Biology subjects

Fischer, R. L.

Publications and source records attributed to Fischer, R. L..

4 recordsLinked to original sources

bioRxiv2023

Embryo-specific epigenetic mechanisms reconstitute the CHH methylation landscape during Arabidopsis embryogenesis

The modification of flowering plant DNA by CHH methylation acts primarily to silence transposable elements, of which many active copies are present in Arabidopsis thaliana. During embryogenesis, the CHH methylation landscape is dramatically reprogrammed, resulting in exceedingly high levels of this modification upon mature embryo formation. The mechanisms constituting the remodeling process, and its function in embryos, are unclear. Here, we isolate embryos from Arabidopsis plants harboring mutations for key components of the pathways that confer CHH methylation, namely RNA-directed DNA methylation (RdDM) and the Chromomethylase 2 (CMT2) pathways. We reveal that embryos are more methylated than leaves at shared CMT2 and RdDM targeting loci, accounting for most embryonic CHH hypermethylation. While the majority of embryo CHH methylated loci overlap with those in somatic tissues, a subset of conventional pericentric CMT2-methylated loci are instead targeted by RdDM in embryos. These loci, termed embRdDM exhibit intermediate H3K9me2 levels, associated with increased chromatin accessibility. Strikingly, more than 50% of the embRdDM loci in pollen vegetative (nurse) cells and ddm1 mutant somatic tissues are also targeted by RdDM, and these tissues were also reported to exhibit increased chromatin accessibility in pericentric heterochromatin. Furthermore, the root columella stem cell niche also displays CHH hypermethylation and an enriched presence of small RNAs at embRdDM loci. Finally, we observe a significant overlap of CHH hypermethylated loci with endosperm DEMETER targeting sites, suggesting that non-cell autonomous communication within the seed may contribute to the epigenetic landscape of the embryo. However, similar overlap with vegetative cell DEMETER targets indicates that the chromatin landscape that allows DEMETER access is mirrored in developing embryos, permitting CHH methylation catalysis at the same loci. Our findings demonstrate that both conserved and embryo-specific epigenetic mechanisms reshape CHH methylation profiles in the dynamic chromatin environment of embryogenesis.

plant biology↗

bioRxiv2023

H2A.X mutants exhibit enhanced DNA demethylation in Arabidopsis thaliana

H2A.X is an H2A variant histone in eukaryotes, unique for its ability to respond to DNA damage, initiating the DNA repair pathway. H2A.X replacement within the histone octamer is mediated by the FAcilitates Chromatin Transactions (FACT) complex, a key chromatin remodeler. FACT is required for DEMETER (DME)-mediated DNA demethylation at certain loci in Arabidopsis thaliana female gametophytes during reproduction, though it is not known how FACT targets DME sites. Here, we investigated whether H2AX is involved in DME- and FACT-mediated DNA demethylation during Arabidopsis reproduction. We show that h2a.x mutants are more sensitive to genotoxic stress, consistent with previous reports. H2A.X fused to the Green Fluorescent Protein (GFP) gene under the H2A.X promoter was highly expressed in newly developing Arabidopsis tissues, including in male and female gametophytes. We examined DNA methylation in h2a.x developing seeds using whole genome bisulfite sequencing, and found that CG DNA methylation in the developing endosperm, but not the embryo, is decreased genome-wide in h2a.x mutants, predominately in transposons and intergenic DNA. Hypomethylated sites overlapped 62 % with canonical DME loci. These data indicate that H2A.X is not required for DME function, but is important for DNA methylation homeostasis in the unique chromatin environment of Arabidopsis endosperm.

plant biology↗

bioRxiv2019

The Catalytic Core of DEMETER Guides Active DNA Demethylation in Arabidopsis

The Arabidopsis DEMETER (DME) DNA glycosylase demethylates the maternal genome in the central cell prior to fertilization, and is essential for seed viability. DME preferentially targets small transposons that flank coding genes, influencing their expression and initiating plant gene imprinting. DME also targets intergenic and heterochromatic regions, and how it is recruited to these differing chromatin landscapes is unknown. The C-terminal DME catalytic core consists of three conserved regions required for catalysis in vitro. We show that the catalytic core of DME guides active demethylation at endogenous targets, rescuing the developmental and genomic hypermethylation phenotypes of DME mutants. However, without the N-terminus, heterochromatin demethylation is significantly impeded, and abundant CG-methylated genic sequences are ectopically demethylated. We used comparative analysis to reveal that the conserved DME N-terminal domains are only present in the flowering plants, whereas the domain architecture of DME-like proteins in non-vascular plants mainly resembles the catalytic core, suggesting that it might represent the ancestral form of the 5mC DNA glycosylase found in all plant lineages. We propose a bipartite model for DME protein action and suggest that the DME N-terminus was acquired late during land plant evolution to improve specificity and facilitate demethylation at heterochromatin targets.

plant biology↗

bioRxiv2017

The FACT complex is required for DNA demethylation at heterochromatin during reproduction in Arabidopsis

The DEMETER (DME) DNA glycosylase catalyzes genome-wide DNA demethylation and is required for endosperm genomic imprinting and embryo viability. Targets of DME-mediated DNA demethylation reside in small, euchromatic, AT-rich transposons and at the boundaries of large transposons, but how DME interacts with these diverse chromatin states is unknown. The STRUCTURE SPECIFIC RECOGNITION PROTEIN 1 (SSRP1), subunit of the chromatin remodeler FAcilitates Chromatin Transactions (FACT), was previously shown to be involved in the DME-dependent regulation of genomic imprinting in Arabidopsis endosperm. Therefore, to investigate the interaction between DME and chromatin, we focused on the activity of the two FACT subunits, SSRP1 and SUPPRESSOR of TY16 (SPT16), during reproduction in Arabidopsis. We find that FACT co-localizes with nuclear DME in vivo, and that DME has two classes of target sites, the first being euchromatic and accessible to DME, but the second, representing over half of DME targets, requiring the action of FACT for DME-mediated DNA demethylation genome-wide. Our results show that the FACT-dependent DME targets are GC-rich heterochromatin domains with high nucleosome occupancy enriched with H3K9me2 and H3K27me1. Further, we demonstrate that heterochromatin-associated linker histone H1 specifically mediates the requirement for FACT at a subset of DME-target loci. Overall, our results demonstrate that FACT is required for DME targeting by facilitating its access to heterochromatin.

plant biology↗