Sequence of the Tomato Chloroplast DNA and Evolutionary Comparison of Solanaceous Plastid Genomes

Chloroplast Genome Variation and Phylogenetic Relationships of Autochthonous Varieties of Vitis vinifera from the Don Valley

The autochthonous grape varieties of the Don Valley, situated in southern Russia, constitute a distinctive element of regional cultural heritage. These varieties have been adapted over centuries to the region's specific local climatic and soil conditions. For the most part, these varieties are not imported from other countries. They are closely related to varieties found in Crimea and the North Caucasus. In this study, we obtained the first complete, unfragmented sequences of the chloroplast genomes of eight autochthonous varieties from the Don Valley and one from Crimea. We also performed a comparative analysis of their genomic features. The size of Vitis vinifera chloroplast genome sequences varied from 160,925 to 160,991 bp, depending on the cultivar, with a uniform GC ratio of 37.38%. Each genome consists of four subregions: a single copy region (LSC) ranging from 89,158 to 89,336 bp, a small single copy region (SSC) ranging from 19,070 to 19,073 bp, and a pair of inverted repeat regions (IRa and IRb) in the range of 26,292 to 26,353 bp. The chloroplast genomes of the studied V. vinifera varieties contained 130 genes, including 85 protein-coding genes, 8 rRNA genes, and 37 tRNA genes. The sequence divergence analysis has enabled the identification of four highly variable regions, which may be utilized as potential markers for phylogenetic analysis. The analysis revealed the presence of 58 to 61 SSRs and multiple long repeated sequences in the chloroplast genomes of these varieties. The phylogenetic analyses of the sequences obtained and complete chloroplast genomes available from public databases indicated that the majority of autochthonous V. vinifera varieties do not have a direct origin from any European variety.

Characterization and expression analysis of a thaumatin-like protein PpTLP1 from ground cherry Physalis pubescens

Ground cherry, Physalis pubescens, is mainly cultivated as a fruit worldwide and popularly used as a food supplement and traditional Chinese medicine. Plants are challenged by external environmental stress and can initiate resistance to the stress through the regulation of pathogenesis-related (PR) proteins. Among PR proteins, PR-5, a thaumatin-like protein (TLP), was identified in many plants and found to be able to enhance stress resistance. However, PR-5 in ground cherry is not characterized and its expression is yet to be understood. In this study, a PR-5 protein PpTLP1 in P. pubescens was firstly identified. Analysis of the amino acid sequences revealed that PpTLP1 was highly similar to PR-NP24 identified in tomato with a difference in only one amino acid. Expression analysis indicated that the PpTLP1 gene was highly expressed in leaf while the PpTLP1 protein was tissue-specifically accumulated in cherry exocarp. Furthermore, the down-regulation of PpTLP1 in ground cherry was induced by NaCl treatment while the up-regulation was promoted by the infection of Sclerotinia sclerotiorum and Botrytis cinerea. This study will provide a new plant resource containing a TLP in Physalis genus and a novel insight for the improvement of postharvest management of ground cherry and other Solanaceae plants.

The tomato chloroplast stromal proteome compendium elucidated by leveraging a plastid protein-localization prediction Atlas

Tomato (Solanum lycopersicum) is a model species for studying fruit development, wounding, herbivory, and pathogen attack. Despite tomato's world-wide economic importance and the role of chloroplasts as metabolic hubs and integrators of environmental cues, little is known about the stromal proteome of tomato. Using a high-yielding protocol for chloroplast and stromal protein isolation, MudPIT nano-LC-MS/MS analyses, a robust in-house protein database (the Atlas) for predicting the plastid localization of tomato proteins, and rigorous selection criteria for inclusion/exclusion in the stromal proteome, we identified 1,278 proteins of the tomato stromal proteome. We provide one of the most robust stromal proteomes available to date with empirical evidence for 545 and 92 proteins not previously described for tomato plastids and the Arabidopsis stroma, respectively. The relative abundance of tomato stromal proteins was determined using the exponentially modified protein abundance index (emPAI). Comparison of the abundance of tomato and Arabidopsis stromal proteomes provided evidence for the species-specific nature of stromal protein homeostasis. The manual curation of the tomato stromal proteome classified proteins into ten functional categories resulting in an accessible compendium of tomato chloroplast proteins. After curation, only 91 proteins remained as unknown, uncharacterized or as enzymes with unknown functions. The curation of the tomato stromal proteins also indicated that tomato has a number of paralogous proteins, not present in Arabidopsis, which accumulated to different levels in chloroplasts. As some of these proteins function in key metabolic pathways or in perceiving or transmitting signals critical for plant adaptation to biotic and abiotic stress, these data suggest that tomato may modulate the bidirectional communication between chloroplasts and nuclei in a novel manner. The stromal proteome provides a fertile ground for future mechanistic studies in the field of tomato chloroplast-nuclear signaling and are foundational for our goal of elucidating the dynamics of the stromal proteome controlled by the solanaceous-specific, stromal, and wound-inducible leucine aminopeptidase A of tomato.

A Comprehensive Evolutionary Study of Chloroplast RNA Editing in Gymnosperms: A Novel Type of G-to-A RNA Editing Is Common in Gymnosperms

Although more than 9100 plant plastomes have been sequenced, RNA editing sites of the whole plastome have been experimentally verified in only approximately 21 species, which seriously hampers the comprehensive evolutionary study of chloroplast RNA editing. We investigated the evolutionary pattern of chloroplast RNA editing sites in 19 species from all 13 families of gymnosperms based on a combination of genomic and transcriptomic data. We found that the chloroplast C-to-U RNA editing sites of gymnosperms shared many common characteristics with those of other land plants, but also exhibited many unique characteristics. In contrast to that noted in angiosperms, the density of RNA editing sites in ndh genes was not the highest in the sampled gymnosperms, and both loss and gain events at editing sites occurred frequently during the evolution of gymnosperms. In addition, GC content and plastomic size were positively correlated with the number of chloroplast RNA editing sites in gymnosperms, suggesting that the increase in GC content could provide more materials for RNA editing and facilitate the evolution of RNA editing in land plants or vice versa. Interestingly, novel G-to-A RNA editing events were commonly found in all sampled gymnosperm species, and G-to-A RNA editing exhibits many different characteristics from C-to-U RNA editing in gymnosperms. This study revealed a comprehensive evolutionary scenario for chloroplast RNA editing sites in gymnosperms, and reported that a novel type of G-to-A RNA editing is prevalent in gymnosperms.

Engineering the plastid and mitochondrial genomes of flowering plants

Engineering the plastid genome based on homologous recombination is well developed in a few model species. Homologous recombination is also the rule in mitochondria, but transformation of the mitochondrial genome has not been realized in the absence of selective markers. The application of transcription activator-like (TAL) effector-based tools brought about a dramatic change because they can be deployed from nuclear genes and targeted to plastids or mitochondria by an N-terminal targeting sequence. Recognition of the target site in the organellar genomes is ensured by the modular assembly of TALE repeats. In this paper, I review the applications of TAL effector nucleases and TAL effector cytidine deaminases for gene deletion, base editing and mutagenesis in plastids and mitochondria. I also review emerging technologies such as post-transcriptional RNA modification to regulate gene expression, Agrobacterium- and nanoparticle-mediated organellar genome transformation, and self-replicating organellar vectors as production platforms.

Comprehensive analysis of plastid gene expression during fruit development and ripening of kiwifruit

Global survey of plastid gene expression during fruit ripening in kiwifruit provides cis-elements for the future engineering of the plastid genome of kiwifruit. A limitation in the application of plastid biotechnology for molecular farming is the low-level expression of transgenes in non-green plastids compared with photosynthetically active chloroplasts. Unlike other fruits, not all chloroplasts are transformed into chromoplasts during ripening of red-fleshed kiwifruit (Actinidia chinensis cv. Hongyang) fruits, which may make kiwifruit an ideal horticultural plant for recombinant protein production by plastid engineering. To identify cis-elements potentially triggering high-level transgene expression in edible tissues of the 'Hongyang' kiwifruit, here we report a comprehensive analysis of kiwifruit plastid gene transcription in green leaves and fruits at three different developmental stages. While transcripts of a few photosynthesis-related genes and most genetic system genes were substantially upregulated in green fruits compared with leaves, nearly all plastid genes were significantly downregulated at the RNA level during fruit development. Expression of a few genes remained unchanged, including psbA, the gene encoding the D1 polypeptide of photosystem II. However, PsbA protein accumulation decreased continuously during chloroplast-to-chromoplast differentiation. Analysis of post-transcriptional steps in mRNA maturation, including intron splicing and RNA editing, revealed that splicing and editing may contribute to regulation of plastid gene expression. Altogether, 40 RNA editing sites were verified, and 5 of them were newly discovered. Taken together, this study has generated a valuable resource for the analysis of plastid gene expression and provides cis-elements for future efforts to engineer the plastid genome of kiwifruit.

Comprehensive analysis of chloroplast genome of Albizia julibrissin Durazz. (Leguminosae sp.)

The Albizia julibrissin chloroplasts have a classical chloroplast genome structure, containing 93 coding genes and 34 non-coding genes. Our research provides basic data for plant phylogenetic evolutionary studies. There is limited genomic information available for the important Chinese herb Albizia julibrissin Durazz. In this study, we constructed the chloroplast (Cp) genome of A. julibrissin. The length of the assembled Cp genome was 175,922 bp consisting of four conserved regions: a 5145 bp small single-copy (SSC) region, a 91,323 bp large single-copy (LSC) region, and two identical length-inverted repeat (IR) regions (39,725 bp). This Cp genome included 34 non-coding RNAs and 93 unique genes, the former contains 30 transfer and 4 ribosomal RNA genes. Gene annotation indicated some of the coding genes (82) in the A. julibrissin Cp genome classified in the Leguminosae family, with some to other related families (11). The results show that low GC content (36.9%) and codon bias towards A- or T-terminal codons may affect the frequency of gene codon usage. The sequence analysis identified 30 forward, 18 palindrome, and 1 reverse repeat > 30 bp length, and 149 simple sequence repeats (SSR). Fifty-five RNA editing sites in the Cp of A. julibrissin were predicted, most of which are C-to-U conversions. Analysis of the reverse repeat expansion or contraction and divergence area between several species, including A. julibrissin, was performed. The phylogenetic tree revealed that A. julibrissin was most closely related to Albizia odoratissima and Albizia bracteata, followed by Samanea saman, forming an evolutionary branch with Mimosa pudica and Leucaena trichandra. The research results are helpful for breeding and genetic improvement of A. julibrissin, and also provide valuable information for understanding the evolution of this plant.

A photosynthesis operon in the chloroplast genome drives speciation in evening primroses

Genetic incompatibility between the cytoplasm and the nucleus is thought to be a major factor in species formation, but mechanistic understanding of this process is poor. In evening primroses (Oenothera spp.), a model plant for organelle genetics and population biology, hybrid offspring regularly display chloroplast-nuclear incompatibility. This usually manifests in bleached plants, more rarely in hybrid sterility or embryonic lethality. Hence, most of these incompatibilities affect photosynthetic capability, a trait that is under selection in changing environments. Here we show that light-dependent misregulation of the plastid psbB operon, which encodes core subunits of photosystem II and the cytochrome b6f complex, can lead to hybrid incompatibility, and this ultimately drives speciation. This misregulation causes an impaired light acclimation response in incompatible plants. Moreover, as a result of their different chloroplast genotypes, the parental lines differ in photosynthesis performance upon exposure to different light conditions. Significantly, the incompatible chloroplast genome is naturally found in xeric habitats with high light intensities, whereas the compatible one is limited to mesic habitats. Consequently, our data raise the possibility that the hybridization barrier evolved as a result of adaptation to specific climatic conditions.

Stable Plastid Transformation of Petunia for Studies in Basic Research

Petunia hybrida is a commercial ornamental plant and is also an important model species for genetic analysis and transgenic research. Here we describe the steps required to isolate stable plastid transformants in P. hybrida using the commercial Pink Wave cultivar. Wave cultivars are popular spreading Petunias sold as ground cover and potted plants. Transgenes introduced into P. hybrida plastids exhibit stable expression over many generations. The development of plastid transformation in P. hybrida provides an enabling technology to bring the benefits of plastid engineering, including maternal inheritance and stable expression of performance-enhancing trait genes, to the important floriculture and horticulture industries.

Plastid Transformation in Tomato: A Vegetable Crop and Model Species

Tomato (Solanum lycopersicum L.), a member of the nightshade family (Solanaceae), is one of the most important vegetable crops and has long been an important model species in plant biology. Plastid biology in tomato is especially interesting due to the chloroplast-to-chromoplast conversion occurring during fruit ripening. Moreover, as tomato represents a major food crop with a fleshy fruit that can be eaten raw, the development of a plastid transformation protocol for tomato was of particular interest to plant biotechnologists. Recent methodological improvements have made tomato plastid transformation more efficient, and facilitated applications in metabolic engineering and molecular farming. This chapter describes the basic methods involved in the generation and analysis of tomato plants with transgenic chloroplast genomes and summarizes recent applications of tomato plastid transformation in plant biotechnology.

Improved genome assembly and pan-genome provide key insights into eggplant domestication and breeding

Eggplant (Solanum melongena L.) is an important horticultural crop and one of the most widely grown vegetables from the Solanaceae family. It was domesticated from a wild, prickly progenitor carrying small, round, non-anthocyanic fruits. We obtained a novel, highly contiguous genome assembly of the eggplant '67/3' reference line, by Hi-C retrofitting of a previously released short read- and optical mapping-based assembly. The sizes of the 12 chromosomes and the fraction of anchored genes in the improved assembly were comparable to those of a chromosome-level assembly. We resequenced 23 accessions of S. melongena representative of the worldwide phenotypic, geographic, and genetic diversity of the species, and one each from the closely related species Solanum insanum and Solanum incanum. The eggplant pan-genome contained approximately 51.5 additional megabases and 816 additional genes compared with the reference genome, while the pan-plastome showed little genetic variation. We identified 53 selective sweeps related to fruit color, prickliness, and fruit shape in the nuclear genome, highlighting selection leading to the emergence of present-day S. melongena cultivars from its wild ancestors. Candidate genes underlying the selective sweeps included a MYBL1 repressor and CHALCONE ISOMERASE (for fruit color), homologs of Arabidopsis GLABRA1 and GLABROUS INFLORESCENCE STEMS2 (for prickliness), and orthologs of tomato FW2.2, OVATE, LOCULE NUMBER/WUSCHEL, SUPPRESSOR OF OVATE, and CELL SIZE REGULATOR (for fruit size/shape), further suggesting that selection for the latter trait relied on a common set of orthologous genes in tomato and eggplant.

Phylogenetic position of Bupleurum sikangense inferred from the complete chloroplast genome sequence

Bupleurum sikangense is an endemic species to China distributed in Xizang (Tibet), which has high saikosaponin content and potential medicinal value. Morphologically, it extremely resembles B. commelynoideum. In order to get a better understanding of the relationship between B. sikangense and B. commelynoideum, and on the phylogenetic status of the two species in the genus, the complete chloroplast (cp) genomes of them were sequenced. The genome organization, repeat sequences, codon usage, RNA-editing sites, and variation of their cp genomes revealed high similarity between the species. Some highly variable regions like trnK-UUU_rps16, rps16_trnQ-UUG, ndhC_trnV-UAC, petA_psbJ, accD_psaI, and petL_psbE were identified, providing potential molecular markers for differentiating the two species. Phylogenetic analysis indicated that B. commelynoideum has a closer relationship to B. chinese than that to B. sikangense. Overall, this study will not only improve our knowledge about cp genomes of these two species, and but also provide data for further research on species identification, safe medical application, conservation genetics, etc., of Bupleurum plants.

GUN control in retrograde signaling: How GENOMES UNCOUPLED proteins adjust nuclear gene expression to plastid biogenesis

Communication between cellular compartments is vital for development and environmental adaptation. Signals emanating from organelles, so-called retrograde signals, coordinate nuclear gene expression with the developmental stage and/or the functional status of the organelle. Plastids (best known in their green photosynthesizing differentiated form, the chloroplasts) are the primary energy-producing compartment of plant cells, and the site for the biosynthesis of many metabolites, including fatty acids, amino acids, nucleotides, isoprenoids, tetrapyrroles, vitamins, and phytohormone precursors. Signals derived from plastids regulate the accumulation of a large set of nucleus-encoded proteins, many of which localize to plastids. A set of mutants defective in retrograde signaling (genomes uncoupled, or gun) was isolated over 25 years ago. While most GUN genes act in tetrapyrrole biosynthesis, resolving the molecular function of GUN1, the proposed integrator of multiple retrograde signals, has turned out to be particularly challenging. Based on its amino acid sequence, GUN1 was initially predicted to be a plastid-localized nucleic acid-binding protein. Only recently, mechanistic information on the function of GUN1 has been obtained, pointing to a role in plastid protein homeostasis. This review article summarizes our current understanding of GUN-related retrograde signaling and provides a critical appraisal of the various proposed roles for GUNs and their respective pathways.

Characterization of the Complete Chloroplast Genome of Buddleja Lindleyana

BACKGROUND

Buddleja lindleyana Fort., which belongs to the Loganiaceae with a distribution throughout the tropics, is widely used as an ornamental plant in China. Buddleja contains several morphologically similar species, which need to be identified by molecular identification. But there is little molecular research on the genus Buddleja.

OBJECTIVE

Using molecular biology techniques to sequence and analyze the complete chloroplast (cp) genome of B. lindleyana.

METHODS

According to next-generation sequencing to sequence the genome data, a series of bioinformatics software were used to assembly and analysis the molecular structure of cp genome of B. lindleyana.

RESULTS

The complete cp genome of B. lindleyana is a circular 154,487-bp-long molecule with a GC content of 38.1%. It has a familiar quadripartite structure, including a large single-copy region (LSC; 85,489 bp), a small single-copy region (SSC; 17,898bp) and a pair of inverted repeats (IRs; 25,550 bp). A total of 133 genes were identified in the genome, including 86 protein-coding genes, 37 tRNA genes, 8 rRNA genes and 2 pseudogenes.

CONCLUSIONS

These results suggested that B. lindelyana cp genome could be used as a potential genomic resource to resolve the phylogenetic positions and relationships of Loganiaceae, and will offer valuable information for future research in the identification of Buddleja species and will conduce to genomic investigations of these species.

Complete Chloroplast Genome of Clethra fargesii Franch., an Original Sympetalous Plant from Central China: Comparative Analysis, Adaptive Evolution, and Phylogenetic Relationships

Clethra fargesii, an essential ecological and endemic woody plant of the genus Clethra in Clethraceae, is widely distributed in Central China. So far, there have been a paucity of studies on its chloroplast genome. In the present study, we sequenced and assembled the complete chloroplast genome of C. fargesii. We also analyzed the chloroplast genome features and compared them to Clethra delavayi and other closely related species in Ericales. The complete chloroplast genome is 157,486 bp in length, including a large single-copy (LSC) region of 87,034 bp and a small single-copy (SSC) region of 18,492 bp, separated by a pair of inverted repeat (IR) regions of 25,980 bp. The GC content of the whole genome is 37.3%, while those in LSC, SSC, and IR regions are 35.4%, 30.7%, and 43.0%, respectively. The chloroplast genome of C. fargesii encodes 132 genes in total, including 87 protein-coding genes (PCGs), 37 tRNA genes, and eight rRNA genes. A total of 26,407 codons and 73 SSRs were identified in C. fargesii chloroplast genome. Additionally, we postulated and demonstrated that the structure of the chloroplast genome in Clethra species may present evolutionary conservation based on the comparative analysis of genome features and genome alignment among eight Ericales species. The low Pi values revealed evolutionary conservation based on the nucleotide diversity analysis of chloroplast genome in two Clethra species. The low selection pressure was shown by a few positively selected genes by adaptive evolution analysis using 80 coding sequences (CDSs) of the chloroplast genomes of two Clethra species. The phylogenetic tree showed that Clethraceae and Ericaceae are sister clades, which reconfirm the previous hypothesis that Clethra is highly conserved in the chloroplast genome using 75 CDSs of chloroplast genome among 40 species. The genome information and analysis results presented in this study are valuable for further study on the intraspecies identification, biogeographic analysis, and phylogenetic relationship in Clethraceae.

Knockdown of the plastid-encoded acetyl-CoA carboxylase gene uncovers functions in metabolism and development

De novo fatty acid biosynthesis in plants relies on a prokaryotic-type acetyl-CoA carboxylase (ACCase) that resides in the plastid compartment. The enzyme is composed of four subunits, one of which is encoded in the plastid genome, whereas the other three subunits are encoded by nuclear genes. The plastid gene (accD) encodes the β-carboxyltransferase subunit of ACCase and is essential for cell viability. To facilitate the functional analysis of accD, we pursued a transplastomic knockdown strategy in tobacco (Nicotiana tabacum). By introducing point mutations into the translational start codon of accD, we obtained stable transplastomic lines with altered ACCase activity. Replacement of the standard initiator codon AUG with UUG strongly reduced AccD expression, whereas replacement with GUG had no detectable effects. AccD knockdown mutants displayed reduced ACCase activity, which resulted in changes in the levels of many but not all species of cellular lipids. Limiting fatty acid availability caused a wide range of macroscopic, microscopic, and biochemical phenotypes, including impaired chloroplast division, reduced seed set, and altered storage metabolism. Finally, while the mutants displayed reduced growth under photoautotrophic conditions, they showed exaggerated growth under heterotrophic conditions, thus uncovering an unexpected antagonistic role of AccD activity in autotrophic and heterotrophic growth.

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.

Genomic Profiling: The Strengths and Limitations of Chloroplast Genome-Based Plant Variety Authentication

Genomic profiling is a suitable tool for variety authentication and has applications in both operational quality and regulatory raw material control. It can be used to differentiate species or varieties and to identify admixtures as well as field contaminants. To establish a molecular profile, reliable and very accurate sequence data are required. As a result of the influence of the pollinator plant, nuclear genome-based authentication is in most cases not suitable for a direct application on the fruit. Sequences must be used that come exclusively from the localized mother plant. Parts of the fruit of maternal origin, e.g., components derived from the blossom, are suitable as a basis for this. Alternatively, DNA from cell organelles that are maternally inherited, such as mitochondria or chloroplasts, can be used. The latter will be discussed in this review in closer detail. Although individual gene segments on the chloroplast genome are already used for species differentiation in barcoding studies on plants, little is known about the usefulness of the entire chloroplast genome for intraspecies differentiation in general and for differentiation between modern varieties in particular. Results from the literature as well as from our own work suggest that chloroplast genome sequences are indeed very well-suited for the differentiation of old varieties. On the other hand, they are less or not suitable for the genetic differentiation of modern cultivars, because they are often too closely related.

Accumulation of the RNA polymerase subunit RpoB depends on RNA editing by OsPPR16 and affects chloroplast development during early leaf development in rice

Plastid-encoded genes are coordinately transcribed by the nucleus-encoded RNA polymerase (NEP) and the plastid-encoded RNA polymerase (PEP). Resulting primary transcripts are frequently subject to RNA editing by cytidine-to-uridine conversions at specific sites. The physiological role of many editing events is largely unknown. Here, we have used the CRISPR/Cas9 technique in rice to knock out a member of the PLS-DYW subfamily of pentatricopeptide repeat (PPR) proteins. We found that OsPPR16 is responsible for a single editing event at position 545 in the chloroplast rpoB messenger RNA (mRNA), resulting in an amino acid change from serine to leucine in the β-subunit of the PEP. In striking contrast to loss-of-function mutations of the putative orthologue in Arabidopsis, which were reported to have no visible phenotype, knockout of OsPPR16 leads to impaired accumulation of RpoB, reduced expression of PEP-dependent genes, and a pale phenotype during early plant development. Thus, by editing the rpoB mRNA, OsPPR16 is required for faithful plastid transcription, which in turn is required for Chl synthesis and efficient chloroplast development. Our results provide new insights into the interconnection of the finely tuned regulatory mechanisms that operate at the transcriptional and post-transcriptional levels of plastid gene expression.

Cultivated Tomato (Solanum lycopersicum L.) Suffered a Severe Cytoplasmic Bottleneck during Domestication: Implications from Chloroplast Genomes

In various crops, genetic bottlenecks occurring through domestication can limit crop resilience to biotic and abiotic stresses. In the present study, we investigated nucleotide diversity in tomato chloroplast genome through sequencing seven plastomes of cultivated accessions from the Campania region (Southern Italy) and two wild species among the closest (Solanum pimpinellifolium) and most distantly related (S. neorickii) species to cultivated tomatoes. Comparative analyses among the chloroplast genomes sequenced in this work and those available in GenBank allowed evaluating the variability of plastomes and defining phylogenetic relationships. A dramatic reduction in genetic diversity was detected in cultivated tomatoes, nonetheless, a few de novo mutations, which still differentiated the cultivated tomatoes from the closest wild relative S. pimpinellifolium, were detected and are potentially utilizable as diagnostic markers. Phylogenetic analyses confirmed that S. pimpinellifolium is the closest ancestor of all cultivated tomatoes. Local accessions all clustered together and were strictly related with other cultivated tomatoes (S. lycopersicum group). Noteworthy, S. lycopersicum var. cerasiforme resulted in a mixture of both cultivated and wild tomato genotypes since one of the two analyzed accessions clustered with cultivated tomato, whereas the other with S. pimpinellifolium. Overall, our results revealed a very reduced cytoplasmic variability in cultivated tomatoes and suggest the occurrence of a cytoplasmic bottleneck during their domestication.

Molecular and functional diversity of organelle RNA editing mediated by RNA recognition motif-containing protein ORRM4 in tomato

Plant organellar RNA editing is a distinct type of post-transcriptional RNA modification that is critical for plant development. We showed previously that the RNA editing factor SlORRM4 is required for mitochondrial function and fruit ripening in tomato (Solanum lycopersicum). However, a comprehensive atlas of the RNA editing mediated by SlORRM4 is lacking. We observed that SlORRM4 is targeted to both chloroplasts and mitochondria, and its knockout results in pale-green leaves and delayed fruit ripening. Using high-throughput sequencing, we identified 12 chloroplast editing sites and 336 mitochondrial editing sites controlled by SlORRM4, accounting for 23% of chloroplast sites in leaves and 61% of mitochondrial sites in fruits, respectively. Analysis of native RNA immunoprecipitation sequencing revealed that SlORRM4 binds to 31 RNA targets; 19 of these targets contain SlORRM4-dependent editing sites. Large-scale analysis of putative SlORRM4-interacting proteins identified SlRIP1b, a RIP/MORF protein. Moreover, functional characterization demonstrated that SlRIP1b is involved in tomato fruit ripening. Our results indicate that SlORRM4 binds to RNA targets and interacts with SlRIP1b to broadly affect RNA editing in tomato organelles. These results provide insights into the molecular and functional diversity of RNA editing factors in higher plants.

Methodology: an optimized, high-yield tomato leaf chloroplast isolation and stroma extraction protocol for proteomics analyses and identification of chloroplast co-localizing proteins

BACKGROUND

Chloroplasts are critical organelles that perceive and convey metabolic and stress signals to different cellular components, while remaining the seat of photosynthesis and a metabolic factory. The proteomes of intact leaves, chloroplasts, and suborganellar fractions of plastids have been evaluated in the model plant Arabidopsis, however fewer studies have characterized the proteomes of plastids in crops. Tomato (Solanum lycopersicum) is an important world-wide crop and a model system for the study of wounding, herbivory and fruit ripening. While significant advances have been made in understanding proteome and metabolome changes in fruit ripening, far less is known about the tomato chloroplast proteome or its subcompartments.

RESULTS

With the long-term goal of understanding chloroplast proteome dynamics in response to stress, we describe a high-yielding method to isolate intact tomato chloroplasts and stromal proteins for proteomic studies. The parameters that limit tomato chloroplast yields were identified and revised to increase yields. Compared to published data, our optimized method increased chloroplast yields by 6.7- and 4.3-fold relative to published spinach and Arabidopsis leaf protocols, respectively; furthermore, tomato stromal protein yields were up to 79-fold higher than Arabidopsis stromal proteins yields. We provide immunoblot evidence for the purity of the stromal proteome isolated using our enhanced methods. In addition, we leverage our nanoliquid chromatography tandem mass spectrometry (nanoLC-MS/MS) data to assess the quality of our stromal proteome. Using strict criteria, proteins detected by 1 peptide spectral match, by one peptide, or were sporadically detected were designated as low-level contaminating proteins. A set of 254 proteins that reproducibly co-isolated with the tomato chloroplast stroma were identified. The subcellular localization, frequency of detection, normalized spectral abundance, and functions of the co-isolating proteins are discussed.

CONCLUSIONS

Our optimized method for chloroplast isolation increased the yields of tomato chloroplasts eightfold enabling the proteomics analysis of the chloroplast stromal proteome. The set of 254 proteins that co-isolate with the chloroplast stroma provides opportunities for developing a better understanding of the extensive and dynamic interactions of chloroplasts with other organelles. These co-isolating proteins also have the potential for expanding our knowledge of proteins that are co-localized in multiple subcellular organelles.

Comparative and Phylogenetic Analysis of the Complete Chloroplast Genomes of Three Paeonia Section Moutan Species (Paeoniaceae)

Analysis of the relationships among wild species of section Moutan in the plant genus Paeonia has traditionally been problematic. Interspecies relationships cannot be effectively determined using phenotypic traits alone or through analysis of nuclear or chloroplast DNA fragments. Elucidation of complete chloroplast genome sequences will aid the identification and phylogeny of these species. In this study, the complete chloroplast genomes of three sect. Moutan plants were sequenced and analyzed. Comparative and phylogenetic analyses of the complete chloroplast genomes of all eight species of sect. Moutan were then conducted. The three complete chloroplast genomes gained in this study showed four-part annular structures, and the genome length, structure, GC content, codon usage, and gene distribution were highly similar. There was greater variation in the noncoding regions of the sequences than in the conserved protein-coding regions. Sequence variations in the small single copy (SSC) regions and large single copy (LSC) regions were considerably greater than those in the inverted repeat (IR) regions. Phylogenetic analysis revealed that the species of sect. Moutan clustered in one branch and then subdivided into smaller branches. As for the three complete chloroplast genome sequences obtained in this study, Paeonia jishanensis clustered with another P. jishanensis sequence from the GenBank database, Paeonia qiui clustered with Paeonia rockii, and Paeonia delavayi var. lutea clustered with Paeonia ludlowii. It was also found that the complete chloroplast genomes, LSC regions, and SSC regions all showed great abilities in identification and phylogenetic analysis of the species of sect. Moutan, while IRs regions and highly variable regions were not suitable for the species of sect. Moutan.

The plastid NAD(P)H dehydrogenase-like complex: structure, function and evolutionary dynamics

The thylakoid NAD(P)H dehydrogenase-like (NDH) complex is a large protein complex that reduces plastoquinone and pumps protons into the lumen generating protonmotive force. In plants, the complex consists of both nuclear and chloroplast-encoded subunits. Despite its perceived importance for stress tolerance and ATP generation, chloroplast-encoded NDH subunits have been lost numerous times during evolution in species occupying seemingly unrelated environmental niches. We have generated a phylogenetic tree that reveals independent losses in multiple phylogenetic lineages, and we use this tree as a reference to discuss possible evolutionary contexts that may have relaxed selective pressure for retention of ndh genes. While we are still yet unable to pinpoint a singular specific lifestyle that negates the need for NDH, we are able to rule out several long-standing explanations. In light of this, we discuss the biochemical changes that would be required for the chloroplast to dispense with NDH functionality with regards to known and proposed NDH-related reactions.

The res (restored cell structure by salinity) tomato mutant reveals the role of the DEAD-box RNA helicase SlDEAD39 in plant development and salt response

Increasing evidences highlight the importance of DEAD-box RNA helicases in plant development and stress responses. In a previous study, we characterized the tomato res mutant (restored cell structure by salinity), showing chlorosis and development alterations that reverted under salt-stress conditions. Map-based cloning demonstrates that RES gene encodes SlDEAD39, a chloroplast-targeted DEAD-box RNA helicase. Constitutive expression of SlDEAD39 complements the res mutation, while the silencing lines had a similar phenotype than res mutant, which is also reverted under salinity. Functional analysis of res mutant proved SlDEAD39 is involved in the in vivo processing of the chloroplast, 23S rRNA, at the hidden break-B site, a feature also supported by in vitro binding experiments of the protein. In addition, our results show that other genes coding for chloroplast-targeted DEAD-box proteins are induced by salt-stress, which might explain the rescue of the res mutant phenotype. Interestingly, salinity restored the phenotype of res adult plants by increasing their sugar content and fruit yield. Together, these results propose an unprecedented role of a DEAD-box RNA helicase in regulating plant development and stress response through the proper ribosome and chloroplast functioning, which, in turn, represents a potential target to improve salt tolerance in tomato crops.

Evolutionary Model of Plastidial RNA Editing in Angiosperms Presumed from Genome-Wide Analysis of Amborella trichopoda

Amborella trichopoda is placed close to the base of the angiosperm lineage (basal angiosperm). By genome-wide RNA sequencing, we identified 184C-to-U RNA editing sites in the plastid genome of Amborella. This number is much higher than that observed in other angiosperms including maize (44 sites), rice (39 sites) and grape (115 sites). Despite the high frequency of RNA editing, the biased distribution of RNA editing sites in the genome, target codon preference and nucleotide preference adjacent to the edited cytidine are similar to that in other angiosperms, suggesting a common editing machinery. Consistent with this idea, the Amborella nuclear genome encodes 2-3 times more of the E- and DYW-subclass members of pentatricopeptide repeat proteins responsible for RNA editing site recognition in plant organelles. Among 165 editing sites in plastid protein coding sequences in Amborella, 100 sites were conserved at least in one out of 38 species selected to represent key branching points of the angiosperm phylogenetic tree. We assume these 100 sites represent at least a subset of the sites in the plastid editotype of ancestral angiosperms. We then mapped the loss and gain of editing sites on the phylogenetic tree of angiosperms. Our results support the idea that the evolution of angiosperms has led to the loss of RNA editing sites in plastids.

Differentiation of chromoplasts and other plastids in plants

Plant cells are characterized by a unique group of interconvertible organelles called plastids, which are descended from prokaryotic endosymbionts. The most studied plastid type is the chloroplast, which carries out the ancestral plastid function of photosynthesis. During the course of evolution, plastid activities were increasingly integrated with cellular metabolism and functions, and plant developmental processes, and this led to the creation of new types of non-photosynthetic plastids. These include the chromoplast, a carotenoid-rich organelle typically found in flowers and fruits. Here, we provide an introduction to non-photosynthetic plastids, and then review the structures and functions of chromoplasts in detail. The role of chromoplast differentiation in fruit ripening in particular is explored, and the factors that govern plastid development are examined, including hormonal regulation, gene expression, and plastid protein import. In the latter process, nucleus-encoded preproteins must pass through two successive protein translocons in the outer and inner envelope membranes of the plastid; these are known as TOC and TIC (translocon at the outer/inner chloroplast envelope), respectively. The discovery of SP1 (suppressor of ppi1 locus1), which encodes a RING-type ubiquitin E3 ligase localized in the plastid outer envelope membrane, revealed that plastid protein import is regulated through the selective targeting of TOC complexes for degradation by the ubiquitin-proteasome system. This suggests the possibility of engineering plastid protein import in novel crop improvement strategies.

The complete organelle genomes of Physochlaina orientalis: Insights into short sequence repeats across seed plant mitochondrial genomes

Short repeats (SR) play an important role in shaping seed plant mitochondrial genomes (mtDNAs). However, their origin, distribution, and relationships across the different plant lineages remain unresolved. We focus on the angiosperm family Solanaceae that shows great variation in repeat content and extend the study to a wide diversity of seed plants. We determined the complete nucleotide sequences of the organellar genomes of the medicinal plant Physochlaina orientalis (Solanaceae), member of the tribe Hyoscyameae. To understand the evolution of the P. orientalis mtDNA we made comparisons with those of five other Solanaceae. P. orientalis mtDNA presents the largest mitogenome (∼685 kb in size) among the Solanaceae and has an unprecedented 8-copy repeat family of ∼8.2 kb in length and a great number of SR arranged in tandem-like structures. We found that the SR in the Solanaceae share a common origin, but these only expanded in members of the tribe Hyoscyameae. We discuss a mechanism that could explain SR formation and expansion in P. orientalis and Hyoscyamus niger. Finally, the great increase in plant mitochondrial data allowed us to systematically extend our repeat analysis to a total of 136 seed plants to characterize and analyze for the first time families of SR among seed plant mtDNAs.

Sequencing and Structural Analysis of the Complete Chloroplast Genome of the Medicinal Plant Lycium chinense Mill

Lycium chinense Mill, an important Chinese herbal medicine, is widely used as a dietary supplement and food. Here the chloroplast (CP) genome of L. chinense was sequenced and analyzed, revealing a size of 155,756 bp and with a 37.8% GC content. The L. chinense CP genome comprises a large single copy region (LSC) of 86,595 bp and a small single copy region (SSC) of 18,209 bp, and two inverted repeat regions (IRa and IRb) of 25,476 bp separated by the single copy regions. The genome encodes 114 genes, 16 of which are duplicated. Most of the 85 protein-coding genes (CDS) had standard ATG start codons, while 3 genes including rps12, psbL and ndhD had abnormal start codons (ACT and ACG). In addition, a strong A/T bias was found in the majority of simple sequence repeats (SSRs) detected in the CP genome. Analysis of the phylogenetic relationships among 16 species revealed that L. chinense is a sister taxon to Lycium barbarum. Overall, the complete sequence and annotation of the L. chinense CP genome provides valuable genetic information to facilitate precise understanding of the taxonomy, species and phylogenetic evolution of the Solanaceae family.

The complete chloroplast genome sequence of watercress (Nasturtium officinale R. Br.): Genome organization, adaptive evolution and phylogenetic relationships in Cardamineae

Watercress (Nasturtium officinale R. Br.), an aquatic leafy vegetable of the Brassicaceae family, is known as a nutritional powerhouse. Here, we de novo sequenced and assembled the complete chloroplast (cp) genome of watercress based on combined PacBio and Illumina data. The cp genome is 155,106 bp in length, exhibiting a typical quadripartite structure including a pair of inverted repeats (IRA and IRB) of 26,505 bp separated by a large single copy (LSC) region of 84,265 bp and a small single copy (SSC) region of 17,831 bp. The genome contained 113 unique genes, including 79 protein-coding genes, 30 tRNAs and 4 rRNAs, with 20 duplicate in the IRs. Compared with the prior cp genome of watercress deposited in GenBank, 21 single nucleotide polymorphisms (SNPs) and 27 indels were identified, mainly located in noncoding sequences. A total of 49 repeat structures and 71 simple sequence repeats (SSRs) were detected. Codon usage showed a bias for A/T-ending codons in the cp genome of watercress. Moreover, 45 RNA editing sites were predicted in 16 genes, all for C-to-U transitions. A comparative plastome study with Cardamineae species revealed a conserved gene order and high similarity of protein-coding sequences. Analysis of the Ka/Ks ratios of Cardamineae suggested positive selection exerted on the ycf2 gene in watercress, which might reflect specific adaptations of watercress to its particular living environment. Phylogenetic analyses based on complete cp genomes and common protein-coding genes from 56 species showed that the genus Nasturtium was a sister to Cardamine in the Cardamineae tribe. Our study provides valuable resources for future evolution, population genetics and molecular biology studies of watercress.

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.

Organellar genome analysis reveals endosymbiotic gene transfers in tomato

We assembled three complete mitochondrial genomes (mitogenomes), two of Solanum lycopersicum and one of Solanum pennellii, and analyzed their intra- and interspecific variations. The mitogenomes were 423,596-446,257 bp in length. Despite numerous rearrangements between the S. lycopersicum and S. pennellii mitogenomes, over 97% of the mitogenomes were similar to each other. These mitogenomes were compared with plastid and nuclear genomes to investigate genetic material transfers among DNA-containing organelles in tomato. In all mitogenomes, 9,598 bp of plastome sequences were found. Numerous nuclear copies of mitochondrial DNA (NUMTs) and plastid DNA (NUPTs) were observed in the S. lycopersicum and S. pennellii nuclear genomes. Several long organellar DNA fragments were tightly clustered in the nuclear genome; however, the NUMT and NUPT locations differed between the two species. Our results demonstrate the recent occurrence of frequent endosymbiotic gene transfers in tomato genomes.

Authentication of Herbal Medicines Dipsacus asper and Phlomoides umbrosa Using DNA Barcodes, Chloroplast Genome, and Sequence Characterized Amplified Region (SCAR) Marker

Dried roots of Dipsacus asper (Caprifoliaceae) are used as important traditional herbal medicines in Korea. However, the roots are often used as a mixture or contaminated with Dipsacus japonicus in Korean herbal markets. Furthermore, the dried roots of Phlomoides umbrosa (Lamiaceae) are used indiscriminately with those of D. asper, with the confusing Korean names of Sok-Dan and Han-Sok-Dan for D. asper and P. umbrosa, respectively. Although D. asper and P. umbrosa are important herbal medicines, the molecular marker and genomic information available for these species are limited. In this study, we analysed DNA barcodes to distinguish among D. asper, D. japonicus, and P. umbrosa and sequenced the chloroplast (CP) genomes of D. asper and D. japonicus. The CP genomes of D. asper and D. japonicus were 160,530 and 160,371 bp in length, respectively, and were highly divergent from those of the other Caprifoliaceae species. Phylogenetic analysis revealed a monophyletic group within Caprifoliaceae. We also developed a novel sequence characterised amplified region (SCAR) markers to distinguish among D. asper, D. japonicus, and P. umbrosa. Our results provide important taxonomic, phylogenetic, and evolutionary information on the Dipsacus species. The SCAR markers developed here will be useful for the authentication of herbal medicines.

A missense mutation of plastid RPS4 is associated with chlorophyll deficiency in Chinese cabbage (Brassica campestris ssp. pekinensis)

BACKGROUND

Plastome mutants are ideal resources for elucidating the functions of plastid genes. Numerous studies have been conducted for the function of plastid genes in barley and tobacco; however, related information is limited in Chinese cabbage.

RESULTS

A chlorophyll-deficient mutant of Chinese cabbage that was derived by ethyl methanesulfonate treatment on isolated microspores showed uniformly pale green inner leaves and slow growth compared with that shown by the wild type "Fukuda 50' ('FT'). Genetic analysis revealed that cdm was cytoplasmically inherited. Physiological and ultrastructural analyses of cdm showed impaired photosynthesis and abnormal chloroplast development. Utilizing next generation sequencing, the complete plastomes of cdm and 'FT' were respectively re-mapped to the reference genome of Chinese cabbage, and an A-to-C base substitution with a mutation ratio higher than 99% was detected. The missense mutation of plastid ribosomal protein S4 led to valine substitution for glycine at residue 193. The expression level of rps4 was analyzed using quantitative real-time PCR and found lower in than in 'FT'. RNA gel-blot assays showed that the abundance of mature 23S rRNA, 16S rRNA, 5S rRNA, and 4.5S rRNA significantly decreased and that the processing of 23S, 16S rRNA, and 4.5S rRNA was seriously impaired, affecting the ribosomal function in cdm.

CONCLUSIONS

These findings indicated that cdm was a plastome mutant and that chlorophyll deficiency might be due to an A-to-C base substitution of the plastome-encoded rps4 that impaired the rRNA processing and affected the ribosomal function.

Mining of potential drug targets through the identification of essential and analogous enzymes in the genomes of pathogens of Glycine max, Zea mays and Solanum lycopersicum

Pesticides are one of the most widely used pest and disease control measures in plant crops and their indiscriminate use poses a direct risk to the health of populations and environment around the world. As a result, there is a great need for the development of new, less toxic molecules to be employed against plant pathogens. In this work, we employed an in silico approach to study the genes coding for enzymes of the genomes of three commercially important plants, soybean (Glycine max), tomato (Solanum lycopersicum) and corn (Zea mays), as well as 15 plant pathogens (4 bacteria and 11 fungi), focusing on revealing a set of essential and non-homologous isofunctional enzymes (NISEs) that could be prioritized as drug targets. By combining sequence and structural data, we obtained an initial set of 568 cases of analogy, of which 97 were validated and further refined, revealing a subset of 29 essential enzymatic activities with a total of 119 different structural forms, most belonging to central metabolic routes, including the carbohydrate metabolism, the metabolism of amino acids, among others. Further, another subset of 26 enzymatic activities possess a tertiary structure specific for the pathogen, not present in plants, men and Apis mellifera, which may be of importance for the development of specific enzymatic inhibitors against plant diseases that are less harmful to humans and the environment.

The chloroplast genome sequence of bittersweet (Solanum dulcamara): Plastid genome structure evolution in Solanaceae

Bittersweet (Solanum dulcamara) is a native Old World member of the nightshade family. This European diploid species can be found from marshlands to high mountainous regions and it is a common weed that serves as an alternative host and source of resistance genes against plant pathogens such as late blight (Phytophthora infestans). We sequenced the complete chloroplast genome of bittersweet, which is 155,580 bp in length and it is characterized by a typical quadripartite structure composed of a large (85,901 bp) and small (18,449 bp) single-copy region interspersed by two identical inverted repeats (25,615 bp). It consists of 112 unique genes from which 81 are protein-coding, 27 tRNA and four rRNA genes. All bittersweet plastid genes including non-functional ones and even intergenic spacer regions are transcribed in primary plastid transcripts covering 95.22% of the genome. These are later substantially edited in a post-transcriptional phase to activate gene functions. By comparing the bittersweet plastid genome with all available Solanaceae sequences we found that gene content and synteny are highly conserved across the family. During genome comparison we have identified several annotation errors, which we have corrected in a manual curation process then we have identified the major plastid genome structural changes in Solanaceae. Interpreted in a phylogenetic context they seem to provide additional support for larger clades. The plastid genome sequence of bittersweet could help to benchmark Solanaceae plastid genome annotations and could be used as a reference for further studies. Such reliable annotations are important for gene diversity calculations, synteny map constructions and assigning partitions for phylogenetic analysis with de novo sequenced plastomes of Solanaceae.

Comparative chloroplast genomics between the invasive weed Mikania micrantha and its indigenous congener Mikania cordata: Structure variation, identification of highly divergent regions, divergence time estimation, and phylogenetic analysis

Mikania micrantha and Mikania cordata are the only two species in genus Mikania (Asteraceae) in China. They share very similar morphological and life-history characteristics but occupy quite different habitats. Most importantly, they generate totally different ecological consequences. While M. micrantha has become an exotic invasive weed, M. cordata exists as an indigenous species with no harmful effects on native plants or habitats. As a continuous study of our previously reported M. micrantha chloroplast (cp) genome, in this study we have further sequenced the M. cordata cp genome to (1) conduct a comparative genome analysis to gain insights into the mechanism of invasiveness; (2) develop cp markers to examine the population genetic adaptation of M. micrantha; and (3) screen variable genome regions of phylogenetic utility. The M. cordata chloroplast genome is 151,984 bp in length and displays a typical quadripartite structure. The number and distribution of protein coding genes, tRNA genes, and rRNA genes of M. cordata are identical to those of M. micrantha. The main difference lays in that the pseudogenization of ndhF and a 118-bp palindromic repeat only arises in M. cordata. Fourteen highly divergent regions, 235 base substitutions, and 58 indels were identified between the two cp genomes. Phylogenetic inferences revealed a sister relationship between M. micrantha and M. cordata whose divergence was estimated to occur around 1.78 million years ago (MYA). Twelve cpSSR loci were detected to be polymorphic and adopted to survey the genetic adaptation of M. micrantha populations. No cpSSR loci were found to undergo selection. Our results build a foundation to examine the invasive mechanism of Mikania weed.

The complete plastome of macaw palm [Acrocomia aculeata (Jacq.) Lodd. ex Mart.] and extensive molecular analyses of the evolution of plastid genes in Arecaceae

The plastome of macaw palm was sequenced allowing analyses of evolution and molecular markers. Additionally, we demonstrated that more than half of plastid protein-coding genes in Arecaceae underwent positive selection. Macaw palm is a native species from tropical and subtropical Americas. It shows high production of oil per hectare reaching up to 70% of oil content in fruits and an interesting plasticity to grow in different ecosystems. Its domestication and breeding are still in the beginning, which makes the development of molecular markers essential to assess natural populations and germplasm collections. Therefore, we sequenced and characterized in detail the plastome of macaw palm. A total of 221 SSR loci were identified in the plastome of macaw palm. Additionally, eight polymorphism hotspots were characterized at level of subfamily and tribe. Moreover, several events of gain and loss of RNA editing sites were found within the subfamily Arecoideae. Aiming to uncover evolutionary events in Arecaceae, we also analyzed extensively the evolution of plastid genes. The analyses show that highly divergent genes seem to evolve in a species-specific manner, suggesting that gene degeneration events may be occurring within Arecaceae at the level of genus or species. Unexpectedly, we found that more than half of plastid protein-coding genes are under positive selection, including genes for photosynthesis, gene expression machinery and other essential plastid functions. Furthermore, we performed a phylogenomic analysis using whole plastomes of 40 taxa, representing all subfamilies of Arecaceae, which placed the macaw palm within the tribe Cocoseae. Finally, the data showed here are important for genetic studies in macaw palm and provide new insights into the evolution of plastid genes and environmental adaptation in Arecaceae.

The Linum usitatissimum L. plastome reveals atypical structural evolution, new editing sites, and the phylogenetic position of Linaceae within Malpighiales

The plastome of Linum usitatissimum was completely sequenced allowing analyses of evolution of genome structure, RNA editing sites, molecular markers, and indicating the position of Linaceae within Malpighiales. Flax (Linum usitatissimum L.) is an economically important crop used as food, feed, and industrial feedstock. It belongs to the Linaceae family, which is noted by high morphological and ecological diversity. Here, we reported the complete sequence of flax plastome, the first species within Linaceae family to have the plastome sequenced, assembled and characterized in detail. The plastome of flax is a circular DNA molecule of 156,721 bp with a typical quadripartite structure including two IRs of 31,990 bp separating the LSC of 81,767 bp and the SSC of 10,974 bp. It shows two expansion events from IRB to LSC and from IRB to SSC, and a contraction event in the IRA-LSC junction, which changed significantly the size and the gene content of LSC, SSC and IRs. We identified 109 unique genes and 2 pseudogenes (rpl23 and ndhF). The plastome lost the conserved introns of clpP gene and the complete sequence of rps16 gene. The clpP, ycf1, and ycf2 genes show high nucleotide and aminoacid divergence, but they still possibly retain the functionality. Moreover, we also identified 176 SSRs, 20 tandem repeats, and 39 dispersed repeats. We predicted in 18 genes a total of 53 RNA editing sites of which 32 were not found before in other species. The phylogenetic inference based on 63 plastid protein-coding genes of 38 taxa supports three major clades within Malpighiales order. One of these clades has flax (Linaceae) sister to Chrysobalanaceae family, differing from earlier studies that included Linaceae into the euphorbioid clade.

Method for the Identification of Taxon-Specific k-mers from Chloroplast Genome: A Case Study on Tomato Plant (Solanum lycopersicum)

Polymerase chain reaction and different barcoding methods commonly used for plant identification from metagenomics samples are based on the amplification of a limited number of pre-selected barcoding regions. These methods are often inapplicable due to DNA degradation, low amplification success or low species discriminative power of selected genomic regions. Here we introduce a method for the rapid identification of plant taxon-specific k-mers, that is applicable for the fast detection of plant taxa directly from raw sequencing reads without aligning, mapping or assembling the reads. We identified more than 800 Solanum lycopersicum specific k-mers (32 nucleotides in length) from 42 different chloroplast genome regions using the developed method. We demonstrated that identified k-mers are also detectable in whole genome sequencing raw reads from S. lycopersicum. Also, we demonstrated the usability of taxon-specific k-mers in artificial mixtures of sequences from closely related species. Developed method offers a novel strategy for fast identification of taxon-specific genome regions and offers new perspectives for detection of plant taxa directly from sequencing raw reads.

The Complete Chloroplast Genomes of Six Ipomoea Species and Indel Marker Development for the Discrimination of Authentic Pharbitidis Semen (Seeds of I. nil or I. purpurea)

Ipomoea L. is the largest genus within the Convolvulaceae and contains 600-700 species. Ipomoea species (morning glories) are economically valuable as horticultural species and scientifically valuable as ecological model plants to investigate mating systems, molecular evolution, and both plant-herbivore and plant-parasite interactions. Furthermore, the dried seeds of I. nil or I. purpurea are used in Korean traditional herbal medicines. In this study, chloroplast (cp) genomes were sequenced from six Ipomoea species, namely, I. nil and I. purpurea and, for the first time, I. triloba, I. lacunosa, I. hederacea, and I. hederacea var. integriuscula. The cp genomes were 161,354-161,750 bp in length and exhibited conserved quadripartite structures. In total, 112 genes were identified, including 78 protein-coding regions, 30 transfer RNA genes, and 4 ribosomal RNA genes. The gene order, content, and orientation of the six Ipomoea cp genomes were highly conserved and were consistent with the general structure of angiosperm cp genomes. Comparison of the six Ipomoea cp genomes revealed locally divergent regions, mainly within intergenic spacer regions (petN-psbM, trnI-CAU-ycf2, ndhH-ndhF, psbC-trnS, and ccsA-ndhD). In addition, the protein-coding genes accD, cemA, and ycf2 exhibited high sequence variability and were under positive selection (Ka/Ks > 1), indicating adaptive evolution to the environment within the Ipomoea genus. Phylogenetic analysis of the six Ipomoea species revealed that these species clustered according to the APG IV system. In particular, I. nil and I. hederacea had monophyletic positions, with I. purpurea as a sister. I. triloba and I. lacunosa in the section Batatas and I. hederacea and I. hederacea var. integriuscula in the section Quamoclit were supported in this study with strong bootstrap values and posterior probabilities. We uncovered high-resolution phylogenetic relationships between Ipomoeeae. Finally, indel markers (IPOTY and IPOYCF) were developed for the discrimination of the important herbal medicine species I. nil and I. purpurea. The cp genomes and analyses in this study provide useful information for taxonomic, phylogenetic, and evolutionary analysis of the Ipomoea genome, and the indel markers will be useful for authentication of herbal medicines.

Optimized Method of Extracting Rice Chloroplast DNA for High-Quality Plastome Resequencing and de Novo Assembly

Chloroplasts, which perform photosynthesis, are one of the most important organelles in green plants and algae. Chloroplasts maintain an independent genome that includes important genes encoding their photosynthetic machinery and various housekeeping functions. Owing to its non-recombinant nature, low mutation rates, and uniparental inheritance, the chloroplast genome (plastome) can give insights into plant evolution and ecology and in the development of biotechnological and breeding applications. However, efficient methods to obtain high-quality chloroplast DNA (cpDNA) are currently not available, impeding powerful sequencing and further functional genomics research. To investigate effects on rice chloroplast genome quality, we compared cpDNA extraction by three extraction protocols: liquid nitrogen coupled with sucrose density gradient centrifugation, high-salt buffer, and Percoll gradient centrifugation. The liquid nitrogen-sucrose gradient method gave a high yield of high-quality cpDNA with reliable purity. The cpDNA isolated by this technique was evaluated, resequenced, and assembled de novo to build a robust framework for genomic and genetic studies. Comparison of this high-purity cpDNA with total DNAs revealed the read coverage of the sequenced regions; next-generation sequencing data showed that the high-quality cpDNA eliminated noise derived from contamination by nuclear and mitochondrial DNA, which frequently occurs in total DNA. The assembly process produced highly accurate, long contigs. We summarize the extent to which this improved method of isolating cpDNA from rice can provide practical progress in overcoming challenges related to chloroplast genomes and in further exploring the development of new sequencing technologies.

The Complete Chloroplast Genome Sequences of the Medicinal Plant Forsythia suspensa (Oleaceae)

Forsythia suspensa is an important medicinal plant and traditionally applied for the treatment of inflammation, pyrexia, gonorrhea, diabetes, and so on. However, there is limited sequence and genomic information available for F. suspensa. Here, we produced the complete chloroplast genomes of F. suspensa using Illumina sequencing technology. F. suspensa is the first sequenced member within the genus Forsythia (Oleaceae). The gene order and organization of the chloroplast genome of F. suspensa are similar to other Oleaceae chloroplast genomes. The F. suspensa chloroplast genome is 156,404 bp in length, exhibits a conserved quadripartite structure with a large single-copy (LSC; 87,159 bp) region, and a small single-copy (SSC; 17,811 bp) region interspersed between inverted repeat (IRa/b; 25,717 bp) regions. A total of 114 unique genes were annotated, including 80 protein-coding genes, 30 tRNA, and four rRNA. The low GC content (37.8%) and codon usage bias for A- or T-ending codons may largely affect gene codon usage. Sequence analysis identified a total of 26 forward repeats, 23 palindrome repeats with lengths >30 bp (identity > 90%), and 54 simple sequence repeats (SSRs) with an average rate of 0.35 SSRs/kb. We predicted 52 RNA editing sites in the chloroplast of F. suspensa, all for C-to-U transitions. IR expansion or contraction and the divergent regions were analyzed among several species including the reported F. suspensa in this study. Phylogenetic analysis based on whole-plastome revealed that F. suspensa, as a member of the Oleaceae family, diverged relatively early from Lamiales. This study will contribute to strengthening medicinal resource conservation, molecular phylogenetic, and genetic engineering research investigations of this species.

The Complete Chloroplast Genome Sequences of Fritillaria ussuriensis Maxim. and Fritillaria cirrhosa D. Don, and Comparative Analysis with Other Fritillaria Species

The genus Fritillaria belongs to the widely distributed Liliaceae. The bulbs of Fritillaria, F. ussuriensis and F. cirrhosa are valuable herbaceous medicinal ingredients. However, they are still used indiscriminately in herbal medicine. Identification and molecular phylogenic analysis of Fritillaria species are therefore required. Here, we report the complete chloroplast (CP) genome sequences of F. ussuriensis and F. cirrhosa. The two Fritillaria CP genomes were 151,524 and 151,083 bp in length, respectively, and each included a pair of inverted repeated regions (52,678 and 52,156 bp) that was separated by a large single copy region (81,732 and 81,390 bp), and a small single copy region (17,114 and 17,537 bp). A total of 111 genes in F. ussuriensis and 112 in F. cirrhosa comprised 77 protein-coding regions in F. ussuriensis and 78 in F. cirrhosa, 30 transfer RNA (tRNA) genes, and four ribosomal RNA (rRNA) genes. The gene order, content, and orientation of the two Fritillaria CP genomes exhibited the general structure of flowering plants, and were similar to those of other Fritillaria species. Comparison of the six Fritillaria species’ CP genomes indicated seven highly divergent regions in intergenic spacers and in the matK, rpoC1, rpoC2, ycf1, ycf2, ndhD, and ndhF coding regions. We established the position of the six species through phylogenic analysis. The complete chloroplast genome sequences of the two Fritillaria species and a comparison study are useful genomic information for identifying and for studying the phylogenetic relationship among Fritillaria species within the Liliaceae.

Cryptochrome 2 extensively regulates transcription of the chloroplast genome in tomato

Light plays a key role in the regulation of many physiological processes required for plant and chloroplast development. Plant cryptochromes (crys) play an important role in monitoring, capturing, and transmitting the light stimuli. In this study, we analyzed the effects of CRY2 overexpression on transcription of tomato chloroplast genome by a tiling array, containing about 90 000 overlapping probes (5‐nucleotide resolution). We profiled transcription in leaves of wild‐type and CRY2‐overexpressing plants grown in a diurnal cycle, to generate a comprehensive map of chloroplast transcription and to monitor potential specific modulations of the chloroplast transcriptome induced by the overexpression of CRY2. Our results demonstrate that CRY2 is a master gene of transcriptional regulation in the tomato chloroplast. In fact, it modulates the day/night mRNA abundance of about 58% of the 114 ORFs. The effect of CRY2 includes a differential extension of some transcripts at their 5′‐end, according to the period of the day. We observed that the influence of CRY2 on chloroplast transcription is not limited to coding RNA; a great number of putative noncoding micro RNA also showed differential accumulation pattern. To our knowledge, this is the first study that highlights how a photoreceptor affects the day/night transcription of the chloroplast genome.

Identification and Analysis of RNA Editing Sites in the Chloroplast Transcripts of Aegilops tauschii L

RNA editing is an important way to convert cytidine (C) to uridine (U) at specific sites within RNA molecules at a post-transcriptional level in the chloroplasts of higher plants. Although it has been systematically studied in many plants, little is known about RNA editing in the wheat D genome donor Aegilops tauschii L. Here, we investigated the chloroplast RNA editing of Ae. tauschii and compared it with other wheat relatives to trace the evolution of wheat. Through bioinformatics prediction, a total of 34 C-to-U editing sites were identified, 17 of which were validated using RT-PCR product sequencing. Furthermore, 60 sites were found by the RNA-Seq read mapping approach, 24 of which agreed with the prediction and six were validated experimentally. The editing sites were biased toward tCn or nCa trinucleotides and 5′-pyrimidines, which were consistent with the flanking bases of editing sites of other seed plants. Furthermore, the editing events could result in the alteration of the secondary structures and topologies of the corresponding proteins, suggesting that RNA editing might impact the function of target genes. Finally, comparative analysis found some evolutionarily conserved editing sites in wheat and two species-specific sites were also obtained. This study is the first to report on RNA editing in Aegilops tauschii L, which not only sheds light on the evolution of wheat from the point of view of RNA editing, but also lays a foundation for further studies to identify the mechanisms of C-to-U alterations.

Abundant RNA editing sites of chloroplast protein-coding genes in Ginkgo biloba and an evolutionary pattern analysis

BACKGROUND

RNA editing is a posttranscriptional modification process that alters the RNA sequence so that it deviates from the genomic DNA sequence. RNA editing mainly occurs in chloroplasts and mitochondrial genomes, and the number of editing sites varies in terrestrial plants. Why and how RNA editing systems evolved remains a mystery. Ginkgo biloba is one of the oldest seed plants and has an important evolutionary position. Determining the patterns and distribution of RNA editing in the ancient plant provides insights into the evolutionary trend of RNA editing, and helping us to further understand their biological significance.

RESULTS

In this paper, we investigated 82 protein-coding genes in the chloroplast genome of G. biloba and identified 255 editing sites, which is the highest number of RNA editing events reported in a gymnosperm. All of the editing sites were C-to-U conversions, which mainly occurred in the second codon position, biased towards to the U_A context, and caused an increase in hydrophobic amino acids. RNA editing could change the secondary structures of 82 proteins, and create or eliminate a transmembrane region in five proteins as determined in silico. Finally, the evolutionary tendencies of RNA editing in different gene groups were estimated using the nonsynonymous-synonymous substitution rate selection mode.

CONCLUSIONS

The G. biloba chloroplast genome possesses the highest number of RNA editing events reported so far in a seed plant. Most of the RNA editing sites can restore amino acid conservation, increase hydrophobicity, and even influence protein structures. Similar purifying selections constitute the dominant evolutionary force at the editing sites of essential genes, such as the psa, some psb and pet groups, and a positive selection occurred in the editing sites of nonessential genes, such as most ndh and a few psb genes.