urf13-T of T cytoplasm maize mitochondria encodes a 13 kD polypeptide

Characterization of the Mitochondrial Genome of a Wheat AL-Type Male Sterility Line and the Candidate CMS Gene

Heterosis utilization is very important in hybrid seed production. An AL-type cytoplasmic male sterile (CMS) line has been used in wheat-hybrid seed production, but its sterility mechanism has not been explored. In the present study, we sequenced and verified the candidate CMS gene in the AL-type sterile line (AL18A) and its maintainer line (AL18B). In the late uni-nucleate stage, the tapetum cells of AL18A showed delayed programmed cell death (PCD) and termination of microspore at the bi-nucleate stage. As compared to AL18B, the AL18A line produced 100% aborted pollens. The mitochondrial genomes of AL18A and AL18B were sequenced using the next generation sequencing such as Hiseq and PacBio. It was found that the mitochondrial genome of AL18A had 99% similarity with that of Triticum timopheevii, AL18B was identical to that of Triticum aestivum cv. Chinese Yumai. Based on transmembrane structure prediction, 12 orfs were selected as candidate CMS genes, including a previously suggested orf256. Only the lines harboring orf279 showed sterility in the transgenic Arabidopsis system, indicating that orf279 is the CMS gene in the AL-type wheat CMS lines. These results provide a theoretical basis and data support to further analyze the mechanism of AL-type cytoplasmic male sterility in wheat.

Genome-wide transcriptome analysis of microspore abortion initiation in radish (Raphanus sativus L.)

The use of male sterile lines is one of the ideal means in hybrid seed production. Despite the widespread application of Ogura cytoplasmic male sterile (CMS) lines, the molecular mechanisms remain largely unknown. In this study, histological analyses of floral buds from a CMS line 40MA and its corresponding maintainer line 40MB were conducted, which indicate that microspore abortion was initiated shortly after the tetrad stage. RNA sequencing was performed to analyze the transcriptomes of floral buds from the tetrad stage and the early microspore stages of these two lines. More than 39 million clean reads were generated for each library, and the portions mapped to the reference genome were all above 70.60%. To further analyze the differentially expressed genes (DEGs), the samples were grouped into four pairs, of which the pair of 40MA and 40MB at the early microspore stage showed the most DEGs (5100 members). According to the abnormal appearance of the tapetum cells in 40MA, a series of tapetum development related genes were screened and analyzed. In addition, a total of 623 genes with differential expressions in the tetrad stage, but not in the early microspore stage between the two lines were filtered as the microspore abortion initiation related candidates. Twelve genes were selected to validate the sequencing result by quantitative RT-PCR. In this study, we identified a number of candidate genes involved in the initiation of microspore degeneration, which may provide a new perspective to unravel the molecular mechanism of Ogura CMS.

Plant genes hijacked by necrotrophic fungal pathogens

Plant fungal pathogens can be classified according to their lifestyles. Biotrophs feed on living tissue and constitute an economically significant group of pathogens historically. Necrotrophs, which feed on dead tissue, have become economically significant over recent decades, especially those of the Dothideomycetes, which produce necrotrophic effectors (NEs) to modulate the host response. Some of these pathogens interact with their hosts in an inverse gene-for-gene manner, where NEs are recognized by specific dominant genes in the host leading to host-mediated programmed cell death allowing the pathogen to cause disease. Whereas the NE genes tend to be unique, several of the plant 'susceptibility' genes belong to the nucleotide-binding leucine-rich repeat class of disease 'resistance' genes, and one is a wall-associated kinase. These susceptible interactions exhibit hallmarks of defense responses to biotrophic pathogens. Therefore, there is now accumulating evidence that many necrotrophic specialists hijack the resistance mechanisms that are effective against biotrophic pathogens.

Intergenomic gene transfer in diploid and allopolyploid Gossypium

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Proteomic analysis reveals strong mitochondrial involvement in cytoplasmic male sterility of pepper (Capsicum annuum L.)

Although cytoplasmic male sterility (CMS) is widely used for developing pepper hybrids, its molecular mechanism remains unclear. In this study, we used a high-throughput proteomics method called label-free to compare protein abundance across a pepper CMS line (A-line) and its isogenic maintainer line (B-line). Data are available via ProteomeXchange with identifier PXD006104. Approximately 324 differentially abundant protein species were identified and quantified; among which, 47 were up-accumulated and 140 were down-accumulated in the A-line; additionally, 75 and 62 protein species were specifically accumulated in the A-line and B-line, respectively. Protein species involved in pollen exine formation, pyruvate metabolic processes, the tricarboxylic acid cycle, the mitochondrial electron transport chain, and oxidative stress response were observed to be differentially accumulated between A-line and B-line, suggesting their potential roles in the regulation of pepper pollen abortion. Based on our data, we proposed a potential regulatory network for pepper CMS that unifies these processes.

BIOLOGICAL SIGNIFICANCE

Artificial emasculation is a major obstacle in pepper hybrid breeding for its high labor cost and poor seed purity. While the use of cytoplasmic male sterility (CMS) in hybrid system is seriously frustrated because a long time is needed to cultivate male sterility line and its isogenic restore line. Transgenic technology is an effective and rapid method to obtain male sterility lines and its widely application has very important significance in speeding up breeding process in pepper. Although numerous studies have been conducted to select the genes related to male sterility, the molecular mechanism of cytoplasmic male sterility in pepper remains unknown. In this study, we used the high-throughput proteomic method called "label-free", coupled with liquid chromatography-quadrupole mass spectrometry (LC-MS/MS), to perform a novel comparison of expression profiles in a CMS pepper line and its maintainer line. Based on our results, we proposed a potential regulated protein network involved in pollen development as a novel mechanism of pepper CMS.

Global invasive Cochliobolus species: cohort of destroyers with implications in food losses and insecurity in the twenty-first century

Matching the global food demand by 2050 and to ensure the stability of food security in over than 99 countries, it is necessary to scale up the production of food such as sorghum, wheat, rice, maize and sugarcane which are however natural hosts of Cochliobolus species. Cochliobolus species major epidemics such as the Great Bengal famine, Southern corn leaf blight, and Northern leaf spot blight were associated with substantial economic losses in the past decades. Thus, there is an urgent need to establish a specific coordinated global surveillance program for the migration of invasive Cochliobolus species, planning contextual control programs engaging all agricultural stakeholders and information sharing in real time for prevention of disastrous Cochliobolus disease outbreak effects. We discuss pertinent outcome of interactions of cash crops with Cochliobolus species having devastating impact on the livelihood of farmers and food security. While post-genomic era elucidated prominent differences among Cochliobolus heterostrophus, C. carbonum, C. victoriae, C. lunatus and C. miyabeanus, their destructive potentials and implications in food losses remained unearthed. Intriguingly, the annual colossal losses caused by Cochliobolus species in the production perspective of sorghum, wheat, rice, maize, cassava and soybean is estimated over 10 billion USD worldwide. This paper provides a comprehensive analysis of the invasive Cochliobolus species distribution and diversity, evolving pathogenicity, persistent diseases, threats and epidemics, consequences on food crops production and increasing global food insecurity issues.

Comparative Transcriptome Profile of the Cytoplasmic Male Sterile and Fertile Floral Buds of Radish (Raphanus sativus L.)

Radish cytoplasmic male sterility (CMS) has been widely used for breeding in Raphanus and Brassica genera. However, the detailed regulation network of the male sterility remains to be determined. Our previous work has shown that the abnormalities in a CMS radish appeared shortly after the tetrad stage when microspores were malformed and the tapetal cells grew abnormally large. In this work, histological analysis shows that anthers are at the tetrad stage when the radish buds are about 1.5 mm in length. Furthermore, a high throughput RNA sequencing technology was employed to characterize the transcriptome of radish buds with length about 1.5 mm from two CMS lines possessing the CMS-inducing orf138 gene and corresponding near-isogenic maintainer lines. A total of 67,140 unigenes were functionally annotated. Functional terms for these genes are significantly enriched in 55 Gene Ontology (GO) groups and 323 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways. The transcriptome detected transcripts for 72 out of a total of 79 protein genes encoded in the chloroplast genome from radish. In contrast, the radish mitochondrial genome contains 34 protein genes, but only 16 protein transcripts were detected from the transcriptome. The transcriptome comparison between CMS and near-isogenic maintainer lines revealed 539 differentially expressed genes (DEGs), indicating that the false positive rate for comparative transcriptome profiling was clearly decreased using two groups of CMS/maintainer lines with different nuclear background. The level of 127 transcripts was increased and 412 transcripts were decreased in the CMS lines. No change in levels of transcripts except CMS-inducing orf138 was identified from the mitochondrial and chloroplast genomes. Some DEGs which would be associated with the CMS, encoding MYB and bHLH transcription factors, pentatricopeptide repeat (PPR) proteins, heat shock transcription factors (HSFs) and heat shock proteins (HSPs), are discussed. The transcriptome dataset and comparative analysis will provide an important resource for further understanding anther development, the CMS mechanism and to improve molecular breeding in radish.

Fine mapping of the first multi-fertility-restoring gene, Rf(multi), of wheat for three Aegilops plasmons, using 1BS-1RS recombinant lines

Fertility-restoring genes, Rfv1, Rfm1 and Rfn1, respectively, for the male sterile cytoplasms of Aegilops kotschyi, Ae. mutica and Ae. uniaristata to common wheat were located on the same locus of Pavon wheat 1BS arm. The male sterile cytoplasm (plasmon) and the fertility-restoring gene are essential genetic components for breeding hybrid seed crops. This article represents information on the genetic similarity of three Aegilops plasmons usable as the male sterile cytoplasm for hybrid wheat and provides an evidence on the possible genetic unity of three fertility-restoring genes reported for these plasmons by their genetic mapping using the 1BS-1RS recombinant lines of Pavon 76 wheat on to a single subsegment of the 1BS chromosome arm less than 2.9 cM in size: the locus is designated Rf (multi) , meaning "Restoration of fertility in multiple CMS systems". Unresolved problems were discussed in the use of the present cytoplasmic male sterility-fertility restoration system for hybrid wheat breeding.

The role of effectors and host immunity in plant-necrotrophic fungal interactions

Fungal diseases pose constant threats to the global economy and food safety. As the largest group of plant fungal pathogens, necrotrophic fungi cause heavy crop losses worldwide. The molecular mechanisms of the interaction between necrotrophic fungi and plants are complex and involve sophisticated recognition and signaling networks. Here, we review recent findings on the roles of phytotoxin and proteinaceous effectors, pathogen-associated molecular patterns (PAMPs), and small RNAs from necrotrophic fungi. We also consider the functions of damage-associated molecular patterns (DAMPs), the receptor-like protein kinase BIK1, and epigenetic regulation in plant immunity to necrotrophic fungi.

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.

Cytoplasmic male sterility in plants: molecular evidence and the nucleocytoplasmic conflict

A much-debated issue in plant evolutionary biology concerns the maintenance of a high frequency of male sterility in natural populations. For the past decade, a theoretical framework has been provided by the concept of nucleocytoplasmic conflict. Recent molecular studies on cytoplasmic male sterility indicate that novel chimeric genes, resulting from duplications and rearrangements of mitochondrial DNA sequences, are involved In its control. Thus, male sterility, which is phenotypically the loss of the male function, is encoded by a new mitochondrial function at the molecular level. Molecular data are in agreement with theoretical models that consider cytoplasmic male sterility as a stage in the coevolution between nucleus and mitochondria, and not simply as a deleterious mitochondrial mutation.

Mitochondrial mutation impairs cytoplasmic male sterility rice in response to H₂O₂ stress

Cytoplasmic male sterility (CMS) is a phenomenon widely observed in various plant species characterized with disrupted anther development caused by mitochondrial mutation. CMS is becoming a model system for the investigations of nucleus-cytoplasmic interaction. To reveal the possible effects of CMS genes on plant growth in adverse environment, plant development and biochemical characters of mitochondria from Honglian (HL)-CMS line Yuetai A and maintainer Yuetai B treated with H(2)O(2) were analyzed. Results showed that 40-60mM H(2)O(2) significantly inhibits rice seedling development and growth. When treated with H(2)O(2), ATP content and mitochondrial membrane potential in Yuetai A decreased significantly faster than those of Yuetai B. These biochemical changes were accompanied by the severe nuclear DNA fragmentation and the release of mitochondrial cytochrome c in the leaf cells of Yuetai A. In addition, the antioxidative enzyme activities and mitochondrial electron transfer chain complexes were significantly down-regulated. Disturbance of the biochemical indexes indicate that HL-CMS line is more susceptible to H(2)O(2) stress than the maintainer line, the deleterious effects caused by the CMS-related ORFH79 peptide compromises the adaptability of HL-CMS line to the adverse environment.

Phytotoxic secondary metabolites and peptides produced by plant pathogenic Dothideomycete fungi

Many necrotrophic plant pathogenic fungi belonging to the class of Dothideomycetes produce phytotoxic metabolites and peptides that are usually required for pathogenicity. Phytotoxins that affect a broad range of plant species are known as non-host-specific toxins (non-HSTs), whereas HSTs affect only a particular plant species or more often genotypes of that species. For pathogens producing HSTs, pathogenicity and host specificity are largely defined by the ability to produce the toxin, while plant susceptibility is dependent on the presence of the toxin target. Non-HSTs are not the main determinants of pathogenicity but contribute to virulence of the producing pathogen. Dothideomycetes are remarkable for the production of toxins, particularly HSTs because they are the only fungal species known so far to produce them. The synthesis, regulation, and mechanisms of action of the most important HSTs and non-HSTs will be discussed. Studies on the mode of action of HSTs have highlighted the induction of programed cell death (PCD) as an important mechanism. We discuss HST-induced PCD and the plant hypersensitive response upon recognition of avirulence factors that share common pathways. In this respect, although nucleotide-binding-site-leucine-rich repeat types of resistance proteins mediate resistance against biotrophs, they can also contribute to susceptibility toward necrotrophs.

Genome barriers between nuclei and mitochondria exemplified by cytoplasmic male sterility

Since plants retain genomes of an extremely large size in mitochondria (200-2,400 kb), and mitochondrial protein complexes are comprised of chimeric structures of nuclear- and mitochondrial-encoded subunits, coordination of gene expression between the nuclei and mitochondria is indispensable for sound plant development. It has been well documented that the nucleus regulates organelle gene expression. This regulation is called anterograde regulation. On the other hand, recent studies have demonstrated that signals emitted from organelles regulate nuclear gene expression. This process is known as retrograde signaling. Incompatibility caused by genome barriers between a nucleus and foreign mitochondria destines the fate of pollen to be dead in cytoplasmic male sterility (CMS), and studies of CMS confirm that pollen fertility is associated with anterograde/retrograde signaling. This review summarizes the current perspectives in CMS and fertility restoration, mainly from the viewpoint of anterograde/retrograde signaling.

Comparisons among two fertile and three male-sterile mitochondrial genomes of maize

We have sequenced five distinct mitochondrial genomes in maize: two fertile cytotypes (NA and the previously reported NB) and three cytoplasmic-male-sterile cytotypes (CMS-C, CMS-S, and CMS-T). Their genome sizes range from 535,825 bp in CMS-T to 739,719 bp in CMS-C. Large duplications (0.5-120 kb) account for most of the size increases. Plastid DNA accounts for 2.3-4.6% of each mitochondrial genome. The genomes share a minimum set of 51 genes for 33 conserved proteins, three ribosomal RNAs, and 15 transfer RNAs. Numbers of duplicate genes and plastid-derived tRNAs vary among cytotypes. A high level of sequence conservation exists both within and outside of genes (1.65-7.04 substitutions/10 kb in pairwise comparisons). However, sequence losses and gains are common: integrated plastid and plasmid sequences, as well as noncoding "native" mitochondrial sequences, can be lost with no phenotypic consequence. The organization of the different maize mitochondrial genomes varies dramatically; even between the two fertile cytotypes, there are 16 rearrangements. Comparing the finished shotgun sequences of multiple mitochondrial genomes from the same species suggests which genes and open reading frames are potentially functional, including which chimeric ORFs are candidate genes for cytoplasmic male sterility. This method identified the known CMS-associated ORFs in CMS-S and CMS-T, but not in CMS-C.

Host-selective toxins and avirulence determinants: what's in a name?

Host-selective toxins, a group of structurally complex and chemically diverse metabolites produced by plant pathogenic strains of certain fungal species, function as essential determinants of pathogenicity or virulence. Investigations into the molecular and biochemical responses to these disease determinants reveal responses typically associated with host defense and incompatibility induced by avirulence determinants. The characteristic responses that unify these disparate disease phenotypes are numerous, yet the evidence implicating a causal relationship of these responses, whether induced by host-selective toxins or avirulence factors, in determining the consequences of the host-pathogen interaction is equivocal. This review summarizes some examples of the action of host-selective toxins to illustrate the similarity in responses with those to avirulence determinants.

Role of Light and Malate in the Decreased Sensitivity of cms-T Cytoplasm Maize Leaves to Bipolaris maydis Race T Toxin

ABSTRACT Leaf segments from Texas male sterile (cms-T) cytoplasm maize isolines exposed to light (50 muM s(-1) m(-2)) for 8 h or more before or after being infiltrated with the Bipolaris maydis race T toxin (T-toxin) leaked significantly less electrolytes when immersed in distilled water (DW) for 24 to 48 h than did dark-treated leaf segments. No comparable effect of light on toxin-induced electrolyte leakage was observed with normal (N) cytoplasm isolines. Toxin-treated cms-T leaf segments incubated in DW for three consecutive 12-h periods of alternating light and dark showed significantly greater electrolyte leakage than leaf segments incubated in continuous light for 36 h and significantly less leakage than segments incubated in continuous dark for 36 h.Exposure of cms-T, but not N, cytoplasm leaves to 25 or 50 muM malic acid decreased their sensitivity to T-toxin in the dark to a level similar to that observed when leaves were incubated in the light without malic acid. The potency of T-toxin appeared to be unaffected by its exposure to light. The loss of electrolytes from T-toxin-treated cms-T cytoplasm leaf segments was at approximately the level seen with light or malate when 25 muM H(2)O(2) was added to the DW bathing solution. Evaluation of the data points to the possibility that H(2)O(2) might be involved with the altered sensitivity of cms-T cytoplasm leaves to T-toxin caused by either light or malate.

The rf2 nuclear restorer gene of male-sterile T-cytoplasm maize

The T cytoplasm of maize serves as a model for the nuclear restoration of cytoplasmic male sterility. The rf2 gene, one of two nuclear genes required for fertility restoration in male-sterile T-cytoplasm (cmsT) maize, was cloned. The protein predicted by the rf2 sequence is a putative aldehyde dehydrogenase, which suggests several mechanisms that might explain Rf2-mediated fertility restoration in cmsT maize. Aldehyde dehydrogenase may be involved in the detoxification of acetaldehyde produced by ethanolic fermentation during pollen development, may play a role in energy metabolism, or may interact with URF13, the mitochondrial protein associated with male sterility in cmsT maize.

Molecular interactions ofBipolaris maydisT-toxin and maize

The toxins (T-toxins) produced by the fungal pathogens Bipolaris maydis race T (BmT) and Phyllosticta maydis (Pm) target the mitochondrial receptor, URF13, in maize (Zea mays L.) plants containing the Texas male-sterile cytoplasm (cms-T). URF13, a 13-kDa protein, is the product of the maize mitochondrial gene T-urf13, which is found only in the mitochondrial genome of cms-T maize and is thought to be responsible for cytoplasmically inherited male sterility and disease susceptibility. Pm-toxin binds specifically to URF13 in a cooperative manner, and Pm- and BmT-toxins compete for the same, or overlapping, binding sites. The binding of T-toxin to URF13 causes rapid permcabilization of the inner mitochondrial membrane, which results in leakage of NAD+and other ions from the matrix. A pore consisting of at least six transmembrane α-helices is required for NAD+leakage. Cross-linking experiments showed that URF13 oligomers are present in the mitochondrial membrane. A model of the secondary structure of URF13 proposes that each monomer contains three transmembrane α-helices. Studies combining site-directed mutagenesis and chemical cross-linking of URF13 expressed by Escherichia coli cells indicate that the oligomers are composed of a central core of helices II that line the center of the URF13 pores. Key words: maize cytoplasmic male sterility, URF13, mitochondrial pores, T-toxin receptor, Bipolaris maydis race T, Phyllosticta maydis, Helminthosporium maydis.

A chimeric gene (orf256) is expressed as protein only in cytoplasmic male-sterile lines of wheat

Mitochondria derived from Triticum timopheevi have a chimeric gene, orf256, immediately upstream from coxI. Antibodies to a peptide corresponding to a part of the encoded amino acid sequence of orf256 detect a 7 kDa protein on western blots of mitochondrial proteins from cytoplasmic male-sterile (cms) wheat (T. aestivum nucleus, T. timopheevi mitochondria) but not in mitochondrial proteins from T. aestivum, T. timopheevi, or cms plants restored to fertility by introduction of nuclear genes for fertility restoration. The 7 kDa protein appears to serve as a marker for cms wheat. Its occurrence as an integral protein of the inner membrane may indicate a cms effect through an influence on mitochondrial membrane function.

Mapping complementary genes in maize: positioning the rf1 and rf2 nuclear-fertility restorer loci of Texas (T) cytoplasm relative to RFLP and visible markers

There are three major groups of cytoplasmic male-sterile cytoplasms in maize; C (Charrua), S (USDA), and T (Texas). These cytoplasms can be classified by the unique nuclear genes that suppress the male-sterility effects of these cytoplasms and restore pollen fertility. Typically, plants that carry Texas (T) cytoplasm are male fertile only if they carry dominant alleles at two unlinked nuclear restorer loci,rf1 andrf2. To facilitate analysis of T-cytoplasm-mediated male sterility and fertility restoration, we have mappedrf1 andrf2 relative to closely-linked RFLP markers using five populations. Therf1 locus and/or linked visible markers were mapped in four populations; therf2 locus was mapped in two of the populations. Data from the individual populations were joined with the aid of JoinMap software. The resulting consensus maps placerf1 between umc97 and umc92 on chromosome 3 andrf2 between umc153 andsus1 on chromosome 9. Markers that flank therf1 andrf2 loci have been used to identify alleles atrf1 andrf2 in segregating populations. These analyses demonstrate the possibility of tracking separate fertility restorer loci that contribute to a single phenotype.

Ogura cytoplasmic male-sterility (CMS)-associated orf138 is translated into a mitochondrial membrane polypeptide in male-sterile Brassica cybrids

Transcription of a putative mitochondrial gene (orf138) has previously been correlated with Ogura cytoplasmic male-sterility (CMS) in rapeseed cybrids. In this paper, studies performed on a Brassica cybrid with a different organization of the orf138 locus confirm this association. We also show that mitochondria isolated from male-sterile rapeseed plants synthesize a polypeptide of 19 kDa, which is absent in fertile revertants. Antibodies against a glutathione S-transferase-ORF138 fusion protein were raised to establish that this 19 kDa polypeptide is the product of orf138. The anti-ORF138 serum was used to demonstrate that the orf138 translation product occurs only in sterile cybrids and co-purifies with the mitochondrial membrane fraction.

Translation in a wheat germ cell-free system of RNA from mitochondria of the normal and Texas male-sterile cytoplasms of maize (Zea mays L.)

RNA isolated from etiolated seedling shoot mitochondria of maize (Zea mays L.) with normal (N) or Texas male-sterile (T) cytoplasm stimulated the incorporation of [35S]-methionine into protein when added to a cell-free protein-synthesizing system from wheat germ. Discrete polypeptides with molecular masses of up to approximately 67 kDa were synthesized, and the pattern of bands was distinct from that obtained with total RNA. Products of translation of T-urf13 RNA were identified by immunoprecipitation, and of atpA, coxI, and coxII RNA by hybrid arrest of translation by the cloned gene. Several polypeptides were differentially synthesized from N and T mitochondrial RNA; these differences were more extensive than those found when isolated, intact, N and T mitochondria are allowed to synthesize proteins.

Characterization of protein synthesis by isolated rice mitochondria

Bacteria-free mitochondria were isolated from aseptically grown, etiolated and green seedlings of both cytoplasmic male-sterile (WA-type) and male-fertile rice (Oryza sativa L.). Protein synthesis in these isolated mitochondria was characterized by gel electrophoresis/fluorography and by the incorporation of [(35)S]-methionine into protein. In the presence of cycloheximide, a set of some 25 discrete polypeptides and an electrophoretically unresolved population were synthesized. This pattern of protein synthesis in organello was essentially the same in mitochondria isolated from both male-fertile and malesterile cytoplasms. Our data does not preclude the possibility, however, that the WA-type CMS possesses a tissue-specific and/or a low abundance mitochondrial protein(s), whose synthesis eluded detection under our experimental conditions. The synthesis of the mitochondria-encoded polypeptides by isolated rice mitochondria was inhibited by chloramphenicol and incompletely inhibited by erythromycin. A minor chloramphenicol-insensitive, cycloheximide-sensitive translation activity was found consistently to copurify with the mitochondria. This activity generated a reproducible electrophoretic profile of a poorly resolved, weakly labelled population of polypeptides and of a few conspicuous polypeptides, including a 42 kDa species.

The fungal genus Cochliobolus and toxin-mediated plant disease

Several times in this century, new and sometimes devastating diseases of cereal crops, caused by fungi in the genus Cochliobolus, have suddenly appeared. In many fungal diseases of plants the factors required for pathogenicity are unknown; in contrast, the key elements in each of several Cochliobolus diseases are known to be host-selective toxins. Recent research on these systems has given surprising insights into the genetic basis of fungal pathogenicity and plant susceptibility to disease.

Mitochondrial dysfunction in yeast expressing the cytoplasmic male sterility T-urf13 gene from maize: analysis at the population and individual cell level

The urf13TW gene, which is derived from the mitochondrial T-urf13 gene responsible for Texas cytoplasmic male sterility in maize, was expressed in Saccharomyces cerevisiae by targeting its translation product into mitochondria. Analysis by oxygraphy at the population level revealed that in the presence of methomyl the oxygen uptake of intact yeast cells carrying the targeted protein is strongly stimulated only with ethanol as respiratory substrate and not with glycerol, lactate, pyruvate, or acetate. When malate is the substrate oxidized by isolated mitochondria, interaction between the targeted protein and methomyl results in significant inhibition of oxygen uptake. This inhibition is eliminated and oxygen uptake is stimulated by subsequent addition of NAD+. Using 3,3'-dihexyloxacarbocyanine iodide [DiOC6(3)] as probe, interactive laser scanning and flow cytometry, which permit analysis at the individual cell level, demonstrated that specific staining of the mitochondrial compartment is obtained and that DiOC6(3) fluorescence serves as a measure of the membrane potential. Finally, it was shown that, as in T cytoplasm maize mitochondria, HmT toxin and methomyl dissipate the membrane potential of yeast mitochondria that carry the foreign protein. Furthermore, the results suggest that the HmT toxin and methomyl response is related to the plasmid copy number per cell and that the deleterious effect induced by HmT toxin is stronger than that of methomyl.

Genetic analysis of nuclear control of T-urf13/orf221 transcription in T cytoplasm maize

The mitochondrial gene T-urf13 in T cytoplasm maize is associated with sensitivity to disease toxins and with cytoplasmic male sterility. T-urf13 is co-transcribed with an open reading frame designated orf221. We have detected alterations in the transcription of the T-urf13/orf221 region that are affected by nuclear genotype. There are multiple mRNA transcripts generated from the T-urf13/orf221 region, one of which is a processed 1538-nucleotide (nt) transcript. This 1538-nt transcript is present in Wf9 (T), but was not found in mitochondrial RNAs (mtRNAs) from maize inbreds B14A (T) and 33-16 (T). For B14A (T) a 1500-nt transcript was detected and for 33-16 (T) a 1400-nt transcript was detected. In F1 progeny of the cross of Wf9 (T) x 33-16 (N), only the 1400-nt transcript was present. Genetic analyses revealed this processing event is nuclear controlled with dominant gene action and is independent of nuclear restorer gene Rf1-associated processing events. T-urf13/orf221 transcriptional patterns were shown to vary in both sterile and fertile states. Segregation analysis of a 1100-nt orf221-specific transcript indicated that the genetic basis of nuclear control for the presence of this transcript was relatively simple. Analysis of the A188 (T4) tissue culture mutant, which has reverted to male fertility but displays the same T-urf13/orf221 transcript pattern as A188 (T), indicated no DNA sequence differences between T4-orf221 and T-orf221. Presence of the nuclear gene Rf2 was not necessary for expression of the T4 cytoplasm-associated malefertile phenotype.

Organization of ATPA coding and 3' flanking sequences associated with cytoplasmic male sterility in Phaseolus vulgaris L

A region of the mitochondrial genome associated with cytoplasmic male sterility (CMS) in Phaseolus vulgaris was flanked by two different repeated sequences designated x and y. The DNA sequence of the CMS-unique region and a portion of each flanking repeat was determined. Repeat x contained a complete coding copy of the F1 ATPase subunit A (atpA) gene, as well as an open reading frame (orf) predicting a protein of 209 amino acids. The TGA termination codon of the atpA gene and the ATG initiation codon of orf209 were overlapping. These reading frames were oriented with their 3' ends proximal to the CMS-unique region. The CMS-unique region of 3736 nucleotides contained numerous orfs. The longest of these predicted proteins being of 239, 98 and 97 amino acids. The 3' coding and 3' flanking regions of orf98 were derived from an internal region of the higher plant chloroplast tRNA alanine intron. The region of repeat y immediately adjacent to the CMS-unique region contained the 111 carboxy-terminal coding residues of the apocytochrome b (cob) gene. This segment was oriented with its 5' end proximal to the CMS-unique region, but cob gene sequences were not fused to an initiation codon within the unique region.

Expression of mitochondrial genes in fertile and sterile sugar beet cytoplasms with different nuclear fertility restorer genes

Variations in mitochondrial genome organization and in its expression between fertile, sterile sugar beet lines and fertile nuclear-restored plants were studied. Southern blot hybridization with COXI, COXII, COB and atpA mitochondrial genes as probes showed that changes in the mitochondrial genome organization of sterile lines are associated with variations in the location of COB, atpA and COXII, but not COXI. When the COXII and atpA genes were used as hybridization probes, differences in the primary structure of mitochondrial DNAs from sterile lines of different origin were revealed. Differences in the transcriptional patterns of the three mitochondrial genes (COXI, COXII and atpA) were observed between fertile and sterile sugar beet lines; COB was the only mitochondrial gene whose transcription was identical in both fertile and sterile cytoplasms. The dominant nuclear fertility restorer genes altered the transcriptional patterns of the COB and atpA without affecting those of the COXI and COXII genes; atp A expression was identical in fertile plants and nuclear-restored plants with sterile cytoplasm.

Structural studies of microsporogenesis in fertile and male-sterile onions (Allium cepa L.) containing the cms-S cytoplasm

The structure of anther tissues has been studied during microsporogenesis in male-sterile and -fertile onions. Three types of abnormal tapetal behaviour have been observed within the single line II/3ms containing the cms-S cytoplasm: type 1, the premature breakdown of the tapetum at the tetrad stage, type 2, the hypertrophy of the tapetum after the diad stage followed by its premature autolysis and, type 3, in which the tapetum remains in good condition but for an abnormally long period of time. Tapetal autolysis proceeds in the same manner in both male-steriles and -fertiles with only the stage at which it occurs differing between the types of plants. Mitochondria were prominent in the tapetal tissue of all onion types throughout all stages of microsporogenesis and were still visible during the last stages of tapetal autolysis. In a detailed study of type 2 behaviour, no differences in mitochondrial volumes were found until the tapetum hypertrophied.

The Texas Cytoplasm of Maize: Cytoplasmic Male Sterility and Disease Susceptibility

The Texas cytoplasm of maize carries two cytoplasmically inherited traits, male sterility and disease susceptibility, which have been of great interest both for basic research and plant breeding. The two traits are inseparable and are associated with an unusual mitochondrial gene, T-urf13, which encodes a 13-kilodalton polypeptide (URF13). An interaction between fungal toxins and URF13, which results in permeabilization of the inner mitochondrial membrane, accounts for the specific susceptibility to the fungal pathogens.

Expression in Saccharomyces cerevisiae of a gene associated with cytoplasmic male sterility from maize: respiratory dysfunction and uncoupling of yeast mitochondria

We asked whether the mitochondrial T-urf13 gene, associated with the male sterility phenotype of T cytoplasm in maize, can be expressed in Saccharomyces cerevisiae and whether this expression can mimic the effects observed in maize. We introduced the universal code equivalent of the T-urf13 gene into the S. cerevisiae nucleus by transformation and directed its translation product into mitochondria by means of a fusion with the targeting presequence from Neurospora crassa AT-Pase subunit 9. We show that expression of the universal code equivalent of the T-urf13 gene in the yeast nucleus does indeed mimic its effects in maize: respiratory growth of yeast is inhibited, respiration-deficient cytoplasmic mutants accumulate and NADH oxidation of isolated mitochondria is uncoupled. All these effects are observed only if the mitochondrial targeting peptide and methomyl or HmT toxin are present.

In vitro selection for methomyl resistance in CMS-T maize

Many plants resistant to methomyl (Lannate), an insecticide which selectively damages maize with the Texas (T) type of cytoplasmic male sterility (CMS-T), were obtained by in vitro selection and also without selection. The selection procedure used 0.6-0.7mM methomyl and callus from CMS-T versions of several field and sweet corn genotypes (W182BN, Wf9, P39, MDM1, SW1 and hybrids of SW1, IL766A1, IL766A2, and 442 with W182BN-N). Addition of 1 mM methomyl to the regeneration medium greatly reduced recovery of methomyl-sensitive escapes. Resistance was linked with reversion to male fertility and maternally inherited. Most progeny of resistant plants exhibited stable maternally inherited resistance for two generations in field tests. First-generation progeny of seven culture-derived plants segregated for resistance and sensitivity; this suggests that ears of these seven regenerants were cytoplasmically chimeral. Resistance to methomyl was associated with resistance to T toxin from Helminthosporium maydis race T and with changes in mitochondrial physiology. Prolonged culture (14-16 months versus 6-8 months) increased the frequency of resistance among both selected and non-selected regenerants. Little or no resistance was found among regenerants from certain genotypes. Selection with methomyl may be useful for production of improved sweet corn lines and as a source of mitochondrial mutants. This system is also convenient for studies of the effects of nuclear background and of culture and selection systems on the generation of cytoplasmic mutants.

On the mechanism of cytoplasmic male sterility in the 447 line of Vicia faba

The trait of cytoplasmic male sterility, expressed in plants bearing the 447 cytoplasm of Vicia faba, is uniquely and positively correlated with the presence of a linear double-stranded RNA molecule (dsRNA) 16.7 kb in size. Restriction enzyme digestion profiles of mitochondrial DNA isolated from fertile and cytoplasmic malesterile (CMS) lines do show a limited number of specific differences in fragment intensities and mobilities. However, mitochondria isolated from the progeny of the cross CMS × Restorer line contain DNA with an identical restriction profile as the male-sterile parent: moreover, subsequent generations are completely and permanently fertile, even upon segregation of the nuclear restoration gene. Southern hybridizations, using cDNA clones as probes, reveal homology between the CMS-associated dsRNA and the nuclear genome of both sterile and fertile lines. The regions cloned, representing approximately 22% of the total dsRNA sequence, show no homology to organelle DNA. We have not been able to stably transmit the dsRNA to fertile lines of V. faba or any other plant species, using a variety of standard virological techniques.

A comparison of cytoplasmic revertants to fertility from different CMS-S maize sources

The mitochondrial genome organizations of a number of independent culture-derived fertile CMS-S revertants with the nuclear genotype W182BN were compared to spontaneous field revertants with the genotypes WF9, M825/Oh07 and 38-11. Regions of the genome around sequences homologous to the terminal repeats of the linear S1 and S2 episomes characteristic of CMS-S mitochondria were used as hybridization probes on Southern blots of BamHI and SalI digested mitochondrial DNA. The results obtained suggest that the nuclear, not the cytoplasmic, genotype of the parent plant affects the type of novel mitochondrial DNA organization found in the revertant. The DNA reorganization during reversion from CMS-S in tissue culture appears to be similar to that observed in spontaneous revertants obtained during the normal plant life-cycle. Unlike the situation for reversion from CMS-T, no common DNA sequence or reading frame appeared to be lost or disrupted in revertants.