Extensive and widespread homologies between mitochondrial DNA and chloroplast DNA in plants

Horizontal gene transfer from chloroplast to mitochondria of seagrasses in the yellow-Bohai seas

Seagrasses are ideal for studying plant adaptation to marine environments. In this study, the mitochondrial (mt) and chloroplast (cp) genomes of Ruppia sinensis were sequenced. The results showed an extensive gene loss in seagrasses, including a complete loss of cp-rpl19 genes in Zosteraceae, most cp-ndh genes in Hydrocharitaceae, and mt-rpl and mt-rps genes in all seagrasses, except for the mt-rpl16 gene in Phyllospadix iwatensis. Notably, most ribosomal protein genes were lost in the mt and cp genomes. The deleted cp genes were not transferred to the mt genomes through horizontal gene transfer. Additionally, a significant DNA transfer between seagrass organelles was found, with the mt genomes of Zostera containing numerous sequences from the cp genome. Rearrangement analyses revealed an unreported inversion of the cp genome in R. sinensis. Moreover, four positively selected genes (atp8, nad5, atp4, and ccmFn) and five variable regions (matR, atp4, atp8, rps7, and ccmFn) were identified.

Complete mitochondrial genome of Mentha spicata L. reveals multiple chromosomal configurations and RNA editing events

Mentha spicata L. is a valuable plant that yields spearmint oil, widely utilized in the pharmaceutical, chemical, and cosmetic industries. The mitochondrial genome (mitogenome) is an essential material for molecular breeding and evolution studies. Here, the mitogenome of M. spicata was assembled by combining Nanopore and Illumina reads. It consisted of a linear chromosome (Ch1) and two circular chromosomes (Ch2 and Ch3). Furthermore, we showed two pairs of repeats (R1 and R2) mediated recombinations resulting in multiple chromosomal configurations. The R1-mediated-recombination generated a large molecule formed by joining Ch2 and Ch1. Similarly, the R2-mediated-recombination generated a large molecule formed by joining Ch3 and Ch1. Then, we identified 17 mitochondrial plastid DNAs (MTPTs) by comparing the mitogenome and cpgenome. The MTPT14 was conserved in multiple species, which has undergone the same evolutionary process as the two organellar genomes among M. spicata, Hesperelaea palmeri and Castilleja paramensis. Based on the RNA-seq reads, 246 RNA editing sites were predicted, resulting in the conversion of cytosine to uracil bases. Furthermore, we successfully validated 40 out of 43 predicted sites. This project reported a complex structure of the M. spicata mitogenome resulting from repeat-mediated recombinations, which will provide valuable information for gene function study and the breeding of different varieties.

Characterisation of the complete mitochondrial genome of Taraxacum mongolicum revealed five repeat-mediated recombinations

We reported the mitochondrial genome of Taraxacum mongolicum for the first time. Five pairs of repeats that can mediate recombination were validated, leading to multiple conformations of genome. Taraxacum mongolicum belongs to the Asteraceae family and has important pharmaceutical value. To explore the possible interaction between the organelle genomes, we assembled the complete mitochondrial genome (mitogenome) of T. mongolicum using Illumina and Oxford Nanopore sequencing data. This genome corresponded to a circular molecule 304,467 bp long. It encodes 52 unique genes including 31 protein-coding, 3 ribosomal RNA (rRNA) and 18 transfer RNA (tRNA) genes. In addition to the single circular conformation, the existence of alternative conformations mediated by five repetitive sequences in the mitogenome was identified and validated. Recombination mediated by the inverted repeats resulted in two conformations. Conversely, recombination mediated by the two direct repeats broke one large circular molecule into two subgenomic circular molecules. Furthermore, we identified 12 homologous fragments by comparing the sequences of mitogenome and plastome, including eight complete tRNA genes. Lastly, we identified a total of 278 RNA-editing sites in protein-coding sequences based on RNA-seq data. Among them, cox1 and nad5 gene has the most sites (21), followed by the nad2 gene with 19 sites. We successfully validated 213 predicted RNA-editing sites using PCR amplification and Sanger sequencing. This project reported the first mitogenome of T. mongolicum and demonstrated its multiple conformations generated by repeat-mediated recombination. This genome could provide critical information for the molecular breeding of T. mongolicum, and also be used as a reference genome for other species of the genus Taraxacum.

Horizontal Gene Transfer Involving Chloroplasts

Horizontal gene transfer (HGT)- is defined as the acquisition of genetic material from another organism. However, recent findings indicate a possible role of HGT in the acquisition of traits with adaptive significance, suggesting that HGT is an important driving force in the evolution of eukaryotes as well as prokaryotes. It has been noted that, in eukaryotes, HGT is more prevalent than originally thought. Mitochondria and chloroplasts lost a large number of genes after their respective endosymbiotic events occurred. Even after this major content loss, organelle genomes still continue to lose their own genes. Many of these are subsequently acquired by intracellular gene transfer from the original plastid. The aim of our review was to elucidate the role of chloroplasts in the transfer of genes. This review also explores gene transfer involving mitochondrial and nuclear genomes, though recent studies indicate that chloroplast genomes are far more active in HGT as compared to these other two DNA-containing cellular compartments.

Wide crossing diversify mitogenomes of rice

BACKGROUND

In most angiosperms, the inheritance of the mitochondria takes place in a typical maternal manner. However, very less information is available about if the existence of structural variations or not in mitochondrial genomes (mitogenomes) between maternal parents and their progenies.

RESULTS

In order to find the answer, a stable rice backcross inbred line (BIL) population was derived from the crosses of Oryza glaberrima/Oryza sativa//Oryza sativa. The current study presents a comparative analysis of the mitogenomes between maternal parents and five BILs. There were recorded universal structural variations such as reversal, translocation, fusion, and fission among the BILs. The repeat-mediated recombination and non-homologous end-joining contributed virtually equal to the rearrangement of mitogenomes. Similarly, the relative order, copy-number, expression level, and RNA-editing rate of mitochondrial genes were also extensively varied among BILs.

CONCLUSIONS

These novel findings unraveled an unusual mystery of the maternal inheritance and possible cause for heterogeneity of mitogenomes in rice population. The current piece of work will greatly develop our understanding of the plant nucleo-cytoplasmic interaction and their potential role in plant growth and developmental processes.

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 and Analysis of Coconut Palm (Cocos nucifera) Mitochondrial Genome

Coconut (Cocos nucifera L.), a member of the palm family (Arecaceae), is one of the most economically important crops in tropics, serving as an important source of food, drink, fuel, medicine, and construction material. Here we report an assembly of the coconut (C. nucifera, Oman local Tall cultivar) mitochondrial (mt) genome based on next-generation sequencing data. This genome, 678,653bp in length and 45.5% in GC content, encodes 72 proteins, 9 pseudogenes, 23 tRNAs, and 3 ribosomal RNAs. Within the assembly, we find that the chloroplast (cp) derived regions account for 5.07% of the total assembly length, including 13 proteins, 2 pseudogenes, and 11 tRNAs. The mt genome has a relatively large fraction of repeat content (17.26%), including both forward (tandem) and inverted (palindromic) repeats. Sequence variation analysis shows that the Ti/Tv ratio of the mt genome is lower as compared to that of the nuclear genome and neutral expectation. By combining public RNA-Seq data for coconut, we identify 734 RNA editing sites supported by at least two datasets. In summary, our data provides the second complete mt genome sequence in the family Arecaceae, essential for further investigations on mitochondrial biology of seed plants.

A complete sequence and transcriptomic analyses of date palm (Phoenix dactylifera L.) mitochondrial genome

Based on next-generation sequencing data, we assembled the mitochondrial (mt) genome of date palm (Phoenix dactylifera L.) into a circular molecule of 715,001 bp in length. The mt genome of P. dactylifera encodes 38 proteins, 30 tRNAs, and 3 ribosomal RNAs, which constitute a gene content of 6.5% (46,770 bp) over the full length. The rest, 93.5% of the genome sequence, is comprised of cp (chloroplast)-derived (10.3% with respect to the whole genome length) and non-coding sequences. In the non-coding regions, there are 0.33% tandem and 2.3% long repeats. Our transcriptomic data from eight tissues (root, seed, bud, fruit, green leaf, yellow leaf, female flower, and male flower) showed higher gene expression levels in male flower, root, bud, and female flower, as compared to four other tissues. We identified 120 potential SNPs among three date palm cultivars (Khalas, Fahal, and Sukry), and successfully found seven SNPs in the coding sequences. A phylogenetic analysis, based on 22 conserved genes of 15 representative plant mitochondria, showed that P. dactylifera positions at the root of all sequenced monocot mt genomes. In addition, consistent with previous discoveries, there are three co-transcribed gene clusters–18S-5S rRNA, rps3-rpl16 and nad3-rps12–in P. dactylifera, which are highly conserved among all known mitochondrial genomes of angiosperms.

Complete chloroplast genome sequence of an orchid model plant candidate: Erycina pusilla apply in tropical Oncidium breeding

Oncidium is an important ornamental plant but the study of its functional genomics is difficult. Erycina pusilla is a fast-growing Oncidiinae species. Several characteristics including low chromosome number, small genome size, short growth period, and its ability to complete its life cycle in vitro make E. pusilla a good model candidate and parent for hybridization for orchids. Although genetic information remains limited, systematic molecular analysis of its chloroplast genome might provide useful genetic information. By combining bacterial artificial chromosome (BAC) clones and next-generation sequencing (NGS), the chloroplast (cp) genome of E. pusilla was sequenced accurately, efficiently and economically. The cp genome of E. pusilla shares 89 and 84% similarity with Oncidium Gower Ramsey and Phalanopsis aphrodite, respectively. Comparing these 3 cp genomes, 5 regions have been identified as showing diversity. Using PCR analysis of 19 species belonging to the Epidendroideae subfamily, a conserved deletion was found in the rps15-trnN region of the Cymbidieae tribe. Because commercial Oncidium varieties in Taiwan are limited, identification of potential parents using molecular breeding method has become very important. To demonstrate the relationship between taxonomic position and hybrid compatibility of E. pusilla, 4 DNA regions of 36 tropically adapted Oncidiinae varieties have been analyzed. The results indicated that trnF-ndhJ and trnH-psbA were suitable for phylogenetic analysis. E. pusilla proved to be phylogenetically closer to Rodriguezia and Tolumnia than Oncidium, despite its similar floral appearance to Oncidium. These results indicate the hybrid compatibility of E. pusilla, its cp genome providing important information for Oncidium breeding.

Plastid sequences contribute to some plant mitochondrial genes

DNA of plastid (chloroplast) origin comprises between 1% and 10% of the mitochondrial genomes of higher plants, but functions are currently considered to be limited to rare instances where plastid tRNA genes have replaced their mitochondrial counterparts, where short patches of mitochondrial genes evolved using their homologous plastidic copies by gene conversion or where a new promoter region is created. Here, we show that, in some angiosperms, plastid-derived DNA in mitochondrial genomes (also called mtpt for mitochondrial plastid DNA) contributes codons to unrelated mitochondrial protein-coding sequences and may also have a role in posttranscriptional RNA processing. We determined that these transfers of plastid DNA occurred a few to 150 Ma and that mtpts can sometimes remain dormant many millions of years before contributing to the mitochondrial proteome.

Characterization of the complete chloroplast genome of Hevea brasiliensis reveals genome rearrangement, RNA editing sites and phylogenetic relationships

Rubber tree (Hevea brasiliensis) is an economical plant and widely grown for natural rubber production. However, genomic research of rubber tree has lagged behind other species in the Euphorbiaceae family. We report the complete chloroplast genome sequence of rubber tree as being 161,191 bp in length including a pair of inverted repeats of 26,810 bp separated by a small single copy region of 18,362 bp and a large single copy region of 89,209 bp. The chloroplast genome contains 112 unique genes, 16 of which are duplicated in the inverted repeat. Of the 112 unique genes, 78 are predicted protein-coding genes, 4 are ribosomal RNA genes and 30 are tRNA genes. Relative to other plant chloroplast genomes, we observed a unique rearrangement in the rubber tree chloroplast genome: a 30-kb inversion between the trnE(UUC)-trnS(GCU) and the trnT(GGU)-trnR(UCU). A comparison between the rubber tree chloroplast genes and cDNA sequences revealed 51 RNA editing sites in which most (48 sites) were located in 26 protein coding genes and the other 3 sites were in introns. Phylogenetic analysis based on chloroplast genes demonstrated a close relationship between Hevea and Manihot in Euphorbiaceae and provided a strong support for a monophyletic group of the eurosid I.

Interorganellar DNA transfer in wheat: dynamics and phylogenetic origin

A homology search of wheat chloroplast (ct) and mitochondrial (mt) genomes identified 54 ctDNA segments that have homology with 66 mtDNA segments. The mtDNA segments were classified according to their origin: orthologs (prokaryotic origin), xenologs (interorganellar DNA transfer origin) and paralogs (intraorganellar DNA amplification origin). The 66 mtDNA sequences with homology to ctDNA segments included 14 paralogs, 18 orthologs and 34 xenologs. Analysis of the xenologs indicated that the DNA transfer occurred unidirectionally from the ct genome to the mt genome. The evolutionary timing of each interorganellar DNA transfer that generated a xenolog was estimated. This analysis showed that 2 xenologs originated early in green plant evolution, 4 in angiosperm evolution, 3 in monocotyledon evolution, 9 during cereal diversification and 8 in the evolution of wheat. Six other xenologs showed recurrent transfer from the ct to mt genomes in more than one taxon. The two remaining xenologs were uninformative on the evolutionary timing of their transfer. The wheat mt nad9 gene was found to be chimeric, consisting of the cereal nad9 gene and its 291 bp 5'-flanking region that included a 58 bp xenolog of the ct-ndhC origin.

On the evolutionary origin of the plant mitochondrion and its genome

Higher plants occupy very different positions in the mitochondrial and nuclear lineages of global phylogenetic trees based on conserved regions of small subunit (SSU) and large subunit (LSU) rRNA sequences. In the nuclear subtree, plants branch off late, at a position reflecting a massive radiation of the major multicellular (and some unicellular) groups; in the mitochondrial subtree, in contrast, plants branch off early, near the point of connection between the mitochondrial and eubacterial lineages. Moreover, in the nuclear lineage, plants branch together with the unicellular green alga Chlamydomonas reinhardtii, whereas in the mitochondrial lineage (in both SSU and LSU trees), metaphytes and chlorophyte branch separately. Statistical evaluation indicates that the anomalous branching position of higher plants in the mitochondrial lineage is not a treeing artifact attributable to the relatively rapid rate of sequence divergence of non-plant mitochondrial rRNA sequences. In considering alternative biological explanations for these results, we are led to propose that the rRNA genes in plant mitochondria may be of more recent evolutionary origin than the rRNA genes in other mitochondria. This proposal has implications for monophyletic vs. polyphyletic scenarios of mitochondrial origin and is consistent with other evidence indicating that plant mtDNA is an evolutionary mosaic.

Ochromonas mitochondria contain a specific chloroplast protein

Antibody raised against the small subunit of ribulose-1,5-bisphosphate carboxylase [3-phospho-D-glycerate carboxy-lyase (dimerizing), EC 4.1.1.39] of Chlamydomonas reinhardtii labeled the mitochondria as well as the chloroplast of the chrysophyte alga Ochromonas danica in sections prepared for immunoelectron microscopy by the protein A-gold technique. The same antibody labeled the chloroplast but not the mitochondria of C. reinhardtii. A quantitative study of labeling in dark-grown, greening (32 hr light), and mature green cells of O. danica revealed that anti-small-subunit staining in the mitochondria increased progressively in the light as it does in the plastid. Antibody to the large subunit of the enzyme did not label the mitochondria of either O. danica or C. reinhardtii. In view of the recent demonstrations of homologous DNA sequences in the mitochondrial and chloroplast genomes of higher plants, we suggest that the DNA sequence coding for the small subunit has migrated to the mitochondria from nucleus or chloroplast and is expressed within the organelle.

Fine-scale mergers of chloroplast and mitochondrial genes create functional, transcompartmentally chimeric mitochondrial genes

The mitochondrial genomes of flowering plants possess a promiscuous proclivity for taking up sequences from the chloroplast genome. All characterized chloroplast integrants exist apart from native mitochondrial genes, and only a few, involving chloroplast tRNA genes that have functionally supplanted their mitochondrial counterparts, appear to be of functional consequence. We developed a novel computational approach to search for homologous recombination (gene conversion) in a large number of sequences and applied it to 22 mitochondrial and chloroplast gene pairs, which last shared common ancestry some 2 billion years ago. We found evidence of recurrent conversion of short patches of mitochondrial genes by chloroplast homologs during angiosperm evolution, but no evidence of gene conversion in the opposite direction. All 9 putative conversion events involve the atp1/atpA gene encoding the alpha subunit of ATP synthase, which is unusually well conserved between the 2 organelles and the only shared gene that is widely sequenced across plant mitochondria. Moreover, all conversions were limited to the 2 regions of greatest nucleotide and amino acid conservation of atp1/atpA. These observations probably reflect constraints operating on both the occurrence and fixation of recombination between ancient homologs. These findings indicate that recombination between anciently related sequences is more frequent than previously appreciated and creates functional mitochondrial genes of chimeric origin. These results also have implications for the widespread use of mitochondrial atp1 in phylogeny reconstruction.

Phylogenetic analysis reveals five independent transfers of the chloroplast gene rbcL to the mitochondrial genome in angiosperms

We used the chloroplast gene rbcL as a model to study the frequency and relative timing of transfer of chloroplast sequences to the mitochondrial genome. Southern blot survey of 20 mitochondrial DNAs confirmed three previously reported groups of plants containing rbcL in their mitochondrion, while PCR studies identified a new mitochondrial rbcL. Published and newly determined mitochondrial and chloroplast rbcL sequences were used to reconstruct rbcL phylogeny. The results imply five or six separate interorganellar transfers of rbcL among the angiosperms examined, and hundreds of successful transfers across all flowering plants. By taxonomic criteria, the crucifer transfer is the most ancient, two separate transfers within the grass family are of intermediate ancestry, and the morning-glory transfer is most recent. All five mitochondrial copies of rbcL examined exhibit insertion and/or deletion events that disrupt the reading frame (three are grossly truncated); and all are elevated in the proportion of nonsynonymous substitutions, providing clear evidence that these sequences are pseudogenes.

Small, repetitive DNAs contribute significantly to the expanded mitochondrial genome of cucumber

Closely related cucurbit species possess eightfold differences in the sizes of their mitochondrial genomes. We cloned mitochondrial DNA (mtDNA) fragments showing strong hybridization signals to cucumber mtDNA and little or no signal to watermelon mtDNA. The cucumber mtDNA clones carried short (30-53 bp), repetitive DNA motifs that were often degenerate, overlapping, and showed no homology to any sequences currently in the databases. On the basis of dot-blot hybridizations, seven repetitive DNA motifs accounted for >13% (194 kb) of the cucumber mitochondrial genome, equaling >50% of the size of the Arabidopsis mitochondrial genome. Sequence analysis of 136 kb of cucumber mtDNA revealed only 11.2% with significant homology to previously characterized mitochondrial sequences, 2.4% to chloroplast DNA, and 15% to the seven repetitive DNA motifs. The remaining 71.4% of the sequence was unique to the cucumber mitochondrial genome. There was <4% sequence colinearity surrounding the watermelon and cucumber atp9 coding regions, and the much smaller watermelon mitochondrial genome possessed no significant amounts of cucumber repetitive DNAs. Our results demonstrate that the expanded cucumber mitochondrial genome is in part due to extensive duplication of short repetitive sequences, possibly by recombination and/or replication slippage.

The localization of mitochondrial sequences to chromosomal DNA in orthopterans

There is growing evidence that the integration of mitochondrial DNA sequences into nuclear and chloroplast genomes of higher organisms may be widespread rather than exceptional. We report the localization of 18S-25S rDNA and mitochondrial DNA sequences to meiotic chromosomes of several orthopteran species using in situ hybridisation. The cytochrome oxidase I (COI) sequence localizes to the centromeric and two telomeric regions of the eight bivalents of Chorthippus parallelus, the telomeric regions in Schistocerca gregaria and is present throughout the genome of Italopodisma sp. (Orthoptera: Acrididae). The control region of the mitochondrion and COI localize to similar chromosomal regions in S. gregaria. These data explain sequencing data that are inconsistent with the COI sequence being solely mitochondrial. The different nuclear locations of mtDNA in the different genera studied suggest that grasshopper mtDNA-like sequences have been inserted into the nuclear genome more than once in Acridid history, and there may have been different mechanisms involved when these events occurred in each of these species.Key words: Schistocerca gregaria, Italopodisma spp., Chorthippus parallelus, in situ hybridisation, mitochondrial DNA, genome organization.

Genomic context influences the activity of maize mitochondrial cox2 promoters

Plant mitochondrial genomes are highly recombinogenic, with a variety of species-specific direct and inverted repeats leading to in vivo accumulation of multiple DNA forms. In maize, the cox2 gene, which encodes subunit II of cytochrome c oxidase, lies immediately downstream of a 0.7-kilobase direct repeat, which is present in two copies in the 570-kilobase master chromosome. Promoters for cox2 exist upstream of both of these copies, in regions we have termed A and B. Three region B promoters are active for cox2 transcription in the master chromosome, whereas two region A promoters are active for cox2 transcription after recombination across the direct repeats. We have measured the proportion of genomes carrying region A or B upstream of cox2 in maize seedlings and found a ratio of approximately 1:6. Promoter strength, based on run-on transcription assays, shows a ratio of 1:4 for region A to region B promoters. These data allowed us to predict the relative contributions of region A and B to mitochondrial transcript accumulation, based on a simple product of genome-form abundance and promoter strength. When promoter use was determined by using quantitative reverse transcriptase-PCR, however, we found that region A promoters were used at an unexpectedly high rate when upstream of cox2 and used less than expected when not upstream of cox2. Thus, the use of this set of promoters seems to respond to genomic context. These results suggest a role for intragenomic and intergenomic recombination in regulating plant mitochondrial gene expression.

Global dinucleotide signatures and analysis of genomic heterogeneity

Early biochemical experiments measuring nearest neighbor frequencies established that the set of dinucleotide relative abundance values (dinucleotide biases) is a remarkably stable property of the DNA of an organism. Analyses of currently available genomic sequence data have extended these earlier results, showing that the dinucleotide biases evaluated for successive 50 kb segments of a genome are significantly more similar to each other than to those of sequences from more distant organisms. From this perspective, the set of dinucleotide biases constitutes a 'genomic signature' that can discriminate sequences from different organisms. The dinucleotide biases appear to reflect species-specific properties of DNA stacking energies, modification, replication, and repair mechanisms. The genomic signature is useful for detecting pathogenicity islands in bacterial genomes.

The role of lipids in plastid protein transport

The elaborate compartmentalization of plant cells requires multiple mechanisms of protein targeting and trafficking. In addition to the organelles found in all eukaryotes, the plant cell contains a semi-autonomous organelle, the plastid. The plastid is not only the most active site of protein transport in the cell, but with its three membranes and three aqueous compartments, it also represents the most topologically complex organelle in the cell. The chloroplast contains both a protein import system in the envelope and multiple protein export systems in the thylakoid. Although significant advances have identified several proteinaceous components of the protein import and export apparatuses, the lipids found within plastid membranes are also emerging as important players in the targeting, insertion, and assembly of proteins in plastid membranes. The apparent affinity of chloroplast transit peptides for chloroplast lipids and the tendency for unsaturated MGDG to adopt a hexagonal II phase organization are discussed as possible mechanisms for initiating the binding and/or translocation of precursors to plastid membranes. Other important roles for lipids in plastid biogenesis are addressed, including the spontaneous insertion of proteins into the outer envelope and thylakoid, the role of cubic lipid structures in targeting and assembly of proteins to the prolamellar body, and the repair process of D1 after photoinhibition. The current progress in the identification of the genes and their associated mutations in galactolipid biosynthesis is discussed. Finally, the potential role of plastid-derived tubules in facilitating macromolecular transport between plastids and other cellular organelles is discussed.

Mitochondrial DNA of the coral Sarcophyton glaucum contains a gene for a homologue of bacterial MutS: a possible case of gene transfer from the nucleus to the mitochondrion

The nucleotide sequences of two segments of 6,737 ntp and 258 nto of the 18.4-kb circular mitochondrial (mt) DNA molecule of the soft coral Sarcophyton glaucum (phylum Cnidaria, class Anthozoa, subclass Octocorallia, order Alcyonacea) have been determined. The larger segment contains the 3' 191 ntp of the gene for subunit 1 of the respiratory chain NADH dehydrogenase (ND1), complete genes for cytochrome b (Cyt b), ND6, ND3, ND4L, and a bacterial MutS homologue (MSH), and the 5' terminal 1,124 ntp of the gene for the large subunit rRNA (1-rRNA). These genes are arranged in the order given and all are transcribed from the same strand of the molecule. The smaller segment contains the 3' terminal 134 ntp of the ND4 gene and a complete tRNA(f-Met) gene, and these genes are transcribed in opposite directions. As in the hexacorallian anthozoan, Metridium senile, the mt-genetic code of S. glaucum is near standard: that is, in contrast to the situation in mt-genetic codes of other invertebrate phyla, AGA and AGG specify arginine, and ATA specifies isoleucine. However, as appears to be universal for metazoan mt-genetic codes, TGA specifies tryptophan rather than termination. Also, as in M. senile the mt-tRNA(f-Met) gene has primary and secondary structural features resembling those of Escherichia coli initiator tRNA, including standard dihydrouridine and T psi C loop sequences, and a mismatched nucleotide pair at the top of the amino-acyl stem. The presence of a mutS gene homologue, which has not been reported to occur in any other known mtDNA, suggests that there is mismatch repair activity in S. glaucum mitochondria. In support of this, phylogenetic analysis of MutS family protein sequences indicates that the S. glaucum mtMSH protein is more closely related to the nuclear DNA-encoded mitochondrial mismatch repair protein (MSH1) of the yeast Saccharomyces cerevisiae than to eukaryotic homologues involved in nuclear function, or to bacterial homologues. Regarding the possible origin of the S. glaucum mtMSH gene, the phylogenetic analysis results, together with comparative base composition considerations, and the absence of an MSH gene in any other known mtDNA best support the hypothesis that S. glaucum mtDNA acquired the mtMSH gene from nuclear DNA early in the evolution of octocorals. The presence of mismatch repair activity in S. glaucum mitochondria might be expected to influence the rate of evolution of this organism's mtDNA.

A chloroplast derived trnH gene is expressed in the mitochondrial genome of gramineous plants

We reported previously that the mitochondrial sequence that contains the chloroplast-derived trnH gene has been highly conserved in the region around one terminus of the junction between chloroplast-derived and mitochondrion-specific sequences in most of the gramineous plants analyzed [15]. The results of RT-PCR, northern hybridization, in vitro capping and ribonuclease protection experiments show that the chloroplast-derived trnH gene is transcribed from a putative promoter that is located in the mitochondrion-specific sequence. Gene expression in this region seems to be correlated with the conservation of the sequence at the junction between the chloroplast-derived fragment and the mitochondrion-specific sequence.

Mitochondrial genome diversity and cytoplasmic male sterility in higher plants

Characteristic differences exist between the mitochondrial genome organization of fertile and cytoplasmic male-sterile (CMS) lines in a range of plant species. Current evidence suggests that these characteristic mitochondrial genotypes arose by aberrant recombination events, generating chimeric mitochondrial DNA sequences which have subsequently become stabilized, possibly by selective amplification. An investigation of the variation in stoichiometry of the fouratpA gene types in maize have suggested evolutionary mechanisms for the generation of mitochondrial genome diversity which are based on amplification of pre-existing, rare recombinant DNA molecules. As with a number of other well-documented examples of genome rearrangement, those involving theatpA gene appear to have no obvious phenotypic significance. However, in a number of cases, recombination events have resulted in either modification of existing mitochondrial genes, leading to the synthesis of a modified polypeptide, e.g. thecoxI gene in the 9E sorghum cytoplasm, or the generation of novel open reading frames. In the latter case the unique open reading frame found in the mitochondrial DNA of CMS-T maize plants encodes a 13 kDa polypeptide, previously identified as a CMS-T-specific mitochondrial translation product. Current studies are directed towards establishing a causal link between the 13 kDa polypeptide, mitochondrial enzyme complexes, and the CMS phenotype.

A chloroplast-derived sequence is utilized as a source of promoter sequences for the gene for subunit 9 of NADH dehydrogenase (nad9) in rice mitochondria

The chloroplast-derived sequence trnS-rps4/ 3'trnL-trnF-ndhJ-ndhK (4066 bases in length) is present in a region that starts 355 bases upstream of the gene for subunit 9 of NADH dehydrogenase (nad9) in the mitochondrial genome of rice. Northern blot hybridization revealed that three large transcripts of 3.05, 1.62 and 1.05 kb hybridized to strand-specific probes for both the nad9 gene and the chloroplast-derived sequence, indicating that the nad9 gene was transcribed together with the chloroplast-derived sequence. From the results of in vitro capping and ribonuclease protection experiments, as well as primer extension analysis, we identified at least seven sites for the initiation of transcription of nad9 in the chloroplast-derived sequence. All of the initiation sites for transcription of the nad9 gene were located in sequences homologous to chloroplast DNA. Two of seven initiation sites were flanked by a sequence homologous to the consensus promoter motif that includes the CRTA motif (where R is A or G) of the rice mitochondrion. However, the sequences surrounding the other five sites showed only limited similarity to the conserved sequence. It is suggested that all the promoters of the rice nad9 gene exist in a sequence that was transferred from the chloroplast during evolution. Thus, the chloroplast-derived sequence has a novel, significant function in the mitochondrial genome of this higher plant.

Inheritance and restriction fragment length polymorphism of chloroplast DNA in the genus Coffea L

CpDNA variation among 52 tree samples belonging to 25 different taxa of Coffea and two species of Psilanthus was assessed by RFLP analysis on both the total chloroplast genome and the atpB-rbcL intergenic region. Twelve variable characters were distinguished allowing the identification of 12 different plastomes. The low sequence divergence observed might suggest that Coffea is a young genus. The results were in contradiction with the present classification into two genera. Additionally, cpDNA inheritance was studied in interspecific hybrids between C. arabica and C. canephora, and in an intraspecific progeny of C. canephora, using PCR-based markers. Both studies showed exclusively maternal inheritance of cpDNA.

Characterisation and expression of the mitochondrial genome of a new type of cytoplasmic male-sterile sunflower

A new cytoplasmic male sterile sunflower, CMS3 [44], was characterised in relation to the Petiolaris (PET1) cytoplasmic male-sterile sunflower, CMS89 [25]. Southern blot analysis showed that the mitochondrial genome of CMS3 contains unique rearrangements in at least five loci (atp6, atp9, atpA, nad1 + 5 and coxIII) compared to the PET1 sterile and the fertile cytoplasms. Transcripts of two (coxIII and atp6) of the five rearranged loci differed in CMS3 when compared to the corresponding loci in the PET1 and fertile cytoplasms. In organello protein synthesis experiments showed that the ca. 15 kDa mitochondrial polypeptide, characteristic of PET1, is not present in the CMS3 line. These data suggest that the molecular basis of male sterility in the CMS3 line differs from that of the PET1 cytoplasm. The nucleotide sequences of the coding and the immediate flanking regions of the coxIII and atp6 genes of CMS3 were compared to the corresponding regions from the fertile sunflower. In CMS3 the ORFB-coxIII locus is located immediately 3' to the atpA gene whereas in the fertile cytoplasm these two loci are ca. 60 kb apart. This DNA rearrangement probably involved a 265 bp repeat which may be implicated in the DNA recombination associated with PET1 CMS. The atp6 gene in CMS3 contains a 5'-terminal extention which results in an extended ORF. The potential involvement of the rearrangements associated with the coxIII and atp6 loci in relation to the CMS phenotype is discussed.

Molecular changes of organelle DNA sequences in rice through dedifferentiation, long-term culture, or the morphogenesis process

Callus-specific rearranged DNA in rice (Oryza sativa L.) was isolated by in-gel reassociation procedure. Southern hybridization experiments revealed that some clones were amplified significantly in primary callus induced from scutellum tissue. Rapid amplification of these clones was observed within 2 days after plating seeds onto callus-induction medium containing 2,4-D. NAA gave no significant effect on DNA amplification event. Colony formation process from isolated protoplasts and plant regeneration process from callus showed clone-specific and process-specific fluctuation patterns of copy number. Sequence analysis of the clones suggested that most of the clones were originated from organelle DNA. Comparison of copy number fluctuation pattern of organelle functional genes with that of the clones suggested multiformity and/or construction-specific amplification of organelle DNA.

Evolutionary variations in DNA sequences transferred from chloroplast genomes to mitochondrial genomes in the Gramineae

The transfer of fragments of DNA from chloroplast genomes to mitochondrial genomes is considered to be a general phenomenon in higher plants. In the present study, Southern hybridization, together with amplification by PCR and DNA sequencing techniques, was used to examine the regions homologous to chloroplast rps19 in the mitochondrial genomes of several gramineous plants. In all the mitochondrial DNAs from the gramineous plants examined, except for that from wheat, the transferred fragments of chloroplast DNA were found to be maintained and the same junctions of mitochondrion-specific and chloroplast-like sequences were found at one terminus. This finding indicates that the transfer of the chloroplast sequence occurred in the distant past during the evolution of gramineous plants. Subsequent analysis revealed that the fragments had been variously rearranged among species with respect to the other terminus. Considering the current diversity of this one particular transferred fragment of chloroplast DNA, we propose that chloroplast-derived DNA sequences that have lost their original functions tend to be rearranged during evolution in mitochondrial genomes.

Relationships among Cichorium species and related genera as determined by analysis of mitochondrial RFLPs

Mitochondrial DNA polymorphism was employed to assess cytoplasmic diversity among cytoypes of the genus Cichorium and related genera of the tribe Lactuceae (Asteraceae). Hybridization patterns of total DNA using six restriction enzymes and five heterologous mtDNA probes were examined. From estimates of mtDNA diversity, Cichorium spinosum appeared as an ecotype of C. intybus rather than a separate species. Interspecific mtDNA polymorphism in the genus Cichorium was higher than that observed in Cicerbita Crepis, Lactuca and Tragopogon. Molecular data seemed to indicate that Catananche is very distant from the other genera examined. Intergeneric comparisons allowed the clustering of Cicerbita, Lactuca and Cichorium, genera which belong to different subtribes. However, further molecular investigations on a larger number of genera are needed to clarify the relationships among genera within and between subtribes of the tribe Lactuceae.

Maternal inheritance of chloroplast genome and paternal inheritance of mitochondrial genome in bananas (Musa acuminata)

Restriction fragment length polymorphisms (RFLPs) were used as markers to determine the transmission of cytoplasmic DNA in diploid banana crosses. Progenies from two controlled crosses were studied with heterologous cytoplasmic probes. This analysis provided evidence for a strong bias towards maternal transmission of chloroplast DNA and paternal transmission of mitochondrial DNA in Musa acuminata. These results suggest the existence of two separate mechanisms of organelle transmission and selection, but no model to explain this can be proposed at the present time. Knowledge of the organelle mode of inheritance constitutes an important point for phylogeny analyses in bananas and may offer a powerful tool to confirm hybrid origins.

Three mitochondrial unassigned open reading frames of Podospora anserina represent remnants of a viral-type RNA polymerase gene

The mitochondrial DNA of Podospora anserina is complex, consisting of a characteristic set of genes with a large number of introns and a substantial amount of sequence of unknown function and origin. In addition, as indicated by various types of reorganization, this genome is highly flexible. Here we report the identification of three unassigned mitochondrial open reading frames (ORF P', ORF Q', ORF 11) as remnants of a rearranged viral-type RNA polymerase gene. These ORFs are not transcribed and may be derived from the integration of a linear plasmid of the type recently identified in a mutant of P. anserina.

Comparative analysis of a recombining-repeat-sequence family in the mitochondrial genomes of wheat (Triticum aestivum L.) and rye (Secale cereale L.)

The mitochondrial genomes of wheat and rye each contain a three-member family of recombining repeat sequences (the "18S/5S repeat") that encode genes for 18S and 5S rRNAs (rrn18 and rrn5) and tRNA(fMet) (trnfM). Here we present, for wheat and rye, the sequence and boundaries of the "common sequence unit" (CSU) that is shared between all three repeat copies in each species. The wheat CSU is 4,429 base-pairs long and contains (in addition to trnfM, rrn18 and rrn5) a putative promoter, three tRNA-like elements ("t-elements"), and part of a pseudogene ("psi atpAc") that is homologous to chloroplast atpA, which encodes the alpha subunit of chloroplast F1 ATPase. The rye CSU is somewhat smaller (2,855 base pairs) but contains much the same genic and other sequence elements as its wheat counterpart, except that two of the three t-elements as well as psi atpAc are found in only one of the three downstream flanks of the 18S/5S repeat, outside the CSU boundaries. In interpreting the sequence data in terms of the evolutionary history of the 18S/5S-repeat family of wheat and rye, we conclude that: (1) the wheat-rye form of the 18S/5S repeat most likely originated between 3 and 14 million years ago, in a lineage that gave rise to wheat and rye but not to barley, oats, rice or maize; (2) the close linkage (1-bp apart) between trnfM and rrn18 is similarly limited in its taxonomic distribution to the wheat/rye lineage; (3) the trnfM-rrn18 pair arose via a single mutation that inserted a sequence block containing trnfM immediately upstream of rrn18; and (4) the presence of a putative promoter upstream of rrn18 in all wheat and rye repeats is consistent with all three repeat copies being transcriptionally active. We discuss these conclusions in the light of the possible functional significance of recombining-repeats in plant mitochondrial genomes.

Identification of the entire set of transferred chloroplast DNA sequences in the mitochondrial genome of rice

The entire set of transferred chloroplast DNA sequences in the mitochondrial genome of rice (Oryza sativa cv. Nipponbare) was identified using clone banks that cover the chloroplast and mitochondrial genomes. The mitochondrial fragments that were homologous to chloroplast DNA were mapped and sequenced. The nucleotide sequences around the termini of integrated chloroplast sequences in the rice mtDNA revealed no common sequences or structures that might enhance the transfer of DNA. Sixteen chloroplast sequences, ranging from 32 bases to 6.8 kb in length, were found to be dispersed throughout the rice mitochondrial genome. The total length of these sequences is equal to approximately 6% (22 kb) of the rice mitochondrial genome and to 19% of the chloroplast genome. The transfer of segments of chloroplast DNA seems to have occurred at different times, both before and after the divergence of rice and maize. The mitochondrial genome appears to have been rearranged after the transfer of chloroplast sequences as a result of recombination at these sequences. The rice mitochondrial DNA contains nine intact tRNA genes and three tRNA pseudogenes derived from the chloroplast genome.

Plastid DNA sequence homologies in the tobacco nuclear genome

The tobacco (Nicotiana tabacum) nuclear genome contains long tracts of DNA (i.e. in excess of 18 kb) with high sequence homology to the tobacco plastid genome. Five lambda clones containing these nuclear DNA sequences encompass more than one-third of the tobacco plastid genome. The absolute size of these five integrants is unknown but potentially includes uninterrupted sequences that are as large as the plastid genome itself. An additional sequence was cloned consisting of both nuclear and plastid-derived DNA sequences. The nuclear component of the clone is part of a family of repeats, which are present in about 400 locations in the nuclear genome. The homologous sequences present in chromosomal DNA were very similar to those of the corresponding sequences in the plastid genome. However significant sequence divergence, including base substitutions, insertions and deletions of up to 41 bp, was observed between these nuclear sequences and the plastid genome. Associated with the larger deletions were sequence motifs suggesting that processes such as DNA replication slippage and excision of hairpin loops may have been involved in deletion formation.

Maternal transmission of mitochondrial DNA in interspecific hybrids of Populus

Restriction fragment analysis was conducted to investigate the mode of inheritance of mitochondrial (mt) DNA in F1 progeny of two P. deltoides x P. deltoides, three P. deltoides x P. nigra, and two P. deltoides x P. maximowiczii controlled crosses, and in Populus x canadensis by using 16 restriction endonucleases and two heterologous probes of cloned mtDNA fragments of maize. Five restriction fragment length polymorphisms (RFLPs) of mtDNA differentiated P. deltoides from P. nigra, whereas three RFLPs of mtDNA separated P. deltoides from P. maximowiczii. In all cases, F1 progeny of P. deltoides x P. nigra, and P. deltoides x P. maximowiczii, crosses had mtDNA restriction fragments of only their maternal P. deltoides parents. P. x canadensis had mtDNA restriction fragments of only P. deltoides. F1 progeny of intraspecific P. deltoides crosses also had the same mtDNA fragments as their maternal parent. The results clearly demonstrate uniparental-maternal inheritance of the mitochondrial genome in F1 interspecific hybrids of P. deltoides with P. nigra and P. maximowiczii.

Chloroplast DNA inheritance in Populus

The inheritance of chloroplast (cp) DNA was examined in F1 hybrid progenies of two Populus deltoides intraspecific controlled crosses and three P. deltoides × P. nigra and two P. deltoides × P. maximowiczii interspecific controlled crosses by restriction fragment analysis. Southern blots of restriction digests of parental and progeny DNAs were hybridized to cloned cpDNA fragments of Petunia hybrida. Sixteen enzymes and five heterologous cpDNA probes were used to screen restriction fragment polymorphisms among the parents. The mode of cpDNA inheritance was demonstrated in progenies of P. deltoides × P. nigra crosses with 26 restriction fragment polymorphisms of cpDNA differentiating P. deltoides from P. nigra, as revealed by 12 enzyme-probe combinations, and in progenies of P. deltoides × P. maximowiczii crosses with 12 restriction fragment polymorphisms separating P. deltoides from P. maximowiczii, as revealed by 7 restriction enzyme-probe combinations. In all cases, F1 offspring of P. deltoides × P. nigra and P. deltoides × P. maximowiczii crosses had cpDNA restriction fragments of only their maternal P. deltoides parent. The results clearly demonstrated uniparental-maternal inheritance of the chloroplast genome in interspecific hybrids of P. deltoides with P. nigra and P. maximowiczii. Intraspecific P. deltoides hybrids also had the same cpDNA restriction fragments as their maternal parent. Maternal inheritance of the chloroplast genome in Populus is in agreement with what has been observed for most other angiosperms.

Expression of Chloroplast and Mitochondrial Genes during Microsporogenesis in Maize

Mitochondrial and plastid gene expression has been examined during maize (Zea mays) microsporogenesis. Accumulation of transcripts was found for three mitochondrial genes studied (cob, atp6, and atp9) at the mid-term of pollen development. In contrast, these mitochondrial transcripts were undetectable in mature pollen. Southern and DNA gel blot experiments showed that the copy number of mitochondrial genes was amplified in microspores at stages preceding the accumulation of these transcripts. Plastid transcripts of the photosynthetic psbA and rbcL genes could not be detected after the two mitoses, whereas precursors of the 16S rRNA are detected at low levels.

Evaluation of the Extent of Homologous Chloroplast DNA Sequences in the Mitochondrial Genome of Cowpea (Vigna unguiculata L.)

Southern blot hybridization techniques were used to estimate the extent of chloroplast DNA sequences present in the mitochondrial genome of cowpea (Vigna unguiculata L.) The entire mitochondrial chromosome was homogeneously labeled and used to probe blotted DNA fragments obtained by extensive restriction of the tobacco chloroplast genome. The strongest cross-homologies were obtained with fragments derived from the inverted repeat and the atpBE cluster regions, although most of the clones tested (spanning 85% of the tobacco plastid genome) hybridized to mitochondrial DNA. Homologous chloroplast DNA restriction fragments represent a total of 30 to 68 kilobase pairs, depending upon the presence or absence of tRNA-encoding fragments. Plastid genes showing homology with mitochondrial DNA include those encoding ribosomal proteins, RNA polymerase, subunits of photosynthetic complexes, and the two major rRNAs.