Comparative analysis of the complete chloroplast genomes of seven Populus species: Insights into alternative female parents of Populus tomentosa

The population chloroplast genomes of Populus reveal the phylogenetic relationship between three new taxa of sect. Leucoides and their parents

BACKGROUND

Poplars are important woody plants, which are widely distributed in the forests from the subtropics to the north of the Northern Hemisphere. Poplars have high ecological and economic value. However, there are frequent interspecific and intraspecific hybrids in Populus, resulting in a large number of intermediate taxa, which makes the morphological identification of Populus very challenging. Plastid genome is an important tool to study the evolutionary relationship of plants. Therefore, comparison and phylogenetic analysis were carried out based on the population chloroplast genomes of 34 individuals from 7 taxa.

RESULTS

In this study, seven newly assembled and annotated chloroplast genomes of Populus were reported. They all had typical quadripartite structures with the same GC content (37.6%), but there were differences within the population, and the genome size ranged from 155,736 bp to 156,812 bp. In all Populus species, 134 genes were identified, including 88 protein coding genes (PCGs), 37 tRNA and 8 rRNA genes. The gene sequences alignment of different taxa showed that the gene sequences and content were relatively conservative, there was no gene rearrangement, and only 3 highly variable regions (psbZ-trnG, ndhC-trnV and trnN-trnR) were identified, which can be used as molecular markers. Most PCGs had high codon usage bias and 3 positive selection genes (rps7, rps12 and rpl16) have been identified. The analysis of population genetic structure and phylogeny showed that the chloroplast genomes supported that Populus was a monophyletic taxon, which could be divided into four sections (Abaso, Turanga, Populus and ATL (Aigeiros, Tacamahaca and Leucoides)). Among them, P. dafengensis, P. butuoensis and P. szechuanica had the closest genetic relationship, P. gonggaensis and P. cathayana had the closest genetic relationship, it was speculated that the taxa of Sect. Tacamahaca may be the main female parent of the three new taxa from Sect. Leucoides.

CONCLUSION

In general, this study provides valuable insights for new species identification, phylogenetic relationships, breeding and resource development, and genetic diversity of Populus.

Comparative plastome analyses and evolutionary relationships of 25 East Asian species within the medicinal plant genus Scrophularia (Scrophulariaceae)

Backgroud

Scrophularia L., a genus of the Scrophulariaceae, is a group of important medicinal plants used for eliminating heat and detoxifying. East Asia has an abundance of potentially medicinal Scrophularia species, and it serves as a secondary diversity center of the genus. However, the genomic resources available for germplasm identification and pharmaceutical exploration of East Asian Scrophularia are insufficient, hindering its commercial and industrial development. Additionally, the interspecific relationships of most East Asian Scrophularia species remain unclear.

Methods

In this study, we sequenced the leaves of 25 East Asian species of the genus Scrophularia, assembled and annotated the complete chloroplast genomes, and subsequently performed comparative and phylogenetic analyses on these genomes.

Results and discussion

The conserved plastome length of these 25 species ranged from 151,582 bp to 153,239 bp, containing a total of 132 coding genes, including 18 duplicated genes and 114 unique genes. Through genome alignment of these 25 species, 38-53 repeated sequences and 7 shared SSRs were identified, along with regions with high nucleotide polymorphism (Pi), which could potentially serve as molecular markers for species identification. The genome structure, gene content, and arrangement showed conservation, while variations were observed in the IR boundary regions and IGS. Phylogenetic inferences based on whole plastomes or on coding sequences (CDS) only yielded congruent results. We categorized the 25 East Asian Scrophularia species into six distinct clades and further explored their interspecies relationships using morphological characteristics, such as flower color, the relative position of stamens and corolla, and plant height. This could lay a genetic basis for future resource development of Scrophularia in East Asia.

Complete chloroplast genomes of three wild perennial Hordeum species from Central Asia: genome structure, mutation hotspot, phylogenetic relationships, and comparative analysis

Hordeum L. is widely distributed in mountain or plateau of subtropical and warm temperate regions around the world. Three wild perennial Hordeum species, including H. bogdanii, H. brevisubulatum, and H. violaceum, have been used as forage and for grassland ecological restoration in high-altitude areas in recent years. To date, the degree of interspecies sequence variation in the three Hordeum species within existing gene pools is still not well-defined. Herein, we sequenced and assembled chloroplast (cp) genomes of the three species. The results revealed that the cp genome of H. bogdanii showed certain sequence variations compared with the cp genomes of the other two species (H. brevisubulatum and H. violaceum), and the latter two were characterized by a higher relative affinity. Parity rule 2 plot (PR2) analysis illuminated that most genes of all ten Hordeum species were concentrated in nucleotide T and G. Numerous single nucleotide polymorphism (SNP) and insertion/deletion (In/Del) events were detected in the three Hordeum species. A series of hotspots regions (tRNA-GGU ~ tRNA-GCA, tRNA-UGU ~ ndhJ, psbE ~ rps18, ndhF ~ tRNA-UAG, etc.) were identified by mVISTA procedures, and the five highly polymorphic genes (tRNA-UGC, tRNA-UAA, tRNA-UUU, tRNA-UAC, and ndhA) were proved by the nucleotide diversity (Pi). Although the distribution and existence of cp simple sequence repeats (cpSSRs) were predicted in the three Hordeum cp genomes, no rearrangement was found between them. A similar phenomenon has been found in the cp genome of the other seven Hordeum species, which has been published so far. In addition, evolutionary relationships were reappraised based on the currently reported cp genome of Hordeum L. This study offers a framework for gaining a better understanding of the evolutionary history of Hordeum species through the re-examination of their cp genomes, and by identifying highly polymorphic genes and hotspot regions that could provide important insights into the genetic diversity and differentiation of these species.

Revisiting chloroplast genomic landscape and annotation towards comparative chloroplast genomes of Rhamnaceae

BACKGROUND

Massive parallel sequencing technologies have enabled the elucidation of plant phylogenetic relationships from chloroplast genomes at a high pace. These include members of the family Rhamnaceae. The current Rhamnaceae phylogenetic tree is from 13 out of 24 Rhamnaceae chloroplast genomes, and only one chloroplast genome of the genus Ventilago is available. Hence, the phylogenetic relationships in Rhamnaceae remain incomplete, and more representative species are needed.

RESULTS

The complete chloroplast genome of Ventilago harmandiana Pierre was outlined using a hybrid assembly of long- and short-read technologies. The accuracy and validity of the final genome were confirmed with PCR amplifications and investigation of coverage depth. Sanger sequencing was used to correct for differences in lengths and nucleotide bases between inverted repeats because of the homopolymers. The phylogenetic trees reconstructed using prevalent methods for phylogenetic inference were topologically similar. The clustering based on codon usage was congruent with the molecular phylogenetic tree. The groups of genera in each tribe were in accordance with tribal classification based on molecular markers. We resolved the phylogenetic relationships among six Hovenia species, three Rhamnus species, and two Ventilago species. Our reconstructed tree provides the most complete and reliable low-level taxonomy to date for the family Rhamnaceae. Similar to other higher plants, the RNA editing mostly resulted in converting serine to leucine. Besides, most genes were subjected to purifying selection. Annotation anomalies, including indel calling errors, unaligned open reading frames of the same gene, inconsistent prediction of intergenic regions, and misannotated genes, were identified in the published chloroplast genomes used in this study. These could be a result of the usual imperfections in computational tools, and/or existing errors in reference genomes. Importantly, these are points of concern with regards to utilizing published chloroplast genomes for comparative genomic analysis.

CONCLUSIONS

In summary, we successfully demonstrated the use of comprehensive genomic data, including DNA and amino acid sequences, to build a reliable and high-resolution phylogenetic tree for the family Rhamnaceae. Additionally, our study indicates that the revision of genome annotation before comparative genomic analyses is necessary to prevent the propagation of errors and complications in downstream analysis and interpretation.

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.

Excellent Anti-lung Cancer Activity of Populus nigra and Phylogenetic Analysis

Lung cancer has the highest incidence rate among malignant tumors all over the world, and it is also the leading cause of death. In this present research, we aimed to evaluate the anti-cancer activity of the Populus nigra extract against the lung cancer and study the genome evolution of the Populus nigra. Firstly, the inhibitory activity of the Populus nigra extract on the NCI-H292 lung cancer cell viability was determined with Cell Counting Kit-8 (CCK-8) assay. The trans-well assay was conducted and the influence of the Populus nigra extract on the NCI-H292 lung cancer cell migration and invasion ability was determined. In addition to this, the chloroplast (cp) genome of Populus nigra was sequenced with high-throughput Illumina pair-end sequencing, which was a classical useful model for genome evolution assessment. The CCK-8 and trans-well assay indicated the Populus nigra extract exhibited excellent inhibitory activity on the NCI-H292 lung cancer cell viability, migration and invasion ability. The circular cp genome of the Populus nigra was 156,354 bp in size, including a large single-copy (LSC) region of 84,528 bp and a small single-copy (SC) region of 16,564 bp, which were separated by two inverted repeat (IR) regions (38,612 bp each). A total of 132 genes were predicted, including 8 ribosomal RNAs (rRNAs), 37 transfer RNAs (tRNAs), and 90 protein-coding genes (PCGs). Furthermore, phylogenetic analysis revealed that Populus nigra has the closest relationship with Populus alba var. pyramidalis. In addition to Populus alba var. pyramidalis, Populus adenopoda and Populus tomentosa are also has closely relationship with Populus nigra.

Complete chloroplast genome of Stephania tetrandra (Menispermaceae) from Zhejiang Province: insights into molecular structures, comparative genome analysis, mutational hotspots and phylogenetic relationships

BACKGROUND

The Stephania tetrandra S. Moore (S. tetrandra) is a medicinal plant belonging to the family Menispermaceae that has high medicinal value and is well worth doing further exploration. The wild resources of S. tetrandra were widely distributed in tropical and subtropical regions of China, generating potential genetic diversity and unique population structures. The geographical origin of S. tetrandra is an important factor influencing its quality and price in the market. In addition, the species relationship within Stephania genus still remains uncertain due to high morphological similarity and low support values of molecular analysis approach. The complete chloroplast (cp) genome data has become a promising strategy to determine geographical origin and understand species evolution for closely related plant species. Herein, we sequenced the complete cp genome of S. tetrandra from Zhejiang Province and conducted a comparative analysis within Stephania plants to reveal the structural variations, informative markers and phylogenetic relationship of Stephania species.

RESULTS

The cp genome of S. tetrandra voucher ZJ was 157,725 bp, consisting of a large single copy region (89,468 bp), a small single copy region (19,685 bp) and a pair of inverted repeat regions (24,286 bp each). A total of 134 genes were identified in the cp genome of S. tetrandra, including 87 protein-coding genes, 8 rRNA genes, 37 tRNA genes and 2 pseudogene copies (ycf1 and rps19). The gene order and GC content were highly consistent in the Stephania species according to the comparative analysis results, with the highest RSCU value in arginine (1.79) and lowest RSCU value in serine of S. tetrandra, respectively. A total of 90 SSRs have been identified in the cp genome of S. tetrandra, where repeats that consisting of A or T bases were much higher than that of G or C bases. In addition, 92 potential RNA editing sites were identified in 25 protein-coding genes, with the most predicted RNA editing sites in ndhB gene. The variations on length and expansion extent to the junction of ycf1 gene were observed between S. tetrandra vouchers from different regions, indicating potential markers for further geographical origin discrimination. Moreover, the values of transition to transversion ratio (Ts/Tv) in the Stephania species were significantly higher than 1 using Pericampylus glaucus as reference. Comparative analysis of the Stephania cp genomes revealed 5 highly variable regions, including 3 intergenic regions (trnH-psbA, trnD-trnY, trnP) and two protein coding genes (rps16 and ndhA). The identified mutational hotspots of Stephania plants exhibited multiple SNP sites and Gaps, as well as different Ka/Ks ratio values. In addition, five pairs of specific primers targeting the divergence regions were accordingly designed, which could be utilized as potential molecular markers for species identification, population genetic and phylogenetic analysis in Stephania species. Phylogenetic tree analysis based on the conserved chloroplast protein coding genes indicated a sister relationship between S. tetrandra and the monophyletic group of S. japonica and S. kwangsiensis with high support values, suggesting a close genetic relationship within Stephania plants. However, two S. tetrandra vouches from different regions failed to cluster into one clade, confirming the occurrences of genetic diversities and requiring further investigation for geographical tracing strategy.

CONCLUSIONS

Overall, we provided comprehensive and detailed information on the complete chloroplast genome and identified nucleotide diversity hotspots of Stephania species. The obtained genetic resource of S. tetrandra from Zhejiang Province would facilitate future studies in DNA barcode, species discrimination, the intraspecific and interspecific variability and the phylogenetic relationships of Stephania plants.

Complete Chloroplast Genomes of Fagus sylvatica L. Reveal Sequence Conservation in the Inverted Repeat and the Presence of Allelic Variation in NUPTs

Growing amounts of genomic data and more efficient assembly tools advance organelle genomics at an unprecedented scale. Genomic resources are increasingly used for phylogenetic analyses of many plant species, but are less frequently used to investigate within-species variability and phylogeography. In this study, we investigated genetic diversity of Fagus sylvatica, an important broadleaved tree species of European forests, based on complete chloroplast genomes of 18 individuals sampled widely across the species distribution. Our results confirm the hypothesis of a low cpDNA diversity in European beech. The chloroplast genome size was remarkably stable (158,428 ± 37 bp). The polymorphic markers, 12 microsatellites (SSR), four SNPs and one indel, were found only in the single copy regions, while inverted repeat regions were monomorphic both in terms of length and sequence, suggesting highly efficient suppression of mutation. The within-individual analysis of polymorphisms showed >9k of markers which were proportionally present in gene and non-gene areas. However, an investigation of the frequency of alternate alleles revealed that the source of this diversity originated likely from nuclear-encoded plastome remnants (NUPTs). Phylogeographic and Mantel correlation analysis based on the complete chloroplast genomes exhibited clustering of individuals according to geographic distance in the first distance class, suggesting that the novel markers and in particular the cpSSRs could provide a more detailed picture of beech population structure in Central Europe.

Genome-wide comparative analyses of GATA transcription factors among seven Populus genomes

GATA transcription factors (TFs) are widespread eukaryotic regulators whose DNA-binding domain is a class IV zinc finger motif (CX2CX17-20CX2C) followed by a basic region. We identified 262 GATA genes (389 GATA TFs) from seven Populus genomes using the pipeline of GATA-TFDB. Alternative splicing forms of Populus GATA genes exhibit dynamics of GATA gene structures including partial or full loss of GATA domain and additional domains. Subfamily III of Populus GATA genes display lack CCT and/or TIFY domains. 21 Populus GATA gene clusters (PCs) were defined in the phylogenetic tree of GATA domains, suggesting the possibility of subfunctionalization and neofunctionalization. Expression analysis of Populus GATA genes identified the five PCs displaying tissue-specific expression, providing the clues of their biological functions. Amino acid patterns of Populus GATA motifs display well conserved manner of Populus GATA genes. The five Populus GATA genes were predicted as membrane-bound GATA TFs. Biased chromosomal distributions of GATA genes of three Populus species. Our comparative analysis approaches of the Populus GATA genes will be a cornerstone to understand various plant TF characteristics including evolutionary insights.

Transcriptome analysis of diploid and triploid Populus tomentosa

Triploid Chinese white poplar (Populus tomentosa Carr., Salicaceae) has stronger advantages in growth and better stress resistance and wood quality than diploid P. tomentosa. Using transcriptome sequencing technology to identify candidate transcriptome-based markers for growth vigor in young tree tissue is of great significance for the breeding of P. tomentosa varieties in the future. In this study, the cuttings of diploid and triploid P. tomentosa were used as plant materials, transcriptome sequencing was carried out, and their tissue culture materials were used for RT-qPCR verification of the expression of genes. The results showed that 12,240 differentially expressed genes in diploid and triploid P. tomentosa transcripts were annotated and enriched into 135 metabolic pathways. The top six pathways that enriched the most significantly different genes were plant-pathogen interaction, phenylpropanoid biosynthesis, MAPK signalling pathway-plant, ascorbate and aldarate metabolism, diterpenoid biosynthesis, and the betalain biosynthesis pathway. Ten growth-related genes were selected from pathways of plant hormone signal transduction and carbon fixation in photosynthetic organisms for RT-qPCR verification. The expression levels of MDH and CYCD3 in tissue-cultured and greenhouse planted triploid P. tomentosa were higher than those in tissue-cultured diploid P. tomentosa, which was consist ent with the TMM values calculated by transcriptome.

Enhancing the expression of ARK1 genes in poplar leads to multiple branches and transcriptomic changes

The ARBORKNOX1 (ARK1) gene is an important gene for regulating plant growth and development; however, transcriptomic responses of enhancing expression of ARK1 gene in poplar are poorly investigated. To provide insight into the gene function of the ARK1 gene in poplar, the ARK1 transgenic poplar '717' and '84 K' lines were obtained, the morphology of transgenic plants was observed, and transcriptome profiles were compared. The results showed that there were multiple branches in ARK1 transgenic seedlings compared with non-transgenic seedlings. The results of transcriptome analysis showed that there were significant differences in transcriptome profiles between the transgenic lines of '717' and '84 K', and between non-transgenic lines (CK) and transgenic plants. The real-time quantitative polymerase chain reaction (RT-qPCR) analysis confirmed the expression levels of the genes involved in the pathway of zeatin biosynthesis and brassinosteroid biosynthesis. The increase in expression levels of AHP and CYCD3 was related to multiple branches. Enhancing the expression of the ARK1 gene in poplar seedlings leads to multiple branches and transcriptomic changes.

Comparison of eight complete chloroplast genomes of the endangered Aquilaria tree species (Thymelaeaceae) and their phylogenetic relationships

Aquilaria tree species are naturally distributed in the Indomalesian region and are protected against over-exploitation. They produce a fragrant non-timber product of high economic value, agarwood. Ambiguous species delimitation and limited genetic information within Aquilaria are among the impediments to conservation efforts. In this study, we conducted comparative analysis on eight Aquilaria species complete chloroplast (cp) genomes, of which seven were newly sequenced using Illumina HiSeq X Ten platform followed by de novo assembly. Aquilaria cp genomes possess a typical quadripartite structure including gene order and genomic structure. The length of each of the cp genome is about 174 kbp and encoded between 89 and 92 proteins, 38 tRNAs, and 8 rRNAs, with 27 duplicated in the IR (inverted repeat) region. Besides, 832 repeats (forward, reverse, palindrome and complement repeats) and nine highly variable regions were also identified. The phylogenetic analysis suggests that the topology structure of Aquilaria cp genomes were well presented with strong support values based on the cp genomes data set and matches their geographic distribution pattern. In summary, the complete cp genomes will facilitate development of species-specific molecular tools to discriminate Aquilaria species and resolve the evolutionary relationships of members of the Thymelaeaceae family.

Comparative analysis of plastid genomes within the Campanulaceae and phylogenetic implications

The conflicts exist between the phylogeny of Campanulaceae based on nuclear ITS sequence and plastid markers, particularly in the subdivision of Cyanantheae (Campanulaceae). Besides, various and complicated plastid genome structures can be found in species of the Campanulaceae. However, limited availability of genomic information largely hinders the studies of molecular evolution and phylogeny of Campanulaceae. We reported the complete plastid genomes of three Cyanantheae species, compared them to eight published Campanulaceae plastomes, and shed light on a deeper understanding of the applicability of plastomes. We found that there were obvious differences among gene order, GC content, gene compositions and IR junctions of LSC/IRa. Almost all protein-coding genes and amino acid sequences showed obvious codon preferences. We identified 14 genes with highly positively selected sites and branch-site model displayed 96 sites under potentially positive selection on the three lineages of phylogenetic tree. Phylogenetic analyses showed that Cyananthus was more closely related to Codonopsis compared with Cyclocodon and also clearly illustrated the relationship among the Cyanantheae species. We also found six coding regions having high nucleotide divergence value. Hotpot regions were considered to be useful molecular markers for resolving phylogenetic relationships and species authentication of Campanulaceae.

Chloroplast (Cp) Transcriptome of P. davidiana Dode×P. bolleana Lauch provides insight into the Cp drought response and Populus Cp phylogeny

Background

Raw second-generation (2G) lignocellulosic biomass materials have the potential for development into a sustainable and renewable source of energy. Poplar is regarded as a promising 2G material (P. davidiana Dode×P. bolleana Lauch, P. bolleana, P. davidiana, P. euphratica, et al). However, their large-scale commercialization still faces many obstacles. For example, drought prevents sufficient irrigation or rainfall, which can reduce soil moisture and eventually destroy the chloroplast, the plant photosynthetic organelle. Heterosis is widely used in the production of drought-tolerant materials, such as the superior clone “Shanxinyang” selected from the offspring of Populus davidiana Dode×Populus bolleana Lauch. Because it produces good wood and is easily genetically transformed, “Shanxinyang” has become a promising material for use in tree genetics. It is also one of the most abundant biofuel plants in northern China. Understanding the genetic features of chloroplasts, the cp transcriptome and physiology is crucial to elucidating the chloroplast drought-response model.

Results

In this study, the whole genome of “Shanxinyang” was sequenced. The chloroplast genome was assembled, and chloroplast structure was analysed and compared with that of other popular plants. Chloroplast transcriptome analysis was performed under drought conditions. The total length of the “Shanxinyang” chloroplast genome was 156,190 bp, the GC content was 36.75%, and the genome was composed of four typical areas (LSC, IRa, IRb, and SSC). A total of 114 simple repeats were detected in the chloroplast genome of “Shanxinyang”. In cp transcriptome analysis, we found 161 up-regulated and 157 down-regulated genes under drought, and 9 cpDEGs was randomly selected to conduct reverse transcription (RT)–qPCR., in which the Log2 (fold change) was significantly consistent with the qPCR results. The analysis of chloroplast transcription under drought provided clues for understanding chloroplast function under drought. The phylogenetic position of “Shanxinyang” within Populus was analysed by using the chloroplast genome sequences of 23 Populus plants, showing that “Shanxinyang” belongs to Sect. Populus and is sister to Populus davidiana. Further, mVISTA analysis showed that the variation in non-coding (regulatory) regions was greater than that in coding regions, which suggests that further attention should be paid to the chloroplast in order to obtain new evolutionary or functional insights related to aspects of plant biology.

Conclusions

Our findings indicate that complex prokaryotic genome regulation occurs when processing transcripts under drought stress. The results not only offer clues for understanding the chloroplast genome and transcription features in woody plants but also serve as a basis for future molecular studies on poplar species.