Mechanism Of Cytoplasmic Male Sterility Research Articles

The mitochondrial orf117Sha gene desynchronizes pollen development and causes pollen abortion in the Arabidopsis Sha CMS.

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 show 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.

The vesicle trafficking gene, OsRab7, is critical for pollen development and male fertility in cytoplasmic male-sterility rice

Rice cytoplasmic male sterility (CMS) provides an exceptional model for studying genetic interaction within plant nuclei given its inheritable trait of non-functional male gametophyte. Gaining a comprehensive understanding of the genes and pathways associated with the CMS mechanism is imperative for improving the vigor of hybrid rice agronomically, such as its productivity. Here, we observed a significant decrease in the expression of a gene named OsRab7 in the anther of the CMS line (SJA) compared to the maintainer line (SJB). OsRab7 is responsible for vesicle trafficking and loss function of OsRab7 significantly reduced pollen fertility and setting rate relative to the wild type. Meanwhile, over-expression of OsRab7 enhanced pollen fertility in the SJA line while a decrease in its expression in the SJB line led to the reduced pollen fertility. Premature tapetum and abnormal development of microspores were observed in the rab7 mutant. The expression of critical genes involved in tapetum development (OsMYB103, OsPTC1, OsEAT1 and OsAP25) and pollen development (OsMSP1, OsDTM1 and OsC4) decreased significantly in the anther of rab7 mutant. Reduced activities of the pDR5::GUS marker in the young panicle and anther of the rab7 mutant were also observed. Furthermore, the mRNA levels of genes involved in auxin biosynthesis (YUCCAs), auxin transport (PINs), auxin response factors (ARFs), and members of the IAA family (IAAs) were all downregulated in the rab7 mutant, indicating its impact on auxin signaling and distribution. In summary, these findings underscore the importance of OsRab7 in rice pollen development and its potential link to cytoplasmic male sterility.

Abnormal Calcium Accumulation and ROS Homeostasis-Induced Tapetal Programmed Cell Death Lead to Pollen Abortion of Petaloid-Type Cytoplasmic Male Sterility in Camellia oleifera

Cytoplasmic male sterility (CMS) plays a crucial role in the utilization of heterosis. The petaloid anther abortion in oil tea (Camellia oleifera Abel.) constitutes a CMS phenomenon, which is of great value for the hybrid breeding of oil tea. However, as the mechanism of its CMS is still poorly understood, it is necessary to study the cytology and physiological characteristics of anther abortion. In this study, a C. oleifera cultivar, Huashuo (HS), and its petalized CMS mutant (HSP) were used as materials to explore this mechanism. Compared with HS, cytological analysis demonstrated that HSP showed early-onset tapetum programmed cell death (PCD) and an organelle disorder phenotype during the tetrad stage. In HSP, anthers exhibited elevated levels of calcium deposition in anther wall tissues, tapetum layers, and microspores, and yet calcium accumulation was abnormal at the later stage. The contents of hydrogen peroxide and MDA in HSP anthers were higher, and the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) were lower than those of HS, which resulted in an excessive accumulation of reactive oxygen species (ROS). Real-time quantitative PCR confirmed that the transcription levels of CoPOD and CoCAT genes encoding key antioxidant enzymes in HSP were downregulated compared with HS in early pollen development; the gene CoCPK, which encodes a calcium-dependent protein kinase associated with antioxidase, was upregulated during the critical period. Thus, we suggest that excessive ROS as a signal breaks the balance of the antioxidant system, and along with an abnormal distribution of calcium ions, leads to the early initiation of PCD in the tapetum, and ultimately leads to pollen abortion for HSP. These results lay a cytological and physiological foundation for further studies on the CMS mechanism, and provide information for breeding male-sterile lines of C. oleifera.

Cloning and Expression Analysis of Onion (Allium cepa L.) MADS-Box Genes and Regulation Mechanism of Cytoplasmic Male Sterility.

Flower organ development is one of the most important processes in plant life. However, onion CMS (cytoplasmic male sterility) shows an abnormal development of floral organs. The regulation of MADS-box transcription factors is important for flower development. To further understand the role of MADS-box transcription factors in the regulation of cytoplasmic male sterility onions. We cloned the full-length cDNA of five MADS-box transcription factors from the flowers of onion using RACE (rapid amplification of cDNA ends) technology. We used bioinformatics methods for sequence analysis and phylogenetic analysis. Real-time quantitative PCR was used to detect the expression patterns of these genes in different onion organs. The relative expression levels of five flower development genes were compared in CMS onions and wild onions. The results showed that the full-length cDNA sequences of the cloned MADS-box genes AcFUL, AcDEF, AcPI, AcAG, and AcSEP3 belonged to A, B, C, and E MADS-box genes, respectively. A phylogenetic tree construction analysis was performed on its sequence. Analysis of MADS-box gene expression in wild onion and CMS onion showed that the formation of CMS onion was caused by down-regulation of AcDEF, AcPI, and AcAG gene expression, up-regulation of AcSEP3 gene expression, and no correlation with AcFUL gene expression. This work laid the foundation for further study of the molecular mechanism of onion flower development and the molecular mechanism of CMS onion male sterility.

Lateral metabolome study reveals the molecular mechanism of cytoplasmic male sterility (CMS) in Chinese cabbage

BackgroundChinese cabbage is one of the most widely grown leafy vegetables in China. Cytoplasmic male sterility (CMS) is a maternally inherited trait that produces abnormal pollen during anther development, which is commonly seen in cruciferous vegetables. However, the molecular mechanism of Chinese cabbage CMS is not clear. In this study, the metabolome and hormone profiles of Chinese cabbage male sterile line (CCR20000) and sterile maintainer line (CCR20001) were analyzed in flower buds during normal stamen development and abnormal stamen development, respectively.ResultsA total of 556 metabolites were detected based on UPLC-MS/MS detection platform and database search, and the changes of hormones such as auxin, cytokinins, abscisic acid, jasmonates, salicylic acid, gibberellin acid and ethylene were analyzed. The results showed that compared with the male fertile line (MF), the male sterile line (MS) significantly decreased the content of flavonoids and phenolamides metabolites in the stamen dysplasia stage, accompanied by a large accumulation of glucosinolate metabolites. Meanwhile, the contents of GA9, GA20, IBA, tZ and other hormones in MS were significantly lower than those in MF strains. Further, by comparing the metabolome changes of MF and MS during stamen dysplasia, it was found that flavonoid metabolites and amino acid metabolites were distinctly different.ConclusionsThese results suggest that flavonoids, phenolamides and glucosinolate metabolites may be closely related to the sterility of MS strains. This study provides an effective basis for further research on the molecular mechanism of CMS in Chinese cabbage.

Integrated transcriptome and proteome analysis provides insights into the mechanism of cytoplasmic male sterility (CMS) in tobacco (Nicotiana tabacum L.)

Cytoplasmic male sterility (CMS) is critical in maximizing crop yield and quality by utilizing tobacco heterosis. However, the mechanism of tobacco CMS formation remains unknown. Using paraffin section observation, transcriptome sequencing, and TMT proteomic analysis, this study describes the differences in expression profiles in morphology, transcription, and translation between the sua-CMS tobacco line (MSYY87) and its corresponding maintainer line (YY87). According to the microspore morphology, MSYY87 began to exhibit abnormal microspore development during the early stages of germination and differentiation (androgynous primordium differentiation stage). According to transcriptomic and proteomic analyses, 17 genes/proteins involved in lipid transport/binding and phenylpropane metabolism were significantly down-regulated at both the mRNA and protein levels. Through further analysis, we identified some key genes that may be involved in tobacco male sterility, including β-GLU related to energy metabolism, 4CL and bHLHs related to anther wall formation, nsLTPs related to pollen germination and anther cuticle, and bHLHs related to pollen tapetum degradation. We speculate that the down-regulation of these genes affects the normal physiological metabolism, making tobacco plants show male sterility. SignificanceCytoplasmic male sterility (CMS) plays a vital role in utilizing tobacco heterosis and enhancing crop yield and quality. We observed paraffin sections and conducted transcriptome sequencing and mitochondrial proteomics to examine the tobacco CMS line Yunyan 87 (MSYY87) and its maintainer line Yunyan 87 (YY87). The down-regulation expression of β-GLU resulted in insufficient ATP supply, which resulted in disordered energy metabolism. The down-regulation expression of 4CL, nsLTPs and bHLHs may affect the formation of anther wall and anther cuticle, pollen germination, as well as the degradation of pollen tapetum. These various abnormal physiological processes, the male sterility of tobacco is finally caused. The findings shed light on the molecular mechanisms of tobacco CMS and serve as a model for fertility research in other flowering plants.

Integrated Methylome and Transcriptome Analysis Provides Insights into the DNA Methylation Underlying the Mechanism of Cytoplasmic Male Sterility in Kenaf (Hibiscus cannabinus L.).

Cytoplasmic male sterility (CMS) is widely exploited in hybrid seed production. Kenaf is an important fiber crop with high heterosis. The molecular mechanism of kenaf CMS remains unclear, particularly in terms of DNA methylation. Here, using the anthers of a kenaf CMS line (P3A) and its maintainer line (P3B), comparative physiological, DNA methylation, and transcriptome analyses were performed. The results showed that P3A had considerably lower levels of IAA, ABA, photosynthetic products and ATP contents than P3B. DNA methylome analysis revealed 650 differentially methylated genes (DMGs) with 313 up- and 337 down methylated, and transcriptome analysis revealed 1788 differentially expressed genes (DEGs) with 558 up- and 1230 downregulated genes in P3A compared with P3B. Moreover, 45 genes were characterized as both DEGs and DMGs, including AUX, CYP, BGL3B, SUS6, AGL30 and MYB21. Many DEGs may be regulated by related DMGs based on methylome and transcriptome studies. These DEGs were involved in carbon metabolism, plant hormone signal transduction, the TCA cycle and the MAPK signaling pathway and were shown to be important for CMS in kenaf. These results provide new insights into the epigenetic mechanism of CMS in kenaf and other crops.

Comparative Analysis of Mitochondrial Genomes between the B-Type Cytoplasmic Male Sterility Line and Its Maintainer Line in Wheat

Cytoplasmic male sterility (CMS) is a complex phenomenon in plants, rendering them unable to produce functional pollen. In general, this is caused by an abnormal or dysfunctional mitochondrial genome. In wheat, however, the systematic structural characteristics of the mitochondrial genome from the CMS line, vis-à-vis its maintainer line, are rarely reported. Here, we identified the morphological characteristics, sequenced, assembled, and characterized the complete mitogenomes of the wheat B-type CMS line (B) and its maintainer line (YS9). The morphological results indicated that the B likely undergoes binucleate microspore abortion. The B and YS9 genomes were assembled into a typical circular molecule 452,794 and 452,453 bp in length, respectively, comprising 34 protein-coding genes (PCGs), 3 ribosomal RNA genes (rRNAs), and 16 transfer RNA genes (tRNAs). The codon usage analysis revealed leucine (Leu) and serine (Ser) as the most frequently used amino acid residues in the B and YS9 mitochondrial proteins. In particular, we uncovered a specific ORF2718, whose length of 501 bp was more 30 bp than that of the atp8 gene in the B genome, which perhaps could affect normal function of ATP8. Further, the existence of SNPs at the atp6 gene is probably associated with the CMS mechanism. This study suggests that sequencing and comparing the genomic features of the B and YS9 mitogenomes provides not only an important opportunity to conduct further genomic breeding studies, but also valuable information for future evolutionary and molecular studies of CMS in wheat.

Orf165 is associated with cytoplasmic male sterility in pepper.

Cytoplasmic male sterility (CMS) is a maternally inherited trait that derives from the inability to produce functional pollen in higher plants. CMS results from recombination of the mitochondrial genome. However, understanding of the molecular mechanism of CMS in pepper is limited. In this study, comparative transcriptomic analyses were performed using a near-isogenic CMS line 14A (CMS-14A) and a maintainer line 14B (ML-14B) as experimental materials. A total of 17,349 differentially expressed genes were detected between CMS-14A and ML-14B at the PMC meiosis stage. Among them, six unigenes associated with CMS and 108 unigenes involved in energy metabolism were identified. The gene orf165 was found in CMS-14A. When orf165 was introduced into ML-14B, almost 30% of transgenic plants were CMS. In addition, orf165 expression in transgenic CMS plants resulted in abnormal function of some genes involved in energy metabolism. When orf165 in transgenic CMS plant was silenced, the resulted orf165-silenced plant was male fertile and the expression patterns of some genes associated with energy metabolism were similar to ML-14B. Thus, we confirmed that orf165 influenced CMS in pepper.

Transcriptomic and proteomic analyses of a new cytoplasmic male sterile line with a wild Gossypium bickii genetic background

BackgroundCotton is an important fiber crop but has serious heterosis effects, and cytoplasmic male sterility (CMS) is the major cause of heterosis in plants. However, to the best of our knowledge,no studies have investigated CMS Yamian A in cotton with the genetic background of Australian wild Gossypium bickii. Conjoint transcriptomic and proteomic analysis was first performed between Yamian A and its maintainer Yamian B.ResultsWe detected 550 differentially expressed transcript-derived fragments (TDFs) and at least 1013 proteins in anthers at various developmental stages. Forty-two TDFs and 11 differentially expressed proteins (DEPs) were annotated by analysis in the genomic databases of G. austral, G. arboreum and G. hirsutum. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses were performed to better understand the functions of these TDFs and DEPs. Transcriptomic and proteomic results showed that UDP-glucuronosyl/UDP-glucosyltransferase, 60S ribosomal protein L13a-4-like, and glutathione S-transferase were upregulated; while heat shock protein Hsp20, ATPase, F0 complex, and subunit D were downregulated at the microspore abortion stage of Yamian A. In addition, several TDFs from the transcriptome and several DEPs from the proteome were detected and confirmed by quantitative real-time PCR as being expressed in the buds of seven different periods of development. We established the databases of differentially expressed genes and proteins between Yamian A and its maintainer Yamian B in the anthers at various developmental stages and constructed an interaction network based on the databases for a comprehensive understanding of the mechanism underlying CMS with a wild cotton genetic background.ConclusionWe first analyzed the molecular mechanism of CMS Yamian A from the perspective of omics, thereby providing an experimental basis and theoretical foundation for future research attempting to analyze the abortion mechanism of new CMS with a wild Gossypium bickii background and to realize three-line matching.

Discovery of Four Novel ORFs Responsible for Cytoplasmic Male Sterility (CMS) in Cotton (Gossypium hirsutum L.) through Comparative Analysis of the Mitochondrial Genomes of Four Isoplasmic Lines

Cytoplasmic male sterility (CMS) is an important feature for achieving heterosis in the development of hybrid crops. Mitochondria contribute to CMS, especially via mitochondrial DNA (mtDNA) rearrangements and chimeric genes. However, the mechanisms of CMS have not been fully elucidated, and the isonuclear alloplasmic lines used in previous studies have limited utility in cotton CMS research. In this study, three CMS lines (J4A-1, J4A-2 and J4A-3) and their isoplasmic maintainer line (J4B) were analyzed for mtDNA structural differences via high-throughput sequencing. The results showed that mtDNA was conserved (with similarities higher than 99%) among the three CMS lines and their isoplasmic maintainer line. All lines harbored 36 known protein-coding genes, 3 rRNAs, and 15 tRNAs. The protein-coding genes with non-synonymous mutations mainly encoded two types of proteins: ATPase and ribosomal proteins. Four new open reading frames (ORFs) (orf116b, orf186a-1, orf186a-2 and orf305a) were identified as candidate ORFs responsible for CMS. Two of the ORFs (orf186a-1 and orf186a-2) were identified as orf4 and orf4-2 of the upland cotton CMS line 2074A (a line with Gossypium harknessii Brandegee CMS-D2-2 cytoplasm), respectively. These findings provide a reference for CMS research in cotton or other crops.

Comparative analysis of mitochondrial genome and expression variation between UG93A and UG93B reveals a candidate gene related to cytoplasmic male sterility in kenaf

Plant cytoplasmic male sterility (CMS) is a maternal genetic trait that results from incompatibilities between the organellar and nuclear genomes which causes pollen abortion. Cytoplasmic male sterility plays an important role in hybrid seed production and heterosis utilization. In kenaf, cytoplasmic male sterility has been exploited in hybrid seed production for several years, but its molecular mechanism has not yet been disclosed. In the present work, the mitochondrial genome of the isonuclear cytoplasmic male sterile line UG93A compared with maintainer line UG93B was described and a total of 398 single nucleotide polymorphisms (SNPs) and 230 insertions/deletions (InDels) were identified in the UG93A mitochondrial DNA. A total of 26 single nucleotide polymorphisms and insertions/deletions were identified in the coding region of atp6, indicating active mitochondrial genome rearrangement. An expression cassette containing atp6 of UG93A without a fused mitochondrial transit peptide induced male sterility in tobacco and Arabidopsis. This indicated abnormal pollen grains and lack of pollen activity. Conclusively, coding sequence of atp6, which contains single nucleotide polymorphism and indel variations may play a crucial role in regulating male sterility in kenaf. These data suggested that atp6 of UG93A may be a candidate gene for kenaf cytoplasmic male sterility and provided basic information for further study of the molecular mechanisms of cytoplasmic male sterility in kenaf.

Advancement in CMS Based Hybrid Development in Cauliflower (Brassica oleracea var. Botrytis)

Among the different mechanism of male sterility operated in the Brassica group crop. Cytoplasmic male sterility mechanism is most suitable for hybrid development in cauliflower because here the curd (intermediate stage) is an edible part of the cauliflower. Further, there is no requirement of restorer line in this case as required in other seed crop. For the multiplication and maintenance of the different lines (A line and B line), sib mating and selfing is not always desirable. In fact, in such situation doubled haploid production through microspore culture is a more appropriate mechanism. Apart from this, the undesirable effect of integration of male sterile cytoplasm can be mitigated by adopting the repeated back crossing, through chloroplast substitution or somatic hybridization mechanism.

Comparative transcriptome analysis in Chinese cabbage (Brassica rapa ssp. pekinesis) for DEGs of Ogura-, Polima-CMS and their shared maintainer.

Cytoplasmic male sterility (CMS) is maternally inherited trait, which hinders the ability to produce viable pollen in plants. It serves as a useful tool for hybrid seed production via exploiting heterosis in crops. The molecular mechanism of CMS and fertility restoration has been investigated in different crops. However, limited number of reports is available on comparison of Ogura- and Polima-CMS with their shared maintainer in Chinese cabbage. We performed transcript profiling of sterile Ogura CMS (Tyms), Polima CMS (22m2) and their shared maintainer line (231-330) with an aim to identify genes associated with male sterility. In this work, we identified 912, 7199 and 6381 DEGs (Differentially Expressed Genes) in 22m2 Vs Tyms, 231-330 VS 22m2 and 231-330 Vs Tyms, respectively. The GO (Gene Ontology) annotation and KEGG pathway analysis suggested that most of the DEGs were involved in pollen development, carbon metabolism, lipase activity, lipid binding, penta-tricopeptide repeat (PPR), citrate cycle and oxidative phosphorylation, which were down-regulated in both CMS lines. This result will provide an important resource for further understanding of functional pollen development, the CMS mechanism and to improve molecular breeding in Chinese cabbage.

Morphological Features of the Anther Development in Tomato Plants with Non-Specific Male Sterility.

The study was devoted to morphological and cytoembryological analysis of disorders in the anther and pollen development of transgenic tomato plants with a normal and abnormal phenotype, which is characterized by the impaired development of generative organs. Various abnormalities in the structural organization of anthers and microspores were revealed. Such abnormalities in microspores lead to the blocking of asymmetric cell division and, accordingly, the male gametophyte formation. Some of the non-degenerated microspores accumulate a large number of storage inclusions, forming sterile mononuclear pseudo-pollen, which is similar in size and appearance to fertile pollen grain (looks like pollen grain). It was discussed that the growth of tapetal cells in abnormal anthers by increasing the size and ploidy level of nuclei contributes to this process. It has been shown that in transgenic plants with a normal phenotype, individual disturbances are also observed in the development of both male and female gametophytes. The reason for the developmental arrest of some ovules was the death of endosperm at different stages of the globular embryo. At the same time, noticeable hypertrophy of endothelial cells performing a secretory function was observed. In the ovules of transgenic plants with abnormalities, the endothelium forms a pseudo-embryo instead of the embryo sac, stimulating the development of parthenocarpic fruits. The data obtained in this study can be useful for a better understanding of the genetic and molecular mechanisms of cytoplasmic male sterility and parthenocarpic fruit development in tomatoes.

Differentially Expressed Genes between Carrot Petaloid Cytoplasmic Male Sterile and Maintainer during Floral Development

Petaloid cytoplasmic male sterility (CMS) is a maternally inherited loss of male fertility due to the complete conversion of stamens into petal-like organs, and CMS lines have been widely utilized in carrot breeding. Petaloid CMS is an ideal model not only for studying the mitochondrial–nuclear interaction but also for discovering genes that are essential for floral organ development. To investigate the comprehensive mechanism of CMS and homeotic organ alternation during carrot flower development, we conducted transcriptome analysis between the petaloid CMS line (P2S) and its maintainer line (P2M) at four flower developmental stages (T1–T4). A total of 2838 genes were found to be differentially expressed, among which 1495 genes were significantly downregulated and 1343 genes were significantly upregulated in the CMS line. Functional analysis showed that most of the differentially expressed genes (DEGs) were involved in protein processing in the endoplasmic reticulum, plant hormone signal transduction, and biosynthesis. A total of 16 MADS-box genes were grouped into class A, B, C, and E, but not class D, genes. Several key genes associated with oxidative phosphorylation showed continuously low expression from stage T2 in P2S, and the expression of DcPI and DcAG-like genes also greatly decreased at stage T2 in P2S. This indicated that energy deficiency might inhibit the expression of B- and C-class MADS-box genes resulting in the conversion of stamens into petals. Stamen petaloidy may act as an intrinsic stress, upregulating the expression of heat shock protein (HSP) genes and MADS-box genes at stages T3 and T4 in P2S, which results in some fertile revertants. This study will provide a better understanding of carrot petaloid CMS and floral development as a basis for further research.

Transcriptome, cytological and biochemical analysis of cytoplasmic male sterility and maintainer line in CMS-D8 cotton.

Key message This research based on RNA-seq, biochemical, and cytological analyses sheds that ROS may serve as important signaling molecules of cytoplasmic male sterility in CMS-D8 cotton. To understand the mechanism of cytoplasmic male sterility in cotton (Gossypium hirsutum), transcriptomic, cytological, and biochemical analysis were performed between the cytoplasmic male sterility CMS-D8 line, Zhong41A, and its maintainer line Zhong41B. A total of 2335 differentially expressed genes (DEGs) were identified in the CMS line at three different stages of anther development. Bioinformatics analysis of these DEGs indicated their relationship to reactive oxygen species (ROS) homeostasis, including reduction-oxidation reactions and the metabolism of glutathione and ascorbate. At the same time, DEGs associated with tapetum development, especially the transition to secretory tapetum, were down-regulated in the CMS line. Biochemical analysis indicated that the ability of the CMS line to eliminate ROS was decreased, which led to the rapid release of H2O2. Cytological analysis revealed that the most crucial defect in the CMS line was the abnormal tapetum. All these results are consistent with the RNA sequencing data. On the basis of our findings, we propose that ROS act as signal molecules, which are released from mitochondria and transferred to the nucleus, triggering the formation of abnormal tapetum.

Recombination Events Involving the atp9 Gene Are Associated with Male Sterility of CMS PET2 in Sunflower.

Cytoplasmic male sterility (CMS) systems represent ideal mutants to study the role of mitochondria in pollen development. In sunflower, CMS PET2 also has the potential to become an alternative CMS source for commercial sunflower hybrid breeding. CMS PET2 originates from an interspecific cross of H. petiolaris and H. annuus as CMS PET1, but results in a different CMS mechanism. Southern analyses revealed differences for atp6, atp9 and cob between CMS PET2, CMS PET1 and the male-fertile line HA89. A second identical copy of atp6 was present on an additional CMS PET2-specific fragment. In addition, the atp9 gene was duplicated. However, this duplication was followed by an insertion of 271 bp of unknown origin in the 5′ coding region of the atp9 gene in CMS PET2, which led to the creation of two unique open reading frames orf288 and orf231. The first 53 bp of orf288 are identical to the 5′ end of atp9. Orf231 consists apart from the first 3 bp, being part of the 271-bp-insertion, of the last 228 bp of atp9. These CMS PET2-specific orfs are co-transcribed. All 11 editing sites of the atp9 gene present in orf231 are fully edited. The anther-specific reduction of the co-transcript in fertility-restored hybrids supports the involvement in male-sterility based on CMS PET2.

Rapid evolutionary divergence of diploid and allotetraploid Gossypium mitochondrial genomes

BackgroundCotton (Gossypium spp.) is commonly grouped into eight diploid genomic groups and an allotetraploid genomic group, AD. The mitochondrial genomes supply new information to understand both the evolution process and the mechanism of cytoplasmic male sterility. Based on previously released mitochondrial genomes of G. hirsutum (AD1), G. barbadense (AD2), G. raimondii (D5) and G. arboreum (A2), together with data of six other mitochondrial genomes, to elucidate the evolution and diversity of mitochondrial genomes within Gossypium.ResultsSix Gossypium mitochondrial genomes, including three diploid species from D and three allotetraploid species from AD genome groups (G. thurberi D1, G. davidsonii D3-d and G. trilobum D8; G. tomentosum AD3, G. mustelinum AD4 and G. darwinii AD5), were assembled as the single circular molecules of lengths about 644 kb in diploid species and 677 kb in allotetraploid species, respectively. The genomic structures of mitochondrial in D group species were identical but differed from the mitogenome of G. arboreum (A2), as well as from the mitogenomes of five species of the AD group. There mainly existed four or six large repeats in the mitogenomes of the A + AD or D group species, respectively. These variations in repeat sequences caused the major inversions and translocations within the mitochondrial genome. The mitochondrial genome complexity in Gossypium presented eight unique segments in D group species, three specific fragments in A + AD group species and a large segment (more than 11 kb) in diploid species. These insertions or deletions were most probably generated from crossovers between repetitive or homologous regions. Unlike the highly variable genome structure, evolutionary distance of mitochondrial genes was 1/6th the frequency of that in chloroplast genes of Gossypium. RNA editing events were conserved in cotton mitochondrial genes. We confirmed two near full length of the integration of the mitochondrial genome into chromosome 1 of G. raimondii and chromosome A03 of G. hirsutum, respectively, with insertion time less than 1.03 MYA.ConclusionTen Gossypium mitochondrial sequences highlight the insights to the evolution of cotton mitogenomes.

Advances in understanding the molecular mechanisms of cytoplasmic male sterility and restoration in rice.

Cytoplasmic male sterility (CMS) in plants is a male reproductive defect determined by mitochondrial genes and inherited maternally. CMS can be suppressed by nuclear restorer of fertility (Rf) genes. Therefore, CMS/Rf systems provide a classic model for the study of mitochondrial-nuclear interactions in plants. Moreover, CMS/Rf systems are economical, effective tools for the production of hybrid seeds. For example, CMS/Rf systems have been applied in over forty countries to breed hybrid rice (Oryza sativa L.) with improved yields due to hybrid vigor. The production of hybrid rice mainly depends on three types of CMS systems, namely Wild-Abortive type CMS (CMS-WA), Hong-Lian type CMS (CMS-HL) and Boro II type CMS (CMS-BT). Understanding the molecular mechanisms underlying these CMS/Rf systems will help us to understand mitochondrial-nuclear interactions, and accelerate the utilization of heterosis for improvement in yield. In the past decades, research benefitting from the availability of the high-quality, annotated mitochondrial and nuclear genome sequences of rice has isolated many CMS genes, identified the cognate nuclear Rf genes and studied the molecular mechanisms underlying CMS and restoration in rice. Here, we focus on recent advances in studies of the three major CMS/Rf systems in rice and discuss the key issues facing basic research and application of CMS/Rf systems in the future.