Nuclear-mitochondrial interactions in cytoplasmic male-sterile sorghum

The CMS-associated 16 kDa protein encoded by orfH522 in the PET1 cytoplasm is also present in other male-sterile cytoplasms of sunflower

In sunflower plants carrying the PET1 cytoplasm male sterility (CMS) is associated with a new open reading frame (orfH522) in the 3'-flanking region of the atpA gene and an additional 16 kDa protein. Twenty-seven male-sterile cytoplasms of different origin were studied for the expression of the 16 kDa protein. In addition to the PET1 cytoplasm nine other male-sterile cytoplasms express the CMS-associated protein. These CMS sources originate from different interspecific crosses, from spontaneously occurring male-sterile plants in wild sunflower and from induced mutagenesis. Polyclonal antisera were raised against fusion proteins which contain 421 bp of the 3'-coding region of orfH522 to verify by immunological methods the identity of the other CMS cytoplasms. The anti-ORFH522 antiserum showed a positive reaction in the immunoblot with all CMS cytoplasms which expressing the 16 kDa protein. Investigations of the mitochondrial DNA demonstrated that all ten CMS cytoplasms which express the 16 kDa protein have the same organization at the atpA locus. OrfH522 as probes gave the same transcript pattern for the investigated CMS cytoplasms, just as for PET1. The MAX1 cytoplasm has an orfH522-related sequence but does not synthesize the 16 kDa protein. Using the sodium carbonate treatment the 16 kDa protein proved to be membrane-bound. Computer analyses predict that the hydrophobic N-terminal region of ORFH522 may form a transmembrane helix functioning as membrane anchor.

Mitochondrial DNA diversity in wild and cultivated sorghum

Cultivated sorghum (Sorghum bicolor ssp. bicolor) is classified into five main races on the basis of spikelet morphology. Isozyme analyses provided new insight into the genetic diversity of sorghum and revealed marked geographic grouping, while nuclear restriction fragment length polymorphisms showed racial differentiation and intraguinea race differentiation. Wild sorghum is diploid or tetraploid and African sorghum (S. bicolor ssp. arundinaceum) is classified into four races, that are considered to be progenitors of cultivated sorghum. We performed mitochondrial DNA analyses to compare the diversity of wild and cultivated sorghum and to study the genetic origin of guinea margaritiferum. The same overall patterns were obtained with the different phenogram construction techniques. Our results confirmed the specificity of guinea margaritiferum and demonstrated the presence of two genetic entities within this subrace. Another guinea group was also noted, which corresponded to Asian guinea roxburghii. In wild sorghum, the arundinaceum race appeared to be homogenous, while the verticilliflorum race was separated into two groups, one of which was associated with the arundinaceum race. The diversity observed in cultivated forms was found to be encompassed within the wild pool, except for one guinea margaritiferum group. There did not seem to be any particular relationship between wild races and cultivated races.

Isolation of mitochondrial DNA sequences that distinguish male-sterility-inducing cytoplasms in Sorghum bicolor (L.) Moench

We have demonstrated that sorghum DNA sequences of mitochondrial origin can be used to distinguish different male-sterility-inducing cytoplasms. Six DNA clones containing single-copy mitochondrial sequences were hybridized on Southern blots to restriction enzyme-digested DNA of 28 sorghum lines representing sources of different cytoplasmic male-sterility (CMS) groups. Four cytoplasmic types were defined on the basis of the pattern of DNA fragments detected. Similar analyses of 50 additional diverse sorghum accessions suggested that three of the four cytoplasmic types may be diagnostic for CMS. Also, three other cytoplasmic types were discovered. These and other mitochondrial DNA clones may be useful molecular tools for "fingerprinting" sterility-inducing cytoplasms in breeding programs, determining cytoplasmic diversity among germ plasm accessions, and identifying new sources of cytoplasm that induce male sterility.

Organ-specific reduction in the abundance of a mitochondrial protein accompanies fertility restoration in cytoplasmic male-sterile radish

The mitochondrial DNA of plants containing the male sterility-causing Ogura cytoplasm of radish contain a novel gene, orf138, that is transcribed as part of a bicistronic mRNA. Genetic studies have previously linked male sterility with the orf138 locus. To determine if orf138 is expressed at the protein level, and investigate the effect of fertility restoration on ORF138 levels, we have raised antibodies to an ORF138-glutathione S-transferase fusion protein. Anti-ORF138 antibodies detect a 20 kDa protein that is associated with the mitochondrial membrane of sterile Ogura radish plants. Nuclear restoration is accompanied by a dramatic reduction in the amount of this protein in mitochondria of flowers and leaves, but not roots of fertile Ogura radish plants. The presence or absence of fertility restoration genes has no detectable effect on the size, abundance, or RNA editing patterns of orf138 transcripts. These results support genetic studies that have implicated orf138 in Ogura cytoplasmic male sterility and suggest that the restorer genes may be affecting either the translation or stability of ORF138.

RFLP analysis of mitochondrial DNA from cytoplasmic male-sterile lines of pearl millet

Mitochondrial DNA (mtDNA) from 13 cytoplasmic male-sterile (cms) lines from diverse sources were characterized by Southern blot hybridization to pearl millet and maize mtDNA probes. Hybridization patterns of mtDNA digested with PstI, BamHI, SmaI or XhoI and probed with 13.6-, 10.9-, 9.7- or 4.7-kb pearl millet mtDNA clones revealed similarities among the cms lines 5141 A and ICMA 1 (classified as the S-A1 type of cytoplasm based on fertility restoration patterns), PMC 30A and ICMA 2. The remaining cms lines formed a distinct group, within which three subgroups were evident. Among the maize mitochondiral gene clones used, the coxI probe revealed two distinct groups of cytoplasms similar to the pearl millet mtDNA clones. The atp9 probe differentiated the cms line 81 A4, derived from P. glaucum subsp. monodii, while the coxII gene probe did not detect any polymorphism among the cms lines studied. MtDNA digested with BamHI, PstI or XhoI and hybridized to the atp6 probe revealed distinct differences among the cms lines. The maize atp6 gene clone identified four distinct cytoplasmic groups and four subgroups within a main group. The mtDNA fragments hybridized to the atp6 gene probe with differing intensities, suggesting the presence of more than one copy of the gene in different stoichiometries. Rearrangements involving the coxI and/or rrn18-rrn5 genes (mapped within the pearl millet clones) probably resulted in the S-A1 type of sterility. Rearrangements involving the atp6 gene (probably resulting in chimeric form) may be responsible for male sterility in other cms lines of pearl millet.

Molecular analysis of cytoplasmic male sterility in chives (Allium schoenoprasum L.)

The mitochondria of chive plants with normal N or male-sterile S cytoplasms have been examined by restriction fragment analysis and Southern hybridizations of mitochondrial DNA (mtDNA) and in organello protein biosynthesis. Restriction fragment patterns of the mtDNA differed extensively between N-and S-cytoplasms. The percentage of fragments with different mobility varied between 44-48% depending on the restriction enzyme used. In contrast to mtDNA, the restriction fragment patterns of the chloropolast DNA from N- and S-cytoplasms were identical. The organization of the analyzed mitochondrial genes coxII, coxIII, nad1 and nad3 was different in N- and S-cytoplasms. Comparison of mitochondrial proteins analyzed by in organello translation revealed an 18-kDa protein present only in S-cytoplasm. The restorer gene X suppressed the synthesis of that protein in S-cytoplasm. Thus, the 18-kDa protein seems to be associated with the cytoplasmic male-sterile phenotype.

Mitochondrial genome variability in Sorghum cell culture protoclones

Sorghum bicolor cv NK300 seedlings, a cell suspension culture, and five protoclone suspension cultures were compared for the occurrence of somaclonal variation by analysis of their mitochondrial DNA (mtDNA). Restriction digests of the mtDNA showed qualitative and quantitative variation of restriction fragments. Southern analyses were performed using a 14.7-kb EcoRI mitochondrial genome fragment and regions carrying mitochondrial protein coding genes, atpA, atp6, cob, and coxI as probes. These analyses revealed part of the 14.7-kb EcoRI region to be present as a repeat in planta, and to be hypervariable when cells were subjected to protoplast culture. All protoclones differed from each other, from the parental cell suspension culture, and from the seedlings in their mitochondrial genome arrangement. Seedlings of five independent sorghum accessions, unrelated to cv NK300, of diverse geographic origin showed conservation of this mitochondrial fragment. Southern analyses of the mtDNA showed no variation for genomic organization of the region carrying coxI, and atpA was identical in all the tissue culture lines. The atp6 gene was present as two copies in the seedlings, and one copy was rearranged upon tissue culture. The region carrying the cob gene was also found to be variant between tissue culture and seedling mtDNA. A substoichiometric 3.3-kb EcoRI cob fragment present in seedlings was amplified in the tissue culture lines. Protoclone S63 differed from the original suspension culture and remaining protoclones in that it had lost the 3.0-kb EcoRI band, the most abundant fragment in seedlings. A new set of fragments was detected in this protoclone. Northern analysis for the cob gene demonstrated altered transcript size in protoclone S63.

Unique patterns of mitochondrial genes, transcripts and proteins in different male-sterile cytoplasms of Daucus carota

Restriction fragment analysis of mitochondrial and chloroplast DNAs from a brown anther and a petaloid cytoplasmic male-sterile (cms) line revealed unique patterns for each cms line distinct from those of normal fertile cytoplasms, but identical restriction fragments for all chloroplast DNAs. The restauration of fertility through the introduction of nuclear restorer genes had no effect on the overall mitochondrial DNA (mtDNA) structure. The genomic environment and transcription patterns of several mitochondrial genes differ between cms and normal cytoplasms, while no difference has so far been detected between cms and the corresponding fertility-restored lines in mitochondrial DNAs and mRNAs. Mitochondrial translation products analysed by in-organello synthesized proteins revealed a number of polypeptides unique to each cytoplasm. Most prominent is a 17-kDa polypeptide that is present in the brown anther cms line but not in fertile mitochondria. Synthesis of this protein was not visibly affected by fertility restauration. The different cms phenotypes in carrot are thus associated with extensive and unique mtDNA rearrangements and distinct alterations in transcription and translation patterns.

Extensive nuclear influence on mitochondrial transcription and genome structure in male-fertile and male-sterile alloplasmic Nicotiana materials

Nuclear influences on mitochondrial transcription and genome organization were analysed in six different male-fertile and male-sterile alloplasmic Nicotiana cultivars. The alloplasmic materials were compared with the corresponding nuclear species (N. tabacum) and cytoplasmic donor species (N. debneyi, N. rapanda or N. suaveolens) in Northern and Southern analyses using twelve different mitochondrial genes as probes. The investigation revealed that the nucleus exerts extensive influence on the expression and structure of the mitochondrial genome. For the majority of the probes, changes in both mitochondrial transcription and DNA patterns in alloplasmic cultivars were detected. Even though changes in transcription patterns, which correlated with male sterility, were detected for three of the probes (atpA, orf25 and coxII), the changes were not consistent for all the male-sterile materials. Likewise, no consistent association between mtDNA restriction patterns and cytoplasmic male sterility (CMS) was detected.

Complex determination of male sterility in Thymus vulgaris L.: genetic and molecular analysis

Nucleocytoplasmic determination of male sterility in Thymus vulgaris L. has been assumed in all papers attempting to explain the remarkably high frequencies of male steriles found in natural populations of this species. This paper provides strong evidence that both nuclear and cytoplasmic genes are involved in the determination of male sterility of this species, giving a complex inheritance. Interpopulation and intrapopulation crosses have shown that the ratio of females versus hermaphrodites among offsprings varied widely from 1∶0 to 1∶1. Furthermore, interpopulation crosses consistently yielded a higher frequency of females than intrapopulation crosses. Nucleocytoplasmic inheritance was demonstrated by an absence of male fertiles in backcrosses and asymmetrical segregation in reciprocal crosses. Molecular analysis of the mitochondrial DNA of some of the parents used in crosses suggested the involvement of different cytoplasms in the inheritance of male steriliy.

Nuclear genotype affects mitochondrial genome organization of CMS-S maize

A WF9 strain of maize with the RD subtype of the S male-sterile cytoplasm (CMS-S) was converted to the inbred M825 nuclear background by recurrent backcrossing. The organization of the mitochondrial genomes of the F1 and succeeding backcross progenies was analyzed and compared with the progenitor RD-WF9 using probes derived from the S1 and S2 mitochondrial episomes, and probes containing the genes for cytochrome c oxidase subunit I (coxI), cytochrome c oxidase subunit II (coxII) and apocytochrome b (cob). Changes in mitochondrial DNA (mtDNA) organization were observed for S1-, S2-, and coxI-homologous sequences that involve loss of homologous restriction enzyme fragments present in the RD-WF9 progenitor. With the coxI probe, the loss of certain fragments was accompanied by the appearance of a fragment not detectable in the progenitor. The changes observed indicate the effect of the nuclear genome on the differential replication of specific mitochondrial subgenomic entities.

Evolution of plant mitochondrial genomes via substoichiometric intermediates

Comparison of the modern fertile maize mitochondrial genome (N) with an ancestral maize mitochondrial genome (RU) reveals a 12 kb duplication (containing the atpA gene) in the modern genome that is absent from the ancestor. Cloning, mapping, and sequencing of the relevant portions of the ancestral genome shows that this duplication probably arose via a three-stage recombination process involving substoichiometric intermediates. Comparison with analogous observations on yeast mitochondrial genomes suggests that this three-stage model of genome reorganization can be generally applied to plant mitochondrial genomes to explain both deletions and the creation of novel repeats, common features of plant mitochondrial genome evolution.

Correction of chlorophyll-defective male-sterile winter oilseed rape (Brassica napus) through organelle exchange: molecular analysis of the cytoplasm of parental lines and corrected progeny

Cytoplasmic differences between male-fertile and male-sterile Brassica napus as well as Raphanus sativus were investigated. Plastids of the male-fertile B. napus were found to differ from those of male-sterile B. napus and R. sativus with respect to DNA restriction enzyme patterns. Differences between male-fertile and male-sterile B. napus mitochondria were detected not only in the restriction fragment patterns of their DNA, but also at the level of expression by in organello translation of mitochondrial polypeptides.The chlorophyll deficiency obtained upon transferral of the male-sterility-conferring radish cytoplasm to a winter variety of B. napus had been corrected earlier through protoplast fusion. The cytoplasmic composition of the corrected lines was analysed using DNA restriction analysis and in organello translation. The stability of the recombined cytoplasm in the corrected lines was confirmed by analysis of the subsequent seed-derived generation.

Dwarfism and male sterility in interspecific hybrids of Epilobium : 2. Expression of mitochondrial genes and structure of the mitochondrial DNA

Mitochondrial DNA (mtDNA) and transcriptional patterns of mitochondrial genes have been examined in dwarf, normal, fertile and male sterile Epilobium hybrids. No alterations or rearrangements of mitochondrial DNA could be detected in the developmentally disturbed hybrids. They exhibit restriction patterns of mtDNA that correspond exactly to those of their female parents. However, the transcription of at least one mitochondrial gene is significantly altered in the male sterile hybrid E. hirsutum x montanum. In normal plants, one mRNA of 1.6 kb hybridizes to the cytochrome c oxidase subunit II gene, while in male sterile plants a transcript of this size is lacking and instead a major transcript of 2.0 kb and two smaller ones occur. The transcript pattern of the F1 ATPase alpha subunit (atpA) gene exhibits slight alterations in sterile plants also. Since these hybrids have the same cytoplasm as normal plants, an incompatibility between the nuclear and the mitochondrial genotype may be responsible for the altered mitochondrial gene expression. No alteration of the transcripts of the mitochondrial genes tested could be detected in dwarf hybrids. The coincidence of male sterility with an altered transcription pattern of mitochondrial genes suggests that the mitochondria are involved in the occurrence of this phenotype.