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1. Natural methylation epialleles correlate with gene expression in maize

   https://doi.org/10.1093/genetics/iyad146  

   Zeng, Yibing; Dawe, R Kelly; Gent, Jonathan I (August 2023, GENETICS)

   Bomblies, K (Ed.)

   Abstract DNA methylation in plants is depleted from cis-regulatory elements in and near genes but is present in some gene bodies, including exons. Methylation in exons solely in the CG context is called gene body methylation (gbM). Methylation in exons in both CG and non-CG contexts is called TE-like methylation (teM). Assigning functions to both forms of methylation in genes has proven to be challenging. Toward that end, we utilized recent genome assemblies, gene annotations, transcription data, and methylome data to quantify common patterns of gene methylation and their relations to gene expression in maize. We found that gbM genes exist in a continuum of CG methylation levels without a clear demarcation between unmethylated genes and gbM genes. Analysis of expression levels across diverse maize stocks and tissues revealed a weak but highly significant positive correlation between gbM and gene expression except in endosperm. gbM epialleles were associated with an approximately 3% increase in steady-state expression level relative to unmethylated epialleles. In contrast to gbM genes, which were conserved and were broadly expressed across tissues, we found that teM genes, which make up about 12% of genes, are mainly silent, are poorly conserved, and exhibit evidence of annotation errors. We used these data to flag teM genes in the 26 NAM founder genome assemblies. While some teM genes are likely functional, these data suggest that the majority are not, and their inclusion can confound the interpretation of whole-genome studies. 

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2. Heritable changes of epialleles near genes in maize can be triggered in the absence of CHH methylation

   https://doi.org/10.1093/plphys/kiad668  

   Liu, Beibei; Yang, Diya; Wang, Dafang; Liang, Chun; Wang, Jianping; Lisch, Damon; Zhao, Meixia (December 2023, Plant Physiology)

   Abstract Trans-chromosomal interactions resulting in changes in DNA methylation during hybridization have been observed in several plant species. However, little is known about the causes or consequences of these interactions. Here, we compared DNA methylomes of F1 hybrids that are mutant for a small RNA biogenesis gene, Mop1 (Mediator of paramutation1), with that of their parents, wild-type siblings, and backcrossed progeny in maize (Zea mays). Our data show that hybridization triggers global changes in both trans-chromosomal methylation (TCM) and trans-chromosomal demethylation (TCdM), most of which involved changes in CHH methylation. In more than 60% of these TCM differentially methylated regions (DMRs) in which small RNAs are available, no significant changes in the quantity of small RNAs were observed. Methylation at the CHH TCM DMRs was largely lost in the mop1 mutant, although the effects of this mutant varied depending on the location of these DMRs. Interestingly, an increase in CHH at TCM DMRs was associated with enhanced expression of a subset of highly expressed genes and suppressed expression of a small number of lowly expressed genes. Examination of the methylation levels in backcrossed plants demonstrates that both TCM and TCdM can be maintained in the subsequent generation, but that TCdM is more stable than TCM. Surprisingly, although increased CHH methylation in most TCM DMRs in F1 plants required Mop1, initiation of a new epigenetic state of these DMRs did not require a functional copy of this gene, suggesting that initiation of these changes is independent of RNA-directed DNA methylation. 

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3. Stable unmethylated DNA demarcates expressed genes and their cis-regulatory space in plant genomes

   https://doi.org/10.1073/pnas.2010250117  

   Crisp, Peter A.; Marand, Alexandre P.; Noshay, Jaclyn M.; Zhou, Peng; Lu, Zefu; Schmitz, Robert J.; Springer, Nathan M. (September 2020, Proceedings of the National Academy of Sciences)

   null (Ed.)

   The genomic sequences of crops continue to be produced at a frenetic pace. It remains challenging to develop complete annotations of functional genes and regulatory elements in these genomes. Chromatin accessibility assays enable discovery of functional elements; however, to uncover the full portfolio of cis-elements would require profiling of many combinations of cell types, tissues, developmental stages, and environments. Here, we explore the potential to use DNA methylation profiles to develop more complete annotations. Using leaf tissue in maize, we define ∼100,000 unmethylated regions (UMRs) that account for 5.8% of the genome; 33,375 UMRs are found greater than 2 kb from genes. UMRs are highly stable in multiple vegetative tissues, and they capture the vast majority of accessible chromatin regions from leaf tissue. However, many UMRs are not accessible in leaf, and these represent regions with potential to become accessible in specific cell types or developmental stages. These UMRs often occur near genes that are expressed in other tissues and are enriched for binding sites of transcription factors. The leaf-inaccessible UMRs exhibit unique chromatin modification patterns and are enriched for chromatin interactions with nearby genes. The total UMR space in four additional monocots ranges from 80 to 120 megabases, which is remarkably similar considering the range in genome size of 271 megabases to 4.8 gigabases. In summary, based on the profile from a single tissue, DNA methylation signatures provide powerful filters to distill large genomes down to the small fraction of putative functional genes and regulatory elements. 

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4. The maize gene maternal derepression of r1 encodes a DNA glycosylase that demethylates DNA and reduces siRNA expression in the endosperm

   https://doi.org/10.1093/plcell/koac199  

   Gent, Jonathan I; Higgins, Kaitlin M; Swentowsky, Kyle W; Fu, Fang-Fang; Zeng, Yibing; Kim, Dong won; Dawe, R Kelly; Springer, Nathan M; Anderson, Sarah N (July 2022, The Plant Cell)

   Abstract Demethylation of transposons can activate the expression of nearby genes and cause imprinted gene expression in the endosperm; this demethylation is hypothesized to lead to expression of transposon small interfering RNAs (siRNAs) that reinforce silencing in the next generation through transfer either into egg or embryo. Here we describe maize (Zea mays) maternal derepression of r1 (mdr1), which encodes a DNA glycosylase with homology to Arabidopsis thaliana DEMETER and which is partially responsible for demethylation of thousands of regions in endosperm. Instead of promoting siRNA expression in endosperm, MDR1 activity inhibits it. Methylation of most repetitive DNA elements in endosperm is not significantly affected by MDR1, with an exception of Helitrons. While maternally-expressed imprinted genes preferentially overlap with MDR1 demethylated regions, the majority of genes that overlap demethylated regions are not imprinted. Double mutant megagametophytes lacking both MDR1 and its close homolog DNG102 result in early seed failure, and double mutant microgametophytes fail pre-fertilization. These data establish DNA demethylation by glycosylases as essential in maize endosperm and pollen and suggest that neither transposon repression nor genomic imprinting is its main function in endosperm. 

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5. Uses and Opportunities for Ethyl Methanesulfonate Mutagenesis in Maize

   https://doi.org/10.1101/pdb.top108504  

   Khangura, Rajdeep S; Best, Norman B; Dilkes, Brian P (April 2026, Cold Spring Harbor Protocols)

   Creating mutations in maize has provided key foundational information for our mechanistic understanding of genetics, evolution, and even the role of chromosomes as units of inheritance. Chemical mutagenesis is used in biological research to create novel genetic variation. Ethyl methanesulfonate (EMS) is an alkylating agent and a highly potent and frequently used mutagen. EMS mutagenesis can be used to identify genes based on phenotypes induced by mutagenesis (forward genetics) and to validate the functions of genes by independently creating multiple mutant alleles in known genes (reverse genetics). Due to our ability to collect huge quantities of maize pollen and to easily apply pollen to the silks of maize ears to conduct pollination and achieve hundreds of fertilization events, pollen EMS mutagenesis is uniquely facile in maize. While pollen EMS mutagenesis is commonly performed, treatment of maize seeds with EMS is also highly effective, and can be used for certain research objectives that are difficult to achieve with pollen mutagenesis, such as recovering mutant sectors. The alkylation of guanine residues by EMS primarily results in G > A or C > T transitions in the DNA, making the molecular profiling of mutations caused by EMS easy, with an extremely low false positive rate. EMS is hydrophilic, has a moderate half-life in water, and is sensitive to light and high temperatures. With appropriate precautions in research settings, EMS can be relatively safe to handle. Here, we provide an introduction to chemical mutagenesis via EMS, including some history on its use in maize and the considerations for the effective and safe design of mutagenesis experiments with EMS in maize. 

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