Transfer of methomyl and HmT-toxin sensitivity from T-cytoplasm maize to tobacco

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.

Expression of mitochondrial gene fragments within the tapetum induce male sterility by limiting the biogenesis of the respiratory machinery in transgenic tobacco

Plant mitochondrial genomes (mtDNAs) are large and undergo frequent recombination events. A common phenotype that emerges as a consequence of altered mtDNA structure is cytoplasmic-male sterility (CMS). The molecular basis for CMS remains unclear, but it seems logical that altered respiration activities would result in reduced pollen production. Analysis of tobacco (Nicotiana tabacum) mtDNAs indicated that CMS-associated loci often contain fragments of known organellar genes. These may assemble with organellar complexes and thereby interfere with normal respiratory functions. Here, we analyzed whether the expression of truncated fragments of mitochondrial genes (i.e. atp4, cox1 and rps3) may induce male sterility by limiting the biogenesis of the respiratory machinery. cDNA fragments corresponding to atp4f, cox1f and rps3f were cloned in-frame to a mitochondrial localization signal and a C-termini HA-tag under a tapetum-specific promoter and introduced to tobacco plants by Agrobacterium-mediated transformation. The constructs were then analyzed for their effect on mitochondrial activity and pollen fertility. Atp4f, Cox1f and Rps3f plants demonstrated male sterility phenotypes, which were tightly correlated with the expression of the recombinant fragments in the floral meristem. Fractionation of native organellar extracts showed that the recombinant ATP4f-HA, COX1f-HA and RPS3f-HA proteins are found in large membrane-associated particles. Analysis of the respiratory activities and protein profiles indicated that organellar complex I was altered in Atp4f, Cox1f and Rps3f plants.

Transformation and analysis of tobacco plant var Petit havana with T-urf13 gene under anther-specific TA29 promoter

T-urf13, a well-documented cms-associated gene from maize, has been shown to render methomyl sensitivity to heterologous systems like rice, yeast and bacteria when expressed constitutively. Since these transgenic plants were fertile, it was hypothesized that T-urf13 gene if expressed in anthers may result in male sterility that could be used for hybrid seed production. Hence, this work was aimed at analysing whether T-urf13 gene when expressed in anthers can result in male sterile plants or requires methomyl treatment to cause male sterility (controllable). This is the first report of transformation of tobacco with T-urf13 gene under anther-specific promoter (TA29) with or without mitochondrial targeting sequence. Most of the transgenic plants obtained were fertile; this was surprising as many male sterile plants were expected as T-urf13 gene is a cms associated gene. Our results suggest that it may not be possible to obtain male sterility by expressing URF13 in the anther by itself or by methomyl application.

Genetic and Genomic Dissection of the Cochliobolus heterostrophus Tox1 Locus Controlling Biosynthesis of the Polyketide Virulence Factor T‐toxin

Fungal pathogenesis to plants is an intricate developmental process requiring biological components found in most fungi, as well as factors that are unique to fungal taxa that participate in particular fungus-plant interactions. The host-selective polyketide toxin known as T-toxin produced by Cochliobolus heterostrophus race T, a highly virulent pathogen of maize, is an intriguing example of the latter type of virulence determinant. The Tox1 locus, which controls biosynthesis of T-toxin, originally defined as a single genetic locus, it is, in fact, two exceedingly complex loci on two chromosomes that are reciprocally translocated with respect to their counterparts in weakly pathogenic race O. Race O lacks the Tox1 locus and does not produce T-toxin. Highly virulent race T was first recognized when it caused an epidemic of Southern Corn Leaf Blight, which devastated the US corn crop in 1970. The evolutionary origin of the Tox1 locus remains unknown.

Mutator-induced mutations of the rf1 nuclear fertility restorer of T-cytoplasm maize alter the accumulation of T-urf13 mitochondrial transcripts

Dominant alleles of the rf1 and rf2 nuclear-encoded fertility restorer genes are necessary for restoration of pollen fertility in T-cytoplasm maize. To further characterize fertility restoration mediated by the Rf1 allele, 123,500 gametes derived from plants carrying the Mutator transposable element family were screened for rf1-mutant alleles (rf1-m) Four heritable rf1-m alleles were recovered from these populations. Three rf1-m alleles were derived from the progenitor allele Rf1-IA153 and one was derived from Rf1-Ky21. Cosegregation analysis revealed 5.5- and 2.4-kb Mu1-hybridizing EcoRI restriction fragments in all of the male-sterile and none of the male-fertile plants in families segregating for rf1-m3207 and rf1-m3310, respectively. Mitochondrial RNA gel blot analyses indicated that all four rf1-m alleles in male-sterile plants cosegregated with the altered steady-state accumulation of 1.6- and 0.6-kb T-urf13 transcripts, demonstrating that these transcripts are Rf1 dependent. Plants carrying a leaky mutant, rf1-m7323, revealed variable levels of Rf1-associated, T-urf13 transcripts and the degree of pollen fertility. The ability to obtain rf1-m derivatives from Rf1 indicates that Rf1 alleles produce a functional gene product necessary for the accumulation of specific T-urf13 transcripts in T-cytoplasm 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.

The relationship between the mitochondrial gene T-urf13 and fungal pathotoxin sensitivity in maize

Mitochondria isolated from maize containing cms-T cytoplasm are specifically sensitive to pathotoxins (T-toxins) produced by the fungi Bipolaris maydis race T and Phyllosticta maydis. T-toxins interact with a 13 kDa membrane-bound toxin receptor protein, URF13, to produce hydrophillic pores in the membrane. Expression of URF13 in Escherichia coli produces bacterial cells that form hydrophillic pores in the plasma membrane when exposed to T-toxin or methomyl. Topological studies have established that URF13 contains three membrane-spanning alpha-helices, two of which are amphipathic and may contribute to pore formation. URF13 specifically binds T-toxin in a cooperative manner. Oligonucleotide-directed mutagenesis of URF13 led to the isolation of methomyl/T-toxin-resistant mutations at 39 separate positions throughout the URF13 primary sequence. Chemical cross-linking of URF13 demonstrated the presence of URF13 oligomers and established that the pore-forming species is oligomeric. The ability of the carboxylate-specific reagent, dicyclohexycarbodiimide to cross-link URF13 has been used in conjunction with site-directed mutagenesis to establish that the URF13 tetramer has a central core consisting of a four-alpha-helical bundle that may undergo a conformational change after T-toxin or methomyl binding. Experimental evidence indicates that URF13 acts as a ligand-gated, pore-forming T-toxin receptor.

Targeting the maize T-urf13 product into tobacco mitochondria confers methomyl sensitivity to mitochondrial respiration

The URF13 protein, which is encoded by the maize mitochondrial T-urf13 gene, is thought to be responsible for pathotoxin and methomyl sensitivity and male sterility. We have investigated whether T-urf13 confers toxin sensitivity and male sterility when expressed in another plant species. The coding sequence of T-urf13 was fused to a mitochondrial targeting presequence, placed under the control of the cauliflower mosaic virus 35S promoter, and introduced into tobacco by Agrobacterium tumefaciens-mediated transformation. Plants expressing high levels of URF13 were methomyl sensitive. Subcellular analysis indicated that URF13 is mainly associated with the mitochondria. Adding methomyl to isolated mitochondria stimulated NADH-linked respiration and uncoupled oxidative phosphorylation, indicating that URF13 was imported into the mitochondria, and conferred toxin sensitivity. Most control plants, which expressed the T-urf13c construct lacking the mitochondrial presequence, were methomyl sensitive and contained URF13 in a membrane fraction. Subcellular fractionation by sucrose gradient centrifugation showed that URF13 sedimented at several positions, suggesting the protein is associated with various organelles, including mitochondria. No methomyl effect was observed in isolated mitochondria, however, indicating that URF13 was not imported and did not confer toxin sensitivity to the mitochondria. Thus, URF13 confers toxin sensitivity to transgenic tobacco with or without import into the mitochondria. There was no correlation between the expression of URF13 and male sterility, suggesting either that URF13 does not cause male sterility in transgenic tobacco or that URF13 is not expressed in sufficient amounts in the appropriate anther cells.

Toxin-deficient mutants from a toxin-sensitive transformant of Cochliobolus heterostrophus

Tox1 is the only genetic element identified which controls production of T-toxin, a linear polyketide involved in the virulence of Cochliobolus heterostrophus to its host plant, corn. Previous attempts to induce toxin-deficient (Tox-) mutants, using conventional mutagenesis and screening procedures, have been unsuccessful. As a strategy to enrich for Tox- mutants, we constructed a Tox1+ strain that carried the corn T-urf13 gene (which confers T-toxin sensitivity) fused to a fungal mitochondrial signal sequence; the fusion was under control of the inducible Aspergillus nidulans pelA promoter which, in both A. nidulans and C. heterostrophus, is repressed by glucose and induced by polygalacturonic acid (PGA). We expected that a transformant carrying this construction would be sensitive to its own toxin when the T-urf13 gene was expressed. Indeed, the strain grew normally on medium containing glucose but was inhibited on medium containing PGA. Conidia of this strain were treated with ethylmethanesulfonate and plated on PGA medium. Among 362 survivors, 9 were defective in T-toxin production. Authenticity of each mutant was established by the presence of the transformation vector, proper mating type, and a restriction fragment length polymorphism tightly linked to the Tox1+ locus. Progeny of each mutant crossed to a Tox1+ tester segregated 1:1 (for wild type toxin production vs. no or reduced toxin production), indicating a single gene mutation in each case. Progeny of each mutant crossed to a Tox1- tester segregated 1:1 (for no toxin production vs. no or reduced toxin production) indicating that each mutation mapped at the Tox1 locus. Availability of Tox- mutants will permit mapping in the Tox1 region without interference from a known Tox1 linked translocation breakpoint.

Baculovirus expression of the maize mitochondrial protein URF13 confers insecticidal activity in cell cultures and larvae

The URF13 protein, which is encoded by the mitochondrial gene T-urf13, is responsible for cytoplasmic male sterility and pathotoxin sensitivity in the Texas male-sterile cytoplasm (cms-T) of maize. Mitochondrial sensitivity to two host-specific fungal toxins (T toxins) is mediated by the interaction of URF13 and T toxins to form pores in the inner mitochondrial membrane. A carbamate insecticide, methomyl, mimics the effects of T toxins on isolated cms-T mitochondria. URF13 was expressed in Spodoptera frugiperda (fall army-worm) cells (Sf9) in culture and in Trichoplusia ni (cabbage looper) larvae with a baculovirus vector. In insect cells, URF13 forms oligomeric structures in the membrane and confers T toxin or methomyl sensitivity. Adding T toxin or methomyl to Sf9 cells producing URF13 causes permeabilization of plasma membranes. In addition, URF13 is toxic to insect cells grown in culture without T toxins or methomyl; even a T-toxin-insensitive mutant form of URF13 is lethal to cell cultures. Baculoviruses expressing URF13 are lethal to T. ni larvae, at times postinjection comparable to those obtained by injecting a baculovirus expressing an insect neurotoxin. This result suggests that URF13 could be useful as a biological control agent for insect pests. Our data indicate that URF13 has two independent mechanisms for toxicity, one that is mediated by T toxin and methomyl and one that is independent of these toxins. Similarly, male sterility and toxin sensitivity in cms-T maize may be due to independent mechanisms.

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.