The mitochondrial genome of soybean reveals complex genome structures and gene evolution at intercellular and phylogenetic levels

Assemble and comparative analysis of the mitochondrial genome of Rhododendron delavayi: Insights into phylogenetic relationships and genomic variations

Rhododendron delavayi, a notable ornamental plant primarily found in regions of China like Yunnan and Guizhou provinces, holds substantial horticultural value. To elucidate the systematic phylogenetic relationships and organelle genomic differences within R. delavayi and related Rhododendron species, we conducted sequencing and assembly of the complete mitochondrial genome of R. delavayi. The full-length mitochondrial genome of it was a singular circular molecule spanning 1,009,263 bp, comprising 53 protein-coding genes, including 18 transfer RNA (tRNA) genes, 3 ribosomal RNA (rRNA) genes, and 32 protein-coding genes. A total of 1,182 simple sequence repeats (SSRs) loci were identified in the R. delavayi mitochondrial genome, primarily consisting of single nucleotide, dinucleotide, and trinucleotide repeats. Nucleotide diversity analysis highlighted five genes (atp6, atp9, cox2, nad1, and rpl10) with the highest diversity within the mitochondrial genomes of Rhododendron genus. Comparative analysis of the mitochondrial genome of R. delavayi with those of four other Rhododendron species indicated complex rearrangements in 21 genes, including rps4, nad6, rps3, atp6, cob, atp9, nad7, among others. The mitochondrial phylogenetic tree revealed a close relationship between R. delavayi and R. decorum, forming a sister clade to R. × pulchrum and R. simsii. Furthermore, 126 plastid-to-mitochondrial gene transfers in R. delavayi were identified, ranging from 30 bp to 19,385 bp. These fragments collectively constituted 47.54 % and 9.52 % of the chloroplast and mitochondrial genomes (202,169 bp), respectively. Complex mitochondrial-to-mitochondrial transfers were also observed, with 843 identified fragments totaling 312,036 bp (30.92 % of the mitochondrial genome). Segments exceeding 10 kb may mediate homologous recombination within the mitochondrial molecules. Remarkably, our study underscores that the mitochondrial genome of R. delavayi was the largest reported within the Rhododendron genus to date. The intricate rearrangements observed in the mitochondrial genomes of Rhododendron species, alone with the identification of five potential molecular marker sites, provided valuable insights for species classification and parentage identification within the Rhododendron genus.

Complete mitochondrial genome of Agropyron cristatum reveals gene transfer and RNA editing events

BACKGROUND

As an important forage in arid and semi-arid regions, Agropyron cristatum provides livestock with exceptionally high nutritional value. Additionally, A. cristatum exhibits outstanding genetic characteristics to endure drought and disease. Therefore, rich genetic diversity serves as a cornerstone for the improvement of major food crops. The purposes of this study were to systematically describe mitogenome of A.cristatum and preliminarily analyze its internal variations.

RESULT

The A. cristatum mitogenome was a single-ring molecular structure of 381,065 bp that comprised 52 genes, including 35 protein-coding, 3 rRNA and 14 tRNA genes. Among these, two pseudoprotein-coding genes and multiple copies of tRNA genes were observed. A total of 320 repetitive sequences was found to cover more than 10% of the mitogenome (105 simple sequences, 185 dispersed and 30 tandem repeats), which led to a large number of fragment rearrangements in the mitogenome of A. cristatum. Leucine was the most frequent amino acid (n = 1087,10.8%) in the protein-coding genes of A. cristatum mitogenome, and the highest usage codon was ATG (initiation codon). The number of A/T changes at the third base of the codon was much higher than that of G/C. Among 23 PCGs, the range of Pi values is from 0.0021 to 0.0539, with an average of 0.013. Additionally, 81 RNA editing sites were predicted, which were considerably fewer than those reported in other plant mitogenomes. Most of the RNA editing site base positions were concentrated at the first and second codon bases, which were C to T transitions. Moreover, we identified 95 sequence fragments (total length of 34, 343 bp) that were transferred from the chloroplast to mitochondria genes, introns, and intergenic regions. The stability of the tRNA genes was maintained during this process. Selection pressure analysis of 23 protein-coding genes shared by 15 Poaceae plants, showed that most genes were subjected to purifying selection during evolution, whereas rps4, cob, mttB, and ccmB underwent positive selection in different plants. Finally, a phylogenetic tree was constructed based on 22 plant mitogenomes, which showed that Agropyron plants have a high degree of independent heritability in Triticeae.

CONCLUSION

The findings of this study provide new data for a better understanding of A. cristatum genes, and demonstrate that mitogenomes are suitable for the study of plant classifications, such as those of Agropyron. Moreover, it provides a reference for further exploration of the phylogenetic relationships within Agropyron species, and establishes a theoretical basis for the subsequent development and utilization of A. cristatum plant germplasm resources.

Characteristics of chloroplast and mitochondrial genomes and intracellular gene transfer in the Korean endemic shrub, Sophora koreensis Nakai (Fabaceae)

Sophora koreensis Nakai, an endemic species distributed only in the Korean Peninsula, is of great geographical, economic, and taxonomic importance. Although its complete chloroplast (cp) genome sequence has been reported, its mitochondrial (mt) genome sequence has not yet been studied. Therefore, in this study, we aimed to investigate its mt genome sequence and compare it with those reported for other Fabaceae species. Total genomic DNA was extracted from fresh S. koreensis leaves collected from natural habitats in Gangwon-do Province, South Korea. This was followed by polymerase chain reaction (PCR) amplification of cpDNA insertions in the mt genome and the detection of microsatellites and dispersed repeats in the cp and mt genomes. Finally, the cp and mt genomes of S. koreensis were compared with those reported for other Fabaceae species. The cp sequence of S. koreensis showed identical gene orders and contents as those previously reported. Only six substitutions and one deletion were detected with 99 % homology. Conversely, the complete mt genome sequence, which was 517,845 bp in length and encoded 61 genes, including 43 protein-coding, 15 transfer RNAs, and 3 ribosomal RNA genes, was considerably different from that of S. japonica in terms of gene order and composition. Further, the mt genome of S. koreensis included ca. 7 and 3 kb insertions, representing an intracellular gene transfer (IGT) event, and the regions with these insertions were determined to be originally present in the cp genome. This IGT event was also confirmed via PCR amplification. IGT events can be induced via biological gene expression control or the use of repetitive sequences, and they provide important insights into the evolutionary lineage of S. koreensis. However, further studies are needed to clarify the gene transfer mechanisms between the two organelles.

Complete mitochondrial genome of the endangered Prunus pedunculata (Prunoideae, Rosaceae) in China: characterization and phylogenetic analysis

Introduction

Prunus pedunculata (Prunoideae: Rosaceae), a relic shrub with strong resistance and multiple application values, is endangered in China. Extensive research had been devoted to gene expression, molecular markers, plastid genome analysis, and genetic background investigations of P. pedunculata. However, the mitochondrial genome of this species has not been systematically described, owing to the complexity of the plant mitogenome.

Methods

In the present research, the complete mitochondrial genome of P. pedunculata was assembled, annotated, and characterized. The genomic features, gene content and repetitive sequences were analyzed. The genomic variation and phylogenetic analysis have been extensively enumerated.

Results and discussion

The P. pedunculata mitogenome is a circular molecule with a total length of 405,855 bp and a GC content of 45.63%, which are the smallest size and highest GC content among the known Prunus mitochondrial genomes. The mitogenome of P. pedunculata encodes 62 genes, including 34 unique protein-coding genes (PCGs, excluding three possible pseudogenes), three ribosomal RNA genes, and 19 transfer RNA genes. The mitogenome is rich in repetitive sequences, counting 112 simple sequence repeats, 15 tandem repeats, and 50 interspersed repetitive sequences, with a total repeat length of 11,793 bp, accounting for 2.91% of the complete genome. Leucine (Leu) was a predominant amino acid in PCGs, with a frequency of 10.67%, whereas cysteine (Cys) and tryptophan (Trp) were the least adopted. The most frequently used codon was UUU (Phe), with a relative synonymous codon usage (RSCU) value of 1.12. Selective pressure was calculated based on 20 shared PCGs in the mitogenomes of the 32 species, most of which were subjected to purifying selection (Ka/Ks < 1), whereas ccmC and ccmFn underwent positive selection. A total of 262 potential RNA editing sites in 26 PCGs were identified. Furthermore, 56 chloroplast-derived fragments were ascertained in the mitogenome, ranging from 30 to 858 bp, and were mainly located across IGS (intergenic spacer) regions or rRNA genes. These findings verify the occurrence of intracellular gene transfer events from the chloroplast to the mitochondria. Furthermore, the phylogenetic relationship of P. pedunculata was supported by the mitogenome data of 30 other taxa of the Rosaceae family. Understanding the mitochondrial genome characteristics of P. pedunculata is of great importance to promote comprehension of its genetic background and this study provides a basis for the genetic breeding of Prunus.

The multi-kingdom microbiome of the goat gastrointestinal tract

BACKGROUND

Goat is an important livestock worldwide, which plays an indispensable role in human life by providing meat, milk, fiber, and pelts. Despite recent significant advances in microbiome studies, a comprehensive survey on the goat microbiomes covering gastrointestinal tract (GIT) sites, developmental stages, feeding styles, and geographical factors is still unavailable. Here, we surveyed its multi-kingdom microbial communities using 497 samples from ten sites along the goat GIT.

RESULTS

We reconstructed a goat multi-kingdom microbiome catalog (GMMC) including 4004 bacterial, 71 archaeal, and 7204 viral genomes and annotated over 4,817,256 non-redundant protein-coding genes. We revealed patterns of feeding-driven microbial community dynamics along the goat GIT sites which were likely associated with gastrointestinal food digestion and absorption capabilities and disease risks, and identified an abundance of large intestine-enriched genera involved in plant fiber digestion. We quantified the effects of various factors affecting the distribution and abundance of methane-producing microbes including the GIT site, age, feeding style, and geography, and identified 68 virulent viruses targeting the methane producers via a comprehensive virus-bacterium/archaea interaction network.

CONCLUSIONS

Together, our GMMC catalog provides functional insights of the goat GIT microbiota through microbiome-host interactions and paves the way to microbial interventions for better goat and eco-environmental qualities. Video Abstract.

Comparative analysis of the complete mitogenome of Geoffroea decorticans: a native tree surviving in the Atacama Desert

Chañar (Geoffroea decorticans (Gill., ex Hook. & Arn.) Burkart) has been highly significant for indigenous people in the Atacama Desert for over 3,000 years. Through evolutionary processes, the G. decorticans mitogenome likely underwent changes facilitating its adaptation to the extreme conditions of the Atacama Desert. Here, we compare the mitochondrial genome of G. decorticans with those of other Papilionoideae family species. The complete mitogenome of G. decorticans was sequenced and assembled, making it the first in the genus Geoffroea. The mitogenome contained 383,963 base pairs, consisting of 33 protein coding genes, 21 transfer RNA genes, and 3 ribosomal RNA genes. The Chañar mitogenome is relatively compact, and has two intact genes (sdh4 and nad1) which were not observed in most other species. Additionally, Chañar possessed the highest amount of mitochondrial DNA of plastid origin among angiosperm species. The phylogenetic analysis of the mitogenomes of Chañar and 12 other taxa displayed a high level of consistency in taxonomic classification, when compared to those of the plastid genome. Atp8 was subjected to positive selection, while the ccmFc and rps1 were subjected to neutral selection. This study provides valuable information regarding its ability to survive the extreme environmental conditions of the Atacama Desert.

In Situ Proteomic Analysis of Herbicide-Resistant Soybean and Hybrid Seeds via Matrix-Assisted Laser Desorption/Ionization-Mass Spectrometry Imaging

Transgenic soybean is the commercial crop with the largest cultivation area worldwide. During transgenic soybean cultivation, exogenous genes may be transferred to wild relatives through gene flow, posing unpredictable ecological risks. Accordingly, an environmental risk assessment should focus on fitness changes and underlying mechanisms in hybrids between transgenic and wild soybeans (Glycine soja). Matrix-assisted laser desorption/ionization-mass spectrometry imaging (MALDI-MSI) was used for in situ detection and imaging of protein changes in the seeds of transgenic herbicide-resistant soybean harboring epsps and pat genes, non-transgenic soybean, wild soybean, and their F₂ hybrid. Protein data clearly distinguished wild soybeans, while the F₂ seeds had protein characteristics of both parents and were distinguished from wild soybean seeds. Using UPLC-Q-TOF-MS, 22 differentially expressed proteins (DEPs) were identified, including 13 specific to wild soybean. Sucrose synthase and stress response-related DEPs were differentially expressed in parental and offspring. Differences in these may underpin the greater adaptability of the latter. MSI revealed DEP distribution in transgenic, wild, and F₂ seeds. Identifying DEPs related to fitness may elucidate mechanisms underlying fitness differences among the studied varieties. Our study shows that MALDI-MSI has the potential to become a visual method for transgenic soybean analysis.

Polymorphism analysis of the chloroplast and mitochondrial genomes in soybean

BACKGROUND

Soybean is an important protein- and oil-rich crop throughout the world. Much attention has been paid to its nuclear genome, which is bi-parentally inherited and associated with many important agronomical traits. However, less is known about the genomes of the semi-autonomous and essential organelles, chloroplasts and mitochondria, of soybean.

RESULTS

Here, through analyzing the polymorphisms of these organelles in 2580 soybean accessions including 107 wild soybeans, we found that the chloroplast genome is more variable than the mitochondrial genome in terms of variant density. Consistent with this, more haplotypes were found in the chloroplast genome (44 haplotypes) than the mitochondrial genome (30 haplotypes). These haplotypes were distributed extremely unevenly with the top two haplotypes (CT1 and CT2 for chloroplasts, MT1 and MT2 for mitochondria) accounting for nearly 70 and 18% of cultivated soybean accessions. Wild soybeans also exhibited more diversity in organelle genomes, harboring 32 chloroplast haplotypes and 19 mitochondrial haplotypes. However, only a small percentage of cultivated soybeans shared cytoplasm with wild soybeans. In particular, the two most frequent types of cytoplasm (CT1/MT1, CT2/MT2) were missing in wild soybeans, indicating that wild soybean cytoplasm has been poorly exploited during breeding. Consistent with the hypothesis that soybean originated in China, we found that China harbors the highest cytoplasmic diversity in the world. The geographical distributions of CT1-CT3 and MT1-MT3 in Northeast China were not significantly different from those in Middle and South China. Two mitochondrial polymorphism sites, p.457333 (T > C) and p.457550 (G > A), were found to be heterozygous in most soybeans, and heterozygosity appeared to be associated with the domestication of cultivated soybeans from wild soybeans, the improvement of landraces to generate elite cultivated soybeans, and the geographic adaptation of soybean.

CONCLUSIONS

The haplotypes of thousands of soybean cultivars should be helpful in evaluating the impact of cytoplasm on soybean performance and in breeding cultivars with the desired cytoplasm. Mitochondrial heterozygosity might be related to soybean adaptation, and this hypothesis needs to be further investigated.

The complete mitochondrial genome of Camellia nitidissima (Theaceae)

The mitochondrial genome of Camellia nitidissima was sequenced by Illumina and Pacbio sequencing. The results of sequences showed that a total length was 949,915 bp, and the GC content was 45.7% in assembled mitochondrial genome of C. nitidissima. 71 unigenes had been found, including 36 coding proteins and 35 non-coding proteins. Subsequently, the phylogenetic tree was built on 24 plants with the maximum-likelihood method, which had high bootstrap value and fited to the angiosperm phylogeny group classification (APG IV). The study's findings unravel the taxonomic status of C. nitidissima and benefit the evolution study.

Assembly and comparative analysis of the complete mitochondrial genome of Salix wilsonii using PacBio HiFi sequencing

Salix L. (willows) is one of the most taxonomically complex genera of flowering plants, including shrubs, tall trees, bushes, and prostrate plants. Despite the high species diversity, only five mitochondrial genomes (mitogenomes) have been released in this genus. Salix wilsonii is an important ornamental and economic willow tree in section Wilsonia of the genus Salix. In this study, the S. wilsonii mitogenome was assembled into a typical circular structure with a size of 711,456 bp using PacBio HiFi sequencing. A total of 58 genes were annotated in the S. wilsonii mitogenome, including 33 protein-coding genes (PCGs), 22 tRNAs, and 3 rRNAs. In the S. wilsonii mitogenome, four genes (mttB, nad3, nad4, and sdh4) were found to play important roles in its evolution through selection pressure analysis. Collinearity analysis of six Salix mitogenomes revealed high structural variability. To determine the evolutionary position of S. wilsonii, we conducted a phylogenetic analysis of the mitogenomes of S. wilsonii and 12 other species in the order Malpighiales. Results strongly supported the segregation of S. wilsonii and other five Salix species with 100% bootstrap support. The comparative analysis of the S. wilsonii mitogenome not only sheds light on the functional and structural features of S. wilsonii but also provides essential information for genetic studies of the genus Salix.

The Mitogenome of Sedum plumbizincicola (Crassulaceae): Insights into RNA Editing, Lateral Gene Transfer, and Phylogenetic Implications

As the largest family within the order Saxifragales, Crassulaceae contains about 34 genera with 1400 species. Mitochondria play a critical role in cellular energy production. Since the first land plant mitogenome was reported in Arabidopsis, more than 400 mitogenomic sequences have been deposited in a public database. However, no entire mitogenome data have been available for species of Crassulaceae to date. To better understand the evolutionary history of the organelles of Crassulaceae, we sequenced and performed comprehensive analyses on the mitogenome of Sedum plumbizincicola. The master mitogenomic circle is 212,159 bp in length, including 31 protein-coding genes (PCGs), 14 tRNA genes, and 3 rRNA genes. We further identified totally 508 RNA editing sites in PCGs, and demonstrated that the second codon positions of mitochondrial genes are most prone to RNA editing events. Notably, by neutrality plot analyses, we observed that the mitochondrial RNA editing events have large effects on the driving forces of plant evolution. Additionally, 4 MTPTs and 686 NUMTs were detected in the mitochondrial and nuclear genomes of S. plumbizincicola, respectively. Additionally, we conducted further analyses on gene transfer, secondary structures of mitochondrial RNAs, and phylogenetic implications. Therefore, the findings presented here will be helpful for future investigations on plant mitogenomes.

Genome sequencing and analysis uncover the regulatory elements involved in the development and oil biosynthesis of Pongamia pinnata (L.) - A potential biodiesel feedstock

Due to rapid industrialization, the consumption of petro-products has increased, while fossil fuel resources have been gradually depleted. There has been a resurgence of interest in plant-derived biofuels as a sustainable alternative to fossil fuels for the purpose of reducing greenhouse gas emissions. Pongamia pinnata L., which is also known as Millettia pinnata is an oil-yielding, leguminous tree with a large and complex genome. Despite its multiple industrial applications, this orphan tree species has inconsistent yields and a limited understanding of its functional genomics. We assessed physiological and morphological characteristics of five high-yielding pongamia accessions and deduced important yield descriptors. Furthermore, we sequenced the genome of this potential biofuel feedstock using Illumina HiSeq, NextSeq, and MiSeq platforms to generate paired-end reads. Around 173 million processed reads amounting to 65.2 Gb were assembled into a 685 Mb genome, with a gap rate of 0.02%. The sequenced scaffolds were used to identify 30,000 gene models, 406,385 Simple-Sequence-Repeat (SSR) markers, and 43.6% of repetitive sequences. We further analyzed the structural information of genes belonging to certain key metabolic pathways, including lipid metabolism, photosynthesis, circadian rhythms, plant-pathogen interactions, and karanjin biosynthesis, all of which are commercially significant for pongamia. A total of 2,219 scaffolds corresponding to 29 transcription factor families provided valuable information about gene regulation in pongamia. Similarity studies and phylogenetic analysis revealed a monophyletic group of Fabaceae members wherein pongamia out-grouped from Glycine max and Cajanus cajan, revealing its unique ability to synthesize oil for biodiesel. This study is the first step toward completing the genome sequence of this imminent biofuel tree species. Further attempts at re-sequencing with different read chemistry will certainly improve the genetic resources at the chromosome level and accelerate the molecular breeding programs.

Assembly and comparative analysis of the first complete mitochondrial genome of Acer truncatum Bunge: a woody oil-tree species producing nervonic acid

BACKGROUND

Acer truncatum (purpleblow maple) is a woody tree species that produces seeds with high levels of valuable fatty acids (especially nervonic acid). The species is admired as a landscape plant with high developmental prospects and scientific research value. The A. truncatum chloroplast genome has recently been reported; however, the mitochondrial genome (mitogenome) is still unexplored.

RESULTS

We characterized the A. truncatum mitogenome, which was assembled using reads from PacBio and Illumina sequencing platforms, performed a comparative analysis against different species of Acer. The circular mitogenome of A. truncatum has a length of 791,052 bp, with a base composition of 27.11% A, 27.21% T, 22.79% G, and 22.89% C. The A. truncatum mitogenome contains 62 genes, including 35 protein-coding genes, 23 tRNA genes and 4 rRNA genes. We also examined codon usage, sequence repeats, RNA editing and selective pressure in the A. truncatum mitogenome. To determine the evolutionary and taxonomic status of A. truncatum, we conducted a phylogenetic analysis based on the mitogenomes of A. truncatum and 25 other taxa. In addition, the gene migration from chloroplast and nuclear genomes to the mitogenome were analyzed. Finally, we developed a novel NAD1 intron indel marker for distinguishing several Acer species.

CONCLUSIONS

In this study, we assembled and annotated the mitogenome of A. truncatum, a woody oil-tree species producing nervonic acid. The results of our analyses provide comprehensive information on the A. truncatum mitogenome, which would facilitate evolutionary research and molecular barcoding in Acer.

Structural variation of mitochondrial genomes sheds light on evolutionary history of soybeans

The architecture and genetic diversity of mitogenome (mtDNA) are largely unknown in cultivated soybean (Glycine max), which is domesticated from the wild progenitor, Glycine soja, 5000 years ago. Here, we de novo assembled the mitogenome of the cultivar 'Williams 82' (Wm82_mtDNA) with Illumina PE300 deep sequencing data, and verified it with polymerase chain reaction (PCR) and Southern blot analyses. Wm82_mtDNA maps as two autonomous circular chromosomes (370 871-bp Chr-m1 and 62 661-bp Chr-m2). Its structure is extensively divergent from that of the mono-chromosomal mitogenome reported in the landrace 'Aiganhuang' (AGH_mtDNA). Synteny analysis showed that the structural variations (SVs) between two genomes are mainly attributed to ectopic and illegitimate recombination. Moreover, Wm82_mtDNA and AGH_mtDNA each possess six and four specific regions, which are absent in their counterparts and likely result from differential sequence-loss events. Mitogenome SV was further studied in 39 wild and 182 cultivated soybean accessions distributed world-widely with PCR/Southern analyses or a comparable in silico analysis. The results classified both wild and cultivated soybeans into five cytoplasmic groups, named as GSa-GSe and G1-G5; 'Williams 82' and 'Aiganhuang' belong to G1 and G5, respectively. Notably, except for members in GSe and G5, all accessions carry a bi-chromosomal mitogenome with a common Chr-m2. Phylogenetic analyses based on mtDNA structures and chloroplast gene sequences both inferred that G1-G3, representing >90% of cultigens, likely inherited cytoplasm from the ancestor of domestic soybean, while G4 and G5 likely inherited cytoplasm from wild soybeans carrying GSa- and GSe-like cytoplasm through interspecific hybridization, offering new insights into soybean cultivation history.

Insights into molecular structure, genome evolution and phylogenetic implication through mitochondrial genome sequence of Gleditsia sinensis

Gleditsia sinensis is an endemic species widely distributed in China with high economic and medicinal value. To explore the genomic evolution and phylogenetic relationships of G. sinensis, the complete mitochondrial (mt) genome of G. sinensis was sequenced and assembled, which was firstly reported in Gleditsia. The mt genome was circular and 594,121 bp in length, including 37 protein-coding genes (PCGs), 19 transfer RNA (tRNA) genes and 3 ribosomal RNA (rRNA) genes. The overall base composition of the G. sinensis mt genome was 27.4% for A, 27.4% for T, 22.6% for G, 22.7% for C. The comparative analysis of PCGs in Fabaceae species showed that most of the ribosomal protein genes and succinate dehydrogenase genes were lost. In addition, we found that the rps4 gene was only lost in G. sinensis, whereas it was retained in other Fabaceae species. The phylogenetic analysis based on shared PCGs of 24 species (22 Fabaceae and 2 Solanaceae) showed that G. sinensis is evolutionarily closer to Senna species. In general, this research will provide valuable information for the evolution of G. sinensis and provide insight into the phylogenetic relationships within the family Fabaceae.

Structural Mutations in the Organellar Genomes of Valeriana sambucifolia f. dageletiana (Nakai. ex Maekawa) Hara Show Dynamic Gene Transfer

Valeriana sambucifolia f. dageletiana (Nakai. ex Maekawa) Hara is a broad-leaved valerian endemic to Ulleung Island, a noted hot spot of endemism in Korea. However, despite its widespread pharmacological use, this plant remains comparatively understudied. Plant cells generally contain two types of organellar genomes (the plastome and the mitogenome) that have undergone independent evolution, which accordingly can provide valuable information for elucidating the phylogenetic relationships and evolutionary histories of terrestrial plants. Moreover, the extensive mega-data available for plant genomes, particularly those of plastomes, can enable researchers to gain an in-depth understanding of the transfer of genes between different types of genomes. In this study, we analyzed two organellar genomes (the 155,179 bp plastome and the 1,187,459 bp mitogenome) of V. sambucifolia f. dageletiana and detected extensive changes throughout the plastome sequence, including rapid structural mutations associated with inverted repeat (IR) contraction and genetic variation. We also described features characterizing the first reported mitogenome sequence obtained for a plant in the order Dipsacales and confirmed frequent gene transfer in this mitogenome. We identified eight non-plastome-originated regions (NPRs) distributed within the plastome of this endemic plant, for six of which there were no corresponding sequences in the current nucleotide sequence databases. Indeed, one of these unidentified NPRs unexpectedly showed certain similarities to sequences from bony fish. Although this is ostensibly difficult to explain, we suggest that this surprising association may conceivably reflect the occurrence of gene transfer from a bony fish to the plastome of an ancestor of V. sambucifolia f. dageletiana mediated by either fungi or bacteria.

Assembly and comparative analysis of the complete mitochondrial genome of Suaeda glauca

Background

Suaeda glauca (S. glauca) is a halophyte widely distributed in saline and sandy beaches, with strong saline-alkali tolerance. It is also admired as a landscape plant with high development prospects and scientific research value. The S. glauca chloroplast (cp) genome has recently been reported; however, the mitochondria (mt) genome is still unexplored.

Results

The mt genome of S. glauca were assembled based on the reads from Pacbio and Illumina sequencing platforms. The circular mt genome of S. glauca has a length of 474,330 bp. The base composition of the S. glauca mt genome showed A (28.00%), T (27.93%), C (21.62%), and G (22.45%). S. glauca mt genome contains 61 genes, including 27 protein-coding genes, 29 tRNA genes, and 5 rRNA genes. The sequence repeats, RNA editing, and gene migration from cp to mt were observed in S. glauca mt genome. Phylogenetic analysis based on the mt genomes of S. glauca and other 28 taxa reflects an exact evolutionary and taxonomic status of S. glauca. Furthermore, the investigation on mt genome characteristics, including genome size, GC contents, genome organization, and gene repeats of S. gulaca genome, was investigated compared to other land plants, indicating the variation of the mt genome in plants. However, the subsequently Ka/Ks analysis revealed that most of the protein-coding genes in mt genome had undergone negative selections, reflecting the importance of those genes in the mt genomes.

Conclusions

In this study, we reported the mt genome assembly and annotation of a halophytic model plant S. glauca. The subsequent analysis provided us a comprehensive understanding of the S. glauca mt genome, which might facilitate the research on the salt-tolerant plant species.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07490-9.

New Insights into Plant Extracellular DNA. A Study in Soybean Root Extracellular Trap

exDNA is found in various organisms, including plants. However, plant exDNA has thus far received little attention related to its origin and role in the RET (root extracellular trap). In this study, we performed the first high-throughput genomic sequencing of plant exDNA from a Fabaceae with worldwide interest: soybean (Glycine max (L.) Merr.). The origin of this exDNA was first investigated in control condition, and the results show high-coverage on organelles (mitochondria/plastid) DNA relative to nuclear DNA, as well as a mix of coding and non-coding sequences. In the second part of this study, we investigated if exDNA release was modified during an elicitation with PEP-13 (a peptide elicitor from oomycete genus Phytophthora). Our results show that treatment of roots with PEP-13 does not affect the composition of exDNA.

Differential RNA Editing and Intron Splicing in Soybean Mitochondria during Nodulation

Nitrogen fixation in soybean consumes a tremendous amount of energy, leading to substantial differences in energy metabolism and mitochondrial activities between nodules and uninoculated roots. While C-to-U RNA editing and intron splicing of mitochondrial transcripts are common in plant species, their roles in relation to nodule functions are still elusive. In this study, we performed RNA-seq to compare transcript profiles and RNA editing of mitochondrial genes in soybean nodules and roots. A total of 631 RNA editing sites were identified on mitochondrial transcripts, with 12% or 74 sites differentially edited among the transcripts isolated from nodules, stripped roots, and uninoculated roots. Eight out of these 74 differentially edited sites are located on the matR transcript, of which the degrees of RNA editing were the highest in the nodule sample. The degree of mitochondrial intron splicing was also examined. The splicing efficiencies of several introns in nodules and stripped roots were higher than in uninoculated roots. These include nad1 introns 2/3/4, nad4 intron 3, nad5 introns 2/3, cox2 intron 1, and ccmFc intron 1. A greater splicing efficiency of nad4 intron 1, a higher NAD4 protein abundance, and a reduction in supercomplex I + III2 were also observed in nodules, although the causal relationship between these observations requires further investigation.

Characterization and Analysis of the Mitochondrial Genome of Common Bean (Phaseolus vulgaris) by Comparative Genomic Approaches

The common bean (Phaseolus vulgaris) is a major source of protein and essential nutrients for humans. To explore the genetic diversity and phylogenetic relationships of P. vulgaris, its complete mitochondrial genome (mitogenome) was sequenced and assembled. The mitogenome is 395,516 bp in length, including 31 unique protein-coding genes (PCGs), 15 transfer RNA (tRNA) genes, and 3 ribosomal RNA (rRNA) genes. Among the 31 PCGs, four genes (mttB, nad1, nad4L, and rps10) use ACG as initiation codons, which are altered to standard initiation codons by RNA editing. In addition, the termination codon CGA in the ccmFC gene is converted to UGA. Selective pressure analysis indicates that the ccmB, ccmFC, rps1, rps10, and rps14 genes were under evolutionary positive selection. The proportions of five amino acids (Phe, Leu, Pro, Arg, and Ser) in the whole amino acid profile of the proteins in each mitogenome can be used to distinguish angiosperms from gymnosperms. Phylogenetic analyses show that P. vulgaris is evolutionarily closer to the Glycininae than other leguminous plants. The results of the present study not only provide an important opportunity to conduct further genomic breeding studies in the common bean, they also provide valuable information for future evolutionary and molecular studies of leguminous plants.

The Development of Highly Specific and Sensitive Primers for the Detection of Potentially Allergenic Soybean (Glycine max) Using Loop-Mediated Isothermal Amplification Combined with Lateral Flow Dipstick (LAMP-LFD)

The soybean (Glycine max) has been recognized as a frequent elicitor of food allergy worldwide. A lack of causative immunotherapy of soybean allergy makes soybean avoidance essential. Therefore, sensitive and specific methods for soybean detection are needed to allow for soybean verification in foods. Loop-mediated isothermal amplification (LAMP) represents a rapid and simple DNA-based detection method principally suitable for field-like applications or on-site analytical screening for allergens during the manufacturing of foods. This work describes the systematic development and selection of suitable LAMP primers based on soybean multicopy genes. The chemistry applied allows for a versatile detection of amplified DNA, using either gel electrophoresis, fluorescence recording, or a simple Lateral Flow Dipstick (LFD). LAMP based on the ORF160b gene was highly specific for the soybean and may allow for a detection level equivalent to approximately 10 mg soy per kg food. Various soybean cultivars were detectable at a comparable level of sensitivity. LAMP combined with LFD-like detection facilitates a simple, highly specific and sensitive detection of the soybean without the need for expensive analytical equipment. In contrast to the majority of antibody-based methods for soybean detection, all identified primer sequences and optimized protocols are disclosed and broadly available to the community.

Intergenomic gene transfer in diploid and allopolyploid Gossypium

BACKGROUND

Intergenomic gene transfer (IGT) between nuclear and organellar genomes is a common phenomenon during plant evolution. Gossypium is a useful model to evaluate the genomic consequences of IGT for both diploid and polyploid species. Here, we explore IGT among nuclear, mitochondrial, and plastid genomes of four cotton species, including two allopolyploids and their model diploid progenitors (genome donors, G. arboreum: A₂ and G. raimondii: D₅).

RESULTS

Extensive IGT events exist for both diploid and allotetraploid cotton (Gossypium) species, with the nuclear genome being the predominant recipient of transferred DNA followed by the mitochondrial genome. The nuclear genome has integrated 100 times more foreign sequences than the mitochondrial genome has in total length. In the nucleus, the integrated length of chloroplast DNA (cpDNA) was between 1.87 times (in diploids) to nearly four times (in allopolyploids) greater than that of mitochondrial DNA (mtDNA). In the mitochondrion, the length of nuclear DNA (nuDNA) was typically three times than that of cpDNA. Gossypium mitochondrial genomes integrated three nuclear retrotransposons and eight chloroplast tRNA genes, and incorporated chloroplast DNA prior to divergence between the diploids and allopolyploid formation. For mitochondrial chloroplast-tRNA genes, there were 2-6 bp conserved microhomologies flanking their insertion sites across distantly related genera, which increased to 10 bp microhomologies for the four cotton species studied. For organellar DNA sequences, there are source hotspots, e.g., the atp6-trnW intergenic region in the mitochondrion and the inverted repeat region in the chloroplast. Organellar DNAs in the nucleus were rarely expressed, and at low levels. Surprisingly, there was asymmetry in the survivorship of ancestral insertions following allopolyploidy, with most numts (nuclear mitochondrial insertions) decaying or being lost whereas most nupts (nuclear plastidial insertions) were retained.

CONCLUSIONS

This study characterized and compared intracellular transfer among nuclear and organellar genomes within two cultivated allopolyploids and their ancestral diploid cotton species. A striking asymmetry in the fate of IGTs in allopolyploid cotton was discovered, with numts being preferentially lost relative to nupts. Our results connect intergenomic gene transfer with allotetraploidy and provide new insight into intracellular genome evolution.

Fluctuations in Fabaceae mitochondrial genome size and content are both ancient and recent

BACKGROUND

Organelle genome studies of Fabaceae, an economically and ecologically important plant family, have been biased towards the plastid genome (plastome). Thus far, less than 15 mitochondrial genome (mitogenome) sequences of Fabaceae have been published, all but four of which belong to the subfamily Papilionoideae, limiting the understanding of size variation and content across the family. To address this, four mitogenomes were sequenced and assembled from three different subfamilies (Cercidoideae, Detarioideae and Caesalpinioideae).

RESULTS

Phylogenetic analysis based on shared mitochondrial protein coding regions produced a fully resolved and well-supported phylogeny that was completely congruent with the plastome tree. Comparative analyses suggest that two kinds of mitogenome expansions have occurred in Fabaceae. Size expansion of four genera (Tamarindus, Libidibia, Haematoxylum, and Leucaena) in two subfamilies (Detarioideae and Caesalpinioideae) occurred in relatively deep nodes, and was mainly caused by intercellular gene transfer and/or interspecific horizontal gene transfer (HGT). The second, more recent expansion occurred in the Papilionoideae as a result of duplication of native mitochondrial sequences. Family-wide gene content analysis revealed 11 gene losses, four (rps2, 7, 11 and 13) of which occurred in the ancestor of Fabaceae. Losses of the remaining seven genes (cox2, rpl2, rpl10, rps1, rps19, sdh3, sdh4) were restricted to specific lineages or occurred independently in different clades. Introns of three genes (cox2, ccmFc and rps10) showed extensive lineage-specific length variation due to large sequence insertions and deletions. Shared DNA analysis among Fabaceae mitogenomes demonstrated a substantial decay of intergenic spacers and provided further insight into HGT between the mimosoid clade of Caesalpinioideae and the holoparasitic Lophophytum (Balanophoraceae).

CONCLUSION

This study represents the most exhaustive analysis of Fabaceae mitogenomes so far, and extends the understanding the dynamic variation in size and gene/intron content. The four newly sequenced mitogenomes reported here expands the phylogenetic coverage to four subfamilies. The family has experienced multiple mitogenome size fluctuations in both ancient and recent times. The causes of these size variations are distinct in different lineages. Fabaceae mitogenomes experienced extensive size fluctuation by recruitment of exogenous DNA and duplication of native mitochondrial DNA.

Understanding the Complexity of Cold Tolerance in White Clover using Temperature Gradient Locations and a GWAS Approach

White clover ( L.) is the most important grazing perennial forage legume in temperate climates. However, its limited capacity to survive and restore growth after low temperatures during winter constrains the productivity and wide adoption of the crop. Despite the importance of cold tolerance for white clover cultivar development, the genetic basis of this trait remains largely unknown. Hence, in this study, we performed the first genome-wide association study (GWAS) analyses in white clover to identify quantitative trait loci (QTL) for cold-tolerance-related traits. Seeds from 192 divergent genotypes from six populations in the Patagonia region of South America were collected and seed-derived plants were further clonally propagated. Clonal trials were established in three locations representing temperature gradient associated with elevation. Given the allotetraploid nature of the white clover genome, distinct genetic models (diploid and tetraploid) were tested. Only the tetraploid parameterization was able to detect the 53 loci associated with cold-tolerance traits. Out of the 53 single nucleotide polymorphism (SNP) trait associations, 17 controlled more than one trait or were stable across multiple sites. This work represents the first report of QTL for cold-tolerance-related traits, providing insights into its genetic basis and candidate genomic regions for further functional validation studies.

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.

PacBio-Based Mitochondrial Genome Assembly of Leucaena trichandra (Leguminosae) and an Intrageneric Assessment of Mitochondrial RNA Editing

Reconstructions of vascular plant mitochondrial genomes (mt-genomes) are notoriously complicated by rampant recombination that has resulted in comparatively few plant mt-genomes being available. The dearth of plant mitochondrial resources has limited our understanding of mt-genome structural diversity, complex patterns of RNA editing, and the origins of novel mt-genome elements. Here, we use an efficient long read (PacBio) iterative assembly pipeline to generate mt-genome assemblies for Leucaena trichandra (Leguminosae: Caesalpinioideae: mimosoid clade), providing the first assessment of non-papilionoid legume mt-genome content and structure to date. The efficiency of the assembly approach facilitated the exploration of alternative structures that are common place among plant mitochondrial genomes. A compact version (729 kbp) of the recovered assemblies was used to investigate sources of mt-genome size variation among legumes and mt-genome sequence similarity to the legume associated root holoparasite Lophophytum. The genome and an associated suite of transcriptome data from select species of Leucaena permitted an in-depth exploration of RNA editing in a diverse clade of closely related species that includes hybrid lineages. RNA editing in the allotetraploid, Leucaena leucocephala, is consistent with co-option of nearly equal maternal and paternal C-to-U edit components, generating novel combinations of RNA edited sites. A preliminary investigation of L. leucocephala C-to-U edit frequencies identified the potential for a hybrid to generate unique pools of alleles from parental variation through edit frequencies shared with one parental lineage, those intermediate between parents, and transgressive patterns.

Complete sequence of kenaf (Hibiscus cannabinus) mitochondrial genome and comparative analysis with the mitochondrial genomes of other plants

Plant mitochondrial (mt) genomes are species specific due to the vast of foreign DNA migration and frequent recombination of repeated sequences. Sequencing of the mt genome of kenaf (Hibiscus cannabinus) is essential for elucidating its evolutionary characteristics. In the present study, single-molecule real-time sequencing technology (SMRT) was used to sequence the complete mt genome of kenaf. Results showed that the complete kenaf mt genome was 569,915 bp long and consisted of 62 genes, including 36 protein-coding, 3 rRNA and 23 tRNA genes. Twenty-five introns were found among nine of the 36 protein-coding genes, and five introns were trans-spliced. A comparative analysis with other plant mt genomes showed that four syntenic gene clusters were conserved in all plant mtDNAs. Fifteen chloroplast-derived fragments were strongly associated with mt genes, including the intact sequences of the chloroplast genes psaA, ndhB and rps7. According to the plant mt genome evolution analysis, some ribosomal protein genes and succinate dehydrogenase genes were frequently lost during the evolution of angiosperms. Our data suggest that the kenaf mt genome retained evolutionarily conserved characteristics. Overall, the complete sequencing of the kenaf mt genome provides additional information and enhances our better understanding of mt genomic evolution across angiosperms.

Assembly and comparative analysis of the complete mitochondrial genome sequence of Sophora japonica 'JinhuaiJ2'

Sophora japonica L. (Faboideae, Leguminosae) is an important traditional Chinese herb with a long history of cultivation. Its flower buds and fruits contain abundant flavonoids, and therefore, the plants are cultivated for the industrial extraction of rutin. Here, we determined the complete nucleotide sequence of the mitochondrial genome of S. japonica ‘JinhuaiJ2’, the most widely planted variety in Guangxi region of China. The total length of the mtDNA sequence is 484,916 bp, with a GC content of 45.4%. Sophora japonica mtDNA harbors 32 known protein-coding genes, 17 tRNA genes, and three rRNA genes with 17 cis-spliced and five trans-spliced introns disrupting eight protein-coding genes. The gene coding and intron regions, and intergenic spacers account for 7.5%, 5.8% and 86.7% of the genome, respectively. The gene profile of S. japonica mitogenome differs from that of the other Faboideae species by only one or two gene gains or losses. Four of the 17 cis-spliced introns showed distinct length variations in the Faboideae, which could be attributed to the homologous recombination of the short repeats measuring a few bases located precisely at the edges of the putative deletions. This reflects the importance of small repeats in the sequence evolution in Faboideae mitogenomes. Repeated sequences of S. japonica mitogenome are mainly composed of small repeats, with only 20 medium-sized repeats, and one large repeat, adding up to 4% of its mitogenome length. Among the 25 pseudogene fragments detected in the intergenic spacer regions, the two largest ones and their corresponding functional gene copies located in two different sets of medium-sized repeats, point to their origins from homologous recombinations. As we further observed the recombined reads associated with the longest repeats of 2,160 bp with the PacBio long read data set of just 15 × in depth, repeat mediated homologous recombinations may play important role in the mitogenomic evolution of S. japonica. Our study provides insightful knowledge to the genetic background of this important herb species and the mitogenomic evolution in the Faboideae species.

The first complete mitochondrial genome of wild soybean (Glycine soja)

Glycine soja (wild soybean) is known as the wild progenitor of cultivated soybean (G. max), representing a valuable genetic resource for soybean breeding programmers. In this study, we determined the complete mitochondrial genome of G. soja. The results revealed that G. soja mt genome is 402,545 bp with 45% GC contents. Furthermore, based on entire mitochondrial genome the evolutionary relationship and phylogenetic analysis revealed that G. soja closely related to G. max.

Ribosome profiling reveals changes in translational status of soybean transcripts during immature cotyledon development

To understand translational capacity on a genome-wide scale across three developmental stages of immature soybean seed cotyledons, ribosome profiling was performed in combination with RNA sequencing and cluster analysis. Transcripts representing 216 unique genes demonstrated a higher level of translational activity in at least one stage by exhibiting higher translational efficiencies (TEs) in which there were relatively more ribosome footprint sequence reads mapping to the transcript than were present in the control total RNA sample. The majority of these transcripts were more translationally active at the early stage of seed development and included 12 unique serine or cysteine proteases and 16 2S albumin and low molecular weight cysteine-rich proteins that may serve as substrates for turnover and mobilization early in seed development. It would appear that the serine proteases and 2S albumins play a vital role in the early stages. In contrast, our investigation of profiles of 19 genes encoding high abundance seed storage proteins, such as glycinins, beta-conglycinins, lectin, and Kunitz trypsin inhibitors, showed that they all had similar patterns in which the TE values started at low levels and increased approximately 2 to 6-fold during development. The highest levels of these seed protein transcripts were found at the mid-developmental stage, whereas the highest ribosome footprint levels of only up to 1.6 TE were found at the late developmental stage. These experimental findings suggest that the major seed storage protein coding genes are primarily regulated at the transcriptional level during normal soybean cotyledon development. Finally, our analyses also identified a total of 370 unique gene models that showed very low TE values including over 48 genes encoding ribosomal family proteins and 95 gene models that are related to energy and photosynthetic functions, many of which have homology to the chloroplast genome. Additionally, we showed that genes of the chloroplast were relatively translationally inactive during seed development.

Genome-wide comparative analysis of DNA methylation between soybean cytoplasmic male-sterile line NJCMS5A and its maintainer NJCMS5B

BACKGROUND

DNA methylation is an important epigenetic modification. It can regulate the expression of many key genes without changing the primary structure of the genomic DNA, and plays a vital role in the growth and development of the organism. The genome-wide DNA methylation profile of the cytoplasmic male sterile (CMS) line in soybean has not been reported so far.

RESULTS

In this study, genome-wide comparative analysis of DNA methylation between soybean CMS line NJCMS5A and its maintainer NJCMS5B was conducted by whole-genome bisulfite sequencing. The results showed 3527 differentially methylated regions (DMRs) and 485 differentially methylated genes (DMGs), including 353 high-credible methylated genes, 56 methylated genes coding unknown protein and 76 novel methylated genes with no known function were identified. Among them, 25 DMRs were further validated that the genome-wide DNA methylation data were reliable through bisulfite treatment, and 9 DMRs were confirmed the relationship between DNA methylation and gene expression by qRT-PCR. Finally, 8 key DMGs possibly associated with soybean CMS were identified.

CONCLUSIONS

Genome-wide DNA methylation profile of the soybean CMS line NJCMS5A and its maintainer NJCMS5B was obtained for the first time. Several specific DMGs which participated in pollen and flower development were further identified to be probably associated with soybean CMS. This study will contribute to further understanding of the molecular mechanism behind soybean CMS.

Unparalleled replacement of native mitochondrial genes by foreign homologs in a holoparasitic plant

Horizontal gene transfer (HGT) among flowering plant mitochondria occurs frequently and, in most cases, leads to nonfunctional transgenes in the recipient genome. Parasitic plants are particularly prone to this phenomenon, but their mitochondrial genomes (mtDNA) have been largely unexplored. We undertook a large-scale mitochondrial genomic study of the holoparasitic plant Lophophytum mirabile (Balanophoraceae). Comprehensive phylogenetic analyses were performed to address the frequency, origin, and impact of HGT. The sequencing of the complete mtDNA of L. mirabile revealed the unprecedented acquisition of host-derived mitochondrial genes, representing 80% of the protein-coding gene content. All but two of these foreign genes replaced the native homologs and are probably functional in energy metabolism. The genome consists of 54 circular-mapping chromosomes, 25 of which carry no intact genes. The likely functional replacement of up to 26 genes in L. mirabile represents a stunning example of the potential effect of rampant HGT on plant mitochondria. The use of host-derived genes may have a positive effect on the host-parasite relationship, but could also be the result of nonadaptive forces.

Assembly and comparative analysis of complete mitochondrial genome sequence of an economic plant Salix suchowensis

Willow is a widely used dioecious woody plant of Salicaceae family in China. Due to their high biomass yields, willows are promising sources for bioenergy crops. In this study, we assembled the complete mitochondrial (mt) genome sequence of S. suchowensis with the length of 644,437 bp using Roche-454 GS FLX Titanium sequencing technologies. Base composition of the S. suchowensis mt genome is A (27.43%), T (27.59%), C (22.34%), and G (22.64%), which shows a prevalent GC content with that of other angiosperms. This long circular mt genome encodes 58 unique genes (32 protein-coding genes, 23 tRNA genes and 3 rRNA genes), and 9 of the 32 protein-coding genes contain 17 introns. Through the phylogenetic analysis of 35 species based on 23 protein-coding genes, it is supported that Salix as a sister to Populus. With the detailed phylogenetic information and the identification of phylogenetic position, some ribosomal protein genes and succinate dehydrogenase genes are found usually lost during evolution. As a native shrub willow species, this worthwhile research of S. suchowensis mt genome will provide more desirable information for better understanding the genomic breeding and missing pieces of sex determination evolution in the future.

Assembly and analysis of the complete Salix purpurea L. (Salicaceae) mitochondrial genome sequence

Plant mitochondrial (mt) genomes possess several complex features, including a variable size, a dynamic genome structure, and complicated patterns of gene loss and gain throughout evolutionary history. Studies of plant mt genomes can, therefore, provide unique insights into organelle evolution. We assembled the complete Salix purpurea L. mt genome by screening genomic sequence reads generated by a Roche-454 pyrosequencing platform. The pseudo-molecule obtained has a typical circular structure 598,970 bp long, with an overall GC content of 55.06%. The S. purpurea mt genome contains 52 genes: 31 protein-coding, 18 tRNAs, and three rRNAs. Eighteen tandem repeats and 404 microsatellites are distributed unevenly throughout the S. purpurea mt genome. A phylogenetic tree of 23 representative terrestrial plants strongly supports S. purpurea inclusion in the Malpighiales clade. Our analysis contributes toward understanding the organization and evolution of organelle genomes in Salicaceae species.

Analysis of the Complete Mitochondrial Genome Sequence of the Diploid Cotton Gossypium raimondii by Comparative Genomics Approaches

Cotton is one of the most important economic crops and the primary source of natural fiber and is an important protein source for animal feed. The complete nuclear and chloroplast (cp) genome sequences of G. raimondii are already available but not mitochondria. Here, we assembled the complete mitochondrial (mt) DNA sequence of G. raimondii into a circular genome of length of 676,078 bp and performed comparative analyses with other higher plants. The genome contains 39 protein-coding genes, 6 rRNA genes, and 25 tRNA genes. We also identified four larger repeats (63.9 kb, 10.6 kb, 9.1 kb, and 2.5 kb) in this mt genome, which may be active in intramolecular recombination in the evolution of cotton. Strikingly, nearly all of the G. raimondii mt genome has been transferred to nucleus on Chr1, and the transfer event must be very recent. Phylogenetic analysis reveals that G. raimondii, as a member of Malvaceae, is much closer to another cotton (G. barbadense) than other rosids, and the clade formed by two Gossypium species is sister to Brassicales. The G. raimondii mt genome may provide a crucial foundation for evolutionary analysis, molecular biology, and cytoplasmic male sterility in cotton and other higher plants.

Analysis of Nuclear Mitochondrial DNA Segments of Nine Plant Species: Size, Distribution, and Insertion Loci

Nuclear mitochondrial DNA segment (Numt) insertion describes a well-known phenomenon of mitochondrial DNA transfer into a eukaryotic nuclear genome. However, it has not been well understood, especially in plants. Numt insertion patterns vary from species to species in different kingdoms. In this study, the patterns were surveyed in nine plant species, and we found some tip-offs. First, when the mitochondrial genome size is relatively large, the portion of the longer Numt is also larger than the short one. Second, the whole genome duplication event increases the ratio of the shorter Numt portion in the size distribution. Third, Numt insertions are enriched in exon regions. This analysis may be helpful for understanding plant evolution.

The Whole Genome Assembly and Comparative Genomic Research of Thellungiella parvula (Extremophile Crucifer) Mitochondrion

The complete nucleotide sequences of the mitochondrial (mt) genome of an extremophile species Thellungiella parvula (T. parvula) have been determined with the lengths of 255,773 bp. T. parvula mt genome is a circular sequence and contains 32 protein-coding genes, 19 tRNA genes, and three ribosomal RNA genes with a 11.5% coding sequence. The base composition of 27.5% A, 27.5% T, 22.7% C, and 22.3% G in descending order shows a slight bias of 55% AT. Fifty-three repeats were identified in the mitochondrial genome of T. parvula, including 24 direct repeats, 28 tandem repeats (TRs), and one palindromic repeat. Furthermore, a total of 199 perfect microsatellites have been mined with a high A/T content (83.1%) through simple sequence repeat (SSR) analysis and they were distributed unevenly within this mitochondrial genome. We also analyzed other plant mitochondrial genomes' evolution in general, providing clues for the understanding of the evolution of organelles genomes in plants. Comparing with other Brassicaceae species, T. parvula is related to Arabidopsis thaliana whose characters of low temperature resistance have been well documented. This study will provide important genetic tools for other Brassicaceae species research and improve yields of economically important plants.

Mitochondrial Genome Analysis of Wild Rice (Oryza minuta) and Its Comparison with Other Related Species

Oryza minuta (Poaceae family) is a tetraploid wild relative of cultivated rice with a BBCC genome. O. minuta has the potential to resist against various pathogenic diseases such as bacterial blight (BB), white backed planthopper (WBPH) and brown plant hopper (BPH). Here, we sequenced and annotated the complete mitochondrial genome of O. minuta. The mtDNA genome is 515,022 bp, containing 60 protein coding genes, 31 tRNA genes and two rRNA genes. The mitochondrial genome organization and the gene content at the nucleotide level are highly similar (89%) to that of O. rufipogon. Comparison with other related species revealed that most of the genes with known function are conserved among the Poaceae members. Similarly, O. minuta mt genome shared 24 protein-coding genes, 15 tRNA genes and 1 ribosomal RNA gene with other rice species (indica and japonica). The evolutionary relationship and phylogenetic analysis revealed that O. minuta is more closely related to O. rufipogon than to any other related species. Such studies are essential to understand the evolutionary divergence among species and analyze common gene pools to combat risks in the current scenario of a changing environment.

The complete mitochondrial genome of Medicago truncatula

The complete mitochondrial genome of Medicago truncatula (M. truncatula) was reported in this study. The mitochondrial genome (mitogenome) was assembled to 271 618 nt. The mitogenome contains 31 protein-coding genes, three rRNA genes and 16 tRNAs. The overall base composition of the mitogenome in descending order is A: 27.21%, C: 22.61%, G: 22.78% and T: 27.40%, and the G + C content is 45.39%. Additionally, 30 exons and 17 introns were identified in eight genes and nine tandem repeats were identified with the period size from 10 nt to 33 nt. Phylogenetic analysis shows that the M. truncatula genome is evolutionarily closest to that of Lotus japonicas. With the complete mitogenome of M. truncatula, it is beneficial to the further research of mitogenome of seed plants, and especially helpful for elucidating vital activities of legumes.

Transcriptome-Wide Identification of Reference Genes for Expression Analysis of Soybean Responses to Drought Stress along the Day

The soybean transcriptome displays strong variation along the day in optimal growth conditions and also in response to adverse circumstances, like drought stress. However, no study conducted to date has presented suitable reference genes, with stable expression along the day, for relative gene expression quantification in combined studies on drought stress and diurnal oscillations. Recently, water deficit responses have been associated with circadian clock oscillations at the transcription level, revealing the existence of hitherto unknown processes and increasing the demand for studies on plant responses to drought stress and its oscillation during the day. We performed data mining from a transcriptome-wide background using microarrays and RNA-seq databases to select an unpublished set of candidate reference genes, specifically chosen for the normalization of gene expression in studies on soybean under both drought stress and diurnal oscillations. Experimental validation and stability analysis in soybean plants submitted to drought stress and sampled during a 24 h timecourse showed that four of these newer reference genes (FYVE, NUDIX, Golgin-84 and CYST) indeed exhibited greater expression stability than the conventionally used housekeeping genes (ELF1-β and β-actin) under these conditions. We also demonstrated the effect of using reference candidate genes with different stability values to normalize the relative expression data from a drought-inducible soybean gene (DREB5) evaluated in different periods of the day.

Bayesian phylogeny of sucrose transporters: ancient origins, differential expansion and convergent evolution in monocots and dicots

Sucrose transporters (SUTs) are essential for the export and efficient movement of sucrose from source leaves to sink organs in plants. The angiosperm SUT family was previously classified into three or four distinct groups, Types I, II (subgroup IIB), and III, with dicot-specific Type I and monocot-specific Type IIB functioning in phloem loading. To shed light on the underlying drivers of SUT evolution, Bayesian phylogenetic inference was undertaken using 41 sequenced plant genomes, including seven basal lineages at key evolutionary junctures. Our analysis supports four phylogenetically and structurally distinct SUT subfamilies, originating from two ancient groups (AG1 and AG2) that diverged early during terrestrial colonization. In both AG1 and AG2, multiple intron acquisition events in the progenitor vascular plant established the gene structures of modern SUTs. Tonoplastic Type III and plasmalemmal Type II represent evolutionarily conserved descendants of AG1 and AG2, respectively. Type I and Type IIB were previously thought to evolve after the dicot-monocot split. We show, however, that divergence of Type I from Type III SUT predated basal angiosperms, likely associated with evolution of vascular cambium and phloem transport. Type I SUT was subsequently lost in monocots along with vascular cambium, and independent evolution of Type IIB coincided with modified monocot vasculature. Both Type I and Type IIB underwent lineage-specific expansion. In multiple unrelated taxa, the newly-derived SUTs exhibit biased expression in reproductive tissues, suggesting a functional link between phloem loading and reproductive fitness. Convergent evolution of Type I and Type IIB for SUT function in phloem loading and reproductive organs supports the idea that differential vascular development in dicots and monocots is a strong driver for SUT family evolution in angiosperms.

Complete mitochondrial genome of Eruca sativa Mill. (Garden rocket)

Eruca sativa (Cruciferae family) is an ancient crop of great economic and agronomic importance. Here, the complete mitochondrial genome of Eruca sativa was sequenced and annotated. The circular molecule is 247 696 bp long, with a G+C content of 45.07%, containing 33 protein-coding genes, three rRNA genes, and 18 tRNA genes. The Eruca sativa mitochondrial genome may be divided into six master circles and four subgenomic molecules via three pairwise large repeats, resulting in a more dynamic structure of the Eruca sativa mtDNA compared with other cruciferous mitotypes. Comparison with the Brassica napus MtDNA revealed that most of the genes with known function are conserved between these two mitotypes except for the ccmFN2 and rrn18 genes, and 27 point mutations were scattered in the 14 protein-coding genes. Evolutionary relationships analysis suggested that Eruca sativa is more closely related to the Brassica species and to Raphanus sativus than to Arabidopsis thaliana.

Assembly and analysis of a male sterile rubber tree mitochondrial genome reveals DNA rearrangement events and a novel transcript

BACKGROUND

The rubber tree, Hevea brasiliensis, is an important plant species that is commercially grown to produce latex rubber in many countries. The rubber tree variety BPM 24 exhibits cytoplasmic male sterility, inherited from the variety GT 1.

RESULTS

We constructed the rubber tree mitochondrial genome of a cytoplasmic male sterile variety, BPM 24, using 454 sequencing, including 8 kb paired-end libraries, plus Illumina paired-end sequencing. We annotated this mitochondrial genome with the aid of Illumina RNA-seq data and performed comparative analysis. We then compared the sequence of BPM 24 to the contigs of the published rubber tree, variety RRIM 600, and identified a rearrangement that is unique to BPM 24 resulting in a novel transcript containing a portion of atp9.

CONCLUSIONS

The novel transcript is consistent with changes that cause cytoplasmic male sterility through a slight reduction to ATP production efficiency. The exhaustive nature of the search rules out alternative causes and supports previous findings of novel transcripts causing cytoplasmic male sterility.

Mitochondrial Genome Sequence of the Legume Vicia faba

The number of plant mitochondrial genomes sequenced exceeds two dozen. However, for a detailed comparative study of different phylogenetic branches more plant mitochondrial genomes should be sequenced. This article presents sequencing data and comparative analysis of mitochondrial DNA (mtDNA) of the legume Vicia faba. The size of the V. faba circular mitochondrial master chromosome of cultivar Broad Windsor was estimated as 588,000 bp with a genome complexity of 387,745 bp and 52 conservative mitochondrial genes; 32 of them encoding proteins, 3 rRNA, and 17 tRNA genes. Six tRNA genes were highly homologous to chloroplast genome sequences. In addition to the 52 conservative genes, 114 unique open reading frames (ORFs) were found, 36 without significant homology to any known proteins and 29 with homology to the Medicago truncatula nuclear genome and to other plant mitochondrial ORFs, 49 ORFs were not homologous to M. truncatula but possessed sequences with significant homology to other plant mitochondrial or nuclear ORFs. In general, the unique ORFs revealed very low homology to known closely related legumes, but several sequence homologies were found between V. faba, Beta vulgaris, Nicotiana tabacum, Vitis vinifera, and even the monocots Oryza sativa and Zea mays. Most likely these ORFs arose independently during angiosperm evolution (Kubo and Mikami, 2007; Kubo and Newton, 2008). Computational analysis revealed in total about 45% of V. faba mtDNA sequence being homologous to the Medicago truncatula nuclear genome (more than to any sequenced plant mitochondrial genome), and 35% of this homology ranging from a few dozen to 12,806 bp are located on chromosome 1. Apparently, mitochondrial rrn5, rrn18, rps10, ATP synthase subunit alpha, cox2, and tRNA sequences are part of transcribed nuclear mosaic ORFs.