Alternative splicing comes in a handful of recognizable flavors. Exon skipping drops an exon located between two others out of the mature message. Mutually exclusive exons keep one exon or group of exons while splicing out a rival, producing similarly shaped proteins with slightly different sequences. Intron retention leaves one or more introns in place, alternative 5′ splice sites and alternative 3′ terminal exons extend or trim exons at their respective ends. The frequencies of these events vary dramatically across species, and that variation tells an evolutionary story. In animals, exon skipping dominates, a pattern that intensified along the bilaterian lineage and is tied to growth, differentiation and movement. In plants, intron retention rules, a characteristic linked to their stationary lifestyle and their need to adapt stress responses in place. The logic is simple: animals can move away from stressors, so they use splicing to diversify their tissues, while plants must stay and endure, so they use splicing to rewire their stress machinery.

The numbers from land plants show just how responsive this system is. In Arabidopsis, roughly 22 percent of genes, or 42 to 61 percent of intron-containing genes, undergo alternative splicing under normal conditions. After treatment with abscisic acid, a compound that mimics a stress signal, that figure can climb to 83.4 percent of intron-containing genes. Wheat edits 11,141 genes in response to salt stress. Tea plants activate an alternatively spliced terpene synthase as a defense against pathogens. Lilies accumulate a spliced variant of the heat stress transcription factor LlHSFA3B, which in turn activates a protein governing tolerance to both high temperature and salinity. Maize and Arabidopsis rely on spliced isoforms of circadian clock-associated genes for drought response. In tea, splicing even processes the structural genes and MYB transcription factors needed for flavonoid synthesis, the chemistry behind tea’s taste.

Microalgae mirror this plant-like pattern, and the model species Chlamydomonas reinhardtii illustrates it in remarkable detail. Its genome is intron-rich, with approximately 8.5 introns per gene and only about 8 percent of genes lacking introns entirely. Between 50 and 55 percent of its genes show alternative site selection, and intron retention is the most prevalent event type, making up roughly 29 to 50 percent of total alternative splicing. Frame-preserving and frame-disrupting splicing occur at similar frequencies, 1412 and 1555 events respectively, but frame-disrupting events show lower percent-spliced-in values. That discrepancy points to two possibilities: either nonsense-mediated decay is removing transcripts with premature termination codons, or frame-disrupting splicing is simply rarer. Nonsense-mediated decay may itself contribute to transcriptional regulation in Chlamydomonas, achieved when alternative splicing introduces an in-frame premature termination codon that triggers the decay pathway.

Splicing in Chlamydomonas fluctuates through every phase of the cell cycle. About 29 percent of events overlap across the light G1, S-M and dark D1 phases, but some are unique to a single phase: roughly 19 percent occur only in S-M, about 17 percent exclusively in dark G1, and nearly 10 percent only in light G1. Most genes show no significant change in expression as they progress through the cycle, indicating that splicing events are regulated separately from transcription. Intron retention in Chlamydomonas can introduce premature termination codons that disrupt RNA recognition motif domains, directly influencing gene expression. Cold stress tells a similar story. Low temperatures induce three splicing factors, including two from the U1 snRNP family, while other factors such as SC35-like protein and H/ACA snoRNP are lost, suggesting the spliceosome machinery is physically rebuilt to adapt. Splicing events also precede structural ribosomal changes in the cytoplasm, mitochondria and plastid, meaning cold reshapes the proteome through translation and possible post-translational processes.

Nitrogen deprivation activates an even broader splicing program. Under these conditions, 3500 genes undergo alternative splicing, totaling 5806 events, heavily concentrated in spliceosome genes and transporter pathways. The events appear to dampen the citrate cycle and fatty acid degradation to help regulate triacylglycerol accumulation, modify membrane lipids through peroxisome-related genes, and induce amino acid and peptide metabolism for protein degradation. Intron-mediated enhancement adds another regulatory layer: the second intron of the RuBisCo small subunit gene boosts nuclear transgene expression 5.5-fold, and exogenous introns with splice sites resembling native ones achieve high enhancement, while foreign introns without recognizable boundaries actually reduce expression. A TFIIS-like protein, conserved from Arabidopsis where it is essential for heat stress adaptation and alternative splicing, may play a similar regulatory role in Chlamydomonas.

Dunaliella salina, a wall-less halotolerant green alga prized for beta-carotene production, takes a different genomic approach. Its genome is 343.7 megabase pairs, roughly three times larger than Chlamydomonas, with 53 percent introns compared to 30 percent. Its mitochondrial DNA carries about 1.5 introns per gene and its plastid DNA about 0.4, with 18 group I introns in the former and 43 introns in the latter, 35 of them group I and eight group II. Under hypersaline stress, the species increases spliceosome processing to manage genes critical for salt response. During palmella formation, a vegetative state induced by salinity changes, RNA processing proteins decline while G-patch domain-containing proteins and DEAD-box helicases shift in abundance, and phosphorylation of the RNA polymerase subunit RPB8 changes in ways that would affect polymerase assembly. The MEK1 gene produces two splice variants: the full-length DsMEK1-X2, preferred under salt stress, drives glycerol accumulation for osmotic regulation, while the truncated DsMEK1-X1, preferred under oxidative stress, features only a partial kinase domain and interacts weakly with partners. Four transcripts encoding the 19-kilodalton subunit of respiratory complex I have also been isolated, though their functions remain unreported.

Haematococcus pluvialis, famous for synthesizing astaxanthin, an antioxidant 10 times more potent than beta-carotene and 550 times more potent than vitamin E, shows intron retention as its dominant splicing mode across a 669-megabase draft genome containing 18,545 genes. Its delta-12 fatty acid desaturase gene HpFAD2, with a 1137-base-pair open reading frame coding for 378 amino acids and nine introns, increases transcription under light stress, possibly implying higher splicing rates. Beyond these models, sequencing advances have revealed 3137 alternative splicing events in the bloom-forming dinoflagellate Akashiwo sanguinea, 2030 new loci in the brown alga Ectocarpus, and significant splicing increases when Auxenochlorella protothecoides shifts from autotrophic to heterotrophic growth, with 705 differentially expressed genes and 660 differentially abundant sequences identified. In Haematococcus lacustris, splicing processes the channelrhodopsin gene into a protein unable to interact with light, while a fragment of the Bracteacoccus aggregatus gene yields a functional light-responsive protein, suggesting splicing may be crucial for light-regulated cellular functions across non-model algae.

Cyanobacteria, photosynthetic prokaryotes, play an entirely different splicing game based on self-splicing catalytic RNAs and proteins. Group I introns, conserved catalytic RNAs with paired segments P1 through P10, splice themselves out. One such intron sits in the ribonucleotide reductase gene of Nostoc punctiforme PCC73102, where splicing is essential to assemble the functional enzyme that catalyzes deoxyribonucleotide production for DNA replication. The Anabaena PCC7120 pre-tRNA-Leu intron becomes a ribozyme after self-splicing, and its shorter 3-base-pair internal guide sequence, compared to the 6-base-pair version in the well-studied Tetrahymena ribozyme, holds substrates more loosely, slowing single-turnover cleavage but enabling more ready splicing under multiple-turnover conditions. Group II introns, regarded as evolutionary ancestors of the eukaryotic spliceosome’s components, excise themselves as lariats with 2′-5′ phosphodiester bonds, and twintrons, introns within introns, require the inner intron to remove itself before the host can be properly spliced. Inteins, mobile protein elements that splice themselves out of flanking exteins, follow a tightly coordinated four-step mechanism: thioester formation at the N-terminal junction, branched intermediate creation, asparagine cyclization and intein release, then a spontaneous acyl shift converting the ester bond between exteins into an amide bond. Split inteins, which reassociate from separate polypeptide halves with high affinity, serve as powerful tools for protein engineering, though divalent copper ions can block their trans-splicing by oxidizing terminal cysteine residues, a block reversible only through chelation and reduction.

The practical implications are considerable. Microalgae can be cultivated at scale in photobioreactors, now increasingly designed with 3D printing flexibility to cater to specific species’ growth requirements, and stress conditions enhance the accumulation of lipids, carotenoids and other prized products. Understanding how alternative splicing mediates stress tolerance could therefore translate directly into productivity gains. CRISPR-Cas9 editing is already delivering results: knocking out the KJFTSY gene in the industrial alga Coccomyxa sp. strain KJ, which can synthesize triacylglycerols comprising more than 60 percent of dry weight under nitrogen deficiency, and editing transcription factors in Nannochloropsis gaditana to produce mutants with twice the lipid content of wild type without slowing growth. Detection methods are advancing too, with short-read and long-read RNA sequencing, percent-spliced-in calculations using tools like rMATS and SUPPA2, and specialized detectors such as IRFinder for intron retention and Whippet for complex events. A ΔPSI greater than 0.1 is typically regarded as a strict criterion for confirming significant splicing events. As sequencing and gene editing mature in tandem with bioreactor design, the molecular editing systems of microalgae and the self-splicing machinery of cyanobacteria may become levers for engineering the photosynthetic cell factories of the future.

Subject of Research: Pre-mRNA splicing regulation in microalgae and cyanobacteria

Article Title: Pre-mRNA splicing and its regulation in microalgae and cyanobacteria

Article References: Do, S., Liu, Y., Huynh, H., Liu, Y., Li, W., Chen, M.-X., & Du, Z.-Y. (2025). Pre-mRNA splicing and its regulation in microalgae and cyanobacteria. Advanced Biotechnology, 3(4), Article 35. https://doi.org/10.1007/s44307-025-00087-3

Image Credits: AI Generated

DOI: 10.1007/s44307-025-00087-3

Keywords: alternative splicing, microalgae, cyanobacteria, intron retention, Chlamydomonas reinhardtii, Dunaliella salina, Haematococcus pluvialis, group I introns, group II introns, inteins, spliceosome, stress response

The story begins with the mutant phenotype itself. Seedlings carrying the yll mutation in the homozygous state show a dramatic reduction in chlorophyll content, the green pigment complexes that harvest light in the chloroplast. Their photosystem II, the protein complex at the heart of the light reactions of photosynthesis, performs poorly, and the plants do not survive past the seedling stage. Because the trait is recessive, plants inheriting only one copy of the mutation look perfectly normal, which is exactly the pattern that makes such mutants so useful for genetic analysis. Chlorophyll-deficient mutants have long served as entry points for dissecting chloroplast biology in crops, and soybean, one of the world’s most important oilseed and protein crops, has relatively few such well-characterized loci.

To find the gene responsible, the researchers turned to classical genetic mapping, but at high resolution. By following the inheritance of molecular markers across segregating populations, they narrowed the yll locus to a remarkably small stretch of DNA: a 165-kilobase interval on chromosome 10. In the reference soybean genome annotation known as Wm82.a6.v1, that interval contains just two genes. This is the kind of genetic compression that mapping projects dream of, because it converts a genome-wide search of tens of thousands of genes into a two-horse race.

Of the two genes in the interval, one emerged as the clear frontrunner. Glyma.10G090700, which the authors designate GmPAO7, encodes a protein annotated as a polyamine oxidase, an enzyme of the PAO1-like family. Several independent lines of evidence converged on it. First, the gene is expressed in leaves, the tissue where the phenotype manifests. Second, its transcript abundance is reduced in yll seedlings compared with their normal siblings. Third, and most strikingly, the mutant plants carry a reproducible splicing defect: a 110-base-pair intron between the first and second exons of the gene is retained in the mature messenger RNA instead of being excised.

The consequences of that retained intron are severe. In molecular terms, the extra sequence introduces a premature termination codon, a stop signal that halts protein synthesis far earlier than it should. Computational prediction suggests that instead of the annotated 500-amino-acid PAO1-like protein, the mutant would produce a truncated peptide of only 32 amino acids. That fragment would lack both of the domains that define the enzyme’s function: the flavin amine oxidase domain and the NAD(P)-binding domain. In other words, if the prediction holds, the mutant plants would be effectively devoid of a functional GmPAO7 protein.

The splicing evidence was verified the old-fashioned way, by direct sequencing. The researchers amplified the relevant region of the gene’s complementary DNA by RT-PCR and read the sequence of the products. Wild-type plants showed the normally spliced transcript, with the intron cleanly removed and the first exon joined directly to the second. Homozygous mutant plants showed the intron-retained sequence. Heterozygous plants, carrying one normal and one mutant copy, displayed overlapping splice forms, with sequencing chromatograms showing mixed signals downstream of the splice junction, consistent with both transcript types being present. The authors are careful to note that this qualitative assay demonstrates the presence of the aberrant transcript but does not establish the relative proportions of the splice forms in any genotype.

To understand what the mutation does to the plant more broadly, the team performed RNA sequencing on mutant and wild-type sibling seedlings at the same developmental stage. The comparison revealed widespread expression differences, with significant enrichment of genes involved in photosynthesis, chloroplast function, and redox metabolism. A protein-protein interaction network built from the differentially expressed genes, using the STRING database with a confidence threshold of 0.700, retained a connected core of 304 nodes and 3,278 predicted interactions, organized into modules covering photosynthesis and chloroplast function, ribosome and translation, RNA processing, redox metabolism, and protein folding. Supplementary heatmaps also documented changes in transcription factor families including MYB, NAC, WRKY, bHLH, ERF/AP2, and bZIP, as well as in genes associated with the major plant hormones: auxin, gibberellin, cytokinin, abscisic acid, and ethylene.

Here the authors exercise an important caution. Because yll seedlings are severely chlorotic and developmentally compromised, the transcriptomic differences they observe cannot be taken as a clean readout of the gene’s primary function. A dying, yellow seedling is a very different biological system from a healthy green one, and many of the expression changes may simply reflect the downstream consequences of a failing chloroplast rather than direct regulatory effects of the candidate gene. The profiles are best interpreted, the authors write, as the transcriptomic state associated with the phenotype. This distinction matters, and it is the kind of nuance that separates careful candidate-gene studies from overreach.

Complementary evidence came from the electron microscope. Transmission electron microscopy of mutant leaves revealed chloroplast ultrastructural abnormalities: grana, the stacked membrane structures where photosystem II concentrates, were poorly developed, and the thylakoid membranes were disorganized. These defects are entirely consistent with the reduced chlorophyll content and impaired photosystem II performance measured physiologically. But as the authors point out, ultrastructural observations, while suggestive, do not by themselves functionally validate the candidate gene. They describe the cellular wreckage without proving which genetic wrench caused it.

That proof remains the outstanding task. Glyma.10G091000, the second gene in the 165-kilobase interval, has not been excluded, and the authors are explicit that functional complementation, introducing a normal copy of GmPAO7 into the mutant to see whether it rescues the phenotype, or the identification of an independent allele of the same gene, will be required to establish causality. They also flag a tempting but untested hypothesis: a connection between polyamine oxidase activity, polyamine metabolism, and reactive oxygen species. Polyamine oxidases are known to participate in redox reactions that can generate hydrogen peroxide, and such a pathway could plausibly tie a defective oxidase to chloroplast damage and cell death. But the authors did not measure PAO enzyme activity, polyamine concentrations, or reactive oxygen species levels in this study, so that link remains a hypothesis to be tested, not a finding.

Even with those caveats, the study is a textbook example of how modern crop genetics proceeds. A lethal phenotype is captured, mapped to a small genomic interval, and a candidate is prioritized by triangulating expression data, transcript abundance, and a concrete molecular lesion, in this case an intron-retention event that would demolish the encoded protein. If GmPAO7 is confirmed, it would add a new entry to the short list of genes whose disruption produces chlorophyll-deficient, seedling-lethal phenotypes in soybean, and it would spotlight alternative splicing, a process often overlooked in crop mutation screens, as a source of catastrophic developmental defects. It would also raise broader questions about what polyamine oxidation contributes to chloroplast maintenance in one of the world’s staple legumes. For now, the strongest candidate stands accused, the evidence is circumstantial but coherent, and the decisive experiment, complementation, is the next act in a story that began with a handful of yellow seedlings that never made it out of the seedling tray.

Subject of Research: Fine mapping and candidate gene analysis of a soybean yellow-leaf lethal mutant implicating aberrantly spliced polyamine oxidase gene GmPAO7

Article Title: Fine mapping of yll prioritizes aberrantly spliced Glyma.10G090700 (GmPAO7) as the strongest candidate in a soybean yellow-leaf lethal mutant

Article References: Chen, L., Feng, J., Ning, J., Shi, X., Wu, X., Che, X., Li, Z., Yu, Z., Wang, X., Chen, Q., Sun, H., Guo, Y., Liu, L., Ma, Z., Li, H., Lin, J., & Zhao, J. (2026). Fine mapping of yll prioritizes aberrantly spliced Glyma.10G090700 (GmPAO7) as the strongest candidate in a soybean yellow-leaf lethal mutant. BMC Plant Biology. https://doi.org/10.1186/s12870-026-10015-9

Image Credits: AI Generated

DOI: 10.1186/s12870-026-10015-9

Keywords: soybean, yellow-leaf lethal mutant, GmPAO7, polyamine oxidase, intron retention, alternative splicing, chloroplast, chlorophyll deficiency, fine mapping, candidate gene, RNA-seq, photosystem II

Lsm8 is the defining subunit of the nuclear Lsm2-8 complex, a heptameric ring of Like-Sm proteins that binds the 3-prime end of U6 small nuclear RNA, the catalytic core of the spliceosome. The spliceosome is the macromolecular machine that removes introns from pre-messenger RNA, and without properly assembled Lsm2-8, U6 is left unprotected and splicing cycles falter. Sequence analysis showed that the Fusarium Lsm8 protein shares roughly half its identity with its yeast counterpart and about 65 percent with the human version, underscoring how ancient this module is. Although Lsm8 lacks a predicted nuclear localization signal, a fluorescent Lsm8-GFP fusion co-localized precisely with a histone marker in fungal nuclei, confirming where the protein operates.

To map the complex’s architecture, the team deployed a battery of interaction assays. Yeast two-hybrid tests revealed direct contacts between Lsm8 and Lsm2 and Lsm3, bimolecular fluorescence complementation captured a nuclear interaction between Lsm8 and Lsm4, and co-immunoprecipitation validated all of these associations in living fungal cells. Most decisively, affinity purification followed by mass spectrometry, using Lsm8-GFP as bait, co-purified all six remaining Lsm subunits. The picture that emerged is of Lsm8 as a central architectural keystone. When the researchers deleted the LSM8 gene, GFP-tagged Lsm2, Lsm3 and Lsm4 abandoned the nucleus entirely and diffused throughout the cytoplasm, demonstrating that Lsm8 is indispensable for both assembling the ring and escorting it to its workplace.

The consequences of losing this keystone were dramatic. Deletion mutants grew slowly on every medium tested, from rich potato dextrose agar to minimal medium. Transcriptome sequencing revealed massive reprogramming: 1,710 genes were significantly upregulated and 1,796 downregulated. The upregulated set was enriched in ribosome biogenesis, spliceosome function and RNA degradation, which the authors interpret as a compensatory feedback response to the missing splicing machinery. The downregulated genes clustered in carbon and nitrogen metabolism, including glycolysis and the pentose phosphate pathway, providing a molecular explanation for the growth collapse. Quantitative PCR confirmed the sequencing results for splicing factors such as PRP4 and PRP5, degradation factors such as RRP6 and XRN2, and glycolytic genes such as FBA1 and ENO1.

The most striking defect, however, lay in splicing itself. Across 13,982 detected introns, the median intron retention rate jumped from 0.011 in the wild type to 0.089 in the mutant, a highly significant eight-fold increase. In total, 2,344 introns from 1,821 genes showed significantly elevated retention, while only 45 introns became better spliced. Crucially, most of the affected genes, 1,270 of them, showed no change in overall transcript abundance, meaning the damage was purely post-transcriptional: transcripts were made normally but never spliced correctly. Retained introns disrupt reading frames, producing truncated or non-functional proteins, or triggering nonsense-mediated decay. The affected genes were enriched in chromatin remodeling, RNA polymerase activity, nucleocytoplasmic transport and DNA repair, processes essential to cellular integrity.

The splicing failures mapped directly onto the fungus’s environmental vulnerabilities. When challenged with a panel of eight stress agents, the mutant proved hypersensitive to all of them, with the strongest effects under osmotic stress from salt and the fungicide fludioxonil, which works by hyperactivating the osmotic stress pathway. The team traced this to the high osmolarity glycerol, or HOG, MAPK cascade. HOG1 carries eight introns, and nearly all of them were inefficiently spliced in the mutant, with defects worsening under stress. Total HOG1 mRNA levels were unchanged, but Hog1 protein levels dropped significantly, showing that the defect lies in protein production rather than transcription. Intriguingly, the residual Hog1 that remained was hyperphosphorylated, and intracellular glycerol accumulated to roughly three times wild-type levels, suggesting distorted pathway feedback. The downstream transcription factor ATF1 also showed splicing abnormalities.

Reproduction fared no better. The mutant produced fewer conidia, the asexual spores that spread infection, and those spores were shorter and abnormally septated. Sexual development was abolished outright: on carrot agar under black light, the mutant formed no perithecia at all, the fruiting bodies that generate overwintering ascospores. The molecular culprits were again splicing defects, this time in two developmental regulators. GPA1, encoding a heterotrimeric G protein alpha subunit, retained all three of its introns at elevated rates, and STE12, a transcription factor essential for sexual reproduction, retained both of its introns. As with HOG1, total mRNA levels were largely unaffected, reinforcing the theme that Lsm8’s influence operates after transcription.

Virulence and mycotoxin synthesis collapsed in parallel. Infection assays on wheat heads, wheat coleoptiles and corn silks showed the mutant was nearly nonpathogenic, and deoxynivalenol production fell sharply. The TRI gene cluster that manufactures the toxin was hit twice: key genes including TRI1, TRI4, TRI5, TRI6, TRI10 and TRI101 were transcriptionally downregulated, and the splicing efficiency of TRI1, TRI4 and TRI5 transcripts was significantly reduced. Microscopy of a Tri1-GFP reporter delivered perhaps the most visually compelling result: in the wild type, Tri1 aggregates into the spherical toxisomes, specialized compartments where deoxynivalenol is synthesized, but in the mutant almost no toxisomes formed and Tri1-GFP protein levels plummeted. Lsm8 thus guards the fungus’s chemical arsenal at every regulatory level simultaneously.

Finally, the study answered a long-standing question about what happens to all those defective transcripts. In eukaryotes, aberrant RNAs are normally cleared by nonsense-mediated decay, the RNA exosome, which degrades transcripts in the 3-prime to 5-prime direction, or by Xrn enzymes working in the opposite direction. Mass spectrometry showed that Lsm8 physically interacts with core exosome subunits including Rrp45, Rrp4 and Dis3, and the mutant compensated by upregulating exosome components. Genetic dissection proved decisive: intron-retained HOG1 transcripts accumulated massively in an exosome-subunit deletion mutant and even more in an Lsm8-exosome double mutant, while accumulation in an Xrn1 mutant was comparatively modest. The exosome, the authors conclude, is the predominant degrader of intron-retained transcripts arising from Lsm2-8 dysfunction, with Xrn1 playing a supporting role. Because both the Lsm2-8 complex and the exosome are conserved across eukaryotes, from yeast and plants to worms and humans, this coupled splicing-and-surveillance module appears to be a fundamental feature of eukaryotic biology. In Candida albicans, a human fungal pathogen, Lsm loss already impairs filamentation and pathogenicity, suggesting the Fusarium findings generalize. Targeting this conserved module, the researchers argue, could yield antifungal strategies that strike at core RNA-processing machinery shared by diverse pathogens, simultaneously curbing crop disease and mycotoxin contamination while sparing the host, whose own cells process RNA through homologous but distinct regulatory contexts.

Subject of Research: RNA splicing fidelity and the Lsm8-exosome surveillance module in fungal stress adaptation and virulence

Article Title: A conserved Lsm8–exosome module maintains RNA splicing fidelity to control fungal stress adaptation and virulence

Article References: Ren, Y., Cheng, H., Han, X., Guo, M., Xu, C., Yan, J., Ge, Z., Ma, Z., & Chen, Y. (2026). A conserved Lsm8–exosome module maintains RNA splicing fidelity to control fungal stress adaptation and virulence. Stress Biology, 6(1), Article 14. https://doi.org/10.1007/s44154-026-00285-6

Image Credits: AI Generated

DOI: 10.1007/s44154-026-00285-6

Keywords: Fusarium graminearum, Lsm8, Lsm2-8 complex, RNA splicing, intron retention, RNA exosome, deoxynivalenol, mycotoxin biosynthesis, HOG-MAPK pathway, fungal virulence, post-transcriptional regulation, antifungal targets

The research, led by Taronish Biayna, Anton Baranovskii and colleagues, provides one of the most detailed portraits yet of how cells manage intron retention, a form of RNA processing control in which an intron, a segment of RNA that is normally spliced out and discarded, is deliberately kept within a mature transcript. Rather than representing a processing error, the study shows that intron retention operates as a genuine regulatory strategy, one that allows cells to pre-manufacture messenger RNAs and hold them in reserve until the precise moment their protein products are needed.

Nuclear speckles, also called interchromatin granule clusters, are dense assemblies of splicing factors and other RNA-processing proteins that punctuate the nucleoplasm. For decades they have been viewed primarily as storage and recycling depots for the molecular machinery that removes introns. The new work adds an unexpected twist: the periphery of these speckles, the thin shell of material surrounding each cluster, functions as a dedicated holding zone for a specific and functionally coherent set of long-lived intron-retained RNAs. The RNAs are not inside the speckles themselves but accumulate in a defined rim around them, suggesting that the boundary between the speckle interior and the surrounding nucleoplasm is an active regulatory interface rather than a passive border.

To reach this conclusion, the team combined super-resolution imaging with single-molecule RNA detection methods, allowing them to see precisely where individual RNA molecules reside relative to speckle markers. They complemented these spatial maps with metabolic labeling approaches that reveal RNA age, showing that the transcripts parked at the speckle periphery are remarkably stable, persisting far longer than typical messenger RNAs, which are usually made, used and degraded within hours. This longevity is a key feature of the compartment: it enables the cell to accumulate a ready supply of transcripts whose final activation can be deferred.

What makes the discovery particularly striking is the identity of the genes whose RNAs are stored in this way. The researchers found that the pooled intron-retained transcripts are disproportionately derived from genes involved in cell cycle regulation and genome maintenance, the very processes a cell must coordinate with exquisite timing as it prepares to divide. By retaining an intron, the cell prevents these RNAs from being translated into protein while keeping them intact and available. The speckle periphery thus acts as a staging ground where the molecular instructions for division and DNA repair are held in check until the proper moment.

That moment, the study shows, arrives at mitosis. When a cell commits to division, its nuclear envelope breaks down, chromosomes condense, and the ordinary architecture of the nucleus dissolves. The researchers observed that the intron-retained RNAs accumulated at the speckle periphery undergo synchronous splicing during this transition, with the retained introns removed en masse as the cell divides. In other words, the entire stockpile is resolved at once, releasing a coordinated wave of newly matured messenger RNAs precisely when the cell reorganizes its contents and re-establishes gene expression in two daughter cells. This synchronized resolution suggests a mechanism by which cells couple RNA maturation directly to the physical events of division.

The finding reframes intron retention from a curiosity of RNA biology into a temporally precise control point. In many previous studies, intron-retained transcripts were viewed mainly as substrates for surveillance pathways that degrade defective RNAs. The new evidence indicates that at least a substantial fraction of retained transcripts are instead functional intermediates, deliberately produced and stored. Their sequestration at the speckle periphery provides a physical address that separates them from the translation machinery and from the splicing factors that would otherwise complete their maturation, while keeping them close to the very machinery they will eventually need.

The spatial logic of the system is as important as the temporal one. By concentrating related RNAs in a shared compartment, the cell may achieve coordination that would be difficult to accomplish with freely diffusing molecules. Grouping cell cycle and genome maintenance transcripts at the speckle rim could allow a single regulatory event, such as the reactivation of splicing during mitosis, to act on many genes simultaneously, ensuring that their protein products appear together rather than in a stochastic trickle. This kind of collective behavior echoes themes familiar from other membrane-less organelles, where concentrated molecules can switch states rapidly in response to cellular cues.

The work also raises new questions that the field is now poised to explore. What molecular signals mark particular intron-retained RNAs for delivery to the speckle periphery, and what prevents their premature splicing during interphase? How is the compartment disassembled and reassembled across successive rounds of the cell cycle, and does the synchronous splicing at mitosis depend on the wholesale reorganization of the nucleus or on dedicated regulatory factors? Answers to these questions could illuminate how cells safeguard the fidelity of division, and how that safeguarding fails in diseases such as cancer, where both splicing patterns and cell cycle control are frequently disrupted.

For now, the study stands as a vivid demonstration that the nucleus is organized with far more purpose than a bag of freely mixing parts. At the edge of each nuclear speckle, cells maintain a carefully curated archive of unfinished messages, genes for division and genome care held in a long-lived, spatially defined waiting room. When mitosis arrives, the doors open all at once, and the archive is converted, in a single synchronized act of splicing, into the working instructions for building two new cells.

Subject of Research: Spatial and temporal regulation of long-lived intron-retained RNAs at nuclear speckle peripheries and their synchronous splicing during mitosis

Article Title: The periphery of nuclear speckles defines a spatially and temporally regulated compartment of long-lived intron-retained RNAs that resolves during mitosis

Article References: Biayna, J., Baranovskii, A., Chaudhuri, A., Paladin, M., Erdem, B., Keller, L.-E., Barutcu, A. R., Dimmeler, S., Marsico, A., & Dumbović, G. (2026). The periphery of nuclear speckles defines a spatially and temporally regulated compartment of long-lived intron-retained RNAs that resolves during mitosis. Nature Cell Biology, 28(9), 1857-1874. https://doi.org/10.1038/s41556-026-02040-5

Image Credits: AI Generated

DOI: 10.1038/s41556-026-02040-5

Keywords: nuclear speckles, intron retention, RNA splicing, mitosis, cell cycle, genome maintenance, RNA processing, nuclear organization, long-lived RNAs, speckle periphery, gene expression, Nature Cell Biology