Figure 3.

24-nt siRNA production during seed development. A, The developing seed is composed of three tissues, with distinct genetic complements. m, maternal or matrigenic; p, patrigenic. B, Pol IV activity in the central cell is hypothesized to establish an epigenetic state that is maintained on matrigenic chromosomes in the endosperm (orange halo). This epigenetic state causes allele-specific siRNA production in the endosperm and might also influence imprinted gene expression. However, whether methylation caused by allele-specific siRNAs reinforces allele-specific gene expression is unclear. (MEG, PEG; filled arrows depict the expressed state while hollow arrows with dashed outline show the non-expressed allele). C, Siren siRNAs are produced in the immature seed coat and trigger DNA methylation at protein-coding genes via DRM proteins. Siren siRNAs might also move intercellularly, resulting in maternally specific accumulation of siRNAs in the endosperm. Siren siRNA methylation of protein-coding genes in the endosperm might influence seed development. D, During embryo development, canonical RdDM is upregulated at many transposons, resulting in hypermethylation of the genome in mature embryos. This methylation is rapidly lost upon germination.