Telomeres, essential for maintaining genomic stability, are typically preserved through the action of telomerase, a ribonucleoprotein complex that synthesizes telomeric DNA. One of its two core components, telomerase RNA (TR), serves as the template for this synthesis, and its evolution across different species is both complex and diverse. This review discusses recent advancements in understanding TR evolution, with a focus on plants (Viridiplantae). Utilizing novel bioinformatic tools and accumulating genomic and transcriptomic data, combined with corresponding experimental validation, researchers have begun to unravel the intricate pathways of TR evolution and telomere maintenance mechanisms. Contrary to previous beliefs, a monophyletic origin of TR has been demonstrated first in land plants and subsequently across the broader phylogenetic megagroup Diaphoretickes. Conversely, the discovery of plant-type TRs in insects challenges assumptions about the monophyletic origin of TRs in animals, suggesting evolutionary innovations coinciding with arthropod divergence. The review also highlights key challenges in TR identification and provides examples of how these have been addressed. Overall, this work underscores the importance of expanding beyond model organisms to comprehend the full complexity of telomerase evolution, with potential applications in agriculture and biotechnology.

• Keywords
• TR evolution, TR identification, non-coding RNA, telomerase RNA, telomere,
• MeSH
• Phylogeny MeSH
• Evolution, Molecular * MeSH
• RNA, Plant * genetics   MeSH
• RNA * genetics   metabolism   MeSH
• Plants * genetics   MeSH
• Telomerase * genetics   metabolism   MeSH
• Telomere * genetics   metabolism   MeSH
• Viridiplantae * genetics   MeSH
• Animals MeSH
• Check Tag
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Review MeSH
• Names of Substances
• RNA, Plant * MeSH
• RNA * MeSH
• Telomerase * MeSH
• telomerase RNA MeSH Browser

The current repertoire of methods available for studying RNA-protein interactions in plants is somewhat limited. Employing an RNA-centric approach, particularly with less abundant RNAs, presents various challenges. Many of the existing methods were initially designed for different model systems, with their application in plants receiving limited attention thus far. The Comprehensive Identification of RNA-Binding Proteins by Mass Spectrometry (ChIRP-MS) technique, initially developed for mammalian cells, has been adapted in this study for application in Arabidopsis thaliana. The procedures have been meticulously modified and optimized for telomerase RNA, a notable example of a low-abundance RNA recently identified. Following these optimization steps, ChIRP-MS can serve as an effective screening method for identifying candidate proteins interacting with any target RNA of interest.

• Keywords
• Arabidopsis thaliana, ChIRP-MS, RNA, RNA-centric methods, RNA-protein interactions, telomerase, telomerase RNA,
• Publication type
• Journal Article MeSH

The enormous sequence heterogeneity of telomerase RNA (TR) subunits has thus far complicated their characterization in a wider phylogenetic range. Our recent finding that land plant TRs are, similarly to known ciliate TRs, transcribed by RNA polymerase III and under the control of the type-3 promoter, allowed us to design a novel strategy to characterize TRs in early diverging Viridiplantae taxa, as well as in ciliates and other Diaphoretickes lineages. Starting with the characterization of the upstream sequence element of the type 3 promoter that is conserved in a number of small nuclear RNAs, and the expected minimum TR template region as search features, we identified candidate TRs in selected Diaphoretickes genomes. Homologous TRs were then used to build covariance models to identify TRs in more distant species. Transcripts of the identified TRs were confirmed by transcriptomic data, RT-PCR and Northern hybridization. A templating role for one of our candidates was validated in Physcomitrium patens. Analysis of secondary structure demonstrated a deep conservation of motifs (pseudoknot and template boundary element) observed in all published TRs. These results elucidate the evolution of the earliest eukaryotic TRs, linking the common origin of TRs across Diaphoretickes, and underlying evolutionary transitions in telomere repeats.

• MeSH
• Transcription, Genetic MeSH
• Nucleic Acid Conformation MeSH
• Evolution, Molecular * MeSH
• Mutation MeSH
• RNA, Plant biosynthesis   chemistry   genetics   MeSH
• RNA Polymerase II metabolism   MeSH
• RNA Polymerase III metabolism   MeSH
• RNA biosynthesis   chemistry   genetics   MeSH
• Sequence Alignment MeSH
• Telomerase biosynthesis   chemistry   genetics   MeSH
• Telomere chemistry   MeSH
• Transcriptome MeSH
• Viridiplantae genetics   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Research Support, U.S. Gov't, Non-P.H.S. MeSH
• Names of Substances
• RNA, Plant MeSH
• RNA Polymerase II MeSH
• RNA Polymerase III MeSH
• RNA MeSH
• Telomerase MeSH
• telomerase RNA MeSH Browser

The canonical DNA polymerases involved in the replication of the genome are unable to fully replicate the physical ends of linear chromosomes, called telomeres. Chromosomal termini thus become shortened in each cell cycle. The maintenance of telomeres requires telomerase-a specific RNA-dependent DNA polymerase enzyme complex that carries its own RNA template and adds telomeric repeats to the ends of chromosomes using a reverse transcription mechanism. Both core subunits of telomerase-its catalytic telomerase reverse transcriptase (TERT) subunit and telomerase RNA (TR) component-were identified in quick succession in Tetrahymena more than 30 years ago. Since then, both telomerase subunits have been described in various organisms including yeasts, mammals, birds, reptiles and fish. Despite the fact that telomerase activity in plants was described 25 years ago and the TERT subunit four years later, a genuine plant TR has only recently been identified by our group. In this review, we focus on the structure, composition and function of telomerases. In addition, we discuss the origin and phylogenetic divergence of this unique RNA-dependent DNA polymerase as a witness of early eukaryotic evolution. Specifically, we discuss the latest information regarding the recently discovered TR component in plants, its conservation and its structural features.

• Keywords
• evolution, plant TERT, plant TR., telomerase, telomerase RNA (TR), telomerase reverse transcriptase (TERT),
• MeSH
• Biological Evolution * MeSH
• History, 20th Century MeSH
• History, 21st Century MeSH
• Eukaryota classification   genetics   metabolism   MeSH
• Phylogeny MeSH
• Humans MeSH
• RNA physiology   MeSH
• Telomerase chemistry   physiology   MeSH
• Telomere metabolism   MeSH
• Animals MeSH
• Check Tag
• History, 20th Century MeSH
• History, 21st Century MeSH
• Humans MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Historical Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Review MeSH
• Names of Substances
• RNA MeSH
• Telomerase MeSH
• telomerase RNA MeSH Browser

To elucidate the molecular nature of evolutionary changes of telomeres in the plant order Asparagales, we aimed to characterize telomerase RNA subunits (TRs) in these plants. The unusually long telomere repeat unit in Allium plants (12 nt) allowed us to identify TRs in transcriptomic data of representative species of the Allium genus. Orthologous TRs were then identified in Asparagales plants harbouring telomere DNA composed of TTAGGG (human type) or TTTAGGG (Arabidopsis-type) repeats. Further, we identified TRs across the land plant phylogeny, including common model plants, crop plants, and plants with unusual telomeres. Several lines of functional testing demonstrate the templating telomerase function of the identified TRs and disprove a functionality of the only previously reported plant telomerase RNA in Arabidopsis thaliana. Importantly, our results change the existing paradigm in plant telomere biology which has been based on the existence of a relatively conserved telomerase reverse transcriptase subunit (TERT) associating with highly divergent TRs even between closely related plant taxa. The finding of a monophyletic origin of genuine TRs across land plants opens the possibility to identify TRs directly in transcriptomic or genomic data and/or predict telomere sequences synthesized according to the respective TR template region.

• MeSH
• Allium genetics   MeSH
• Arabidopsis genetics   MeSH
• Asparagales genetics   MeSH
• Phylogeny * MeSH
• Genome, Plant genetics   MeSH
• Humans MeSH
• Evolution, Molecular * MeSH
• RNA genetics   MeSH
• Telomerase genetics   MeSH
• Telomere genetics   MeSH
• Embryophyta genetics   MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• RNA MeSH
• Telomerase MeSH
• telomerase RNA MeSH Browser

Parallel research on multiple model organisms shows that while some principles of telomere biology are conserved among all eukaryotic kingdoms, we also find some deviations that reflect different evolutionary paths and life strategies, which may have diversified after the establishment of telomerase as a primary mechanism for telomere maintenance. Much more than animals, plants have to cope with environmental stressors, including genotoxic factors, due to their sessile lifestyle. This is, in principle, made possible by an increased capacity and efficiency of the molecular systems ensuring maintenance of genome stability, as well as a higher tolerance to genome instability. Furthermore, plant ontogenesis differs from that of animals in which tissue differentiation and telomerase silencing occur during early embryonic development, and the "telomere clock" in somatic cells may act as a preventive measure against carcinogenesis. This does not happen in plants, where growth and ontogenesis occur through the serial division of apical meristems consisting of a small group of stem cells that generate a linear series of cells, which differentiate into an array of cell types that make a shoot and root. Flowers, as generative plant organs, initiate from the shoot apical meristem in mature plants which is incompatible with the human-like developmental telomere shortening. In this review, we discuss differences between human and plant telomere biology and the implications for aging, genome stability, and cell and organism survival. In particular, we provide a comprehensive comparative overview of telomere proteins acting in humans and in Arabidopsis thaliana model plant, and discuss distinct epigenetic features of telomeric chromatin in these species.

• Keywords
• Arabidopsis, aging, chromatin, epigenetics, human, review, telomerase, telomere,
• MeSH
• Chromatin metabolism   MeSH
• Epigenesis, Genetic MeSH
• Humans MeSH
• Plants metabolism   MeSH
• Cellular Senescence genetics   MeSH
• Telomerase metabolism   MeSH
• Telomere metabolism   MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Review MeSH
• Names of Substances
• Chromatin MeSH
• Telomerase MeSH

Standard pathways involved in the regulation of telomere stability do not contribute to gradual telomere elongation observed in the course of A. thaliana calli propagation. Genetic and epigenetic changes accompanying the culturing of plant cells have frequently been reported. Here we aimed to characterize the telomere homeostasis during long term callus propagation. While in Arabidopsis thaliana calli gradual telomere elongation was observed, telomeres were stable in Nicotiana tabacum and N. sylvestris cultures. Telomere elongation during callus propagation is thus not a general feature of plant cells. The long telomere phenotype in Arabidopsis calli was correlated neither with changes in telomerase activity nor with activation of alternative mechanisms of telomere elongation. The dynamics of telomere length changes was maintained in mutant calli with loss of function of important epigenetic modifiers but compromised in the presence of epigenetically active drug zebularine. To examine whether the cell culture-induced disruption of telomere homeostasis is associated with the modulated structure of chromosome ends, epigenetic properties of telomere chromatin were analysed. Albeit distinct changes in epigenetic modifications of telomere histones were observed, these were broadly stochastic. Our results show that contrary to animal cells, the structure and function of plant telomeres is not determined significantly by the epigenetic character of telomere chromatin. Set of differentially transcribed genes was identified in calli, but considering the known telomere- or telomerase-related functions of respective proteins, none of these changes per se was apparently related to the elongated telomere phenotype. Based on our data, we propose that the disruption in telomere homeostasis in Arabidopsis calli arises from the interplay of multiple factors, as a part of reprogramming of plant cells to long-term culture conditions.

• Keywords
• Arabidopsis thaliana, Callus, Chromosome stability, Epigenetics, Regenerated plants, Telomere,
• MeSH
• Arabidopsis drug effects   genetics   metabolism   MeSH
• Chromatin genetics   MeSH
• Cytidine analogs & derivatives   pharmacology   MeSH
• Species Specificity MeSH
• Ecotype MeSH
• Epigenesis, Genetic drug effects   MeSH
• Histones metabolism   MeSH
• Telomere Homeostasis * drug effects   MeSH
• RNA, Messenger genetics   metabolism   MeSH
• Mutation genetics   MeSH
• Arabidopsis Proteins metabolism   MeSH
• Regeneration drug effects   MeSH
• Genes, Plant MeSH
• Nicotiana genetics   MeSH
• Tissue Culture Techniques * MeSH
• Telomerase metabolism   MeSH
• Telomere metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Chromatin MeSH
• Cytidine MeSH
• Histones MeSH
• RNA, Messenger MeSH
• Arabidopsis Proteins MeSH
• pyrimidin-2-one beta-ribofuranoside MeSH Browser
• Telomerase MeSH

In tobacco, three sequence variants of the TERT gene have been described. We revealed unbalanced levels of TERT variant transcripts in vegetative tobacco tissues and enhanced TERT transcription and telomerase activity in reproductive tissues. Telomerase is a ribonucleoprotein complex responsible for the maintenance of telomeres, structures delimiting ends of linear eukaryotic chromosomes. In the Nicotiana tabacum (tobacco) allotetraploid plant, three sequence variants (paralogs) of the gene coding for the telomerase reverse transcriptase subunit (TERT) have been described, two of them derived from the maternal N. sylvestris genome (TERT_Cs, TERT_D) and one originated from the N. tomentosiformis paternal genome (TERT_Ct). In this work, we analyzed the transcription of TERT variants in correlation with telomerase activity in tobacco tissues. High and approximately comparable levels of TERT_Ct and TERT_Cs transcripts were detected in seedlings, roots, flower buds and leaves, while the transcript of the TERT_D variant was markedly underrepresented. Similarly, in N. sylvestris tissues, TERT_Cs transcript significantly predominated. A specific pattern of TERT transcripts was found in samples of tobacco pollen with the TERT_Cs variant clearly dominating particularly at the early stage of pollen development. Detailed analysis of TERT_C variants representation in functionally distinct fractions of pollen transcriptome revealed their prevalence in large ribonucleoprotein particles encompassing translationally silent mRNA; only a minority of TERT_Ct and TERT_Cs transcripts were localized in actively translated polysomes. Histones of the TERT_C chromatin were decorated predominantly with the euchromatin-specific epigenetic modification in both telomerase-positive and telomerase-negative tobacco tissues. We conclude that the existence and transcription pattern of tobacco TERT paralogs represents an interesting phenomenon and our results indicate its functional significance. Nicotiana species have again proved to be appropriate and useful model plants in telomere biology studies.

• Keywords
• Gene sequence variant, Pollen, Polyploids, Telomerase, Telomere, Transcription,
• MeSH
• Cell Nucleus genetics   MeSH
• Chromatin Immunoprecipitation MeSH
• Euchromatin metabolism   MeSH
• Transcription, Genetic MeSH
• Genetic Variation * MeSH
• Histones metabolism   MeSH
• RNA, Messenger genetics   metabolism   MeSH
• Organ Specificity genetics   MeSH
• Polyribosomes metabolism   MeSH
• Protein Processing, Post-Translational MeSH
• Pollen Tube growth & development   MeSH
• Gene Expression Regulation, Plant * MeSH
• Nicotiana genetics   MeSH
• Telomerase genetics   metabolism   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Euchromatin MeSH
• Histones MeSH
• RNA, Messenger MeSH
• Telomerase MeSH

Telomeres, as physical ends of linear chromosomes, are targets of a number of specific proteins, including primarily telomerase reverse transcriptase. Access of proteins to the telomere may be affected by a number of diverse factors, e.g., protein interaction partners, local DNA or chromatin structures, subcellular localization/trafficking, or simply protein modification. Knowledge of composition of the functional nucleoprotein complex of plant telomeres is only fragmentary. Moreover, the plant telomeric repeat binding proteins that were characterized recently appear to also be involved in non-telomeric processes, e.g., ribosome biogenesis. This interesting finding was not totally unexpected since non-telomeric functions of yeast or animal telomeric proteins, as well as of telomerase subunits, have been reported for almost a decade. Here we summarize known facts about the architecture of plant telomeres and compare them with the well-described composition of telomeres in other organisms.

• Keywords
• plant, shelterin, telomerase, telomere, telomeric proteins, telomeric repeat binding (TRB),
• Publication type
• Journal Article MeSH
• Review MeSH

Telomeres are nucleoprotein structures that distinguish native chromosomal ends from double-stranded breaks. They are maintained by telomerase that adds short G-rich telomeric repeats at chromosomal ends in most eukaryotes and determines the TnAmGo sequence of canonical telomeres. We employed an experimental approach that was based on detection of repeats added by telomerase to identify the telomere sequence type forming the very ends of chromosomes. Our previous studies that focused on the algal order Chlamydomonadales revealed several changes in telomere motifs that were consistent with the phylogeny and supported the concept of the Arabidopsis-type sequence being the ancestral telomeric motif for green algae. In addition to previously described independent transitions to the Chlamydomonas-type sequence, we report that the ancestral telomeric motif was replaced by the human-type sequence in the majority of algal species grouped within a higher order clade, Caudivolvoxa. The Arabidopsis-type sequence was apparently retained in the Polytominia clade. Regarding the telomere sequence, the Chlorogonia clade within Caudivolvoxa bifurcates into two groups, one with the human-type sequence and the other group with the Arabidopsis-type sequence that is solely formed by the Chlorogonium species. This suggests that reversion to the Arabidopsis-type telomeric motif occurred in the common ancestral Chlorogonium species. The human-type sequence is also synthesized by telomerases of algal strains from Arenicolinia, Dunaliellinia and Stephanosphaerinia, except a distinct subclade within Stephanosphaerinia, where telomerase activity was not detected and a change to an unidentified telomeric motif might arise. We discuss plausible reasons why changes in telomeric motifs were tolerated during evolution of green algae.

• Keywords
• 18S rDNA phylogeny, Green algae, TRAP, Telomerase activity, Telomere evolution,
• MeSH
• Amino Acid Motifs genetics   MeSH
• Phylogeny MeSH
• Repetitive Sequences, Nucleic Acid genetics   MeSH
• DNA, Ribosomal genetics   MeSH
• RNA, Ribosomal, 18S genetics   MeSH
• Base Sequence MeSH
• Sequence Analysis, DNA MeSH
• Telomerase genetics   MeSH
• Telomere genetics   MeSH
• Volvocida genetics   MeSH
• Telomere Shortening genetics   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• DNA, Ribosomal MeSH
• RNA, Ribosomal, 18S MeSH
• Telomerase MeSH