Epigenetic reprogramming in plant reproductive lineages

Monoecious flowering plants produce both microgametophytes (pollen) and megagametophytes (embryo sacs) containing the male and female gametes, respectively, which participate in double fertilization. Much is known about cellular and developmental processes giving rise to these reproductive structures and the formation of gametes. However, little is known about the role played by changes in the epigenome in dynamically shaping these defining events during plant sexual reproduction. This has in part been hampered by the inaccessibility of these structures-especially the female gametes, which are embedded within the female reproductive tissues of the plant sporophyte. However, with the recent development of new cellular isolation technologies that can be coupled to next-generation sequencing, a new wave of epigenomic studies indicate that an intricate epigenetic regulation takes place during the formation of male and female reproductive lineages. In this mini review, we assess the fast growing body of evidence for the epigenetic regulation of the developmental fate and function of plant gametes. We describe how small interfereing RNAs and DNA methylation machinery play a part in setting up unique epigenetic landscapes in different gametes, which may be responsible for their different fates and functions during fertilization. Collectively these studies will shed light on the dynamic epigenomic landscape of plant gametes or 'epigametes' and help to answer important unresolved questions on the sexual reproduction of flowering plants, especially those underpinning the formation of two products of fertilization, the embryo and the endosperm.