Substoichiometric shifting in the plant mitochondrial genome is influenced by a gene homologous to MutS

Cytoplasmic inheritance: The transmission of plastid and mitochondrial genomes across cells and generations

In photosynthetic organisms, genetic material is stored in the nucleus and the two cytoplasmic organelles: plastids and mitochondria. While both the nuclear and cytoplasmic genomes are essential for survival, the inheritance of these genomes is subject to distinct laws. Cytoplasmic inheritance differs fundamentally from nuclear inheritance through two unique processes: vegetative segregation and uniparental inheritance. To illustrate the significance of these processes in shaping cytoplasmic inheritance, I will trace the journey of plastid and mitochondrial genomes, following their transmission from parents to progeny. The cellular and molecular mechanisms regulating their transmission along the path are explored. By providing a framework that encompasses the inheritance of both plastid and mitochondrial genomes across cells and generations, I aim to present a comprehensive overview of cytoplasmic inheritance and highlight the intricate interplay of cellular processes that determine inheritance patterns. I will conclude this review by summarizing recent breakthroughs in the field that have significantly advanced our understanding of cytoplasmic inheritance. This knowledge has paved the way for achieving the first instance of controlled cytoplasmic inheritance in plants, unlocking the potential to harness cytoplasmic genetics for crop improvement.

Disruption of recombination machinery alters the mutational landscape in plant organellar genomes

Land plant organellar genomes have extremely low rates of point mutation yet also experience high rates of recombination and genome instability. Characterizing the molecular machinery responsible for these patterns is critical for understanding the evolution of these genomes. While much progress has been made toward understanding recombination activity in land plant organellar genomes, the relationship between recombination pathways and point mutation rates remains uncertain. The organellar-targeted mutS homolog MSH1 has previously been shown to suppress point mutations as well as non-allelic recombination between short repeats in Arabidopsis thaliana. We therefore implemented high-fidelity Duplex Sequencing to test if other genes that function in recombination and maintenance of genome stability also affect point mutation rates. We found small to moderate increases in the frequency of single nucleotide variants (SNVs) and indels in mitochondrial and/or plastid genomes of A. thaliana mutant lines lacking radA, recA1, or recA3. In contrast, osb2 and why2 mutants did not exhibit an increase in point mutations compared to wild-type (WT) controls. In addition, we analyzed the distribution of SNVs in previously generated Duplex Sequencing data from A. thaliana organellar genomes and found unexpected strand asymmetries and large effects of flanking nucleotides on mutation rates in WT plants and msh1 mutants. Finally, using long-read Oxford Nanopore sequencing, we characterized structural variants in organellar genomes of the mutant lines and show that different short repeat sequences become recombinationally active in different mutant backgrounds. Together, these complementary sequencing approaches shed light on how recombination may impact the extraordinarily low point mutation rates in plant organellar genomes.

Flipping the switch on some of the slowest mutating genomes: Direct measurements of plant mitochondrial and plastid mutation rates in msh1 mutants

Plant mitochondrial and plastid genomes have exceptionally slow rates of sequence evolution, and recent work has identified an unusual member of the MutS gene family ("plant MSH1") as being instrumental in preventing point mutations in these genomes. However, the effects of disrupting MSH1-mediated DNA repair on "germline" mutation rates have not been quantified. Here, we used Arabidopsis thaliana mutation accumulation (MA) lines to measure mutation rates in msh1 mutants and matched wild type (WT) controls. We detected 124 single nucleotide variants (SNVs: 49 mitochondrial and 75 plastid) and 668 small insertions and deletions (indels: 258 mitochondrial and 410 plastid) in msh1 MA lines. In striking contrast, we did not find any organelle mutations in the WT MA lines, and reanalysis of data from a much larger WT MA experiment also failed to detect any variants. The observed number of SNVs in the msh1 MA lines corresponds to estimated mutation rates of 6.1×10-7 and 3.2 ×10-6 per bp per generation in mitochondrial and plastid genomes, respectively. These rates exceed those of species known to have very high mitochondrial mutation rates (e.g., nematodes and fruit flies) by an order of magnitude or more and are on par with estimated rates in humans despite the generation times of A. thaliana being nearly 100-fold shorter. Therefore, disruption of a single plant-specific genetic factor in A. thaliana is sufficient to erase or even reverse the enormous difference in organelle mutation rates between plants and animals.

TALE-based C-to-T base editor for multiple homologous genes with flexible precision

Recently a cytidine deaminase-based method for highly efficient C-to-T targeted base editing was developed and has been used with CRISPR-mediated systems. It is a powerful method for genome engineering, although it is prone to off-target effects and has a limited targeting scope. Transcription activator-like effector (TALE)-based tools which allow longer recognition sequences than do CRISPR/Cas9 systems, can also be used for targeted C-to-T base editing. Here, we describe a method that efficiently achieved targeted C-to-T substitutions in Arabidopsis nuclear genes using cytidine deaminase fused to a TALE DNA-binding domain. We used a single pair of TALEs with a novel TALE-repeat unit that can recognize all four DNA bases, especially to allow for variations in the third base of codons in homologous genes. This targeting strategy makes it possible to simultaneously base edit almost identical sites in multiple isoforms of a gene while suppressing off-target substitutions.

Methods for assembling complex mitochondrial genomes in land plants

The large size and complex structural rearrangements inherent in the mitochondrial genomes of land plants pose challenges for their sequencing. Originally, the assembly of these genomes required the cloning of mitochondrial DNA fragments followed by Sanger sequencing. Subsequently, the advent of next-generation sequencing significantly expedited the process. This review highlights examples of plant mitochondrial genome assembly employing various technologies, including 454 sequencing, Illumina short sequencing reads, and Pacific Biosciences or Oxford Nanopore Technology long sequencing reads. The combination of short and long reads in hybrid assembly has proven to be the most efficient approach for achieving reliable assemblies of land plant mitochondrial genomes.

Rice floury endosperm26 encoding a mitochondrial single-stranded DNA-binding protein is essential for RNA-splicing of mitochondrial genes and endosperm development

Endosperm, the major storage organ in cereal grains, determines the grain yield and quality. Mitochondria provide the energy for dry matter accumulation, in the endosperm development. Although mitochondrial single-stranded DNA-binding proteins (mtSSBs) play a canonical role in the maintenance of single-stranded mitochondrial DNA, their molecular functions in RNA processing and endosperm development remain obscure. Here, we report a defective rice endosperm mutant, floury endosperm26 (flo26), which develops abnormal starch grains in the endosperm. Map-based cloning and complementation experiments showed that FLO26 allele encodes a mitochondrial single-stranded DNA-binding protein, named as mtSSB1.1. Loss of function of mtSSB1.1 affects the transcriptional level of many mitochondrially-encoded genes and RNA splicing of nad1, a core component of respiratory chain complex I in mitochondria. As a result, dysfunctional mature nad1 led to dramatically decreased complex I activity, thereby reducing ATP production. Our results reveal that mtSSB1.1 plays an important role in the maintenance of mitochondrial function and endosperm development by stabilizing the splicing of mitochondrial RNA in rice.

Evolution and maintenance of mtDNA gene content across eukaryotes

Across eukaryotes, most genes required for mitochondrial function have been transferred to, or otherwise acquired by, the nucleus. Encoding genes in the nucleus has many advantages. So why do mitochondria retain any genes at all? Why does the set of mtDNA genes vary so much across different species? And how do species maintain functionality in the mtDNA genes they do retain? In this review, we will discuss some possible answers to these questions, attempting a broad perspective across eukaryotes. We hope to cover some interesting features which may be less familiar from the perspective of particular species, including the ubiquity of recombination outside bilaterian animals, encrypted chainmail-like mtDNA, single genes split over multiple mtDNA chromosomes, triparental inheritance, gene transfer by grafting, gain of mtDNA recombination factors, social networks of mitochondria, and the role of mtDNA dysfunction in feeding the world. We will discuss a unifying picture where organismal ecology and gene-specific features together influence whether organism X retains mtDNA gene Y, and where ecology and development together determine which strategies, importantly including recombination, are used to maintain the mtDNA genes that are retained.

A Systematic Review and Developmental Perspective on Origin of CMS Genes in Crops

Cytoplasmic male sterility (CMS) arises from the incompatibility between the nucleus and cytoplasm as typical representatives of the chimeric structures in the mitochondrial genome (mitogenome), which has been extensively applied for hybrid seed production in various crops. The frequent occurrence of chimeric mitochondrial genes leading to CMS is consistent with the mitochondrial DNA (mtDNA) evolution. The sequence conservation resulting from faithfully maternal inheritance and the chimeric structure caused by frequent sequence recombination have been defined as two major features of the mitogenome. However, when and how these chimeric mitochondrial genes appear in the context of the highly conserved reproduction of mitochondria is an enigma. This review, therefore, presents the critical view of the research on CMS in plants to elucidate the mechanisms of this phenomenon. Generally, distant hybridization is the main mechanism to generate an original CMS source in natural populations and in breeding. Mitochondria and mitogenomes show pleomorphic and dynamic changes at key stages of the life cycle. The promitochondria in dry seeds develop into fully functioning mitochondria during seed imbibition, followed by massive mitochondria or mitogenome fusion and fission in the germination stage along with changes in the mtDNA structure and quantity. The mitogenome stability is controlled by nuclear loci, such as the nuclear gene Msh1. Its suppression leads to the rearrangement of mtDNA and the production of heritable CMS genes. An abundant recombination of mtDNA is also often found in distant hybrids and somatic/cybrid hybrids. Since mtDNA recombination is ubiquitous in distant hybridization, we put forward a hypothesis that the original CMS genes originated from mtDNA recombination during the germination of the hybrid seeds produced from distant hybridizations to solve the nucleo-cytoplasmic incompatibility resulting from the allogenic nuclear genome during seed germination.

Expansion of the MutS Gene Family in Plants

The MutS gene family is distributed across the tree of life and is involved in recombination, DNA repair, and protein translation. Multiple evolutionary processes have expanded the set of MutS genes in plants relative to other eukaryotes. Here, we investigate the origins and functions of these plant-specific genes. Land plants, green algae, red algae, and glaucophytes share cyanobacterial-like MutS1 and MutS2 genes that presumably were gained via plastid endosymbiotic gene transfer. MutS1 was subsequently lost in some taxa, including seed plants, whereas MutS2 was duplicated in Viridiplantae (i.e., land plants and green algae) with widespread retention of both resulting paralogs. Viridiplantae also have two anciently duplicated copies of the eukaryotic MSH6 gene (i.e., MSH6 and MSH7) and acquired MSH1 via horizontal gene transfer - potentially from a nucleocytovirus. Despite sharing the same name, "plant MSH1" is not directly related to the gene known as MSH1 in some fungi and animals, which may be an ancestral eukaryotic gene acquired via mitochondrial endosymbiosis and subsequently lost in most eukaryotic lineages. There has been substantial progress in understanding the functions of MSH1 and MSH6/MSH7 in plants, but the roles of the cyanobacterial-like MutS1 and MutS2 genes remain uncharacterized. Known functions of bacterial homologs and predicted protein structures, including fusions to diverse nuclease domains, provide hypotheses about potential molecular mechanisms. Because most plant-specific MutS proteins are targeted to the mitochondria and/or plastids, the expansion of this family appears to have played a large role in shaping plant organelle genetics.

Disruption of recombination machinery alters the mutational landscape in plant organellar genomes

Land plant organellar genomes have extremely low rates of point mutation yet also experience high rates of recombination and genome instability. Characterizing the molecular machinery responsible for these patterns is critical for understanding the evolution of these genomes. While much progress has been made towards understanding recombination activity in land plant organellar genomes, the relationship between recombination pathways and point mutation rates remains uncertain. The organellar targeted mutS homolog MSH1 has previously been shown to suppress point mutations as well as non-allelic recombination between short repeats in Arabidopsis thaliana. We therefore implemented high-fidelity Duplex Sequencing to test if other genes that function in recombination and maintenance of genome stability also affect point mutation rates. We found small to moderate increases in the frequency of single nucleotide variants (SNVs) and indels in mitochondrial and/or plastid genomes of A. thaliana mutant lines lacking radA, recA1, or recA3. In contrast, osb2 and why2 mutants did not exhibit an increase in point mutations compared to wild type (WT) controls. In addition, we analyzed the distribution of SNVs in previously generated Duplex Sequencing data from A. thaliana organellar genomes and found unexpected strand asymmetries and large effects of flanking nucleotides on mutation rates in WT plants and msh1 mutants. Finally, using long-read Oxford Nanopore sequencing, we characterized structural variants in organellar genomes of the mutant lines and show that different short repeat sequences become recombinationally active in different mutant backgrounds. Together, these complementary sequencing approaches shed light on how recombination may impact the extraordinarily low point mutation rates in plant organellar genomes.

Plant Organellar MSH1 Is a Displacement Loop-Specific Endonuclease

MutS HOMOLOG 1 (MSH1) is an organellar-targeted protein that obstructs ectopic recombination and the accumulation of mutations in plant organellar genomes. MSH1 also modulates the epigenetic status of nuclear DNA, and its absence induces a variety of phenotypic responses. MSH1 is a member of the MutS family of DNA mismatch repair proteins but harbors an additional GIY-YIG nuclease domain that distinguishes it from the rest of this family. How MSH1 hampers recombination and promotes fidelity in organellar DNA inheritance is unknown. Here, we elucidate its enzymatic activities by recombinantly expressing and purifying full-length MSH1 from Arabidopsis thaliana (AtMSH1). AtMSH1 is a metalloenzyme that shows a strong binding affinity for displacement loops (D-loops). The DNA-binding abilities of AtMSH1 reside in its MutS domain and not in its GIY-YIG domain, which is the ancillary nickase of AtMSH1. In the presence of divalent metal ions, AtMSH1 selectively executes multiple incisions at D-loops, but not other DNA structures including Holliday junctions or dsDNA, regardless of the presence or absence of mismatches. The selectivity of AtMSH1 to dismantle D-loops supports the role of this enzyme in preventing recombination between short repeats. Our results suggest that plant organelles have evolved novel DNA repair routes centered around the anti-recombinogenic activity of MSH1.

Collective mitochondrial dynamics resolve conflicting cellular tensions: From plants to general principles

Mitochondria play diverse and essential roles in eukaryotic cells, and plants are no exception. Plant mitochondria have several differences from their metazoan and fungal cousins: they often exist in a fragmented state, move rapidly on actin rather than microtubules, have many plant-specific metabolic features and roles, and usually contain only a subset of the complete mtDNA genome, which itself undergoes frequent recombination. This arrangement means that exchange and complementation is essential for plant mitochondria, and recent work has begun to reveal how their collective dynamics and resultant "social networks" of encounters support this exchange, connecting plant mitochondria in time rather than in space. This review will argue that this social network perspective can be extended to a "societal network", where mitochondrial dynamics are an essential part of the interacting cellular society of organelles and biomolecules. Evidence is emerging that mitochondrial dynamics allow optimal resolutions to competing cellular priorities; we will survey this evidence and review potential future research directions, highlighting that plant mitochondria can help reveal and test principles that apply across other kingdoms of life. In parallel with this fundamental cell biology, we also highlight the translational "One Health" importance of plant mitochondrial behaviour - which is exploited in the production of a vast amount of crops consumed worldwide - and the potential for multi-objective optimisation to understand and rationally re-engineer the evolved resolutions to these tensions.

Stochastic organelle genome segregation through Arabidopsis development and reproduction

Organelle DNA (oDNA) in mitochondria and plastids is vital for plant (and eukaryotic) life. Selection against damaged oDNA is mediated in part by segregation - sorting different oDNA types into different cells in the germline. Plants segregate oDNA very rapidly, with oDNA recombination protein MSH1 a key driver of this segregation, but we have limited knowledge of the dynamics of this segregation within plants and between generations. Here, we reveal how oDNA evolves through Arabidopsis thaliana development and reproduction. We combine stochastic modelling, Bayesian inference, and model selection with new and existing tissue-specific oDNA measurements from heteroplasmic Arabidopsis plant lines through development and between generations. Segregation proceeds gradually but continually during plant development, with a more rapid increase between inflorescence formation and the next generation. When MSH1 is compromised, the majority of observed segregation can be achieved through partitioning at cell divisions. When MSH1 is functional, mtDNA segregation is far more rapid; we show that increased oDNA gene conversion is a plausible mechanism quantitatively explaining this acceleration. These findings reveal the quantitative, time-dependent details of oDNA segregation in Arabidopsis. We also discuss the support for different models of the plant germline provided by these observations.

Bioinformatics Analysis of MSH1 Genes of Green Plants: Multiple Parallel Length Expansions, Intron Gains and Losses, Partial Gene Duplications, and Alternative Splicing

MutS homolog 1 (MSH1) is involved in the recombining and repairing of organelle genomes and is essential for maintaining their stability. Previous studies indicated that the length of the gene varied greatly among species and detected species-specific partial gene duplications in Physcomitrella patens. However, there are critical gaps in the understanding of the gene size expansion, and the extent of the partial gene duplication of MSH1 remains unclear. Here, we screened MSH1 genes in 85 selected species with genome sequences representing the main clades of green plants (Viridiplantae). We identified the MSH1 gene in all lineages of green plants, except for nine incomplete species, for bioinformatics analysis. The gene is a singleton gene in most of the selected species with conserved amino acids and protein domains. Gene length varies greatly among the species, ranging from 3234 bp in Ostreococcus tauri to 805,861 bp in Cycas panzhihuaensis. The expansion of MSH1 repeatedly occurred in multiple clades, especially in Gymnosperms, Orchidaceae, and Chloranthus spicatus. MSH1 has exceptionally long introns in certain species due to the gene length expansion, and the longest intron even reaches 101,025 bp. And the gene length is positively correlated with the proportion of the transposable elements (TEs) in the introns. In addition, gene structure analysis indicated that the MSH1 of green plants had undergone parallel intron gains and losses in all major lineages. However, the intron number of seed plants (gymnosperm and angiosperm) is relatively stable. All the selected gymnosperms contain 22 introns except for Gnetum montanum and Welwitschia mirabilis, while all the selected angiosperm species preserve 21 introns except for the ANA grade. Notably, the coding region of MSH1 in algae presents an exceptionally high GC content (47.7% to 75.5%). Moreover, over one-third of the selected species contain species-specific partial gene duplications of MSH1, except for the conserved mosses-specific partial gene duplication. Additionally, we found conserved alternatively spliced MSH1 transcripts in five species. The study of MSH1 sheds light on the evolution of the long genes of green plants.

Identification, cloning and characterization of AcMSH1 from Onion (Allium cepa L.)

BACKGROUND

MSH1 (MutS homolog1) is a nuclear-encoded protein that plays a crucial role in maintaining low mutation rates and stability of the organellar genome. While plastid MSH1 maintains nuclear epigenome plasticity and affects plant development patterns, mitochondrial MSH1 suppresses illegitimate recombination within the mitochondrial genome, affects mitochondrial genome substoichiometric shifting activity and induces cytoplasmic male sterility (CMS) in crops. However, a detailed functional investigation of onion MSH1 has yet to be achieved.

MATERIALS AND RESULTS

The homology analysis of onion genome database identified a single copy of the AcMSH1 gene in the onion cv. Bhima Super. In silico analysis of AcMSH1 protein sequence revealed the presence of 6 conserved functional domains including a unique MSH1-specific GIY-YIG endonuclease domain at the C-terminal end. At N-terminal end, it has signal peptide sequences targeting chloroplast and mitochondria. The concentration of AcMSH1 was found to be highest in isolated mitochondria, followed by chloroplasts, and negligible in the cytoplasmic fraction; which proved its localization to the mitochondria and chloroplasts. Quantitative expression analysis revealed that AcMSH1 protein levels were highest in leaves, followed by flower buds, root tips, flowers, and umbels, with the lowest amount found in callus tissue.

CONCLUSION

Onion genome has single copy of MSH1, with characteristic GIY-YIG endonuclease domain. AcMSH1 targeted towards both chloroplasts and mitochondria. The identification and characterisation of AcMSH1 may provide valuable insights into the development of CMS lines in onion.

Mitochondrial Genome Sequence of Salvia officinalis (Lamiales: Lamiaceae) Suggests Diverse Genome Structures in Cogeneric Species and Finds the Stop Gain of Genes through RNA Editing Events

Our previous study was the first to confirm that the predominant conformation of mitochondrial genome (mitogenome) sequence of Salvia species contains two circular chromosomes. To further understand the organization, variation, and evolution of Salvia mitogenomes, we characterized the mitogenome of Salvia officinalis. The mitogenome of S. officinalis was sequenced using Illumina short reads and Nanopore long reads and assembled using a hybrid assembly strategy. We found that the predominant conformation of the S. officinalis mitogenome also had two circular chromosomes that were 268,341 bp (MC1) and 39,827 bp (MC2) in length. The S. officinalis mitogenome encoded an angiosperm-typical set of 24 core genes, 9 variable genes, 3 rRNA genes, and 16 tRNA genes. We found many rearrangements of the Salvia mitogenome through inter- and intra-specific comparisons. A phylogenetic analysis of the coding sequences (CDs) of 26 common protein-coding genes (PCGs) of 11 Lamiales species and 2 outgroup taxa strongly indicated that the S. officinalis was a sister taxon to S. miltiorrhiza, consistent with the results obtained using concatenated CDs of common plastid genes. The mapping of RNA-seq data to the CDs of PCGs led to the identification of 451 C-to-U RNA editing sites from 31 PCGs of the S. officinalis mitogenome. Using PCR amplification and Sanger sequencing methods, we successfully validated 113 of the 126 RNA editing sites from 11 PCGs. The results of this study suggest that the predominant conformation of the S. officinalis mitogenome are two circular chromosomes, and the stop gain of rpl5 was found through RNA editing events of the Salvia mitogenome.

Comparative analysis of mitochondrial genomes of Broussonetia spp. (Moraceae) reveals heterogeneity in structure, synteny, intercellular gene transfer, and RNA editing

The genus Broussonetia (Moraceae) is comprised of three non-hybrid recognized species that all produce high quality fiber essential in the development of papermaking and barkcloth-making technology. In addition, these species also have medicinal value in several countries. Despite their important economical, medicinal, and ecological values, the complete mitogenome of Broussonetia has not been reported and investigated, which would greatly facilitate molecular phylogenetics, species identification and understanding evolutionary processes. Here, we assembled the first-reported three complete Broussonetia (B. papyrifera, B. kaempferi, and B. monoica) mitochondrial genomes (mitogenome) based on a hybrid strategy using Illumina and Oxford Nanopore Technology sequencing data, and performed comprehensive comparisons in terms of their structure, gene content, synteny, intercellular gene transfer, phylogeny, and RNA editing. Our results showed their huge heterogeneities among the three species. Interestingly, the mitogenomes of B. monoica and B. papyrifera consisted of a single circular structure, whereas the B. kaempferi mitogenome was unique and consisted of a double circular structure. Gene content was consistent except for a few transfer RNA (tRNA) genes. The Broussonetia spp. mitogenomes had high sequence conservation but B. monoica with B. kaempferi contained more synteny blocks and were more related, a finding that was well-supported in organellar phylogeny. Fragments that had been transferred between mitogenomes were detected at plastome hotspots that had integrated under potential mediation of tRNA genes. In addition, RNA editing sites showed great differences in abundance, type, location and efficiency among species and tissues. The availability of these complete gap-free mitogenomes of Broussonetia spp. will provide a valuable genetic resource for evolutionary research and understanding the communications between the two organelle genomes.

The mitochondrial genome sequence of Abies alba Mill. reveals a high structural and combinatorial variation

BACKGROUND

Plant mitogenomes vary widely in size and genomic architecture. Although hundreds of plant mitogenomes of angiosperm species have already been sequence-characterized, only a few mitogenomes are available from gymnosperms. Silver fir (Abies alba) is an economically important gymnosperm species that is widely distributed in Europe and occupies a large range of environmental conditions. Reference sequences of the nuclear and chloroplast genome of A. alba are available, however, the mitogenome has not yet been assembled and studied.

RESULTS

Here, we used paired-end Illumina short reads generated from a single haploid megagametophyte in combination with PacBio long reads from high molecular weight DNA of needles to assemble the first mitogenome sequence of A. alba. Assembly and scaffolding resulted in 11 mitogenome scaffolds, with the largest scaffold being 0.25 Mbp long. Two of the scaffolds displayed a potential circular structure supported by PCR. The total size of the A. alba mitogenome was estimated at 1.43 Mbp, similar to the size (1.33 Mbp) of a draft assembly of the Abies firma mitogenome. In total, 53 distinct genes of known function were annotated in the A. alba mitogenome, comprising 41 protein-coding genes, nine tRNA, and three rRNA genes. The proportion of highly repetitive elements (REs) was 0.168. The mitogenome seems to have a complex and dynamic structure featured by high combinatorial variation, which was specifically confirmed by PCR for the contig with the highest mapping coverage. Comparative analysis of all sequenced mitogenomes of gymnosperms revealed a moderate, but significant positive correlation between mitogenome size and proportion of REs.

CONCLUSIONS

The A. alba mitogenome provides a basis for new comparative studies and will allow to answer important structural, phylogenetic and other evolutionary questions. Future long-read sequencing with higher coverage of the A. alba mitogenome will be the key to further resolve its physical structure. The observed positive correlation between mitogenome size and proportion of REs will be further validated once available mitogenomes of gymnosperms would become more numerous. To test whether a higher proportion of REs in a mitogenome leads to an increased recombination and higher structural complexity and variability is a prospective avenue for future research.

Fine Mapping of BoVl Conferring the Variegated Leaf in Ornamental Kale (Brassica oleracea var. acephala)

Ornamental kale, as a burgeoning landscaping plant, is gaining popularity for its rich color patterns in leaf and cold tolerance. Leaf variegation endows ornamental kale with unique ornamental characters, and the mutants are ideal materials for exploring the formation mechanisms of variegated phenotype. Herein, we identified a novel variegated leaf kale mutant ‘JC007-2B’ with green margins and white centers. Morphological observations and physiological determinations of the green leaf stage (S1), albino stage (S2) and variegated leaf stage (S3) demonstrated that the chloroplast structure and photosynthetic pigment content in the white sectors (S3_C) of variegated leaves were abnormal. Genetic analysis revealed that a single dominant nuclear gene (BoVl) controlled the variegated leaf trait of ‘JC007-2B’, and three candidate genes for BoVl were fine-mapped to a 6.74 Kb interval on chromosome C03. Multiple sequence alignment among the green-leaf mapping parent ‘BS’, recombinant individuals, mutant parent ‘JC007-2B’ and its same originated DH line population established that the mutation sites in Bo3g002080 exhibited a complete consensus. Bo3g002080, homologous to Arabidopsis MED4, was identified as the candidate gene for BoVl. Expression analysis showed that Bo3g002080 displayed a 2158.85-fold higher expression at albino stage than that in green leaf stage. Transcriptome analysis showed that related pathways of photosynthesis and chloroplast development were significantly enriched in the white sectors, and relevant DEGs involved in these pathways were almost down-regulated. Overall, our study provides a new gene resource for cultivar breeding in ornamental kale and contributes to uncovering the molecular genetic mechanism underlying the variegated leaf formation.

Long-read sequencing characterizes mitochondrial and plastid genome variants in Arabidopsis msh1 mutants

The abundant repeats in plant mitochondrial genomes can cause rapid genome rearrangements and are also a major obstacle in short-read sequencing studies. Nuclear-encoded proteins such as MSH1 are known to suppress the generation of repeat-associated mitochondrial genome variants, but our understanding of these mechanisms has been constrained by the limitations of short-read technologies. Here, we used highly accurate long-read sequencing (PacBio HiFi) to characterize mitochondrial and plastid genome variants in Arabidopsis thaliana msh1 mutant individuals. The HiFi reads provided a global view of recombination dynamics with detailed quantification of parental and crossover recombination products for both large and small repeats. We found that recombination breakpoints were distributed relatively evenly across the length of repeated sequences and detected widespread internal exchanges of sequence variants between pairs of imperfect repeats in the mitochondrial genome of msh1 mutants. Long-read assemblies of mitochondrial genomes from seven other A. thaliana wild-type accessions differed by repeat-mediated structural rearrangements similar to those observed in msh1 mutants, but they were all in a simple low-heteroplasmy state. The Arabidopsis plastid genome generally lacks small repeats and exhibited a very different pattern of variant accumulation in msh1 mutants compared with the mitochondrial genome. Our data illustrate the power of HiFi technology in studying repeat-mediated recombination in plant organellar genomes and improved the sequence resolution for recombinational processes suppressed by MSH1. Plant organellar genomes can undergo rapid rearrangements. Long-read sequencing provides a detailed and quantitative view of mitochondrial and plastid genome variants normally suppressed by MSH1, advancing our understanding of plant organellar genome dynamics.

Organelle bottlenecks facilitate evolvability by traversing heteroplasmic fitness valleys

Bioenergetic organelles-mitochondria and plastids-retain their own genomes (mtDNA and ptDNA), and these organelle DNA (oDNA) molecules are vital for eukaryotic life. Like all genomes, oDNA must be able to evolve to suit new environmental challenges. However, mixed oDNA populations in cells can challenge cellular bioenergetics, providing a penalty to the appearance and adaptation of new mutations. Here we show that organelle "bottlenecks," mechanisms increasing cell-to-cell oDNA variability during development, can overcome this mixture penalty and facilitate the adaptation of beneficial mutations. We show that oDNA heteroplasmy and bottlenecks naturally emerge in evolutionary simulations subjected to fluctuating environments, demonstrating that this evolvability is itself evolvable. Usually thought of as a mechanism to clear damaging mutations, organelle bottlenecks therefore also resolve the tension between intracellular selection for pure cellular oDNA populations and the "bet-hedging" need for evolvability and adaptation to new environments. This general theory suggests a reason for the maintenance of organelle heteroplasmy in cells, and may explain some of the observed diversity in organelle maintenance and inheritance across taxa.

Arabidopsis mitochondrial single-stranded DNA-binding proteins SSB1 and SSB2 are essential regulators of mtDNA replication and homologous recombination

Faithful DNA replication is one of the most essential processes in almost all living organisms. However, the proteins responsible for organellar DNA replication are still largely unknown in plants. Here, we show that the two mitochondrion-targeted single-stranded DNA-binding (SSB) proteins SSB1 and SSB2 directly interact with each other and act as key factors for mitochondrial DNA (mtDNA) maintenance, as their single or double loss-of-function mutants exhibit severe germination delay and growth retardation. The mtDNA levels in mutants lacking SSB1 and/or SSB2 function were two- to four-fold higher than in the wild-type (WT), revealing a negative role for SSB1/2 in regulating mtDNA replication. Genetic analysis indicated that SSB1 functions upstream of mitochondrial DNA POLYMERASE IA (POLIA) or POLIB in mtDNA replication, as mutation in either gene restored the high mtDNA copy number of the ssb1-1 mutant back to WT levels. In addition, SSB1 and SSB2 also participate in mitochondrial genome maintenance by influencing mtDNA homologous recombination (HR). Additional genetic analysis suggested that SSB1 functions upstream of ORGANELLAR SINGLE-STRANDED DNA-BINDING PROTEIN1 (OSB1) during mtDNA replication, while SSB1 may act downstream of OSB1 and MUTS HOMOLOG1 for mtDNA HR. Overall, our results yield new insights into the roles of the plant mitochondrion-targeted SSB proteins and OSB1 in maintaining mtDNA stability via affecting DNA replication and DNA HR.

Altered collective mitochondrial dynamics in the Arabidopsis msh1 mutant compromising organelle DNA maintenance

Mitochondria form highly dynamic populations in the cells of plants (and almost all eukaryotes). The characteristics and benefits of this collective behaviour, and how it is influenced by nuclear features, remain to be fully elucidated. Here, we use a recently developed quantitative approach to reveal and analyse the physical and collective 'social' dynamics of mitochondria in an Arabidopsis msh1 mutant where the organelle DNA maintenance machinery is compromised. We use a newly created line combining the msh1 mutant with mitochondrially targeted green fluorescent protein (GFP), and characterize mitochondrial dynamics with a combination of single-cell time-lapse microscopy, computational tracking, and network analysis. The collective physical behaviour of msh1 mitochondria is altered from that of the wild type in several ways: mitochondria become less evenly spread, and networks of inter-mitochondrial encounters become more connected, with greater potential efficiency for inter-organelle exchange-reflecting a potential compensatory mechanism for the genetic challenge to the mitochondrial DNA population, supporting more inter-organelle exchange. We find that these changes are similar to those observed in friendly, where mitochondrial dynamics are altered by a physical perturbation, suggesting that this shift to higher connectivity may reflect a general response to mitochondrial challenges, where physical dynamics of mitochondria may be altered to control the genetic structure of the mtDNA population.

Social networks in the single cell

This article comments on: Chustecki JM, Etherington RD, Gibbs DJ, Johnston IG. 2022. Altered collective mitochondrial dynamics in the Arabidopsis msh1 mutant compromising organelle DNA maintenance. Journal of Experimental Botany 73, 5428-5439. Plant mitochondrial DNA (mtDNA) can become damaged in many ways. A major repair mechanism is homologous recombination, which requires an undamaged DNA template. Presumably, this template comes from a different mitochondrion in the same cell. Plant mitochondria undergo fission and fusion to form transient networks which could allow the exchange of genetic information. To test this hypothesis, Chustecki et al. (2022) used msh1 mutants with defective DNA repair, and showed that mitochondrial interactions increased, revealing a link between the physical and genetic behavior of mitochondria.

Sorting of mitochondrial and plastid heteroplasmy in Arabidopsis is extremely rapid and depends on MSH1 activity

The fate of new mitochondrial and plastid mutations depends on their ability to persist and spread among the numerous organellar genome copies within a cell (heteroplasmy). The extent to which heteroplasmies are transmitted across generations or eliminated through genetic bottlenecks is not well understood in plants, in part because their low mutation rates make these variants so infrequent. Disruption of MutS Homolog 1 (MSH1), a gene involved in plant organellar DNA repair, results in numerous de novo point mutations, which we used to quantitatively track the inheritance of single nucleotide variants in mitochondrial and plastid genomes in Arabidopsis. We found that heteroplasmic sorting (the fixation or loss of a variant) was rapid for both organelles, greatly exceeding rates observed in animals. In msh1 mutants, plastid variants sorted faster than those in mitochondria and were typically fixed or lost within a single generation. Effective transmission bottleneck sizes (N) for plastids and mitochondria were N ∼ 1 and 4, respectively. Restoring MSH1 function further increased the rate of heteroplasmic sorting in mitochondria (N ∼ 1.3), potentially because of its hypothesized role in promoting gene conversion as a mechanism of DNA repair, which is expected to homogenize genome copies within a cell. Heteroplasmic sorting also favored GC base pairs. Therefore, recombinational repair and gene conversion in plant organellar genomes can potentially accelerate the elimination of heteroplasmies and bias the outcome of this sorting process.

Transcriptome Analysis of Air Space-Type Variegation Formation in Trifolium pratense

Air space-type variegation is the most diverse among the species of known variegated leaf plants and is caused by conspicuous intercellular spaces between the epidermal and palisade cells and among the palisade cells at non-green areas. Trifolium pratense, a species in Fabaceae with V-shaped air space-type variegation, was selected to explore the application potential of variegated leaf plants and accumulate basic data on the molecular regulatory mechanism and evolutionary history of leaf variegation. We performed comparative transcriptome analysis on young and adult leaflets of variegated and green plants and identified 43 candidate genes related to air space-type variegation formation. Most of the genes were related to cell-wall structure modification (CESA, CSL, EXP, FLA, PG, PGIP, PLL, PME, RGP, SKS, and XTH family genes), followed by photosynthesis (LHCB subfamily, RBCS, GOX, and AGT family genes), redox (2OG and GSH family genes), and nitrogen metabolism (NodGS family genes). Other genes were related to photooxidation, protein interaction, and protease degradation systems. The downregulated expression of light-responsive LHCB subfamily genes and the upregulated expression of the genes involved in cell-wall structure modification were important conditions for air space-type variegation formation in T. pratense. The upregulated expression of the ubiquitin-protein ligase enzyme (E3)-related genes in the protease degradation systems were conducive to air space-type variegation formation. Because these family genes are necessary for plant growth and development, the mechanism of the leaf variegation formation in T. pratense might be a widely existing regulation in air space-type variegation in nature.

The Barley Chloroplast Mutator (cpm) Mutant: All Roads Lead to the Msh1 Gene

The barley chloroplast mutator (cpm) is a nuclear gene mutant that induces a wide spectrum of cytoplasmically inherited chlorophyll deficiencies. Plastome instability of cpm seedlings was determined by identification of a particular landscape of polymorphisms that suggests failures in a plastome mismatch repair (MMR) protein. In Arabidopsis, MSH genes encode proteins that are in charge of mismatch repair and have anti-recombination activity. In this work, barley homologs of these genes were identified, and their sequences were analyzed in control and cpm mutant seedlings. A substitution, leading to a premature stop codon and a truncated MSH1 protein, was identified in the Msh1 gene of cpm plants. The relationship between this mutation and the presence of chlorophyll deficiencies was established in progenies from crosses and backcrosses. These results strongly suggest that the mutation identified in the Msh1 gene of the cpm mutant is responsible for the observed plastome instabilities. Interestingly, comparison of mutant phenotypes and molecular changes induced by the barley cpm mutant with those of Arabidopsis MSH1 mutants revealed marked differences.

Implementation of Epigenetic Variation in Sorghum Selection and Implications for Crop Resilience Breeding

Crop resilience and yield stability are complex traits essential for food security. Sorghum bicolor is an important grain crop that shows promise for its natural resilience to drought and potential for marginal land production. We have developed sorghum lines in the Tx430 genetic background suppressed for MSH1 expression as a means of inducing de novo epigenetic variation, and have used these materials to evaluate changes in plant growth vigor. Plant crossing and selection in two distinct environments revealed features of phenotypic plasticity derived from MSH1 manipulation. Introduction of an epigenetic variation to an isogenic sorghum population, in the absence of selection, resulted in 10% yield increase under ideal field conditions and 20% increase under extreme low nitrogen conditions. However, incorporation of early-stage selection amplified these outcomes to 36% yield increase under ideal conditions and 64% increase under marginal field conditions. Interestingly, the best outcomes were derived by selecting mid-range performance early-generation lines rather than highest performing. Data also suggested that phenotypic plasticity derived from the epigenetic variation was non-uniform in its response to environmental variability but served to reduce genotype × environment interaction. The MSH1-derived growth vigor appeared to be associated with enhanced seedling root growth and altered expression of auxin response pathways, and plants showed evidence of cold tolerance, features consistent with observations made previously in Arabidopsis. These data imply that the MSH1 system is conserved across plant species, pointing to the value of parallel model plant studies to help devise effective plant selection strategies for epigenetic breeding in multiple crops.

Exploiting sterility and fertility variation in cytoplasmic male sterile vegetable crops

Cytoplasmic male sterility (CMS) has long been used to economically produce hybrids that harness growth vigor through heterosis. Yet, how CMS systems operate within commercially viable seed production strategies in various economically important vegetable crops, and their underlying molecular mechanisms, are often overlooked details that could expand the utility of CMS as a cost-effective and stable system. We provide here an update on the nature of cytoplasmic-nuclear interplay for pollen sterility and fertility transitions in vegetable crops, based on the discovery of components of nuclear fertility restoration and reversion determinants. Within plant CMS systems, pollen fertility can be rescued by the introduction of nuclear fertility restorer genes (Rfs), which operate by varied mechanisms to countermand the sterility phenotype. By understanding these systems, it is now becoming feasible to achieve fertility restoration with Rfs designed for programmable CMS-associated open reading frames (ORFs). Likewise, new opportunities exist for targeted disruption of CMS-associated ORFs by mito-TALENs in crops where natural Rfs have not been readily identified, providing an alternative approach to recovering fertility of cytoplasmic male sterile lines in crops. Recent findings show that facultative gynodioecy, as a reproductive strategy, can coordinate the sterility and fertility transition in response to environmental cues and/or metabolic signals that reflect ecological conditions of reproductive isolation. This information is important to devising future systems that are more inherently stable.

Plant mitochondria - past, present and future

The study of plant mitochondria started in earnest around 1950 with the first isolations of mitochondria from animal and plant tissues. The first 35 years were spent establishing the basic properties of plant mitochondria and plant respiration using biochemical and physiological approaches. A number of unique properties (compared to mammalian mitochondria) were observed: (i) the ability to oxidize malate, glycine and cytosolic NAD(P)H at high rates; (ii) the partial insensitivity to rotenone, which turned out to be due to the presence of a second NADH dehydrogenase on the inner surface of the inner mitochondrial membrane in addition to the classical Complex I NADH dehydrogenase; and (iii) the partial insensitivity to cyanide, which turned out to be due to an alternative oxidase, which is also located on the inner surface of the inner mitochondrial membrane, in addition to the classical Complex IV, cytochrome oxidase. With the appearance of molecular biology methods around 1985, followed by genomics, further unique properties were discovered: (iv) plant mitochondrial DNA (mtDNA) is 10-600 times larger than the mammalian mtDNA, yet it only contains approximately 50% more genes; (v) plant mtDNA has kept the standard genetic code, and it has a low divergence rate with respect to point mutations, but a high recombinatorial activity; (vi) mitochondrial mRNA maturation includes a uniquely complex set of activities for processing, splicing and editing (at hundreds of sites); (vii) recombination in mtDNA creates novel reading frames that can produce male sterility; and (viii) plant mitochondria have a large proteome with 2000-3000 different proteins containing many unique proteins such as 200-300 pentatricopeptide repeat proteins. We describe the present and fairly detailed picture of the structure and function of plant mitochondria and how the unique properties make their metabolism more flexible allowing them to be involved in many diverse processes in the plant cell, such as photosynthesis, photorespiration, CAM and C4 metabolism, heat production, temperature control, stress resistance mechanisms, programmed cell death and genomic evolution. However, it is still a challenge to understand how the regulation of metabolism and mtDNA expression works at the cellular level and how retrograde signaling from the mitochondria coordinates all those processes.

An Insight Into the Mechanism of Plant Organelle Genome Maintenance and Implications of Organelle Genome in Crop Improvement: An Update

Besides the nuclear genome, plants possess two small extra chromosomal genomes in mitochondria and chloroplast, respectively, which contribute a small fraction of the organelles’ proteome. Both mitochondrial and chloroplast DNA have originated endosymbiotically and most of their prokaryotic genes were either lost or transferred to the nuclear genome through endosymbiotic gene transfer during the course of evolution. Due to their immobile nature, plant nuclear and organellar genomes face continuous threat from diverse exogenous agents as well as some reactive by-products or intermediates released from various endogenous metabolic pathways. These factors eventually affect the overall plant growth and development and finally productivity. The detailed mechanism of DNA damage response and repair following accumulation of various forms of DNA lesions, including single and double-strand breaks (SSBs and DSBs) have been well documented for the nuclear genome and now it has been extended to the organelles also. Recently, it has been shown that both mitochondria and chloroplast possess a counterpart of most of the nuclear DNA damage repair pathways and share remarkable similarities with different damage repair proteins present in the nucleus. Among various repair pathways, homologous recombination (HR) is crucial for the repair as well as the evolution of organellar genomes. Along with the repair pathways, various other factors, such as the MSH1 and WHIRLY family proteins, WHY1, WHY2, and WHY3 are also known to be involved in maintaining low mutation rates and structural integrity of mitochondrial and chloroplast genome. SOG1, the central regulator in DNA damage response in plants, has also been found to mediate endoreduplication and cell-cycle progression through chloroplast to nucleus retrograde signaling in response to chloroplast genome instability. Various proteins associated with the maintenance of genome stability are targeted to both nuclear and organellar compartments, establishing communication between organelles as well as organelles and nucleus. Therefore, understanding the mechanism of DNA damage repair and inter compartmental crosstalk mechanism in various sub-cellular organelles following induction of DNA damage and identification of key components of such signaling cascades may eventually be translated into strategies for crop improvement under abiotic and genotoxic stress conditions. This review mainly highlights the current understanding as well as the importance of different aspects of organelle genome maintenance mechanisms in higher plants.

Epigenetics for Crop Improvement in Times of Global Change

Epigenetics has emerged as an important research field for crop improvement under the on-going climatic changes. Heritable epigenetic changes can arise independently of DNA sequence alterations and have been associated with altered gene expression and transmitted phenotypic variation. By modulating plant development and physiological responses to environmental conditions, epigenetic diversity-naturally, genetically, chemically, or environmentally induced-can help optimise crop traits in an era challenged by global climate change. Beyond DNA sequence variation, the epigenetic modifications may contribute to breeding by providing useful markers and allowing the use of epigenome diversity to predict plant performance and increase final crop production. Given the difficulties in transferring the knowledge of the epigenetic mechanisms from model plants to crops, various strategies have emerged. Among those strategies are modelling frameworks dedicated to predicting epigenetically controlled-adaptive traits, the use of epigenetics for in vitro regeneration to accelerate crop breeding, and changes of specific epigenetic marks that modulate gene expression of traits of interest. The key challenge that agriculture faces in the 21st century is to increase crop production by speeding up the breeding of resilient crop species. Therefore, epigenetics provides fundamental molecular information with potential direct applications in crop enhancement, tolerance, and adaptation within the context of climate change.

OMICs, Epigenetics, and Genome Editing Techniques for Food and Nutritional Security

The incredible success of crop breeding and agricultural innovation in the last century greatly contributed to the Green Revolution, which significantly increased yields and ensures food security, despite the population explosion. However, new challenges such as rapid climate change, deteriorating soil, and the accumulation of pollutants require much faster responses and more effective solutions that cannot be achieved through traditional breeding. Further prospects for increasing the efficiency of agriculture are undoubtedly associated with the inclusion in the breeding strategy of new knowledge obtained using high-throughput technologies and new tools in the future to ensure the design of new plant genomes and predict the desired phenotype. This article provides an overview of the current state of research in these areas, as well as the study of soil and plant microbiomes, and the prospective use of their potential in a new field of microbiome engineering. In terms of genomic and phenomic predictions, we also propose an integrated approach that combines high-density genotyping and high-throughput phenotyping techniques, which can improve the prediction accuracy of quantitative traits in crop species.

Mitochondrial Genomic Landscape: A Portrait of the Mitochondrial Genome 40 Years after the First Complete Sequence

Notwithstanding the initial claims of general conservation, mitochondrial genomes are a largely heterogeneous set of organellar chromosomes which displays a bewildering diversity in terms of structure, architecture, gene content, and functionality. The mitochondrial genome is typically described as a single chromosome, yet many examples of multipartite genomes have been found (for example, among sponges and diplonemeans); the mitochondrial genome is typically depicted as circular, yet many linear genomes are known (for example, among jellyfish, alveolates, and apicomplexans); the chromosome is normally said to be “small”, yet there is a huge variation between the smallest and the largest known genomes (found, for example, in ctenophores and vascular plants, respectively); even the gene content is highly unconserved, ranging from the 13 oxidative phosphorylation-related enzymatic subunits encoded by animal mitochondria to the wider set of mitochondrial genes found in jakobids. In the present paper, we compile and describe a large database of 27,873 mitochondrial genomes currently available in GenBank, encompassing the whole eukaryotic domain. We discuss the major features of mitochondrial molecular diversity, with special reference to nucleotide composition and compositional biases; moreover, the database is made publicly available for future analyses on the MoZoo Lab GitHub page.

Intraspecific variations of the cytoplasmic male sterility genes orf108 and orf117 in Brassica maurorum and Moricandia arvensis, and the specificity of the mRNA processing

The mitochondrial gene orf108 co-transcribed with atp1 and causes cytoplasmic male sterility in Brassica crops, is widely distributed across wild species and genera of Brassicaceae. However, intraspecific variations in the presence of orf108 have not yet been studied, and the mechanisms for the wide distribution of the gene remain unclear. We analyzed the presence and sequence variations of orf108 in two wild species, Brassica maurorum and Moricandia arvensis. After polymerase chain reaction amplification of the 5' region of atp1 and the coding sequence of orf108, we determined the DNA sequences. B. maurorum and M. arvensis showed variations for the presence of orf108 or orf117 (orf108^V117) both between and within accessions, and were not fixed to the mitochondrial type having the male sterile genes. Sequencing of the amplicons clarified that B. maurorum has orf108^V117 instead of orf108. Sequencing also indicated mitochondrial heteroplasmy in the two species; particularly, in B. maurorum, one plant possessed both the orf108 and orf108^V117 sequences. The results suggested that substoichiometric shifting of the mitochondrial genomes leads to the acquisition or loss of orf108. Furthermore, fertility restorer genes of the two species were involved in the processing of the mRNA of the male sterility genes at different sites.

Complete Plastid and Mitochondrial Genomes of Aeginetia indica Reveal Intracellular Gene Transfer (IGT), Horizontal Gene Transfer (HGT), and Cytoplasmic Male Sterility (CMS)

Orobanchaceae have become a model group for studies on the evolution of parasitic flowering plants, and Aeginetia indica, a holoparasitic plant, is a member of this family. In this study, we assembled the complete chloroplast and mitochondrial genomes of A. indica. The chloroplast and mitochondrial genomes were 56,381 bp and 401,628 bp long, respectively. The chloroplast genome of A. indica shows massive plastid genes and the loss of one IR (inverted repeat). A comparison of the A. indica chloroplast genome sequence with that of a previous study demonstrated that the two chloroplast genomes encode a similar number of proteins (except atpH) but differ greatly in length. The A. indica mitochondrial genome has 53 genes, including 35 protein-coding genes (34 native mitochondrial genes and one chloroplast gene), 15 tRNA (11 native mitochondrial genes and four chloroplast genes) genes, and three rRNA genes. Evidence for intracellular gene transfer (IGT) and horizontal gene transfer (HGT) was obtained for plastid and mitochondrial genomes. ψndhB and ψcemA in the A. indica mitogenome were transferred from the plastid genome of A. indica. The atpH gene in the plastid of A. indica was transferred from another plastid angiosperm plastid and the atpI gene in mitogenome A. indica was transferred from a host plant like Miscanthus siensis. Cox2 (orf43) encodes proteins containing a membrane domain, making ORF (Open Reading Frame) the most likely candidate gene for CMS development in A. indica.

Ultra-deep sequencing reveals dramatic alteration of organellar genomes in Physcomitrella patens due to biased asymmetric recombination

Destabilization of organelle genomes causes organelle dysfunction that appears as abnormal growth in plants and diseases in human. In plants, loss of the bacterial-type homologous recombination repair (HRR) factors RECA and RECG induces organelle genome instability. In this study, we show the landscape of organelle genome instability in Physcomitrella patens HRR knockout mutants by deep sequencing in combination with informatics approaches. Genome-wide maps of rearrangement positions in the organelle genomes, which exhibited prominent mutant-specific patterns, were highly biased in terms of direction and location and often associated with dramatic variation in read depth. The rearrangements were location-dependent and mostly derived from the asymmetric products of microhomology-mediated recombination. Our results provide an overall picture of organelle-specific gross genomic rearrangements in the HRR mutants, and suggest that chloroplasts and mitochondria share common mechanisms for replication-related rearrangements.

Masaki Odahara and Kensuke Nakamura et al. use deep paired-end sequencing to examine organellar genome recombination when homologous recombination repair genes are individually knocked out in the moss, Physcomitrella patens. Their results suggest that chloroplasts and mitochondria share a common mechanism for replication-related rearrangements.

Avoiding organelle mutational meltdown across eukaryotes with or without a germline bottleneck

Mitochondrial DNA (mtDNA) and plastid DNA (ptDNA) encode vital bioenergetic apparatus, and mutations in these organelle DNA (oDNA) molecules can be devastating. In the germline of several animals, a genetic “bottleneck” increases cell-to-cell variance in mtDNA heteroplasmy, allowing purifying selection to act to maintain low proportions of mutant mtDNA. However, most eukaryotes do not sequester a germline early in development, and even the animal bottleneck remains poorly understood. How then do eukaryotic organelles avoid Muller’s ratchet—the gradual buildup of deleterious oDNA mutations? Here, we construct a comprehensive and predictive genetic model, quantitatively describing how different mechanisms segregate and decrease oDNA damage across eukaryotes. We apply this comprehensive theory to characterise the animal bottleneck with recent single-cell observations in diverse mouse models. Further, we show that gene conversion is a particularly powerful mechanism to increase beneficial cell-to-cell variance without depleting oDNA copy number, explaining the benefit of observed oDNA recombination in diverse organisms which do not sequester animal-like germlines (for example, sponges, corals, fungi, and plants). Genomic, transcriptomic, and structural datasets across eukaryotes support this mechanism for generating beneficial variance without a germline bottleneck. This framework explains puzzling oDNA differences across taxa, suggesting how Muller’s ratchet is avoided in different eukaryotes.

A comprehensive model for mitochondrial and plasmid DNA segregation, supported by with genomic, transcriptomic, and single-cell data, shows how the attritional effects of Muller’s ratchet can be avoided in the organelles of diverse eukaryotes.

HERMES: An improved method to test mitochondrial genome molecular synapomorphies among clades

Mitochondrial chromosomes have diversified among eukaryotes and many different architectures and features are now acknowledged for this genome. Here we present the improved HERMES index, which can measure and quantify the amount of molecular change experienced by mitochondrial genomes. We test the improved approach with ten molecular phylogenetic studies based on complete mitochondrial genomes, representing six bilaterian Phyla. In most cases, HERMES analysis spotted out clades or single species with peculiar molecular synapomorphies, allowing to identify phylogenetic and ecological patterns. The software presented herein handles linear, circular, and multi-chromosome genomes, thus widening the HERMES scope to the complete eukaryotic domain.

MutS HOMOLOG1 mediates fertility reversion from cytoplasmic male sterile Brassica juncea in response to environment

Spontaneous fertility reversion has been documented in cytoplasmic male sterile (CMS) plants of several species, influenced in frequency by nuclear genetic background. In this study, we found that MutS HOMOLOG1 (MSH1) mediates fertility reversion via substoichiometric shifting (SSS) of the CMS-associated mitochondrial Open Reading Frame 220 (ORF220), a process that may be regulated by pollination signalling in Brassica juncea. We show that plants adjust their growth and development in response to unsuccessful pollination. Measurable decrease in MSH1 transcript levels and evidence of ORF220 SSS under non-pollination conditions suggest that this nuclear-mitochondrial interplay influences fertility reversion in CMS plants in response to physiological signals. Suppression of MSH1 expression induced higher frequency SSS in CMS plants than occurs normally. Transcriptional analysis of floral buds under pollination and non-pollination conditions, and the response of MSH1 expression to different sugars, supports the hypothesis that carbon flux is involved in the pollination signalling of fertility reversion in CMS plants. Our findings suggest that facultative gynodioecy as a reproductive strategy may incorporate environmentally responsive genes like MSH1 as an "on-off" switch for sterility-fertility transition under ecological conditions of reproductive isolation.

MSH1 is required for maintenance of the low mutation rates in plant mitochondrial and plastid genomes

Mitochondrial and plastid genomes in land plants exhibit some of the slowest rates of sequence evolution observed in any eukaryotic genome, suggesting an exceptional ability to prevent or correct mutations. However, the mechanisms responsible for this extreme fidelity remain unclear. We tested seven candidate genes involved in cytoplasmic DNA replication, recombination, and repair (POLIA, POLIB, MSH1, RECA3, UNG, FPG, and OGG1) for effects on mutation rates in the model angiosperm Arabidopsis thaliana by applying a highly accurate DNA sequencing technique (duplex sequencing) that can detect newly arisen mitochondrial and plastid mutations even at low heteroplasmic frequencies. We find that disrupting MSH1 (but not the other candidate genes) leads to massive increases in the frequency of point mutations and small indels and changes to the mutation spectrum in mitochondrial and plastid DNA. We also used droplet digital PCR to show transmission of de novo heteroplasmies across generations in msh1 mutants, confirming a contribution to heritable mutation rates. This dual-targeted gene is part of an enigmatic lineage within the mutS mismatch repair family that we find is also present outside of green plants in multiple eukaryotic groups (stramenopiles, alveolates, haptophytes, and cryptomonads), as well as certain bacteria and viruses. MSH1 has previously been shown to limit ectopic recombination in plant cytoplasmic genomes. Our results point to a broader role in recognition and correction of errors in plant mitochondrial and plastid DNA sequence, leading to greatly suppressed mutation rates perhaps via initiation of double-stranded breaks and repair pathways based on faithful homologous recombination.

MSH1 is required for maintenance of the low mutation rates in plant mitochondrial and plastid genomes

Mitochondrial and plastid genomes in land plants exhibit some of the slowest rates of sequence evolution observed in any eukaryotic genome, suggesting an exceptional ability to prevent or correct mutations. However, the mechanisms responsible for this extreme fidelity remain unclear. We tested seven candidate genes involved in cytoplasmic DNA replication, recombination, and repair (POLIA, POLIB, MSH1, RECA3, UNG, FPG, and OGG1) for effects on mutation rates in the model angiosperm Arabidopsis thaliana by applying a highly accurate DNA sequencing technique (duplex sequencing) that can detect newly arisen mitochondrial and plastid mutations even at low heteroplasmic frequencies. We find that disrupting MSH1 (but not the other candidate genes) leads to massive increases in the frequency of point mutations and small indels and changes to the mutation spectrum in mitochondrial and plastid DNA. We also used droplet digital PCR to show transmission of de novo heteroplasmies across generations in msh1 mutants, confirming a contribution to heritable mutation rates. This dual-targeted gene is part of an enigmatic lineage within the mutS mismatch repair family that we find is also present outside of green plants in multiple eukaryotic groups (stramenopiles, alveolates, haptophytes, and cryptomonads), as well as certain bacteria and viruses. MSH1 has previously been shown to limit ectopic recombination in plant cytoplasmic genomes. Our results point to a broader role in recognition and correction of errors in plant mitochondrial and plastid DNA sequence, leading to greatly suppressed mutation rates perhaps via initiation of double-stranded breaks and repair pathways based on faithful homologous recombination.

The Tempo and Mode of Angiosperm Mitochondrial Genome Divergence Inferred from Intraspecific Variation in Arabidopsis thaliana

The mechanisms of sequence divergence in angiosperm mitochondrial genomes have long been enigmatic. In particular, it is difficult to reconcile the rapid divergence of intergenic regions that can make non-coding sequences almost unrecognizable even among close relatives with the unusually high levels of sequence conservation found in genic regions. It has been hypothesized that different mutation and repair mechanisms act on genic and intergenic sequences or alternatively that mutational input is relatively constant but that selection has strikingly different effects on these respective regions. To test these alternative possibilities, we analyzed mtDNA divergence within Arabidopsis thaliana, including variants from the 1001 Genomes Project and changes accrued in published mutation accumulation (MA) lines. We found that base-substitution frequencies are relatively similar for intergenic regions and synonymous sites in coding regions, whereas indel and nonsynonymous substitutions rates are greatly depressed in coding regions, supporting a conventional model in which mutation/repair mechanisms are consistent throughout the genome but differentially filtered by selection. Most types of sequence and structural changes were undetectable in 10-generation MA lines, but we found significant shifts in relative copy number across mtDNA regions for lines grown under stressed vs. benign conditions. We confirmed quantitative variation in copy number across the A. thaliana mitogenome using both whole-genome sequencing and droplet digital PCR, further undermining the classic but oversimplified model of a circular angiosperm mtDNA structure. Our results suggest that copy number variation is one of the most fluid features of angiosperm mitochondrial genomes.

Evolving mtDNA populations within cells

Mitochondrial DNA (mtDNA) encodes vital respiratory machinery. Populations of mtDNA molecules exist in most eukaryotic cells, subject to replication, degradation, mutation, and other population processes. These processes affect the genetic makeup of cellular mtDNA populations, changing cell-to-cell distributions, means, and variances of mutant mtDNA load over time. As mtDNA mutant load has nonlinear effects on cell functionality, and cell functionality has nonlinear effects on tissue performance, these statistics of cellular mtDNA populations play vital roles in health, disease, and inheritance. This mini review will describe some of the better-known ways in which these populations change over time in different organisms, highlighting the importance of quantitatively understanding both mutant load mean and variance. Due to length constraints, we cannot attempt to be comprehensive but hope to provide useful links to some of the many excellent studies on these topics.

Mitochondrial DNA Repair in an Arabidopsis thaliana Uracil N-Glycosylase Mutant

Substitution rates in plant mitochondrial genes are extremely low, indicating strong selective pressure as well as efficient repair. Plant mitochondria possess base excision repair pathways; however, many repair pathways such as nucleotide excision repair and mismatch repair appear to be absent. In the absence of these pathways, many DNA lesions must be repaired by a different mechanism. To test the hypothesis that double-strand break repair (DSBR) is that mechanism, we maintained independent self-crossing lineages of plants deficient in uracil-N-glycosylase (UNG) for 11 generations to determine the repair outcomes when that pathway is missing. Surprisingly, no single nucleotide polymorphisms (SNPs) were fixed in any line in generation 11. The pattern of heteroplasmic SNPs was also unaltered through 11 generations. When the rate of cytosine deamination was increased by mitochondrial expression of the cytosine deaminase APOBEC3G, there was an increase in heteroplasmic SNPs but only in mature leaves. Clearly, DNA maintenance in reproductive meristem mitochondria is very effective in the absence of UNG while mitochondrial genomes in differentiated tissue are maintained through a different mechanism or not at all. Several genes involved in DSBR are upregulated in the absence of UNG, indicating that double-strand break repair is a general system of repair in plant mitochondria. It is important to note that the developmental stage of tissues is critically important for these types of experiments.

Factors Affecting Organelle Genome Stability in Physcomitrella patens

Organelle genomes are essential for plants; however, the mechanisms underlying the maintenance of organelle genomes are incompletely understood. Using the basal land plant Physcomitrella patens as a model, nuclear-encoded homologs of bacterial-type homologous recombination repair (HRR) factors have been shown to play an important role in the maintenance of organelle genome stability by suppressing recombination between short dispersed repeats. In this review, I summarize the factors and pathways involved in the maintenance of genome stability, as well as the repeats that cause genomic instability in organelles in P. patens, and compare them with findings in other plant species. I also discuss the relationship between HRR factors and organelle genome structure from the evolutionary standpoint.

Emerging Genome Engineering Tools in Crop Research and Breeding

Recent advances in genome engineering are revolutionizing crop research and plant breeding. The ability to make specific modifications to a plant's genetic material creates opportunities for rapid development of elite cultivars with desired traits. The plant genome can be altered in several ways, including targeted introduction of nucleotide changes, deleting DNA segments, introducing exogenous DNA fragments and epigenetic modifications. Targeted changes are mediated by sequence specific nucleases (SSNs), such as zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and CRISPR (clustered regularly interspersed short palindromic repeats)-Cas (CRISPR associated protein) systems. Recent advances in engineering chimeric Cas nucleases fused to base editing enzymes permit for even greater precision in base editing and control over gene expression. In addition to gene editing technologies, improvement in delivery systems of exogenous DNA into plant cells have increased the rate of successful gene editing events. Regeneration of fertile plants containing the desired edits remains challenging; however, manipulation of embryogenesis-related genes such as BABY BOOM (BBM) has been shown to facilitate regeneration through tissue culture, often a major hurdle in recalcitrant cultivars. Epigenome reprogramming for improved crop performance is another possibility for future breeders, with recent studies on MutS HOMOLOG 1 (MSH1) demonstrating epigenetic-dependent hybrid vigor in several crops. While these technologies offer plant breeders new tools in creating high yielding, better adapted crop varieties, constantly evolving government policy regarding the cultivation of plants containing transgenes may impede the widespread adoption of some of these techniques. This chapter summarizes advances in genome editing tools and discusses the future of these techniques for crop improvement.

Organellar protein multi-functionality and phenotypic plasticity in plants

With the increasing impact of climate instability on agricultural and ecological systems has come a heightened sense of urgency to understand plant adaptation mechanisms in more detail. Plant species have a remarkable ability to disperse their progeny to a wide range of environments, demonstrating extraordinary resiliency mechanisms that incorporate epigenetics and transgenerational stability. Surprisingly, some of the underlying versatility of plants to adapt to abiotic and biotic stress emerges from the neofunctionalization of organelles and organellar proteins. We describe evidence of possible plastid specialization and multi-functional organellar protein features that serve to enhance plant phenotypic plasticity. These features appear to rely on, for example, spatio-temporal regulation of plastid composition, and unusual interorganellar protein targeting and retrograde signalling features that facilitate multi-functionalization. Although we report in detail on three such specializations, involving MSH1, WHIRLY1 and CUE1 proteins in Arabidopsis, there is ample reason to believe that these represent only a fraction of what is yet to be discovered as we begin to elaborate cross-species diversity. Recent observations suggest that plant proteins previously defined in one context may soon be rediscovered in new roles and that much more detailed investigation of proteins that show subcellular multi-targeting may be warranted.

This article is part of the theme issue ‘Linking the mitochondrial genotype to phenotype: a complex endeavour’.