Biology subjects

Nelms, B.

Publications and source records attributed to Nelms, B..

9 recordsLinked to original sources

bioRxiv2026

Constitutive and tissue-specific target site preferences of the maize Mutator transposon

Transposable elements insert non-randomly into host genomes, generating mutations whose consequences depend on the tissue and developmental stage in which transposition occurs. We analyzed 3.2 million de novo Mutator (Mu) insertions across maize leaf, root, pollen, and endosperm to map target site preferences and their tissue specificity at fine-scale resolution. Insertions clustered into 29,408 genomic hotspots near gene 5' ends, collectively capturing 85% of events while covering just 1.9% of the genome. The vast majority of hotspots were consistently targeted across tissues, indicating that Mu insertion-site preference is largely constitutive rather than tissue-regulated. A minority ([~]5%) showed significant tissue-associated variation in Mu activity; this variation was correlated with tissue-specific gene expression, but the relationship was imperfect: some tissue-associated hotspots overlapped broadly expressed genes, while other constitutively active hotspots overlapped genes with tissue-specific expression. These discrepancies indicate that gene expression alone is insufficient to predict Mu insertion-site preference, pointing to additional genomic or chromatin determinants. Together, our results provide a genome-wide, locus-level map of Mu targeting and a quantitative framework for understanding how developmental and genomic context shapes transposon target site preference.

plant biology↗

bioRxiv2026

A distributive germline restricts the spread of new mutations

Genetic bottlenecks in the germline can amplify the impact of mutations, increasing the likelihood that new mutations are transmitted to multiple offspring. Here, we evaluate the timing and consequences of bottlenecks leading to maize pollen. By tracking transposon-induced mutations across tissues, we find that pollen derives from multiple early embryonic cell lineages, maintained by radial symmetries throughout development. In controlled crosses, offspring from bulk pollen or apical branches rarely share mutations, whereas lateral branches sample fewer lineages and produce offspring with a 6.2-fold increase in mutation sharing. Thus, the persistence of multiple, early-diverged cell lineages into the germline reduces mutation recurrence without altering the underlying mutation rate. Similar principles may apply in animals, where diverse mechanisms ensure that multiple cell lineages contribute to the germline.

genetics↗

bioRxiv2025

Nanorate sequencing reveals the Arabidopsis somatic mutation landscape

The rate and spectrum of somatic mutations can diverge from that of germline mutations. This is because somatic tissues experience different mutagenic processes than germline tissues. Here, we use nanorate sequencing (NanoSeq) to identify somatic mutations in Arabidopsis shoots with high sensitivity. We report a somatic mutation rate of 3.6x10-8 mutations/bp, [~]4-5x the germline mutation rate. Somatic mutations displayed elevated signatures consistent with oxidative damage, UV damage, and transcription-coupled nucleotide excision repair. Both somatic and germline mutations were enriched in transposable elements and depleted in genes, but this depletion was greater in germline mutations. Somatic mutation rate correlated with proximity to the centromere, DNA methylation, chromatin accessibility, and gene/TE content, properties which were also largely true of germline mutations. We note DNA methylation and chromatin accessibility have different predicted effects on mutation rate for genic and non-genic regions; DNA methylation associates with a greater increase in mutation rate when in non-genic regions, and accessible chromatin associates with a lower mutation rate in non-genic regions but a higher mutation rate in genic regions. Together, these results characterize key differences and similarities in the genomic distribution of somatic and germline mutations.

genomics↗

bioRxiv2025

FX-Cell: a method for single-cell RNA sequencing on difficult-to-digest and cryopreserved plant samples

Single-cell RNA sequencing (scRNA-seq) in plants requires the isolation of high-quality protoplasts-- cells devoid of cell walls. However, many plant tissues and organs are resistant to enzymatic digestion, posing a significant barrier to advancing single-cell multi-omics research in plants. Furthermore, for many field-grown crops, the lack of immediate laboratory access presents another major challenge for protoplast preparation. To address these limitations, we developed the FX-Cell method and its derivatives, FXcryo-Cell and cryoFX-Cell, to enable scRNA-seq with both difficult-to-digest and cryopreserved plant samples. By optimizing the fixation buffer and minimizing RNA degradation, our approach ensures efficient cell wall digestion at high temperatures while maintaining high-quality single cells, even after long-term storage at -80{degrees}C, and circumvents use of nuclei, which are not representative of the pool of translatable mRNAs. Using these methods, we successfully constructed high-quality cell atlases for rice tiller nodes, rhizomes of wild rice, and maize crown roots grown in field conditions. Moreover, these methods enable the accurate reconstruction of plant acute wounding responses at single-cell resolution. Collectively, these advancements expand the applicability of plant single-cell genomics across a wider range of species and tissues, paving the way for comprehensive Plant Cell Atlases for plant species.

plant biology↗

bioRxiv2025

Ultra-deep sequencing of somatic mutations induced by a maize transposon

Cells accumulate mutations throughout development, contributing to cancer, aging, and evolution. Quantitative data on the abundance of de novo mutations within plants or animals are limited, as new mutations are often rare within a tissue and fall below the limits of current sequencing depths and error rates. Here, we show that mutations induced by the maize Mutator (Mu) transposon can be reliably quantified down to a detection limit of 1 part in 16,000. We measured the abundance of millions of de novo Mu insertions across four tissue types. Within a tissue, the distribution of de novo Mu allele frequencies was highly reproducible between plants, showing that, despite the stochastic nature of mutation, repeated statistical patterns of mutation abundance emerge. In contrast, there were significant differences in the allele frequency distribution between tissues. At the extremes, root was dominated by a small number of highly abundant de novo insertions, while endosperm was characterized by thousands of insertions at low allele frequencies. Finally, we used the measured pollen allele frequencies to reinterpret a classic genetic experiment, showing that evidence for late Mu activity in pollen are better explained by cell division statistics. These results provide insight into the complexity of mutation accumulation in multicellular organisms and a system to interrogate the factors that shape mutation abundance. SignificanceNew mutations provide the raw material for evolution and contribute to cancer, aging, and genetic diseases. It has been challenging to follow the origin and spread of new mutations because they can be exceptionally rare and difficult to detect. By focusing on a class of mutation that can be detected more readily - Mu transposon insertions - we followed the abundance of new mutations in multiple maize tissues. We find that the Mu has broad activity across tissues, but with significant tissue-specific differences in how abundant individual new mutations become. Most mutations were below the detection limit available for other classes of mutation. These results provide a glimpse into the complexity of mutation within multicellular organisms.

plant biology↗

bioRxiv2024

Potent pollen gene regulation by DNA glycosylases in maize

Although DNA methylation primarily represses TEs, it also represses select genes that are methylated in plant body tissues but demethylated by DNA glycosylases (DNGs) in endosperm or pollen. Activity of either one of two DNGs, MDR1 or DNG102, is essential for pollen viability in maize. Using single-pollen mRNA sequencing on pollen segregating mutations in both genes, we identified 58 candidate DNG target genes that account for 11.1% of the wild-type transcriptome but are silent or barely detectable in the plant body (sporophyte). They are unusual in their tendency to lack introns but even more so in their having TE-like methylation in their CDS. The majority have predicted functions in cell wall modification, and they likely support the rapid tip growth characteristic of pollen tubes. These results suggest a critical role for DNA methylation and demethylation in regulating maize genes with potential for extremely high expression in pollen but constitutive silencing elsewhere.

genetics↗

bioRxiv2021

Quantitative cell release from plant tissues for single-cell genomics

Single-cell RNA-sequencing (scRNA-seq) can provide invaluable insight into cell development, cell type identification, and plant evolution. However, the resilience of the cell wall makes it difficult to dissociate plant tissues and release individual cells for single-cell analysis. Here, we show that plant organs can be rapidly and quantitatively dissociated into cells if fixed prior to enzymatic digestion. Fixation enables digestion at high temperatures at which enzymatic activity is optimal and stabilizes the plant cell cytoplasm, rendering cells resistant to mechanical shear force while maintaining high quality RNA. This protocol, FX-Cell, releases four to ten-fold more recoverable cells than optimized protoplasting methods applied to maize anthers or root tips with no cell type biases and can be readily applied to a variety of plant taxa and tissues with no optimization. FX-Cell and scRNA-seq analysis were applied to maize anthers for which 95% of the cells were dispersed and provided suitable scRNA-seq data for the identification of anther cell types with marker genes and well-understood biological functions, including rare meiocytes ([~]1% anther cells). In addition, the scRNA-seq data provided putative marker genes and gene ontology information for the identification of unknown cell types. FX-Cell also preserves the morphology of the isolated cells, permitting cell type identification without staining. Ultimately, FX-Cell can be applied to a range of plant species and tissues with minimal to no optimization paving the way for plant scRNA-seq analyses in non-model taxa and tissues.

plant biology↗

bioRxiv2021

Gametophyte genome activation occurs at pollen mitosis I in maize

Flowering plants alternate between multicellular haploid (gametophyte) and diploid (sporophyte) generations. One consequence of this life cycle is that plants face substantial selection during the haploid phase1-3. Pollen actively transcribes its haploid genome4, providing phenotypic diversity even among pollen grains from a single plant. Currently, the timing that pollen precursors first establish this independence is unclear. Starting with an endowment of transcripts from the diploid parent, when do haploid cells generated by meiosis begin to express genes? Here, we follow the shift to haploid expression in maize pollen using allele-specific RNA-sequencing (RNA-Seq) of single pollen precursors. We observe widespread biallelic expression for 11 days after meiosis, indicating that transcripts synthesized by the diploid sporophyte persist long into the haploid phase. Subsequently, there was a rapid and global conversion to monoallelic expression at pollen mitosis I (PMI), driven by active new transcription from the haploid genome. Genes expressed during the haploid phase showed reduced rates of nonsynonymous relative to synonymous substitutions (dn/ds) if they were expressed after PMI, but not before, consistent with purifying selection acting on the haploid gametophyte. This work establishes the timing with which haploid selection may act in pollen and provides a detailed time-course of gene expression during pollen development.

plant biology↗

bioRxiv2018

Defining the developmental program leading to meiosis in maize

In multicellular organisms, the entry into meiosis is a complex process characterized by increasing levels of meiotic specialization. We used single-cell RNA-sequencing to reconstruct the developmental program into meiosis in maize. We observed a smooth continuum of expression stages leading up to meiosis, followed by a sharp reorganization of the transcriptome in early meiotic prophase. This latter transcriptional shift was dramatic, with 26.7% of expressed genes changing by 2 fold or more, and occurred just prior to a proposed cell cycle checkpoint. Changes in cell physiology accompanied the nuclear events of meiosis, including a decrease in protein translation capacity and increase in membrane-bound organelles. We further identified differences in gene expression between the mitotic and meiotic cell cycles. Our results uncover a multi-step pathway into meiosis and highlight the power of single cell RNA-seq to define developmental transitions.

plant biology↗