Mitochondrial sensitivity to Drechslera maydis T-toxin and the synthesis of a variant mitochondrial polypeptide in plants derived from maize tissue cultures with texas male-sterile cytoplasm

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

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.

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.

Study of two different recombination events in maize cmsT-regenerated plants during reversion to fertility

The change of phenotype from sterility to fertility for some cmsT callus tissue culture regenerated plants and their progenies has been correlated with changes in their mitochondrial genome. Those changes that have been analyzed here are the result of recombination events. Two different sets of repeated sequences have been found to be involved in those recombination events. The most common one is a recombination through a 127-bp repeat between various independently isolated revertants. The second one is a recombination through a 58-bp repeat. In every case the products of recombination containing the urf13 gene have been deleted.

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.

Tomato resistance to Alternaria stem canker: localization in host genotypes and functional expression compared to non-host resistance

The Alternaria stem canker resistance locus (Asc-locus), involved in resistance to the fungal pathogen Alternaria alternata f. sp. lycopersici and in insensitivity to host-specific toxins (AAL-toxins) produced by the pathogen, was genetically mapped on the tomato genome. Susceptibility and resistance were assayed by testing a segregating F2 population for sensitivity to AAL-toxins in leaf bioassays. Linkage was observed to phenotypic markers solanifolium and sunny, both on chromosome 3. For the Asc-locus, a distance of 18 centiMorgan to solanifolium was calculated, corresponding to position 93 on chromosome 3. This map position of the resistance locus turned out to be the same in three different resistant tomato accessions, one Dutch and two American, that are at least 40 years apart. AAL-toxin sensitivity in susceptible and resistant tomato genotypes was compared with AAL-toxin sensitivity in a non-host Nicotiana tabacum during different levels of plant cell development. In susceptible and resistant tomato genotypes, inhibitory effects were demonstrated at all levels, except for leaves of resistant genotypes. However, during pollen and root development, inhibitory effects on susceptible genotypes were larger than on resistant genotypes. In the non-host Nicotiana tabacum, hardly any effects of AAL-toxins were demonstrated. Apparently, a cellular target site is present in tomato, but not in Nicotiana tabacum. It was concluded that three levels of AAL-toxin sensitivity exist: (1) a susceptible host sensitivity, (2) a resistant host sensitivity, (3) a non-host sensitivity, and that the resistance mechanism operating in tomato is different from that operating in Nicotiana tabacum.

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.

Loss of CMS-specific mitochondrial DNA arrangement in fertile segregants of Petunia hybrids

The progeny of somatic hybrid Petunia plants derived from the fusion of a male-fertile line and a cytoplasmic male-sterile (cms) line were examined. Male-fertile progeny derived from three different male-sterile somatic hybrid plants did not exhibit the mitochondrial DNA (mtDNA) arrangement which has previously been correlated with cms in Petunia. The cms-associated mtDNA arrangement was present in the male-sterile predecessors of these fertile revertants. Thus, it is concluded that the loss of this mtDNA arrangement is associated with reversion to fertility in the progeny of the unstable somatic hybrid petunia plants.

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

Polyspecific antibody to a 17 amino acid synthetic peptide from the maize T-cytoplasm urf13-T mitochondrial open reading frame immunoprecipitated a 13 kD polypeptide from (35)S-methionine incorporations of T cytoplasm maize. Male-fertile, toxin-insensitive mutants in which the urf13-T sequence is deleted do not synthesize the 13 kD polypeptide. A mutant designated T-4, which carries a 5 bp insertion and a premature stop codon, synthesizes a truncated polypeptide, corresponding to an open reading frame of 8.3 kD. Thus the 13 kD polypeptide is trunctated or absent in mutants expressing male fertility and toxin insensitivity in T-cytoplasm maize.

Nuclear-mitochondrial interactions in cytoplasmic male-sterile sorghum

Variation in mitochondrial genome organization and expression between male fertile and sterile nuclear-cytoplasmic combinations of sorghum has been examined. Cytoplasmic genotypes were classified into eleven groups on the basis of restriction endonuclease digestion of mitochondrial DNA (mtDNA) and five groups on the basis of mitochondrial translation products. These cytoplasms were further characterized by hybridization of specific gene probes to Southern blots of EcoRI digested mtDNA, and identification of the fragment location of four mitochondrial genes. Variation was observed in the genomic location and copy number of the F1 ATPase α-subunit gene, as well as the genomic location and gene product of the cytochrome c oxidase subunit I gene. The effect of nuclear genotype on mitochondrial genome organization, expression and the presence of two linear plasmid-like mtDNA molecules was examined. Our results indicate that nuclear-mitochondrial interactions are required for regulation of mitochondrial gene expression. When a cytoplasm is transferred from its natural to a foreign nuclear background some changes in the products of in organello mitochondrial protein synthesis occur. In a number of cytoplasmic genotypes these changes correlate with the expression of cytoplasmic male sterile phenotype, suggesting a possible molecular basis for this mutation.

Structural alterations in a transcribed region of the T type cytoplasmic male sterile maize mitochondrial genome

H. maydis toxin resistance, and cytoplasmic reversion from sterility to fertility in Zea mays T type cytoplasm are correlated with sequence changes in a 6.6 kb XhoI fragment of T mitochondrial DNA. Comparative Northern blot analysis of N (normal male fertile), and cmsT (cytoplasmic male sterile) mitochondrial RNAs using subclones of the 6.6 kb Xho I region of cmsT as probes, reveals different sized transcripts. In cmsT these RNA coding sequences have been mapped onto a 1.5 kb AvaI fragment completely internal to the 6.6 kb XhoI region. Comparative Southern analysis of AvaI digested mitochondrial DNA from cmsT, N and a T-type cytoplasm which has reverted to fertility (designated V3) (Brettel et al. 1979), positions the RNA coding sequences on a 1.5 kb, 2.1 kb and 2.1 kb fragment respectively. These sequence rearrangements in the fertile T revertants create a novel mtRNA transcript. Southern hybridization experiments of other higher plant mitochondrial DNAs using the 1.5 kb Ava I fragment from cmsT mtDNA as a probe, indicate the presence of homologous sequences.

Molecular analysis of mitochondria from a fertility restorer line of maize

Inbred line Ky21 carries nuclear genes which restore fertility to all three cytoplasmic male sterile (cms) types of maize: T, C and S. By substituting the Ky21 cytoplasm into a nuclear background lacking all of the nuclear restorer genes, we have demonstrated that Ky21 contains a fertile (normal) cytoplasm. Gel electrophoresis of mitochondrial DNA from Ky21 demonstrated an approximately 2.1 kb plasmid and no evidence for a 2.35 kb plasmid found in many normal cytoplasms of North American lines of maize. A 2.1 kb plasmid had been reported to be diagnostic for the T-type cms. However, the restriction endonuclease digestion pattern of Ky21 mtDNA more closely resembled that of normal lines than T-cms. Furthermore, mitochondria of Ky21 plants did not synthesize a 13 kilodalton polypeptide, which has only been found to be synthesized by T-type mitochondria. From these molecular criteria, as well as from the genetic analysis, we conclude that the mitochondria of the Ky21 universal restorer line are normal. In having a shorter form of a linear mtDNA plasmid, Ky21 resembles cytoplasms found in Mexican races of maize.

Microanalysis of plant mitochondrial protein synthesis products: : Detection of variant polypeptides associated with cytoplasmic male sterility

A mitochondrial fraction obtained from 0.5 g of leaves was purified on a 0.75 ml Percoll gradient and used for an in vitro mitochondrial protein synthesis assay in the presence of [(35)S] methionine. A set of 15 to 20 labeled polypeptides were revealed by autoradiography after sodium dodecylsulfate polyacrylamide gel electrophoresis. This could be applied at an early growth stage by using a few leaves from individual seedlings. It revealed the presence of variant mitochondrially translated polypeptides in green leaves of cytoplasmic male sterile lines from various cultivated plants of large economic importance: maize, wheat, sugar beet, tobacco and faba bean. This non-destructive microanalysis is thus of general use and opens new possibilities for rapid and large mass screening of mitochondrial parameters such as male sterility.