Russell, S. D. | Search bioRxiv

bioRxiv2020

Resetting of 24-nt siRNA landscape is initiated in the unicellular zygote in rice

BackgroundThe zygote, a totipotent stem cell, constitutes a critical stage of the life cycle of sexually reproducing organisms. It is produced by the fusion of two differentiated cells -- the egg and sperm, which in plants have radically different siRNA transcriptomes from each other and from multicellular embryos. Due to technical challenges, the epigenetic changes that accompany the zygotic transition are poorly understood. ResultsHere, we characterized the small RNA transcriptome of rice zygotes. We found widespread redistribution of 24-nt siRNAs relative to gametes, including absence of sperm signature siRNAs, reduction at egg signature siRNA loci, and upregulation at seedling signature siRNA loci. Loci with reduced siRNAs in zygote relative to egg were gene-distal and heterochromatic, while loci with increased siRNAs relative to egg had a similar genomic distribution to canonical siRNA loci. Although both egg and zygote siRNA loci had higher mCHH level in wildtype than in drm2 embryo, zygote but not egg siRNA loci were associated with hypermethylation in mature embryo. A small fraction of siRNA loci (~1%) called siren loci accounted for 60% of all siRNAs within zygote siRNA loci, that likely arose from maternal carryover as they had similarly abundant siRNAs in egg; these siren loci were not associated with embryo hypermethylation. ConclusionsTaken together, our results indicate re-distribution of siRNAs in rice zygotes towards the canonical vegetative profile, that are consistent with the initiation of resetting of the gametic epigenome before the first embryonic division.

plant biology↗

bioRxiv2019

Reprogramming of 24nt siRNAs in rice gametes

Gametes constitute a critical stage of the plant life cycle, during which the genome undergoes reprogramming in preparation for embryogenesis. Here we characterized the small RNA transcriptomes of egg cells and sperm cells from rice to elucidate genome-wide distributions of 24nt siRNAs, which are a hallmark of RNA-directed DNA methylation (RdDM) in plants and are typically concentrated at boundaries of heterochromatin. We found that 24nt siRNAs were depleted from heterochromatin boundaries in both gametes, reminiscent of siRNA patterns in DDM1-type nucleosome remodeler mutants. In sperm, 24nt siRNAs were spread across broad heterochromatic regions, while in eggs, 24nt siRNAs were concentrated at a smaller number of heterochromatic loci throughout the genome, which were shared with vegetative tissues and sperm. In both gametes, patterns of CHH methylation, typically a strong indicator of RdDM, were similar to vegetative tissues, although lower in magnitude. These findings indicate that the small RNA transcriptome undergoes large-scale re-programming in both male and female gametes, which is not correlated with recruitment of DNA methyltransferases in gametes and suggestive of unexplored regulatory activities of gamete small RNAs in seeds after fertilization.

molecular biology↗