Two distinct plastid genome configurations and unprecedented intraspecies length variation in the accD coding region in Medicago truncatula

A comparative plastome approach enhances the assessment of genetic variation in the Melilotus genus

BACKGROUND

Melilotus, a member of the Fabaceae family, is a pivotal forage crop that is extensively cultivated in livestock regions globally due to its notable productivity and ability to withstand abiotic stress. However, the genetic attributes of the chloroplast genome and the evolutionary connections among different Melilotus species remain unresolved.

RESULTS

In this study, we compiled the chloroplast genomes of 18 Melilotus species and performed a comprehensive comparative analysis. Through the examination of protein-coding genes, we successfully established a robust phylogenetic tree for these species. This conclusion is further supported by the phylogeny derived from single-nucleotide polymorphisms (SNPs) across the entire chloroplast genome. Notably, our findings revealed that M. infestus, M. siculus, M. sulcatus, and M. speciosus formed a distinct subgroup within the phylogenetic tree. Additionally, the chloroplast genomes of these four species exhibit two shared inversions. Moreover, inverted repeats were observed to have reemerged in six species within the IRLC. The distribution patterns of single-nucleotide polymorphisms (SNPs) and insertions/deletions (InDels) within protein-coding genes indicated that ycf1 and ycf2 accumulated nonconservative alterations during evolutionary development. Furthermore, an examination of the evolutionary rate of protein-coding genes revealed that rps18, rps7, and rpl16 underwent positive selection specifically in Melilotus.

CONCLUSIONS

We present a comparative analysis of the complete chloroplast genomes of Melilotus species. This study represents the most thorough and detailed exploration of the evolution and variability within the genus Melilotus to date. Our study provides valuable chloroplast genomic information for improving phylogenetic reconstructions and making biogeographic inferences about Melilotus and other Papilionoideae species.

Plastid phylogenomics and cytonuclear discordance in Rubioideae, Rubiaceae

In this study of evolutionary relationships in the subfamily Rubioideae (Rubiaceae), we take advantage of the off-target proportion of reads generated via previous target capture sequencing projects based on nuclear genomic data to build a plastome phylogeny and investigate cytonuclear discordance. The assembly of off-target reads resulted in a comprehensive plastome dataset and robust inference of phylogenetic relationships, where most intratribal and intertribal relationships are resolved with strong support. While the phylogenetic results were mostly in agreement with previous studies based on plastome data, novel relationships in the plastid perspective were also detected. For example, our analyses of plastome data provide strong support for the SCOUT clade and its sister relationship to the remaining members of the subfamily, which differs from previous results based on plastid data but agrees with recent results based on nuclear genomic data. However, several instances of highly supported cytonuclear discordance were identified across the Rubioideae phylogeny. Coalescent simulation analysis indicates that while ILS could, by itself, explain the majority of the discordant relationships, plastome introgression may be the better explanation in some cases. Our study further indicates that plastomes across the Rubioideae are, with few exceptions, highly conserved and mainly conform to the structure, gene content, and gene order present in the majority of the flowering plants.

Leaf variegation caused by plastome structural variation: an example from Dianella tasmanica

Variegated plants often exhibit plastomic heteroplasmy due to single-nucleotide mutations or small insertions/deletions in their albino sectors. Here, however, we identified a plastome structural variation in albino sectors of the variegated plant Dianella tasmanica (Asphodelaceae), a perennial herbaceous plant widely cultivated as an ornamental in tropical Asia. This structural variation, caused by intermolecular recombination mediated by an 11-bp inverted repeat flanking a 92-bp segment in the large single-copy region (LSC), generates a giant plastome (228 878 bp) with the largest inverted repeat of 105 226 bp and the smallest LSC of 92 bp known in land plants. It also generates an ~7-kb deletion on the boundary of the LSC, which eliminates three protein coding genes (psbA, matK, and rps16) and one tRNA gene (trnK). Albino sectors exhibit dramatic changes in expression of many plastid genes, including negligible expression of psbA, matK, and rps16, reduced expression of photosynthesis-related genes, and increased expression of genes related to the translational apparatus. Microscopic and ultrastructure observations showed that albino tissues were present in both green and albino sectors of the variegated individuals, and chloroplasts were poorly developed in the mesophyll cells of the albino tissues of the variegated individuals. These poorly developed chloroplasts likely carry the large and rearranged plastome, which is likely responsible for the loss of photosynthesis and albinism in the leaf margins. Considering that short repeats are relatively common in plant plastomes and that photosynthesis is not necessary for albino sectors, structural variation of this kind may not be rare in the plastomes of variegated plants.

Extensive characterization of 28 complete chloroplast genomes of Hydrangea species: A perspective view of their organization and phylogenetic and evolutionary relationships

The tribe Hydrangeeae displays a unique, distinctive disjunct distribution encompassing East Asia, North America and Hawaii. Despite its complex trait variations and polyphyletic nature, comprehensive phylogenomic and biogeographical studies on this tribe have been lacking. To address this gap, we sequenced and characterized 28 plastomes of Hydrangeeae. Our study highlights the highly conserved nature of Hydrangeaceae chloroplast (cp) genomes in terms of gene content and arrangement. Notably, synapomorphic characteristics of tandem repeats in the conserved domain of accD were observed in the Macrophyllae, Chinenses, and Dichroa sections within the Hydrangeeae tribe. Additionally, we found lower expression of accD in these sections using structure prediction and quantitative real-time PCR analysis. Phylogenomic analyses revealed the subdivision of the Hydrangeeae tribe into two clades with robust support values. Consistent with polyphyletic relationships, sect. Broussaisia was identified as the basal group in the tribe Hydrangeeae. Our study also provides insights into the phylogenetic relationships of Hydrangea petiolaris in the Jeju and Ulleung Island populations, suggesting the need for further studies with more samples and molecular data. Divergence time estimation and biogeographical analyses suggested that the common ancestors of the tribe Hydrangeeae likely originated from North America and East Asia during the Paleocene period via the Bering Land Bridge, potentially facilitating migration within the tribe between these regions. In conclusion, this study enhances our understanding of the evolutionary history and biogeography of the tribe Hydrangeeae, shedding light on the dispersal patterns and origins of this intriguing plant group with its unique disjunct distribution.

Plastid phylogenomics uncovers multiple species in Medicago truncatula (Fabaceae) germplasm accessions

Medicago truncatula is a model legume that has been extensively investigated in diverse subdisciplines of plant science. Medicago littoralis can interbreed with M. truncatula and M. italica; these three closely related species form a clade, i.e. TLI clade. Genetic studies have indicated that M. truncatula accessions are heterogeneous but their taxonomic identities have not been verified. To elucidate the phylogenetic position of diverse M. truncatula accessions within the genus, we assembled 54 plastid genomes (plastomes) using publicly available next-generation sequencing data and conducted phylogenetic analyses using maximum likelihood. Five accessions showed high levels of plastid DNA polymorphism. Three of these highly polymorphic accessions contained sequences from both M. truncatula and M. littoralis. Phylogenetic analyses of sequences placed some accessions closer to distantly related species suggesting misidentification of source material. Most accessions were placed within the TLI clade and maximally supported the interrelationships of three subclades. Two Medicago accessions were placed within a M. italica subclade of the TLI clade. Plastomes with a 45-kb (rpl20-ycf1) inversion were placed within the M. littoralis subclade. Our results suggest that the M. truncatula accession genome pool represents more than one species due to possible mistaken identities and gene flow among closely related species.

Complete chloroplast genomes of two medicinal Swertia species: the comparative evolutionary analysis of Swertia genus in the Gentianaceae family

The complete chloroplast genome of Swertia kouitchensis has been sequenced and assembled, compared with that of S. bimaculata to determine the evolutionary relationships among species of the Swertia in the Gentianaceae family. Swertia kouitchensis and S. bimaculata are from the Gentianaceae family. The complete chloroplast genome of S. kouitchensis was newly assembled, annotated, and analyzed by Illumina Hiseq 2500 platform. The chloroplast genomes of the two species encoded a total of 133, 134 genes, which included 88-89 protein-coding genes, 37 transfer RNA (tRNA) genes, and 8 ribosomal RNA genes. One intron was contained in each of the eight protein-coding genes and eight tRNA-coding genes, whereas two introns were found in two genes (ycf3 and clpP). The most abundant codon of the two species was for isoleucine, and the least abundant codon was for cysteine. The number of microsatellite repeat sequences was twenty-eight and thirty-two identified in the chloroplast genomes of S. kouitchensis and S. bimaculata, respectively. A total of 1127 repeat sequences were identified in all the 23 Swertia chloroplast genomes, and they fell into four categories. Furthermore, five divergence hotspot regions can be applied to discriminate these 23 Swertia species through genomes comparison. One pair of genus-specific DNA barcodes primer has been accurately identified. Therefore, the diverse regions cloned by a specific primer may become an effective and powerful molecular marker for the identification of Swertia genus. Moreover, four genes (ccsA, ndhK, rpoC1, and rps12) were positive selective pressure. The phylogenetic tree showed that the 23 Swertia species were clustered into a large clade including four evident subbranches, whereas the two species of S. kouitchensis and S. bimaculata were separately clustered into the diverse but correlated species group.

Born in the mitochondrion and raised in the nucleus: evolution of a novel tandem repeat family in Medicago polymorpha (Fabaceae)

Plant nuclear genomes harbor sequence elements derived from the organelles (mitochondrion and plastid) through intracellular gene transfer (IGT). Nuclear genomes also show a dramatic range of repeat content, suggesting that any sequence can be readily amplified. These two aspects of plant nuclear genomes are well recognized but have rarely been linked. Through investigation of 31 Medicago taxa we detected exceptionally high post-IGT amplification of mitochondrial (mt) DNA sequences containing rps10 in the nuclear genome of Medicago polymorpha and closely related species. The amplified sequences were characterized as tandem arrays of five distinct repeat motifs (2157, 1064, 987, 971, and 587 bp) that have diverged from the mt genome (mitogenome) in the M. polymorpha nuclear genome. The mt rps10-like arrays were identified in seven loci (six intergenic and one telomeric) of the nuclear chromosome assemblies and were the most abundant tandem repeat family, representing 1.6-3.0% of total genomic DNA, a value approximately three-fold greater than the entire mitogenome in M. polymorpha. Compared to a typical mt gene, the mt rps10-like sequence coverage level was 691.5-7198-fold higher in M. polymorpha and closely related species. In addition to the post-IGT amplification, our analysis identified the canonical telomeric repeat and the species-specific satellite arrays that are likely attributable to an ancestral chromosomal fusion in M. polymorpha. A possible relationship between chromosomal instability and the mt rps10-like tandem repeat family in the M. polymorpha clade is discussed.

The plastome of the arctic Oxytropis arctobia (Fabaceae) has large differences compared with that of O. splendens and those of related species

Anatomical and physiological specializations for plant adaptation to harsh climates are the results of molecular mechanisms that can be nuclear or organellar encoded. In this study, the complete plastomes of an arctic species, Oxytropis arctobia Bunge (Fabaceae,) and a closely related temperate species, O. splendens Douglas ex Hook., were assembled, annotated and analyzed to search for differences that might help explain their adaptation to different environments. Consistently with the previously sequenced O. bicolor DC. and O. glabra plastomes, the O. arctobia and O. splendens plastomes both have the common features of the inverted repeat-lacking clade (IRLC), as well as the atpF intron loss, which is unique to the genus. However, significant differences distinguishes the O. arctobia from O. splendens and other closely related plastomes (Oxytropis spp. and Astragalus spp.), including a 3 kb inversion, two large insertions (>1 kb), significant modifications of the accD gene, and an overall larger size.

The complete chloroplast genome of Onobrychis gaubae (Fabaceae-Papilionoideae): comparative analysis with related IR-lacking clade species

BACKGROUND

Plastome (Plastid genome) sequences provide valuable markers for surveying evolutionary relationships and population genetics of plant species. Papilionoideae (papilionoids) has different nucleotide and structural variations in plastomes, which makes it an ideal model for genome evolution studies. Therefore, by sequencing the complete chloroplast genome of Onobrychis gaubae in this study, the characteristics and evolutionary patterns of plastome variations in IR-loss clade were compared.

RESULTS

In the present study, the complete plastid genome of O. gaubae, endemic to Iran, was sequenced using Illumina paired-end sequencing and was compared with previously known genomes of the IRLC species of legumes. The O. gaubae plastid genome was 122,688 bp in length and included a large single-copy (LSC) region of 81,486 bp, a small single-copy (SSC) region of 13,805 bp and one copy of the inverted repeat (IR_b) of 29,100 bp. The genome encoded 110 genes, including 76 protein-coding genes, 30 transfer RNA (tRNA) genes and four ribosome RNA (rRNA) genes and possessed 83 simple sequence repeats (SSRs) and 50 repeated structures with the highest proportion in the LSC. Comparative analysis of the chloroplast genomes across IRLC revealed three hotspot genes (ycf1, ycf2, clpP) which could be used as DNA barcode regions. Moreover, seven hypervariable regions [trnL(UAA)-trnT(UGU), trnT(GGU)-trnE(UUC), ycf1, ycf2, ycf4, accD and clpP] were identified within Onobrychis, which could be used to distinguish the Onobrychis species. Phylogenetic analyses revealed that O. gaubae is closely related to Hedysarum. The complete O. gaubae genome is a valuable resource for investigating evolution of Onobrychis species and can be used to identify related species.

CONCLUSIONS

Our results reveal that the plastomes of the IRLC are dynamic molecules and show multiple gene losses and inversions. The identified hypervariable regions could be used as molecular markers for resolving phylogenetic relationships and species identification and also provide new insights into plastome evolution across IRLC.

Mahonia vs. Berberis Unloaded: Generic Delimitation and Infrafamilial Classification of Berberidaceae Based on Plastid Phylogenomics

The early-diverging eudicot family Berberidaceae is composed of a morphologically diverse assemblage of disjunctly distributed genera long praised for their great horticultural and medicinal values. However, despite century-long studies, generic delimitation of Berberidaceae remains controversial and its tribal classification has never been formally proposed under a rigorous phylogenetic context. Currently, the number of accepted genera in Berberidaceae ranges consecutively from 13 to 19, depending on whether to define Berberis, Jeffersonia, and Podophyllum broadly, or to segregate these three genera further and recognize Alloberberis, Mahonia, and Moranothamnus, Plagiorhegma, and Dysosma, Diphylleia, and Sinopodophyllum, respectively. To resolve Berberidaceae's taxonomic disputes, we newly assembled 23 plastomes and, together with 85 plastomes from the GenBank, completed the generic sampling of the family. With 4 problematic and 14 redundant plastome sequences excluded, robust phylogenomic relationships were reconstructed based on 93 plastomes representing all 19 genera of Berberidaceae and three outgroups. Maximum likelihood phylogenomic relationships corroborated with divergence time estimation support the recognition of three subfamilies Berberidoideae, Nandinoideae, and Podophylloideae, with tribes Berberideae and Ranzanieae, Leonticeae and Nandineae, and Podophylleae, Achlydeae, Bongardieae tr. nov., Epimedieae, and Jeffersonieae tr. nov. in the former three subfamilies, respectively. By applying specifically stated criteria, our phylogenomic data also support the classification of 19 genera, recognizing Alloberberis, Mahonia, and Moranothamnus, Plagiorhegma, and Diphylleia, Dysosma, and Sinopodophyllum that are morphologically and evolutionarily distinct from Berberis, Jeffersonia, and Podophyllum, respectively. Comparison of plastome structures across Berberidaceae confirms inverted repeat expansion in the tribe Berberideae and reveals substantial length variation in accD gene caused by repeated sequences in Berberidoideae. Comparison of plastome tree with previous studies and nuclear ribosomal DNA (nrDNA) phylogeny also reveals considerable conflicts at different phylogenetic levels, suggesting that incomplete lineage sorting and/or hybridization had occurred throughout the evolutionary history of Berberidaceae and that Alloberberis and Moranothamnus could have resulted from reciprocal hybridization between Berberis and Mahonia in ancient times prior to the radiations of the latter two genera.

Genome structure and diversity among Cynanchum wilfordii accessions

BACKGROUND

Cynanchum wilfordii (Cw) and Cynanchum auriculatum (Ca) have long been used in traditional medicine and as functional food in Korea and China, respectively. They have diverse medicinal functions, and many studies have been conducted, including pharmaceutical efficiency and metabolites. Especially, Cw is regarded as the most famous medicinal herb in Korea due to its menopausal symptoms relieving effect. Despite the high demand for Cw in the market, both species are cultivated using wild resources with rare genomic information.

RESULTS

We collected 160 Cw germplasm from local areas of Korea and analyzed their morphological diversity. Five Cw and one Ca of them, which were morphologically diverse, were sequenced, and nuclear ribosomal DNA (nrDNA) and complete plastid genome (plastome) sequences were assembled and annotated. We investigated the genomic characteristics of Cw as well as the genetic diversity of plastomes and nrDNA of Cw and Ca. The Cw haploid nuclear genome was approximately 178 Mbp. Karyotyping revealed the juxtaposition of 45S and 5S nrDNA on one of 11 chromosomes. Plastome sequences revealed 1226 interspecies polymorphisms and 11 Cw intraspecies polymorphisms. The 160 Cw accessions were grouped into 21 haplotypes based on seven plastome markers and into 108 haplotypes based on seven nuclear markers. Nuclear genotypes did not coincide with plastome haplotypes that reflect the frequent natural outcrossing events.

CONCLUSIONS

Cw germplasm had a huge morphological diversity, and their wide range of genetic diversity was revealed through the investigation with 14 molecular markers. The morphological and genomic diversity, chromosome structure, and genome size provide fundamental genomic information for breeding of undomesticated Cw plants.

Plastome Structural Evolution and Homoplastic Inversions in Neo-Astragalus (Fabaceae)

The plastid genomes of photosynthetic green plants have largely maintained conserved gene content and order as well as structure over hundreds of millions of years of evolution. Several plant lineages, however, have departed from this conservation and contain many plastome structural rearrangements, which have been associated with an abundance of repeated sequences both overall and near rearrangement endpoints. We sequenced the plastomes of 25 taxa of Astragalus L. (Fabaceae), a large genus in the inverted repeat-lacking clade of legumes, to gain a greater understanding of the connection between repeats and plastome inversions. We found plastome repeat structure has a strong phylogenetic signal among these closely related taxa mostly in the New World clade of Astragalus called Neo-Astragalus. Taxa without inversions also do not differ substantially in their overall repeat structure from four taxa each with one large-scale inversion. For two taxa with inversion endpoints between the same pairs of genes, differences in their exact endpoints indicate the inversions occurred independently. Our proposed mechanism for inversion formation suggests the short inverted repeats now found near the endpoints of the four inversions may be there as a result of these inversions rather than their cause. The longer inverted repeats now near endpoints may have allowed the inversions first mediated by shorter microhomologous sequences to propagate, something that should be considered in explaining how any plastome rearrangement becomes fixed regardless of the mechanism of initial formation.

Lineage-Specific Variation in IR Boundary Shift Events, Inversions, and Substitution Rates among Caprifoliaceae s.l. (Dipsacales) Plastomes

Caprifoliaceae s.l. plastid genomes (plastomes) show that one inversion and two inverted repeat boundary shifts occurred in the common ancestor of this family, after which the plastomes are generally conserved. This study reports plastome sequences of five additional species, Fedia cornucopiae, Valeriana fauriei, and Valerianella locusta from the subfamily Valerianoideae, as well as Dipsacus japonicus and Scabiosa comosa from the subfamily Dipsacoideae. Combined with the published plastomes, these plastomes provide new insights into the structural evolution of plastomes within the family. Moreover, the three plastomes from the subfamily Valerianoideae exhibited accelerated nucleotide substitution rates, particularly at synonymous sites, across the family. The patterns of accD sequence divergence in the family are dynamic with structural changes, including interruption of the conserved domain and increases in nonsynonymous substitution rates. In particular, the Valeriana accD gene harbors a large insertion of amino acid repeat (AAR) motifs, and intraspecific polymorphism with a variable number of AARs in the Valeriana accD gene was detected. We found a correlation between intron losses and increased ratios of nonsynonymous to synonymous substitution rates in the clpP gene with intensified positive selection. In addition, two Dipsacoideae plastomes revealed the loss of the plastid-encoded rps15, and a potential functional gene transfer to the nucleus was confirmed.

Heterogeneous Genetic Diversity Estimation of a Promising Domestication Medicinal Motherwort Leonurus Cardiaca Based on Chloroplast Genome Resources

Leonurus cardiaca has a long history of use in western herbal medicine and is applied for the treatment of gynaecological conditions, anxiety, and heart diseases. Because of its botanical relationship to the primary Chinese species, L. japonicus, and extensive medical indications that go beyond the traditional indications for the Chinese species, it is a promising medicinal resource. Therefore, the features of genetic diversity and variability in the species have been prioritized. To explore these issues, we sequenced the chloroplast genomes of 22 accessions of L. cardiaca from different geographical locations worldwide using high-throughput sequencing. The results indicate that L. cardiaca has a typical quadripartite structure and range from 1,51,236 bp to 1,51,831 bp in size, forming eight haplotypes. The genomes all contain 114 distinct genes, including 80 protein-coding genes, 30 transfer RNA genes and four ribosomal RNA genes. Comparative analysis showed abundant diversity of single nucleotide polymorphisms (SNPs), indels, simple sequence repeats (SSRs) in 22 accessions. Codon usage showed highly similar results for L. cardiaca species. The phylogenetic and network analysis indicated 22 accessions forming four clades that were partly related to the geographical distribution. In summary, our study highlights the advantage of chloroplast genome with large data sets in intraspecific diversity evaluation and provides a new tool to facilitate medicinal plant conservation and domestication.

Extensive genomic rearrangements mediated by repetitive sequences in plastomes of Medicago and its relatives

BACKGROUND

Although plastomes are highly conserved with respect to gene content and order in most photosynthetic angiosperms, extensive genomic rearrangements have been reported in Fabaceae, particularly within the inverted repeat lacking clade (IRLC) of Papilionoideae. Two hypotheses, i.e., the absence of the IR and the increased repeat content, have been proposed to affect the stability of plastomes. However, this is still unclear for the IRLC species. Here, we aimed to investigate the relationships between repeat content and the degree of genomic rearrangements in plastomes of Medicago and its relatives Trigonella and Melilotus, which are nested firmly within the IRLC.

RESULTS

We detected abundant repetitive elements and extensive genomic rearrangements in the 75 newly assembled plastomes of 20 species, including gene loss, intron loss and gain, pseudogenization, tRNA duplication, inversion, and a second independent IR gain (IR ~ 15 kb in Melilotus dentata) in addition to the previous first reported cases in Medicago minima. We also conducted comparative genomic analysis to evaluate plastome evolution. Our results indicated that the overall repeat content is positively correlated with the degree of genomic rearrangements. Some of the genomic rearrangements were found to be directly linked with repetitive sequences. Tandem repeated sequences have been detected in the three genes with accelerated substitution rates (i.e., accD, clpP, and ycf1) and their length variation could be explained by the insertions of tandem repeats. The repeat contents of the three localized hypermutation regions around these three genes with accelerated substitution rates are also significantly higher than that of the remaining plastome sequences.

CONCLUSIONS

Our results suggest that IR reemergence in the IRLC species does not ensure their plastome stability. Instead, repeat-mediated illegitimate recombination is the major mechanism leading to genome instability, a pattern in agreement with recent findings in other angiosperm lineages. The plastome data generated herein provide valuable genomic resources for further investigating the plastome evolution in legumes.

Unprecedented Intraindividual Structural Heteroplasmy in Eleocharis (Cyperaceae, Poales) Plastomes

Plastid genomes (plastomes) of land plants have a conserved quadripartite structure in a gene-dense unit genome consisting of a large inverted repeat that separates two single copy regions. Recently, alternative plastome structures were suggested in Geraniaceae and in some conifers and Medicago the coexistence of inversion isomers has been noted. In this study, plastome sequences of two Cyperaceae, Eleocharis dulcis (water chestnut) and Eleocharis cellulosa (gulf coast spikerush), were completed. Unlike the conserved plastomes in basal groups of Poales, these Eleocharis plastomes have remarkably divergent features, including large plastome sizes, high rates of sequence rearrangements, low GC content and gene density, gene duplications and losses, and increased repetitive DNA sequences. A novel finding among these features was the unprecedented level of heteroplasmy with the presence of multiple plastome structural types within a single individual. Illumina paired-end assemblies combined with PacBio single-molecule real-time sequencing, long-range polymerase chain reaction, and Sanger sequencing data identified at least four different plastome structural types in both Eleocharis species. PacBio long read data suggested that one of the four E. dulcis plastome types predominates.

Plastid Genomes of Flowering Plants: Essential Principles

The plastid genome (plastome ) has proved a valuable source of data for evaluating evolutionary relationships among angiosperms. Through basic and applied approaches, plastid transformation technology offers the potential to understand and improve plant productivity, providing food, fiber, energy, and medicines to meet the needs of a burgeoning global population. The growing genomic resources available to both phylogenetic and biotechnological investigations is allowing novel insights and expanding the scope of plastome research to encompass new species. In this chapter, we present an overview of some of the seminal and contemporary research that has contributed to our current understanding of plastome evolution and attempt to highlight the relationship between evolutionary mechanisms and the tools of plastid genetic engineering.

Caught in the Act: Variation in plastid genome inverted repeat expansion within and between populations of Medicago minima

The inverted repeat (IR) lacking clade (IRLC) is a monophyletic group within the Papilionoideae subfamily of Fabaceae where plastid genomes (plastomes) do not contain the large IR typical of land plants. Recently, an IRLC legume, Medicago minima, was found to have regrown a ~9 kb IR that contained a number of canonical IR genes, and closely related M. lupulina contained an incomplete IR of ~425 bp. Complete plastomes were generated for seven additional species, putative members of the M. minima clade. Polymerase chain reaction was employed to investigate the presence of the IR across M. minima and M. lupulina including individuals of nine and eight Eurasian and North African accessions and 15 and 14 Texas populations, respectively. While no sequence similar to the ~9 kb IR was detected among the seven newly sequenced plastomes, all Eurasian and North African accessions of M. minima contained the IR. Variation in IR extent was detected within and between the Texas populations. Expansions of 13 bp and 11 bp occurred at the boundaries of both IR/small single-copy regions, and populations had one or the other expansion, but not both. Expansion of the IR was not detected in the accessions from Eurasia and North Africa suggesting recent mutations yielded at least two additional plastid haplotypes in M. minima.

Two Independent Plastid accD Transfers to the Nuclear Genome of Gnetum and Other Insights on Acetyl-CoA Carboxylase Evolution in Gymnosperms

Acetyl-CoA carboxylase (ACCase) is the key regulator of fatty acid biosynthesis. In most plants, ACCase exists in two locations (cytosol and plastids) and in two forms (homomeric and heteromeric). Heteromeric ACCase comprises four subunits, three of them (ACCA-C) are nuclear encoded (nr) and the fourth (ACCD) is usually plastid encoded. Homomeric ACCase is encoded by a single nr-gene (ACC). We investigated the ACCase gene evolution in gymnosperms by examining the transcriptomes of newly sequenced Gnetum ula, combined with 75 transcriptomes and 110 plastomes of other gymnosperms. AccD-coding sequences are elongated through the insertion of repetitive DNA in four out of five cupressophyte families (except Sciadopityaceae) and were functionally transferred to the nucleus of gnetophytes and Sciadopitys. We discovered that, among the three genera of gnetophytes, only Gnetum has two copies of nr-accD. Furthermore, using protoplast transient expression assays, we experimentally verified that the nr-accD precursor proteins in Gnetum and Sciadopitys can be delivered to the plastids. Of the two nr-accD copies of Gnetum, one dually targets plastids and mitochondria, whereas the other potentially targets plastoglobuli. The distinct transit peptides, gene architectures, and flanking sequences between the two Gnetum accDs suggest that they have independent origins. Our findings are the first account of two distinctly targeted nr-accDs of any green plants and the most comprehensive analyses of ACCase evolution in gymnosperms to date.

The plastomes of Astrocaryum aculeatum G. Mey. and A. murumuru Mart. show a flip-flop recombination between two short inverted repeats

The plastomes of Astrocaryum murumuru and A. aculeatum revealed a lineage-specific structural feature originated by flip-flop recombination, non-synonymous substitutions in conserved genes and several molecular markers. Astrocaryum murumuru Mart. and A. aculeatum G.Mey. are two palm species of Amazon forest that are economically important as source of food, oil and raw material for several applications. Genetic studies aiming to establish strategies for conservation and domestication of both species are still in the beginning given that the exploitation is mostly by extractive activity. The identification and characterization of molecular markers are essential to assess the genetic diversity of natural populations of both species. Therefore, we sequenced and characterized in detail the plastome of both species. We compared both species and identified 32 polymorphic SSR loci, 150 SNPs, 46 indels and eight hotspots of nucleotide diversity. Additionally, we reported a specific RNA editing site found in the ccsA gene, which is exclusive to A. murumuru. Moreover, the structural analysis in the plastomes of both species revealed a 4.6-kb inversion encompassing a set of genes involved in chlororespiration and plastid translation. This 4.6-kb inversion is a lineage-specific structural feature of the genus Astrocaryum originated by flip-flop recombination between two short inverted repeats. Furthermore, our phylogenetic analysis using whole plastomes of 39 Arecaceae species placed the Astrocaryum species sister to Acrocomia within the tribe Cocoseae. Finally, our data indicated substantial changes in the plastome structure and sequence of both species of the genus Astrocaryum, bringing new molecular markers, several structural and evolving features, which can be applied in several areas such as genetic, evolution, breeding, phylogeny and conservation strategies for both species.

Thirteen New Plastid Genomes from Mixotrophic and Autotrophic Species Provide Insights into Heterotrophy Evolution in Neottieae Orchids

Mixotrophic species use both organic and mineral carbon sources. Some mixotrophic plants combine photosynthesis and a nutrition called mycoheterotrophy, where carbon is obtained from fungi forming mycorrhizal symbiosis with their roots. These species can lose photosynthetic abilities and evolve full mycoheterotrophy. Besides morphological changes, the latter transition is associated with a deep alteration of the plastid genome. Photosynthesis-related genes are lost first, followed by housekeeping genes, eventually resulting in a highly reduced genome. Whether relaxation of selective constraints already occurs for the plastid genome of mixotrophic species, which remain photosynthetic, is unclear. This is partly due to the difficulty of comparing plastid genomes of autotrophic, mixotrophic, and mycoheterotrophic species in a narrow phylogenetic framework. We address this question in the orchid tribe Neottieae, where this large assortment of nutrition types occurs. We sequenced 13 new plastid genomes, including 9 mixotrophic species and covering all 6 Neottieae genera. We investigated selective pressure on plastid genes in each nutrition type and conducted a phylogenetic inference of the group. Surprisingly, photosynthesis-related genes did not experience selection relaxation in mixotrophic species compared with autotrophic relatives. Conversely, we observed evidence for selection intensification for some plastid genes. Photosynthesis is thus still under purifying selection, maybe because of its role in fruit formation and thus reproductive success. Phylogenetic analysis resolved most relationships, but short branches at the base of the tree suggest an evolutionary radiation at the beginning of Neottieae history, which, we hypothesize, may be linked to mixotrophy emergence.

Correction to: Comparison of efficiency and time to regeneration of Agrobacterium-mediated transformation methods in Medicago truncatula

[This corrects the article DOI: 10.1186/s13007-019-0404-1.].

Intergeneric Relationships within the Family Salicaceae s.l. based on Plastid Phylogenomics

Many Salicaceae s.l. plants are recognized for their important role in the production of products such as wood, oils, and medicines, and as a model organism in life studies. However, the difference in plastid sequence, phylogenetic relationships, and lineage diversification of the family Salicaceae s.l. remain poorly understood. In this study, we compare 24 species representing 18 genera of the family. Simple sequence repeats (SSRs) are considered effective molecular markers for plant species identification and population genetics. Among them, a total of 1798 SSRs were identified, among which mononucleotide repeat was the most common with 1455 accounts representing 80.92% of the total. Most of the SSRs are located in the non-coding region. We also identified five other types of repeats, including 1750 tandems, 434 forward, 407 palindromic, 86 reverse, and 30 complementary repeats. The species in Salicaceae s.l. have a conserved plastid genome. Each plastome presented a typical quadripartite structure and varied in size due to the expansion and contraction of the inverted repeat (IR) boundary, lacking major structural variations, but we identified six divergence hotspot regions. We obtained phylogenetic relationships of 18 genera in Salicaceae s.l. and the 24 species formed a highly supported lineage. Casearia was identified as the basal clade. The divergence time between Salicaceae s.l. and the outgroup was estimated as ~93 Mya; Salix, and Populus diverged around 34 Mya, consistent with the previously reported time. Our research will contribute to a better understanding of the phylogenetic relationships among the members of the Salicaceae s.l.

Lost and Found: Return of the Inverted Repeat in the Legume Clade Defined by Its Absence

The plant genome comprises a coevolving, integrated genetic system housed in three subcellular compartments: the nucleus, mitochondrion, and the plastid. The typical land plant plastid genome (plastome) comprises the sum of repeating units of 130–160 kb in length. The plastome inverted repeat (IR) divides each plastome monomer into large and small single copy regions, an architecture highly conserved across land plants. There have been varying degrees of expansion or contraction of the IR, and in a few distinct lineages, including the IR-lacking clade of papilionoid legumes, one copy of the IR has been lost. Completion of plastome sequencing and assembly for 19 Medicago species and Trigonella foenum-graceum and comparative analysis with other IR-lacking clade taxa revealed modest divergence with regard to structural organization overall. However, one clade contained unique variation suggesting an ancestor had experienced repeat-mediated changes in plastome structure. In Medicago minima, a novel IR of ∼9 kb was confirmed and the role of repeat-mediated, recombination-dependent replication in IR reemergence is discussed.

Incongruence between gene trees and species trees and phylogenetic signal variation in plastid genes

The current classification of angiosperms is based primarily on concatenated plastid markers and maximum likelihood (ML) inference. This approach has been justified by the assumption that plastid DNA (ptDNA) is inherited as a single locus and that its individual genes produce congruent trees. However, structural and functional characteristics of ptDNA suggest that plastid genes may not evolve as a single locus and are experiencing different evolutionary forces. To examine this idea, we produced new complete plastid genome (plastome) sequences of 27 species and combined these data with publicly available sequences to produce a final dataset that includes 78 plastid genes for 89 species of rosids and five outgroups. We used four data matrices (i.e., gene, exon, codon-aligned, and amino acid) to infer species and gene trees using ML and multispecies coalescent (MSC) methods. Rosids include about one third of all angiosperms and their two major clades, fabids and malvids, were recovered in almost all analyses. However, we detected incongruence between species trees inferred with different matrices and methods and previously published plastid and nuclear phylogenies. We visualized and tested the significance of incongruence between gene trees and species trees. We then measured the distribution of phylogenetic signal across sites and genes supporting alternative placements of five controversial nodes at different taxonomic levels. Gene trees inferred with plastid data often disagree with species trees inferred using both ML (with unpartitioned or partitioned data) and MSC. Species trees inferred with both methods produced alternative topologies for a few taxa. Our results show that, in a phylogenetic context, plastid protein-coding genes may not be fully linked and behaving as a single locus. Furthermore, concatenated matrices may produce highly supported phylogenies that are discordant with individual gene trees. We also show that phylogenies inferred with MSC are accurate. We therefore emphasize the importance of considering variation in phylogenetic signal across plastid genes and the exploration of plastome data to increase accuracy of estimating relationships. We also support the use of MSC with plastome matrices in future phylogenomic investigations.

Allelic Diversity of Acetyl Coenzyme A Carboxylase accD/bccp Genes Implicated in Nuclear-Cytoplasmic Conflict in the Wild and Domesticated Pea (Pisum sp.)

Reproductive isolation is an important component of species differentiation. The plastid accD gene coding for the acetyl-CoA carboxylase subunit and the nuclear bccp gene coding for the biotin carboxyl carrier protein were identified as candidate genes governing nuclear-cytoplasmic incompatibility in peas. We examined the allelic diversity in a set of 195 geographically diverse samples of both cultivated (Pisum sativum, P. abyssinicum) and wild (P. fulvum and P. elatius) peas. Based on deduced protein sequences, we identified 34 accD and 31 bccp alleles that are partially geographically and genetically structured. The accD is highly variable due to insertions of tandem repeats. P. fulvum and P. abyssinicum have unique alleles and combinations of both genes. On the other hand, partial overlap was observed between P. sativum and P. elatius. Mapping of protein sequence polymorphisms to 3D structures revealed that most of the repeat and indel polymorphisms map to sequence regions that could not be modeled, consistent with this part of the protein being less constrained by requirements for precise folding than the enzymatically active domains. The results of this study are important not only from an evolutionary point of view but are also relevant for pea breeding when using more distant wild relatives.

Genetic, evolutionary and phylogenetic aspects of the plastome of annatto (Bixa orellana L.), the Amazonian commercial species of natural dyes

The plastome of B. orellana reveals specific evolutionary features, unique RNA editing sites, molecular markers and the position of Bixaceae within Malvales. Annatto (Bixa orellana L.) is a native species of tropical Americas with center of origin in Brazilian Amazonia. Its seeds accumulate the apocarotenoids, bixin and norbixin, which are only found in high content in this species. The seeds of B. orellana are commercially valued by the food industry because its dyes replace synthetic ones from the market due to potential carcinogenic risks. The increasing consumption of B. orellana seeds for dye extraction makes necessary the increase of productivity, which is possible accessing the genetic basis and searching for elite genotypes. The identification and characterization of molecular markers are essential to analyse the genetic diversity of natural populations and to establish suitable strategies for conservation, domestication, germplasm characterization and genetic breeding. Therefore, we sequenced and characterized in detail the plastome of B. orellana. The plastome of B. orellana is a circular DNA molecule of 159,708 bp with a typical quadripartite structure and 112 unique genes. Additionally, a total of 312 SSR loci were identified in the plastome of B. orellana. Moreover, we predicted in 23 genes a total of 57 RNA-editing sites of which 11 are unique for B. orellana. Furthermore, our plastid phylogenomic analyses, using the plastome sequences available in the plastid database belonging to species of order Malvales, indicate a closed relationship between Bixaceae and Malvaceae, which formed a sister group to Thymelaeaceae. Finally, our study provided useful data to be employed in several genetic and biotechnological approaches in B. orellana and related species of the family Bixaceae.

Green Biotechnology: A Brief Update on Plastid Genome Engineering

Plant genetic engineering has become an inevitable tool in the molecular breeding of crops. Significant progress has been made in the generation of novel plastid transformation vectors and optimized transformation protocols. There are several advantages of plastid genome engineering over conventional nuclear transformation. Some of the advantages include multigene engineering by expression of biosynthetic pathway genes as operons, extremely high-level expression of protein accumulation, lack of transgene silencing, etc. Transgene containment owing to maternal inheritance is another important advantage of plastid genome engineering. Chloroplast genome modification usually results in alteration of several thousand plastid genome copies in a cell. Several therapeutic proteins, edible vaccines, antimicrobial peptides, and industrially important enzymes have been successfully expressed in chloroplasts so far. Here, we critically recapitulate the latest developments in plastid genome engineering. Latest advancements in plastid genome sequencing are briefed. In addition, advancement of extending the toolbox for plastid engineering for selected applications in the area of molecular farming and production of industrially important enzyme is briefed.

Cryptic divergences in the genus Pisum L. (peas), as revealed by phylogenetic analysis of plastid genomes

Organellar genomes may shed light on complicated patterns of plant evolution at inter- and intraspecies level. Primary structure of plastid genomes sequenced in this study and taken from public databases was characterised and compared in 22 diverse, mostly wild representatives of the genus Pisum (peas). Phylogenetic trees reconstructed via Bayesian approach on the basis of entire plastid genomes resembled those reconstructed on the basis of a nuclear gene His5 coding for a minor histone H1 subtype. They reveal Pisum fulvum as an early divergence of the genus but do not support other taxonomical subdivisions. The positions of three accessions, classified as P. sativum subsp. elatius (the wild subspecies of the common pea), appeared quite unexpected. On the entire plastid genome tree, two accessions, from the Black Sea area of Turkey and Georgia, clustered with representatives of another species, P. fulvum, while the other, from Greece, was the first divergence of the P. sativum branch. We suppose these unusual plastid genomes to be ancient lineages ascending to a 'missing link' between P. fulvum and P. sativum, represented by accession Pe 013 from Turkey. Accessions with common pea appearance but deeply diverged plastids could occur through occasional crossing of diverged pea lines in the past and biparental plastid inheritance, both events being possible in peas.

Reconfiguration of the plastid genome in Lamprocapnos spectabilis: IR boundary shifting, inversion, and intraspecific variation

We generated a complete plastid genome (plastome) sequence for Lamprocapnos spectabilis, providing the first complete plastome from the subfamily Fumarioideae (Papaveraceae). The Lamprocapnos plastome shows large differences in size, structure, gene content, and substitution rates compared with two sequenced Papaveraceae plastomes. We propose a model that explains the major rearrangements observed, involving at least six inverted repeat (IR) boundary shifts and five inversions, generating a number of gene duplications and relocations, as well as a two-fold expansion of the IR and miniaturized small single-copy region. A reduction in the substitution rates for genes transferred from the single-copy regions to the IR was observed. Accelerated substitution rates of plastid accD and clpP were detected in the Lamprocapnos plastome. The accelerated substitution rate for the accD gene was correlated with a large insertion of amino acid repeat (AAR) motifs in the middle region, but the forces driving the higher substitution rate of the clpP gene are unclear. We found a variable number of AARs in Lamprocapnos accD and ycf1 genes within individuals, and the repeats were associated with coiled-coil regions. In addition, comparative analysis of three Papaveraceae plastomes revealed loss of rps15 in Papaver, and functional replacement to the nucleus was identified.

Plastome characteristics of Cannabaceae

Cannabaceae is an economically important family that includes ten genera and ca. 117 accepted species. To explore the structure and size variation of their plastomes, we sequenced ten plastomes representing all ten genera of Cannabaceae. Each plastome possessed the typical angiosperm quadripartite structure and contained a total of 128 genes. The Inverted Repeat (IR) regions in five plastomes had experienced small expansions (330-983 bp) into the Large Single-Copy (LSC) region. The plastome of Chaetachme aristata has experienced a 942-bp IR contraction and lost rpl22 and rps19 in its IRs. The substitution rates of rps19 and rpl22 decreased after they shifted from the LSC to IR. A 270-bp inversion was detected in the Parasponia rugosa plastome, which might have been mediated by 18-bp inverted repeats. Repeat sequences, simple sequence repeats, and nucleotide substitution rates varied among these plastomes. Molecular markers with more than 13% variable sites and 5% parsimony-informative sites were identified, which may be useful for further phylogenetic analysis and species identification. Our results show strong support for a sister relationship between Gironniera and Lozanell (BS = 100). Celtis, Cannabis-Humulus, Chaetachme-Pteroceltis, and Trema-Parasponia formed a strongly supported clade, and their relationships were well resolved with strong support (BS = 100). The availability of these ten plastomes provides valuable genetic information for accurately identifying species, clarifying taxonomy and reconstructing the intergeneric phylogeny of Cannabaceae.

The Repeat Sequences and Elevated Substitution Rates of the Chloroplast accD Gene in Cupressophytes

The plastid accD gene encodes a subunit of the acetyl-CoA carboxylase (ACCase) enzyme. The length of accD gene has been supposed to expand in Cryptomeria japonica, Taiwania cryptomerioides, Cephalotaxus, Taxus chinensis, and Podocarpus lambertii, and the main reason for this phenomenon was the existence of tandemly repeated sequences. However, it is still unknown whether the accD gene length in other cupressophytes has expanded. Here, in order to investigate how widespread this phenomenon was, 18 accD sequences and its surrounding regions of cupressophyte were sequenced and analyzed. Together with 39 GenBank sequence data, our taxon sampling covered all the extant gymnosperm orders. The repetitive elements and substitution rates of accD among 57 gymnosperm species were analyzed, the results show: (1) Reading frame length of accD gene in 18 cupressophytes species has also expanded. (2) Many repetitive elements were identified in accD gene of cupressophyte lineages. (3) The synonymous and non-synonymous substitution rates of accD were accelerated in cupressophytes. (4) accD was located in rearrangement endpoints. These results suggested that repetitive elements may mediate the chloroplast genome rearrangement and accelerated the substitution rates.

Plastid Genome Evolution in the Early-Diverging Legume Subfamily Cercidoideae (Fabaceae)

The subfamily Cercidoideae is an early-branching legume lineage, which consists of 13 genera distributed in the tropical and warm temperate Northern Hemisphere. A previous study detected two plastid genomic variations in this subfamily, but the limited taxon sampling left the overall plastid genome (plastome) diversification across the subfamily unaddressed, and phylogenetic relationships within this clade remained unresolved. Here, we assembled eight plastomes from seven Cercidoideae genera and conducted phylogenomic-comparative analyses in a broad evolutionary framework across legumes. The plastomes of Cercidoideae all exhibited a typical quadripartite structure with a conserved gene content typical of most angiosperm plastomes. Plastome size ranged from 151,705 to 165,416 bp, mainly due to the expansion and contraction of inverted repeat (IR) regions. The order of genes varied due to the occurrence of several inversions. In Tylosema species, a plastome with a 29-bp IR-mediated inversion was found to coexist with a canonical-type plastome, and the abundance of the two arrangements of isomeric molecules differed between individuals. Complete plastome data were much more efficient at resolving intergeneric relationships of Cercidoideae than the previously used selection of only a few plastid or nuclear loci. In sum, our study revealed novel insights into the structural diversification of plastomes in an early-branching legume lineage, and, thus, into the evolutionary trajectories of legume plastomes in general.

Genus-Wide Screening Reveals Four Distinct Types of Structural Plastid Genome Organization in Pelargonium (Geraniaceae)

Geraniaceae are known for their unusual plastid genomes (plastomes), with the genus Pelargonium being most conspicuous with regard to plastome size and gene organization as judged by the sequenced plastomes of P. x hortorum and P. alternans. However, the hybrid origin of P. x hortorum and the uncertain phylogenetic position of P. alternans obscure the events that led to these extraordinary plastomes. Here, we examine all plastid reconfiguration hotspots for 60 Pelargonium species across all subgenera using a PCR and sequencing approach. Our reconstruction of the rearrangement history revealed four distinct plastome types. The ancestral plastome configuration in the two subgenera Magnipetala and Pelargonium is consistent with that of the P. alternans plastome, whereas that of the subgenus Parvulipetala deviates from this organization by one synapomorphic inversion in the trnNGUU–ndhF region. The plastome of P. x hortorum resembles those of one group of the subgenus Paucisignata, but differs from a second group by another inversion in the psaI–psaJ region. The number of microstructural changes and amount of repetitive DNA are generally elevated in all inverted regions. Nucleotide substitution rates correlate positively with the number of indels in all regions across the different subgenera. We also observed lineage- and species-specific changes in the gene content, including gene duplications and fragmentations. For example, the plastid rbcL–psaI region of Pelargonium contains a highly variable accD-like region. Our results suggest alternative evolutionary paths under possibly changing modes of plastid transmission and indicate the non-functionalization of the plastid accD gene in Pelargonium.

Total duplication of the small single copy region in the angiosperm plastome: Rearrangement and inverted repeat instability in Asarum

PREMISE OF THE STUDY

As more plastomes are assembled, it is evident that rearrangements, losses, intergenic spacer expansion and contraction, and syntenic breaks within otherwise functioning plastids are more common than was thought previously, and such changes have developed independently in disparate lineages. However, to date, the magnoliids remain characterized by their highly conserved plastid genomes (plastomes).

METHODS

Illumina HiSeq and MiSeq platforms were used to sequence the plastomes of Saruma henryi and those of representative species from each of the six taxonomic sections of Asarum. Sequenced plastomes were compared in a phylogenetic context provided by maximum likelihood and parsimony inferences made using an additional 18 publicly available plastomes from early-diverging angiosperm lineages.

KEY RESULTS

In contrast to previously published magnoliid plastomes and the newly sequenced Saruma henryi plastome published here, Asarum plastomes have undergone extensive disruption and contain extremely lengthy AT-repeat regions. The entirety of the small single copy region (SSC) of A. canadense and A. sieboldii var. sieboldii has been incorporated into the inverted repeat regions (IR), and the SSC of A. delavayi is only 14 bp long. All sampled Asarum plastomes share an inversion of a large portion of the large single copy region (LSC) such that trnE-UUC is adjacent to the LSC-IR boundary.

CONCLUSIONS

Plastome divergence in Asarum appears to be consistent with trends seen in highly rearranged plastomes of the monocots and eudicots. We propose that plastome instability in Asarum is due to repetitive motifs that serve as recombinatory substrates and reduce genome stability.

Medicago truncatula: Genetic and Genomic Resources

Medicago truncatula was chosen by the legume community, along with Lotus japonicus, as a model plant to study legume biology. Since then, numerous resources and tools have been developed for M. truncatula. These include, for example, its genome sequence, core ecotype collections, transformation/regeneration methods, extensive mutant collections, and a gene expression atlas. This review aims to describe the different genetic and genomic tools and resources currently available for M. truncatula. We also describe how these resources were generated and provide all the information necessary to access these resources and use them from a practical point of view. © 2017 by John Wiley & Sons, Inc.

Contrasting Patterns of Nucleotide Substitution Rates Provide Insight into Dynamic Evolution of Plastid and Mitochondrial Genomes of Geranium

Geraniaceae have emerged as a model system for investigating the causes and consequences of variation in plastid and mitochondrial genomes. Incredible structural variation in plastid genomes (plastomes) and highly accelerated evolutionary rates have been reported in selected lineages and functional groups of genes in both plastomes and mitochondrial genomes (mitogenomes), and these phenomena have been implicated in cytonuclear incompatibility. Previous organelle genome studies have included limited sampling of Geranium, the largest genus in the family with over 400 species. This study reports on rates and patterns of nucleotide substitutions in plastomes and mitogenomes of 17 species of Geranium and representatives of other Geraniaceae. As detected across other angiosperms, substitution rates in the plastome are 3.5 times higher than the mitogenome in most Geranium. However, in the branch leading to Geranium brycei/Geranium incanum mitochondrial genes experienced significantly higher dN and dS than plastid genes, a pattern that has only been detected in one other angiosperm. Furthermore, rate accelerations differ in the two organelle genomes with plastomes having increased dN and mitogenomes with increased dS. In the Geranium phaeum/Geranium reflexum clade, duplicate copies of clpP and rpoA genes that experienced asymmetric rate divergence were detected in the single copy region of the plastome. In the case of rpoA, the branch leading to G. phaeum/G. reflexum experienced positive selection or relaxation of purifying selection. Finally, the evolution of acetyl-CoA carboxylase is unusual in Geraniaceae because it is only the second angiosperm family where both prokaryotic and eukaryotic ACCases functionally coexist in the plastid.

Insights into the Existence of Isomeric Plastomes in Cupressoideae (Cupressaceae)

The cypress family (Cupressaceae) possesses highly rearranged plastomes that lack a pair of large inverted repeats typically found in land plants. A few cypress species have been reported to contain isomeric plastomes, but whether the existence of isomeric plastomes is ubiquitous in the family remains to be investigated with a broader taxon sampling. In this study, we sequenced the complete plastomes of ten species in Cupressoideae, the largest cypress subfamily. Cupressoideae showed relatively accelerated rates of substitutions at both nonsynonymous and synonymous sites as compared with other subfamilies of Cupressaceae. Our PCR and read mapping analyses together suggested the existence of isomeric plastomes in eight of the ten sequenced Cupressoideae species. The isomeric plastomes were also detected in 176 individuals from nine wild populations of four Cupressoideae species. Within Calocedrus macrolepis, we discovered a new type of isomeric plastomes that was likely derived from homologous recombination mediated by an 11-bp repeat. We conclude that isomeric plastomes are commonly present in Cupressoideae, thereby contributing to increased plastomic complexity.

Recombination-dependent replication and gene conversion homogenize repeat sequences and diversify plastid genome structure

PREMISE OF THE STUDY

There is a misinterpretation in the literature regarding the variable orientation of the small single copy region of plastid genomes (plastomes). The common phenomenon of small and large single copy inversion, hypothesized to occur through intramolecular recombination between inverted repeats (IR) in a circular, single unit-genome, in fact, more likely occurs through recombination-dependent replication (RDR) of linear plastome templates. If RDR can be primed through both intra- and intermolecular recombination, then this mechanism could not only create inversion isomers of so-called single copy regions, but also an array of alternative sequence arrangements.

METHODS

We used Illumina paired-end and PacBio single-molecule real-time (SMRT) sequences to characterize repeat structure in the plastome of Monsonia emarginata (Geraniaceae). We used OrgConv and inspected nucleotide alignments to infer ancestral nucleotides and identify gene conversion among repeats and mapped long (>1 kb) SMRT reads against the unit-genome assembly to identify alternative sequence arrangements.

RESULTS

Although M. emarginata lacks the canonical IR, we found that large repeats (>1 kilobase; kb) represent ∼22% of the plastome nucleotide content. Among the largest repeats (>2 kb), we identified GC-biased gene conversion and mapping filtered, long SMRT reads to the M. emarginata unit-genome assembly revealed alternative, substoichiometric sequence arrangements.

CONCLUSION

We offer a model based on RDR and gene conversion between long repeated sequences in the M. emarginata plastome and provide support that both intra-and intermolecular recombination between large repeats, particularly in repeat-rich plastomes, varies unit-genome structure while homogenizing the nucleotide sequence of repeats.

Enhancing faba bean (Vicia faba L.) genome resources

Grain legume improvement is currently impeded by a lack of genomic resources. The paucity of genome information for faba bean can be attributed to the intrinsic difficulties of assembling/annotating its giant (~13 Gb) genome. In order to address this challenge, RNA-sequencing analysis was performed on faba bean (cv. Wizard) leaves. Read alignment to the faba bean reference transcriptome identified 16 300 high quality unigenes. In addition, Illumina paired-end sequencing was used to establish a baseline for genomic information assembly. Genomic reads were assembled de novo into contigs with a size range of 50-5000 bp. Over 85% of sequences did not align to known genes, of which ~10% could be aligned to known repetitive genetic elements. Over 26 000 of the reference transcriptome unigenes could be aligned to DNA-sequencing (DNA-seq) reads with high confidence. Moreover, this comparison identified 56 668 potential splice points in all identified unigenes. Sequence length data were extended at 461 putative loci through alignment of DNA-seq contigs to full-length, publicly available linkage marker sequences. Reads also yielded coverages of 3466× and 650× for the chloroplast and mitochondrial genomes, respectively. Inter- and intraspecies organelle genome comparisons established core legume organelle gene sets, and revealed polymorphic regions of faba bean organelle genomes.

Different cytokinin histidine kinase receptors regulate nodule initiation as well as later nodule developmental stages in Medicago truncatula

Legume plants adapt to low nitrogen by developing an endosymbiosis with nitrogen-fixing soil bacteria to form a new specific organ: the nitrogen-fixing nodule. In the Medicago truncatula model legume, the MtCRE1 cytokinin receptor is essential for this symbiotic interaction. As three other putative CHASE-domain containing histidine kinase (CHK) cytokinin receptors exist in M. truncatula, we determined their potential contribution to this symbiotic interaction. The four CHKs have extensive redundant expression patterns at early nodulation stages but diverge in differentiated nodules, even though MtCHK1/MtCRE1 has the strongest expression at all stages. Mutant and knock-down analyses revealed that other CHKs than MtCHK1/CRE1 are positively involved in nodule initiation, which explains the delayed nodulation phenotype of the chk1/cre1 mutant. In addition, cre1 nodules exhibit an increased growth, whereas other chk mutants have no detectable phenotype, and the maintained nitrogen fixation capacity in cre1 requires other CHK genes. Interestingly, an AHK4/CRE1 genomic locus from the aposymbiotic Arabidopsis plant rescues nodule initiation but not the nitrogen fixation capacity. This indicates that different CHK cytokinin signalling pathways regulate not only nodule initiation but also later developmental stages, and that legume-specific determinants encoded by the MtCRE1 gene are required for later nodulation stages than initiation.

Revisiting the Plastid Phylogenomics of Pinaceae with Two Complete Plastomes of Pseudolarix and Tsuga

Phylogeny of the ten Pinaceous genera has long been contentious. Plastid genomes (plastomes) provide an opportunity to resolve this problem because they contain rich evolutionary information. To comprehend the plastid phylogenomics of all ten Pinaceous genera, we sequenced the plastomes of two previously unavailable genera, Pseudolarix amabilis (122,234 bp) and Tsuga chinensis (120,859 bp). Both plastomes share similar gene repertoire and order. Here for the first time we report a unique insertion of tandem repeats in accD of T. chinensis From the 65 plastid protein-coding genes common to all Pinaceous genera, we re-examined the phylogenetic relationship among all Pinaceous genera. Our two phylogenetic trees are congruent in an identical tree topology, with the five genera of the Abietoideae subfamily constituting a monophyletic clade separate from the other three subfamilies: Pinoideae, Piceoideae, and Laricoideae. The five genera of Abietoideae were grouped into two sister clades consisting of (1) Cedrus alone and (2) two sister subclades of Pseudolarix-Tsuga and Abies-Keteleeria, with the former uniquely losing the gene psaM and the latter specifically excluding the 3 psbA from the residual inverted repeat.

Cell-to-cell movement of mitochondria in plants

We report cell-to-cell movement of mitochondria through a graft junction. Mitochondrial movement was discovered in an experiment designed to select for chloroplast transfer from Nicotiana sylvestris into Nicotiana tabacum cells. The alloplasmic N. tabacum line we used carries Nicotiana undulata cytoplasmic genomes, and its flowers are male sterile due to the foreign mitochondrial genome. Thus, rare mitochondrial DNA transfer from N. sylvestris to N. tabacum could be recognized by restoration of fertile flower anatomy. Analyses of the mitochondrial genomes revealed extensive recombination, tentatively linking male sterility to orf293, a mitochondrial gene causing homeotic conversion of anthers into petals. Demonstrating cell-to-cell movement of mitochondria reconstructs the evolutionary process of horizontal mitochondrial DNA transfer and enables modification of the mitochondrial genome by DNA transmitted from a sexually incompatible species. Conversion of anthers into petals is a visual marker that can be useful for mitochondrial transformation.

Evolutionary dynamics of the plastid inverted repeat: the effects of expansion, contraction, and loss on substitution rates

Rates of nucleotide substitution were previously shown to be several times slower in the plastid inverted repeat (IR) compared with single-copy (SC) regions, suggesting that the IR provides enhanced copy-correction activity. To examine the generality of this synonymous rate dependence on the IR, we compared plastomes from 69 pairs of closely related species representing 52 families of angiosperms, gymnosperms, and ferns. We explored the breadth of IR boundary shifts in land plants and demonstrate that synonymous substitution rates are, on average, 3.7 times slower in IR genes than in SC genes. In addition, genes moved from the SC into the IR exhibit lower synonymous rates consistent with other IR genes, while genes moved from the IR into the SC exhibit higher rates consistent with other SC genes. Surprisingly, however, several plastid genes from Pelargonium, Plantago, and Silene have highly accelerated synonymous rates despite their IR localization. Together, these results provide strong evidence that the duplicative nature of the IR reduces the substitution rate within this region. The anomalously fast-evolving genes in Pelargonium, Plantago, and Silene indicate localized hypermutation, potentially induced by a higher level of error-prone double-strand break repair in these regions, which generates substitutional rate variation.

Nuclear-cytoplasmic conflict in pea (Pisum sativum L.) is associated with nuclear and plastidic candidate genes encoding acetyl-CoA carboxylase subunits

In crosses of wild and cultivated peas (Pisum sativum L.), nuclear-cytoplasmic incompatibility frequently occurs manifested as decreased pollen fertility, male gametophyte lethality, sporophyte lethality. High-throughput sequencing of plastid genomes of one cultivated and four wild pea accessions differing in cross-compatibility was performed. Candidate genes for involvement in the nuclear-plastid conflict were searched in the reconstructed plastid genomes. In the annotated Medicago truncatula genome, nuclear candidate genes were searched in the portion syntenic to the pea chromosome region known to harbor a locus involved in the conflict. In the plastid genomes, a substantial variability of the accD locus represented by nucleotide substitutions and indels was found to correspond to the pattern of cross-compatibility among the accessions analyzed. Amino acid substitutions in the polypeptides encoded by the alleles of a nuclear locus, designated as Bccp3, with a complementary function to accD, fitted the compatibility pattern. The accD locus in the plastid genome encoding beta subunit of the carboxyltransferase of acetyl-coA carboxylase and the nuclear locus Bccp3 encoding biotin carboxyl carrier protein of the same multi-subunit enzyme were nominated as candidate genes for main contribution to nuclear-cytoplasmic incompatibility in peas. Existence of another nuclear locus involved in the accD-mediated conflict is hypothesized.

Plastid genomics in horticultural species: importance and applications for plant population genetics, evolution, and biotechnology

During the evolution of the eukaryotic cell, plastids, and mitochondria arose from an endosymbiotic process, which determined the presence of three genetic compartments into the incipient plant cell. After that, these three genetic materials from host and symbiont suffered several rearrangements, bringing on a complex interaction between nuclear and organellar gene products. Nowadays, plastids harbor a small genome with ∼130 genes in a 100-220 kb sequence in higher plants. Plastid genes are mostly highly conserved between plant species, being useful for phylogenetic analysis in higher taxa. However, intergenic spacers have a relatively higher mutation rate and are important markers to phylogeographical and plant population genetics analyses. The predominant uniparental inheritance of plastids is like a highly desirable feature for phylogeny studies. Moreover, the gene content and genome rearrangements are efficient tools to capture and understand evolutionary events between different plant species. Currently, genetic engineering of the plastid genome (plastome) offers a number of attractive advantages as high-level of foreign protein expression, marker gene excision, gene expression in operon and transgene containment because of maternal inheritance of plastid genome in most crops. Therefore, plastid genome can be used for adding new characteristics related to synthesis of metabolic compounds, biopharmaceutical, and tolerance to biotic and abiotic stresses. Here, we describe the importance and applications of plastid genome as tools for genetic and evolutionary studies, and plastid transformation focusing on increasing the performance of horticultural species in the field.

Evolutionary origin of highly repetitive plastid genomes within the clover genus (Trifolium)

Background

Some clover species, particularly Trifolium subterraneum, have previously been reported to have highly unusual plastomes, relative to closely related legumes, enlarged with many duplications, gene losses and the presence of DNA unique to Trifolium, which may represent horizontal transfer. In order to pinpoint the evolutionary origin of this phenomenon within the genus Trifolium, we sequenced and assembled the plastomes of eight additional Trifolium species widely sampled from across the genus.

Results

The Trifolium plastomes fell into two groups: those of Trifolium boissieri, T. strictum and T. glanduliferum (representing subgenus Chronosemium and subg. Trifolium section Paramesus) were tractable, assembled readily and were not unusual in the general context of Fabeae plastomes. The other Trifolium species (“core Trifolium”) proved refractory to assembly mainly because of numerous short duplications. These species form a single clade, which we call the “refractory clade” (comprising subg, Trifolium sections Lupinaster, Trifolium, Trichocephalum, Vesicastrum and Trifoliastrum). The characteristics of the refractory clade are the presence of numerous short duplications and 7-15% longer genomes than the tractable species. Molecular dating estimates that the origin of the most recent common ancestor (MRCA) of the refractory clade is approximately 13.1 million years ago (MYA). This is considerably younger than the estimated MRCA ages of Trifolium (c. 18.6 MYA) and Trifolium subg. Trifolium (16.1 MYA).

Conclusions

We conclude that the unusual repetitive plastome type previously characterized in Trifolium subterraneum had a single origin within Trifolium and is characteristic of most (but not all) species of subgenus Trifolium. It appears that an ancestral plastome within Trifolium underwent an evolutionary change resulting in plastomes that either actively promoted, were permissive to, or were unable to control, duplications within the genome. The precise mechanism of this important change in the mode and tempo of plastome evolution deserves further investigation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-014-0228-6) contains supplementary material, which is available to authorized users.