mammals, in part because mammals erase epigenetic information each generation by germline reprogramming. To assess the extent of germline reprogramming in plants, we sequenced the methylome  from sperm cells (SC), the vegetative nucleus (VN), and the precursor microspore from developing haploid pollen. We found that asymmetric CHH methylation is lost in microspores and sperm cells, but restored in the VN and in fertilized seed, guided by 24nt small interfering RNA (siRNA). In the VN symmetric CG methylation is lost from targets of the DNA glycosylases DEMETER (DME) and REPRESSOR OF SILENCING 1 (ROS1). Methylation at these targets is retained in sperm cells, and many accumulate corresponding siRNA. These targets include maternally expressed imprinted genes, and recurrent epialleles that are pre-methylated in sperm and inherited in the next generation. Thus genome reprogramming in pollen contributes to epigenetic inheritance, transposon silencing, and imprinting, guided by small RNA.