Abstract Detail

Functional Genetics/Genomics

Russell, Scott [1], Li, Chenxin [2], Xu, Hengping [3], Fu, Hong [3], Fu, Fang-Fang [4], Gent, Jonathan [5], Sundaresan, Venkatesan [6].

Genome-wide redistribution, setting of 24-nt siRNAs in rice gametes and subsequent resetting of the 24-nt siRNA landscape in rice zygotes.

Gametes consist of maternal and paternal cell lineages that initiate the life cycle of plant cells. During this phase of the life cycle, the gametic genome undergoes reprogramming in preparation for genomic embryogenesis. We chose rice (Oryza sativa L) as a model for the genomic changes in expression that relate to the control of heterochromatin expression and the maturation of the embryo upon fertilization, in association with chromosomal and epigenetic labeling. We found that 24-nt siRNAs, which are a hallmark of RNA-directed DNA methylation (RdDM) in plants, were depleted from heterochromatin boundaries in both male and female gametes relative to vegetative tissues. This pattern is reminiscent of siRNA patterns in DDM1-type nucleosome remodeler mutants. In sperm cells, 24-nt siRNAs spread across heterochromatic regions, whereas in egg cells, 24-nt siRNAs were concentrated in a smaller number of heterochromatic loci throughout the genome—especially at loci which also produced siRNAs in other tissues. In both gametes, patterns of CHH methylation, which are 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 redistribution in both male and female gametes, which is not correlated with recruitment of DNA methyltransferases in gametes and is suggestive of unexplored regulatory activities of gametic small RNAs. The zygote as a totipotent stem cell is crucial to the life cycle of sexually-reproducing organisms and is produced by the fusion of two differentiated cells—the egg and sperm—which in plants have siRNA transcriptomes that radically differ from one other and from later multicellular embryos. Owing to technical challenges, the epigenetic changes that accompany the transition from differentiated gametes to totipotent zygote are poorly understood. Because siRNAs serve as both regulators and outputs of the epigenome, we characterized small RNA transcriptomes of zygotes from rice. Zygotically expressed small RNAs exhibit extensive maternal carryover and display an apparent lack of paternal contributions, as indicated by the absence of sperm signature siRNAs. Zygote formation is accompanied by widespread redistribution of 24-nt siRNAs relative to gametes, such that ∼70% of the zygote siRNA loci do not overlap any egg cell siRNA loci. The newly-detected siRNA loci in zygote are gene-proximal and are not associated with centromeric heterochromatin, in sharp contrast to gametic siRNA loci, which are gene-distal and heterochromatic. In addition, zygotes but not egg siRNA loci are associated with high DNA methylation in the mature embryo. Thus, the zygote begins its transition before the first embryonic division to an siRNA profile, which is associated with future RdDM in embryogenesis. These findings indicate that, in addition to changes in gene expression, the transition to totipotency in the plant zygote is accompanied by resetting of the epigenetic reprogramming that occurred during gamete formation.

1 - University Of Oklahoma, Department Of Microbiology And Plant Biology, 770 Van Vleet Oval, Norman, OK, 73019, United States
2 - University of California, Davis, Department of Plant Biology, Davis, CA
3 - University of Oklahoma, Department of Microbiology and Plant Biology, Norman, OK
4 - University of Georgia, Department of Plant Biology, Athens, GA
5 - University of Georgia, , Department of Plant Biology, Athens, GA
6 - University of California, Department of Plant Biology, Davis, CA

Keywords:
none specified

Presentation Type: Oral Paper
Number: FGG2004
Abstract ID:1072
Candidate for Awards:Margaret Menzel Award