The Ogura sterility-inducing protein forms a large complex without interfering with the oxidative phosphorylation components in rapeseed mitochondria

The mitochondrial orf117Sha gene desynchronizes pollen development and causes pollen abortion in Arabidopsis Sha cytoplasmic male sterility

Cytoplasmic male sterility (CMS) is of major agronomical relevance in hybrid breeding. In gametophytic CMS, abortion of pollen is determined by the grain genotype, while in sporophytic CMS, it is determined by the mother plant genotype. While several CMS mechanisms have been dissected at the molecular level, gametophytic CMS has not been straightforwardly accessible. We used the gametophytic Sha-CMS in Arabidopsis to characterize the cause and process of pollen abortion by implementing in vivo biosensing in single pollen and mitoTALEN mutagenesis. We obtained conclusive evidence that orf117Sha is the CMS-causing gene, despite distinct characteristics from other CMS genes. We measured the in vivo cytosolic ATP content in single pollen, followed pollen development, and analyzed pollen mitochondrial volume in two genotypes that differed only by the presence of the orf117Sha locus. Our results showed that the Sha-CMS is not triggered by ATP deficiency. Instead, we observed desynchronization of a pollen developmental program. Pollen death occurred independently in pollen grains at diverse stages and was preceded by mitochondrial swelling. We conclude that pollen death is grain-autonomous in Sha-CMS and propose that mitochondrial permeability transition, which was previously described as a hallmark of developmental and environmental-triggered cell death programs, precedes pollen death in Sha-CMS.

Molecular genetics of cytoplasmic male sterility and restorer-of-fertility for the fine tuning of pollen production in crops

Cytoplasmic male sterility (CMS) is an increasingly important issue within the context of hybrid seed production. Its genetic framework is simple: S-cytoplasm for male sterility induction and dominant allele of the restorer-of-fertility gene (Rf) for suppression of S. However, breeders sometimes encounter a phenotype of CMS plants too complex to be explained via this simple model. The molecular basis of CMS provides clue to the mechanisms that underlie the expression of CMS. Mitochondria have been associated with S, and several unique ORFs to S-mitochondria are thought to be responsible for the induction of male sterility in various crops. Their functions are still the subject of debate, but they have been hypothesized to emit elements that trigger sterility. Rf suppresses the action of S by various mechanisms. Some Rfs, including those that encode the pentatricopeptide repeat (PPR) protein and other proteins, are now considered members of unique gene families that are specific to certain lineages. Additionally, they are thought to be complex loci in which several genes in a haplotype simultaneously counteract an S-cytoplasm and differences in the suite of genes in a haplotype can lead to multiple allelism including strong and weak Rf at phenotypic level. The stability of CMS is influenced by factors such as the environment, cytoplasm, and genetic background; the interaction of these factors is also important. In contrast, unstable CMS becomes inducible CMS if its expression can be controlled. CMS becomes environmentally sensitive in a genotype-dependent manner, suggesting the feasibility of controlling the expression of CMS.

Transcriptomic and Proteomic Analyses of Celery Cytoplasmic Male Sterile Line and Its Maintainer Line

Male sterility is a common phenomenon in the plant kingdom and based on the organelles harboring the male-sterility genes, it can be classified into the genic male sterility (GMS) and the cytoplasmic male sterility (CMS). In every generation, CMS can generate 100% male-sterile population, which is very important for the breeders to take advantage of the heterosis and for the seed producers to guarantee the seed purity. Celery is a cross-pollinated plant with the compound umbel type of inflorescence which carries hundreds of small flowers. These characteristics make CMS the only option to produce the commercial hybrid celery seeds. In this study, transcriptomic and proteomic analyses were performed to identify genes and proteins that are associated with celery CMS. A total of 1255 differentially expressed genes (DEGs) and 89 differentially expressed proteins (DEPs) were identified between the CMS and its maintainer line, then 25 genes were found to differentially expressed at both the transcript and protein levels. Ten DEGs involved in the fleece layer and outer pollen wall development were identified by Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses, most of which were down-regulated in the sterile line W99A. These DEGs and DEPs were mainly enriched in the pathways of "phenylpropanoid/sporopollenin synthesis/metabolism", "energy metabolism", "redox enzyme activity" and "redox processes". Results obtained in this study laid a foundation for the future investigation of mechanisms of pollen development as well as the reasons for the CMS in celery.

Mechanism and Utilization of Ogura Cytoplasmic Male Sterility in Cruciferae Crops

Hybrid production using lines with cytoplasmic male sterility (CMS) has become an important way to utilize heterosis in vegetables. Ogura CMS, with the advantages of complete pollen abortion, ease of transfer and a progeny sterility rate reaching 100%, is widely used in cruciferous crop breeding. The mapping, cloning, mechanism and application of Ogura CMS and fertility restorer genes in Brassica napus, Brassica rapa, Brassica oleracea and other cruciferous crops are reviewed herein, and the existing problems and future research directions in the application of Ogura CMS are discussed.

Integrative Analysis of Transcriptomic and Proteomic Changes Related to Cytoplasmic Male Sterility in Spring Stem Mustard (Brassica juncea var. tumida Tsen et Lee)

The development of flower and pollen is a complex biological process that involves multiple metabolic pathways in plants. In revealing novel insights into flower and pollen development underlying male sterility (MS), we conducted an integrated profiling of gene and protein activities in developing buds in cytoplasmic male sterile (CMS) mutants of mustard (Brassica juncea). Using RNA-Seq and label-free quantitative proteomics, 11,832 transcripts and 1780 protein species were identified with significant differential abundance between the male sterile line 09-05A and its maintainer line 09-05B at the tetrad stage and bi-nucleate stage of B. juncea. A large number of differentially expressed genes (DEGs) and differentially abundant proteins (DAPs) involved in carbohydrate and energy metabolism, including starch and sucrose metabolism, tricarboxylic acid (TCA) cycle, glycolysis, and oxidoreductase activity pathways, were significantly downregulated in 09-05A buds. The low expression of these DEGs or functional loss of DAPs, which can lead to an insufficient supply of critical substrates and ATP, could be associated with flower development, pollen development, and changes in fertility in B. juncea. Therefore, this study provided transcriptomic and proteomic information of pollen abortion for B. juncea and a basis for further research on the molecular regulatory mechanism of MS in plants.

Combined Transcriptome and Proteome Analysis of Anthers of AL-type Cytoplasmic Male Sterile Line and Its Maintainer Line Reveals New Insights into Mechanism of Male Sterility in Common Wheat

Cytoplasmic male sterility (CMS) plays an essential role in hybrid seeds production. In wheat, orf279 was reported as a CMS gene of AL-type male sterile line (AL18A), but its sterility mechanism is still unclear. Therefore, transcriptomic and proteomic analyses of the anthers of AL18A and its maintainer line (AL18B) were performed to interpret the sterility mechanism. Results showed that the electron transport chain and ROS scavenging enzyme expression levels changed in the early stages of the anther development. Biological processes, i.e., fatty acid synthesis, lipid transport, and polysaccharide metabolism, were abnormal, resulting in pollen abortion in AL18A. In addition, we identified several critical regulatory genes related to anther development through combined analysis of transcriptome and proteome. Most of the genes were enzymes or transcription factors, and 63 were partially homologous to the reported genic male sterile (GMS) genes. This study provides a new perspective of the sterility mechanism of AL18A and lays a foundation to study the functional genes of anther development.

Current understanding of male sterility systems in vegetable Brassicas and their exploitation in hybrid breeding

Overview of the current status of GMS and CMS systems available in Brassica vegetables, their molecular mechanism, wild sources of sterile cytoplasm and exploitation of male sterility in hybrid breeding. The predominantly herbaceous family Brassicaceae (crucifers or mustard family) encompasses over 3700 species, and many of them are scientifically and economically important. The genus Brassica is an economically important genus within the tribe Brassicaceae that comprises important vegetable, oilseed and fodder crops. Brassica vegetables display strong hybrid vigor, and heterosis breeding is the integral part in their improvement. Commercial production of F₁ hybrid seeds in Brassica vegetables requires an effective male sterility system. Among the available male sterility systems, cytoplasmic male sterility (CMS) is the most widely exploited in Brassica vegetables. This system is maternally inherited and studied intensively. A limited number of reports about the genic male sterility (GMS) are available in Brassica vegetables. The GMS system is reported to be dominant, recessive and trirecessive in nature in different species. In this review, we discuss the available male sterility systems in Brassica vegetables and their potential use in hybrid breeding. The molecular mechanism of mt-CMS and causal mitochondrial genes of CMS has been discussed in detail. Finally, the exploitation of male sterility system in heterosis breeding of Brassica vegetables, future prospects and need for further understanding of these systems are highlighted.

Abnormal tapetum development and energy metabolism associated with sterility in SaNa-1A CMS of Brassica napus L

Abnormal tapetum degradation and anther development in cytoplasmic male sterility SaNa-1A are the main reasons for the anther abortion. SaNa-1A is a novel cytoplasmic male sterility (CMS) line of Brassica napus derived from somatic hybrids of B. napus-Sinapis alba, and SaNa-1B is the corresponding maintainer line. Ultrastructural comparison between developing anthers of sterile and maintainer lines revealed abnormal subcellular structure of pollen mother cells (PMCs) in the CMS line. The PMC volume and size of nucleus and nucleolus in the CMS line were smaller than those in the maintainer line. The abnormal tapetum cell development and delayed tapetum degradation inhibited microspore development. Finally, anther abortion in the CMS line occurred. Physiological and biochemical analyses of developing anthers and mitochondria revealed that over-accumulation of reactive oxygen species (ROS) in the SaNa-1A and deficiency in antioxidant enzyme system aggravated the oxidization of membrane lipids, resulting in malondialdehyde (MDA) accumulation in anthers. High MDA content in the CMS line was toxic to the cells. ROS accumulation in SaNa-1A also affected anther development. Abnormal structure and function of terminal oxidase, which participates in the electron transport chain of mitochondrial membrane, were observed and affected the activity of cytochrome c oxidase and F₁F₀-ATPase, which inhibited ATP biosynthesis. Proline deficiency in SaNa-1A also affected anther development. Few hybridization signals of programmed cell death (PCD) in tetrads of SaNa-1A were identified using TdT-mediated dUTP Nick-End Labeling assay. PCD was not obvious in tapetum cells of SaNa-1A until the unicellular stage. These results validated the cytological differences mentioned above, and proved that abnormal tapetum degradation and anther development in SaNa-1A were the main reasons for the anther abortion.

Proportional subcellular localization of Arabidopsis thaliana RabA1a

Subcellular localization of trafficking proteins in a single cell affects the assembly of trafficking machinery between organelles and vesicles throughout the targeting pathway. RabGTPase is one of the regulators to direct specific targeting of cargo molecules depending on GDP/GTP bound status. We have recently determined the crystal structures of GDP-bound inactive and both GTP- and GppNHp-bound active forms of Arabidopsis RabA1a. It is notable that the switch regions of RabA1a exhibit conformational changes derived by GDP or GTP binding. However, it was not clear that where the GDP- or GTP-bound RabA1a is localized at the subcellular level in a cell. Here we demonstrate that the distinct proportion of subcellular localization of RabA1a depends on its site-specific mutation as the GDP- or GTP-bound form. RabA1a proteins located at the plasma membrane, endosomes, and cytosol. While the GDP-bound form of RabA1a^S27N located more at endosomes than the plasma membrane compared to the proportions of RabA1a wild-type, and the GTP-bound RabA1a^Q72L located mainly at the plasma membrane in comparison to RabA1a wild-type and RabA1a^S27N. These distinct proportional localizations of RabA1a enable a cognate interaction between inactive/active RabA1 and effector molecules to direct specific targeting of its cargo molecules.

Identification and characterization of a semi-dominant restorer-of-fertility 1 allele in sugar beet (Beta vulgaris)

The sugar beet Rf1 locus has a number of molecular variants. We found that one of the molecular variants is a weak allele of a previously identified allele. Male sterility (MS) caused by nuclear-mitochondrial interaction is called cytoplasmic male sterility (CMS) in which MS-inducing mitochondria are suppressed by a nuclear gene, restorer-of-fertility. Rf and rf are the suppressing and non-suppressing alleles, respectively. This dichotomic view, however, seems somewhat unsatisfactory to explain the recently discovered molecular diversity of Rf loci. In the present study, we first identified sugar beet line NK-305 as a new source of Rf1. Our crossing experiment revealed that NK-305 Rf1 is likely a semi-dominant allele that restores partial fertility when heterozygous but full fertility when homozygous, whereas Rf1 from another sugar beet line appeared to be a dominant allele. Proper degeneration of anther tapetum is a prerequisite for pollen development; thus, we compared tapetal degeneration in the NK-305 Rf1 heterozygote and the homozygote. Degeneration occurred in both genotypes but to a lesser extent in the heterozygote, suggesting an association between NK-305 Rf1 dose and incompleteness of tapetal degeneration leading to partial fertility. Our protein analyses revealed a quantitative correlation between NK-305 Rf1 dose and a reduction in the accumulation of a 250 kDa mitochondrial protein complex consisting of a CMS-specific mitochondrial protein encoded by MS-inducing mitochondria. The abundance of Rf1 transcripts correlated with NK-305 Rf1 dose. The molecular organization of NK-305 Rf1 suggested that this allele evolved through intergenic recombination. We propose that the sugar beet Rf1 locus has a series of multiple alleles that differ in their ability to restore fertility and are reflective of the complexity of Rf evolution.

iTRAQ-Based Proteomic Analysis of Ogura-CMS Cabbage and Its Maintainer Line

Ogura cytoplasmic male sterility (CMS) contributes considerably to hybrid seed production in Brassica crops. To detect the key protein species and pathways involved in Ogura-CMS, we analysed the proteome of the cabbage Ogura-CMS line CMS01-20 and its corresponding maintainer line F01-20 using the isobaric tags for the relative and absolute quantitation (iTRAQ) approach. In total, 162 differential abundance protein species (DAPs) were identified between the two lines, of which 92 were down-accumulated and 70 were up-accumulated in CMS01-20. For energy metabolism in the mitochondrion, eight DAPs involved in oxidative phosphorylation were down-accumulated in CMS01-20, whereas in the tricarboxylic acid (TCA) cycle, five DAPs were up-accumulated, which may compensate for the decreased respiration capacity and may be associated with the elevated O2 consumption rate in Ogura-CMS plants. Other key protein species and pathways involved in pollen wall assembly and programmed cell death (PCD) were also identified as being male-sterility related. Transcriptome profiling revealed 3247 differentially expressed genes between the CMS line and the fertile line. In a conjoint analysis of the proteome and transcriptome data, 30 and 9 protein species/genes showed the same and opposite accumulation patterns, respectively. Nine noteworthy genes involved in sporopollenin synthesis, callose wall degeneration, and oxidative phosphorylation were presumably associated with the processes leading to male sterility, and their expression levels were validated by qRT-PCR analysis. This study will improve our understanding of the protein species involved in pollen development and the molecular mechanisms underlying Ogura-CMS.

Transcript levels of orf288 are associated with the hau cytoplasmic male sterility system and altered nuclear gene expression in Brassica juncea

Cytoplasmic male sterility (CMS) is primarily caused by chimeric genes located in the mitochondrial genomes. In Brassica juncea, orf288 has been identified as a CMS-associated gene in the hau CMS line; however, neither the specific abortive stage nor the molecular function of the gene have been determined. We therefore characterized the hau CMS line, and found that defective mitochondria affect the development of archesporial cells during the L2 stage, leading to male sterility. The expression level of the orf288 transcript was higher in the male-sterility line than in the fertility-restorer line, although no significant differences were apparent at the protein level. The toxicity region of ORF288 was found to be located near the N-terminus and repressed growth of Escherichia coli. However, transgenic expression of different portions of ORF288 indicated that the region that causes male sterility resides between amino acids 73 and 288, the expression of which in E. coli did not result in growth inhibition. Transcriptome analysis revealed a wide range of genes involved in anther development and mitochondrial function that were differentially expressed in the hau CMS line. This study provides new insights into the hau CMS mechanism by which orf288 affects the fertility of Brassica juncea.

The Role of Non-Coding RNAs in Cytoplasmic Male Sterility in Flowering Plants

The interactions between mitochondria and nucleus substantially influence plant development, stress response and morphological features. The prominent example of a mitochondrial-nuclear interaction is cytoplasmic male sterility (CMS), when plants produce aborted anthers or inviable pollen. The genes responsible for CMS are located in mitochondrial genome, but their expression is controlled by nuclear genes, called fertility restorers. Recent explosion of high-throughput sequencing methods enabled to study transcriptomic alterations in the level of non-coding RNAs under CMS biogenesis. We summarize current knowledge of the role of nucleus encoded regulatory non-coding RNAs (long non-coding RNA, microRNA as well as small interfering RNA) in CMS. We also focus on the emerging data of non-coding RNAs encoded by mitochondrial genome and their possible involvement in mitochondrial-nuclear interactions and CMS development.

Non-coding RNA may be associated with cytoplasmic male sterility in Silene vulgaris

Cytoplasmic male sterility (CMS) is a widespread phenomenon in flowering plants caused by mitochondrial (mt) genes. CMS genes typically encode novel proteins that interfere with mt functions and can be silenced by nuclear fertility-restorer genes. Although the molecular basis of CMS is well established in a number of crop systems, our understanding of it in natural populations is far more limited. To identify CMS genes in a gynodioecious plant, Silene vulgaris, we constructed mt transcriptomes and compared transcript levels and RNA editing patterns in floral bud tissue from female and hermaphrodite full siblings. The transcriptomes from female and hermaphrodite individuals were very similar overall with respect to variation in levels of transcript abundance across the genome, the extent of RNA editing, and the order in which RNA editing and intron splicing events occurred. We found only a single genomic region that was highly overexpressed and differentially edited in females relative to hermaphrodites. This region is not located near any other transcribed elements and lacks an open-reading frame (ORF) of even moderate size. To our knowledge, this transcript would represent the first non-coding mt RNA associated with CMS in plants and is, therefore, an important target for future functional validation studies.

Involvement of a universal amino acid synthesis impediment in cytoplasmic male sterility in pepper

To explore the mechanisms of pepper (Capsicum annuum L.) cytoplasmic male sterility (CMS), we studied the different maturation processes of sterile and fertile pepper anthers. A paraffin section analysis of the sterile anthers indicated an abnormality of the tapetal layer and an over-vacuolization of the cells. The quantitative proteomics results showed that the expression of histidinol dehydrogenase (HDH), dihydroxy-acid dehydratase (DAD), aspartate aminotransferase (ATAAT), cysteine synthase (CS), delta-1-pyrroline-5-carboxylate synthase (P5CS), and glutamate synthetase (GS) in the amino acid synthesis pathway decreased by more than 1.5-fold. Furthermore, the mRNA and protein expression levels of DAD, ATAAT, CS and P5CS showed a 2- to 16-fold increase in the maintainer line anthers. We also found that most of the amino acid content levels decreased to varying degrees during the anther tapetum period of the sterile line, whereas these levels increased in the maintainer line. The results of our study indicate that during pepper anther development, changes in amino acid synthesis are significant and accompany abnormal tapetum maturity, which is most likely an important cause of male sterility in pepper.

Post-translational mechanisms are associated with fertility restoration of cytoplasmic male sterility in sugar beet (Beta vulgaris)

Genetic conflict between cytoplasmically inherited elements and nuclear genes arising from their different transmission patterns can be seen in cytoplasmic male sterility (CMS), the mitochondrion-encoded inability to shed functional pollen. CMS is associated with a mitochondrial open reading frame (ORF) that is absent from non-sterility inducing mitochondria (S-orf). Nuclear genes that suppress CMS are called restorer-of-fertility (Rf) genes. Post-transcriptional and translational repression of S-orf mediates the molecular action of Rf that encodes a class of RNA-binding proteins with pentatricopeptide repeat (PPR) motifs. Besides the PPR-type of Rfs, there are also non-PPR Rfs, but the molecular interactions between non-PPR Rf and S-orf have not been described. In this study, we investigated the interaction of bvORF20, a non-PPR Rf from sugar beet (Beta vulgaris), with preSatp6, the S-orf from sugar beet. Anthers expressing bvORF20 contained a protein that interacted with preSATP6 protein. Analysis of anthers and transgenic calli expressing a FLAG-tagged bvORF20 suggested the binding of preSATP6 to bvORF20. To see the effect of bvORF20 on preSATP6, which exists as a 250-kDa protein complex in CMS plants, signal bands of preSATP6 in bvORF20-expressing and non-expressing anthers were compared by immunoblotting combined with Blue Native polyacrylamide gel electrophoresis. The signal intensity of the 250-kDa band decreased significantly, and 200- and 150-kDa bands appeared in bvORF20-expressing anthers. Transgenic callus expressing bvORF20 also generated the 200- and 150-kDa bands. The 200-kDa complex is likely to include both preSATP6 and bvORF20. Post-translational interaction between preSATP6 and bvORF20 appears to alter the higher order structure of preSATP6 that may lead to fertility restoration in sugar beet.

In vivo functional analysis of a nuclear restorer PPR protein

BACKGROUND

Nuclear restorers of cytoplasmic male fertility (CMS) act to suppress the male sterile phenotype by down-regulating the expression of novel CMS-specifying mitochondrial genes. One such restorer gene is Rfo, which restores fertility to the radish Ogura or ogu CMS. Rfo, like most characterized restorers, encodes a pentatricopeptide repeat (PPR) protein, a family of eukaryotic proteins characterized by tandem repeats of a 35 amino acid motif. While over 400 PPR genes are found in characterized plant genomes and the importance of this gene family in organelle gene expression is widely recognized, few detailed in vivo assessments of primary structure-function relationships in this protein family have been conducted.

RESULTS

In contrast to earlier studies, which identified 16 or 17 PPR domains in the Rfo protein, we now find, using a more recently developed predictive tool, that Rfo has 18 repeat domains with the additional domain N-terminal to the others. Comparison of transcript sequences from pooled rfo/rfo plants with pooled Rfo/Rfo plants of a mapping population led to the identification of a non-restoring rfo allele with a 12 bp deletion in the fourth domain. Introduction into ogu CMS plants of a genetic construct in which this deletion had been introduced into Rfo led to a partial loss in the capacity to produce viable pollen, as assessed by vital staining, pollen germination and the capacity for seed production following pollination of CMS plants. The degree of viable pollen production among different transgenic plants roughly correlated with the copy number of the introduced gene and with the reduction of the levels of the ORF138 CMS-associated protein. All other constructs tested, including one in which only the C-terminal PPR repeat was deleted and another in which this repeat was replaced by the corresponding domain of the related, non-restoring gene, PPR-A, failed to result in any measure of fertility restoration.

CONCLUSIONS

The identification of the additional PPR domain in Rfo indicates that the protein, apart from its N-terminal mitochondrial targeting presequence, consists almost entirely of PPR repeats. The newly identified rfo allele carries the same 4 amino acid deletion as that found in the neighboring, related, non-restoring PPR gene, PPR-A. Introduction of this four amino acid deletion into a central domain the Rfo protein, however, only partially reduces its restoration capacity, even though this alteration might be expected to alter the spacing between the adjoining repeats. All other tested alterations, generated by deleting specific PPR repeats or exchanging repeats with corresponding domains of PPR-A, led to a complete loss of restorer function. Overall we demonstrate that introduction of targeted alterations of Rfo into ogu CMS plants provides a sensitive in vivo readout for analysis of the relationship between primary structure and biological function in this important family of plant proteins.

Cytoplasmic male sterility and mitochondrial metabolism in plants

Cytoplasmic male sterility (CMS) is a common feature encountered in plant species. It is the result of a genomic conflict between the mitochondrial and the nuclear genomes. CMS is caused by mitochondrial encoded factors which can be counteracted by nuclear encoded factors restoring male fertility. Despite extensive work, the molecular mechanism of male sterility still remains unknown. Several studies have suggested the involvement of respiration on the disruption of pollen production through an energy deficiency. By comparing recent works on CMS and respiratory mutants, we suggest that the "ATP hypothesis" might not be as obvious as previously suggested.

Mitochondrion role in molecular basis of cytoplasmic male sterility

Cytoplasmic male sterility and its fertility restoration via nuclear genes offer the possibility to understand the role of mitochondria during microsporogenesis. In most cases rearrangements in the mitochondrial DNA involving known mitochondrial genes as well as unknown sequences result in the creation of new chimeric open reading frames, which encode proteins containing transmembrane domains. So far, most of the CMS systems have been characterized via restriction fragment polymorphisms followed by transcript analysis. However, whole mitochondrial genome sequence analyses comparing male sterile and fertile cytoplasm open options for deeper insights into mitochondrial genome rearrangements. We more and more start to unravel how mitochondria are involved in triggering death of the male reproductive organs. Reduced levels of ATP accompanied by increased concentrations of reactive oxygen species, which are produced more under conditions of mitochondrial dysfunction, seem to play a major role in the fate of pollen production. Nuclear genes, so called restorer-of-fertility are able to restore the male fertility. Fertility restoration can occur via pentatricopeptide repeat (PPR) proteins or via different mechanisms involving non-PPR proteins.

Male sterility and fertility restoration in crops

In plants, male sterility can be caused either by mitochondrial genes with coupled nuclear genes or by nuclear genes alone; the resulting conditions are known as cytoplasmic male sterility (CMS) and genic male sterility (GMS), respectively. CMS and GMS facilitate hybrid seed production for many crops and thus allow breeders to harness yield gains associated with hybrid vigor (heterosis). In CMS, layers of interaction between mitochondrial and nuclear genes control its male specificity, occurrence, and restoration of fertility. Environment-sensitive GMS (EGMS) mutants may involve epigenetic control by noncoding RNAs and can revert to fertility under different growth conditions, making them useful breeding materials in the hybrid seed industry. Here, we review recent research on CMS and EGMS systems in crops, summarize general models of male sterility and fertility restoration, and discuss the evolutionary significance of these reproductive systems.

ORFH79 impairs mitochondrial function via interaction with a subunit of electron transport chain complex III in Honglian cytoplasmic male sterile rice

Cytoplasmic male sterility (CMS) has attracted great interest because of its application in crop breeding. Despite increasing knowledge of CMS, not much is understood about its molecular mechanisms. Previously, orfH79 was cloned and identified as the CMS gene in Honglian rice, but how the ORFH79 protein causes pollen abortion is still unknown. Through bacterial two-hybrid library screening, P61, a subunit of the mitochondrial electron transport chain (ETC) complex III, was selected as a candidate that interacts with ORFH79. Bimolecular fluorescence complementation (BiFC) and coimmunoprecipitation (coIP) assays verified their interaction inside mitochondria. Blue native polyacrylamide gel electrophoresis (BN-PAGE) and western blotting showed ORF79 and P61 colocalized in mitochondrial ETC complex III of CMS lines. Compared with the maintainer line, Yuetai B (YB), a significant decrease of enzyme activity was detected in mitochondrial complex III of the CMS line, Yuetai A (YA), which resulted in decreased ATP concentrations and an increase in the reactive oxygen species (ROS) content. We propose that the CMS protein, ORFH79, can bind to complex III and decrease its enzyme activity through interaction with P61. This defect results in energy production dysfunction and oxidative stress in mitochondria, which may work as retrograde signals that lead to abnormal pollen development.

Ogura-CMS in Chinese cabbage (Brassica rapa ssp. pekinensis) causes delayed expression of many nuclear genes

We investigated the mechanism regulating cytoplasmic male sterility (CMS) in Brassica rapa ssp. pekinensis using floral bud transcriptome analyses of Ogura-CMS Chinese cabbage and its maintainer line in B. rapa 300-K oligomeric probe (Br300K) microarrays. Ogura-CMS Chinese cabbage produced few and infertile pollen grains on indehiscent anthers. Compared to the maintainer line, CMS plants had shorter filaments and plant growth, and delayed flowering and pollen development. In microarray analysis, 4646 genes showed different expression, depending on floral bud size, between Ogura-CMS and its maintainer line. We found 108 and 62 genes specifically expressed in Ogura-CMS and its maintainer line, respectively. Ogura-CMS line-specific genes included stress-related, redox-related, and B. rapa novel genes. In the maintainer line, genes related to pollen coat and germination were specifically expressed in floral buds longer than 3mm, suggesting insufficient expression of these genes in Ogura-CMS is directly related to dysfunctional pollen. In addition, many nuclear genes associated with auxin response, ATP synthesis, pollen development and stress response had delayed expression in Ogura-CMS plants compared to the maintainer line, which is consistent with the delay in growth and development of Ogura-CMS plants. Delayed expression may reduce pollen grain production and/or cause sterility, implying that mitochondrial, retrograde signaling delays nuclear gene expression.

Alterations of mitochondrial protein assembly and jasmonic acid biosynthesis pathway in Honglian (HL)-type cytoplasmic male sterility rice

It has been suggested that the mitochondrial chimeric gene orfH79 is the cause for abortion of microspores in Honglian cytoplasmic male sterile rice, yet little is known regarding its mechanism of action. In this study, we used a mass spectrometry-based quantitative proteomics strategy to compare the mitochondrial proteome between the sterile line Yuetai A and its fertile near-isogenic line Yuetai B. We discovered a reduced quantity of specific proteins in mitochondrial complexes in Yuetai A compared with Yuetai B, indicating a defect in mitochondrial complex assembly in the sterile line. Western blotting showed that ORFH79 protein and ATP1 protein, an F(1) sector component of complex V, are both associated with large protein complexes of similar size. Respiratory complex activity assays and transmission electron microscopy revealed functional and morphological defects in the mitochondria of Yuetai A when compared with Yuetai B. In addition, we identified one sex determination TASSELSEED2-like protein increased in Yuetai A, leading to the discovery of an aberrant variation of the jasmonic acid pathway during the development of microspores.

A male sterility-associated cytotoxic protein ORF288 in Brassica juncea causes aborted pollen development

Cytoplasmic male sterility (CMS) is a widespread phenomenon in higher plants, and several studies have established that this maternally inherited defect is often associated with a mitochondrial mutant. Approximately 10 chimeric genes have been identified as being associated with corresponding CMS systems in the family Brassicaceae, but there is little direct evidence that these genes cause male sterility. In this study, a novel chimeric gene (named orf288) was found to be located downstream of the atp6 gene and co-transcribed with this gene in the hau CMS sterile line. Western blotting analysis showed that this predicted open reading frame (ORF) was translated in the mitochondria of male-sterile plants. Furthermore, the growth of Escherichia coli was significantly repressed in the presence of ORF288, which indicated that this protein is toxic to the E. coli host cells. To confirm further the function of orf288 in male sterility, the gene was fused to a mitochondrial-targeting pre-sequence under the control of the Arabidopsis APETALA3 promoter and introduced into Arabidopsis thaliana. Almost 80% of transgenic plants with orf288 failed to develop anthers. It was also found that the independent expression of orf288 caused male sterility in transgenic plants, even without the transit pre-sequence. Furthermore, transient expression of orf288 and green fluorescent protein (GFP) as a fused protein in A. thaliana protoplasts showed that ORF288 was able to anchor to mitochondria even without the external mitochondrial-targeting peptide. These observations provide important evidence that orf288 is responsible for the male sterility of hau CMS in Brassica juncea.

Cost of Having the Largest Mitochondrial Genome: Evolutionary Mechanism of Plant Mitochondrial Genome

The angiosperm mitochondrial genome is the largest and least gene-dense among the eukaryotes, because its intergenic regions are expanded. There seems to be no functional constraint on the size of the intergenic regions; angiosperms maintain the large mitochondrial genome size by a currently unknown mechanism. After a brief description of the angiosperm mitochondrial genome, this review focuses on our current knowledge of the mechanisms that control the maintenance and alteration of the genome. In both processes, the control of homologous recombination is crucial in terms of site and frequency. The copy numbers of various types of mitochondrial DNA molecules may also be controlled, especially during transmission of the mitochondrial genome from one generation to the next. An important characteristic of angiosperm mitochondria is that they contain polypeptides that are translated from open reading frames created as byproducts of genome alteration and that are generally nonfunctional. Such polypeptides have potential to evolve into functional ones responsible for mitochondrially encoded traits such as cytoplasmic male sterility or may be remnants of the former functional polypeptides.