Restoration of fertility by antisense RNA in genetically engineered male sterile tobacco plants

Evolvement of transgenic male-sterility and fertility-restoration system in rice for production of hybrid varieties

We have developed a unique male-sterility and fertility-restoration system in rice by combining Brassica napus cysteine-protease gene (BnCysP1) with anther-specific P12 promoter of rice for facilitating production of hybrid varieties. In diverse crop plants, male-sterility has been exploited as a useful approach for production of hybrid varieties to harness the benefits of hybrid vigour. The promoter region of Os12bglu38 gene of rice has been isolated from the developing panicles and was designated as P12. The promoter was fused with gusA reporter gene and was expressed in Arabidopsis and rice systems. Transgenic plants exhibited GUS activity in tapetal cells and pollen of the developing anthers indicating anther/pollen-specific expression of the promoter. For engineering nuclear male sterility, the coding region of Brassica napus cysteine protease1 (BnCysP1) was isolated from developing seeds and fused to P12 promoter. Transgenic rice plants obtained with P12-BnCysP1 failed to produce functional pollen grains. The F₁ seeds obtained from BnCysP1 male-sterile plants and untransformed controls showed 1:1 (tolerant:sensitive) ratio when germinated on the MS medium supplemented with phosphinothricin (5 mg/l), confirming that the male sterility has been successfully engineered in rice. For male fertility restoration, transgenic rice plants carrying BnCysP1Si silencing system were developed. The pollination of BnCysP1 male-sterile (female-fertile) plants with BnCysP1Si pollen resulted in normal grain filling. The F₁ seeds of BnCysP1 × BnCysP1Si when germinated on the MS basal medium containing PPT (5 mg/l) and hygromycin (70 mg/l) exhibited 1:1 (tolerant:sensitive) ratio and the tolerant plants invariably showed normal grain filling. The overall results clearly suggest that the customized male-sterility & fertility-restoration system can be exploited for quality hybrid seed production in various crops.

A novel male sterility-fertility restoration system in plants for hybrid seed production

Hybrid seeds are used for stimulated crop production, as they harness heterosis. The achievement of complete male-sterility in the female-parent and the restored-fertility in F1-hybrids are the major bottlenecks in the commercial hybrid seed production. Here, we report a male sterility-fertility restoration system by engineering the in most nutritive anther wall layer tapetum of female and male parents. In the female parent, high-level, and stringent expression of Arabidopsis autophagy-related gene BECLIN1 was achieved in the tapetum, which altered the tapetal degeneration program, leading to male sterility. This works on our previously demonstrated expression cassette based on functional complementation of TATA-box mutant (TGTA) promoter and TATA-binding protein mutant3 (TBPm3), with modification by conjugating Long Hypocotyle in Far-Red1 fragment (HFR1(NT131)) with TBPm3 (HFR1(NT131)-TBPm3) to exercise regulatory control over it. In the male parent, tapetum-specific Constitutive photo-morphogenesis1 (COP1) was expressed. The F1 obtained by crossing these engineered parents showed decreased BECLIN1 expression, which was further completely abolished when COP1-mutant (COP1(L105A)) was used as a male parent, leading to normal tapetal development and restored fertility. The system works on COP1-HFR1 interaction and COP1-mediated degradation of TBPm3 pool (HFR1(NT131)-TBPm3). The system can be deployed for hybrid seed production in agricultural crops.

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.

Intron hairpin and transitive RNAi mediated silencing of orfH522 transcripts restores male fertility in transgenic male sterile tobacco plants expressing orfH522

The present work was aimed at developing vector construct(s) suitable for restoring fertility in transgenic male sterile tobacco plants expressing male-sterility-inducing ORFH522 in tapetal cell layer (Nizampatnam et al. Planta 229:987-1001, 2009). PTGS vectors that could produce either intron spliced hairpin RNA against the orfH522 or induce silencing of orfH522 by heterologous 3'UTR region were developed using the selected 316 bp (orf316) fragment of orfH522. The constructs were independently mobilized into Agrobacterium and used for transforming tobacco. The T(1) generation plants carrying the restorer gene cassettes in homozygous condition were identified and crossed with the male sterile transgenic tobacco plants to obtain the hybrid seeds. PCR analysis of hybrid plants indicated segregation for the sterility inducing cassette while all the plants carried the restorer cassette. Hybrid plants produced fertile pollen grains and formed normal capsules upon selfing. Further molecular analyses of these hybrid plants with RT-PCR, Northern blotting and siRNA detection, revealed that intron interrupted hairpin RNA (ihp-RNA) mediated gene silencing was more effective compared to silencing by heterologous 3'UTR (SHUTR) as indicated by the complete degradation of orfH522 transcripts and formation of higher levels of orf316 specific siRNA molecules in plants carrying ihp-RNA restorer construct. Segregation analyses of F(2) (selfed hybrid) plants confirmed the co-segregation of gene cassettes and the traits in Mendelian di-hybrid ratio (9:3:3:1). Taken together, the results established that intron hairpin and transitive RNAi mediated silencing of orfH522 transcripts restored fertility in transgenic male sterile tobacco plants expressing orfH522 and ihp-RNA was more efficient in silencing orfH522 transcripts.

Expression of sunflower cytoplasmic male sterility-associated open reading frame, orfH522 induces male sterility in transgenic tobacco plants

Sterility in the universally exploited PET1-CMS system of sunflower is associated with the expression of orfH522, a novel mitochondrial gene. Definitive evidence that ORFH522 is directly responsible for male sterility is lacking. To test the hypothesis that ORFH522 is sufficient to induce male sterility, a set of chimeric constructs were developed. The cDNA of orfH522 was cloned in-frame with yeast coxIV pre-sequence, and was expressed under tapetum-specific promoter TA29 (construct designated as TCON). For developing control vectors, orfH522 was cloned without the transit peptide under TA29 promoter (TON) or orfH522 was cloned with or without transit peptide under the constitutive CaMV35S promoter (SCOP and SOP). Among several independent transformants obtained with each of the gene cassettes, one third of the transgenics (6/17) with TCON were completely male sterile while more than 10 independent transformants obtained with each of the control vectors were fertile. The male sterile plants were morphologically similar to fertile plants, but had anthers that remained below the stigmatic surface at anthesis. RT-PCR analysis of the sterile plants confirmed the anther-specific expression of orfH522 and bright-field microscopy demonstrated ablation of the tapetal cell layer. Premature DNA fragmentation and programmed cell death was observed at meiosis stage in the anthers of sterile plants. Stable transmission of induced male sterility trait was confirmed in test cross progeny. This constitutes the first report at demonstrating the induction of male sterility by introducing orfH522 gene that could be useful for genetic engineering of male sterility.

Towards male sterility in Pinus radiata--a stilbene synthase approach to genetically engineer nuclear male sterility

A male cone-specific promoter from Pinus radiata D. Don (radiata pine) was used to express a stilbene synthase gene (STS) in anthers of transgenic Nicotiana tabacum plants, resulting in complete male sterility in 70% of transformed plants. Three plants were 98%-99.9% male sterile, as evidenced by pollen germination. To identify the stage at which transgenic pollen first developed abnormally, tobacco anthers from six different developmental stages were assayed microscopically. Following the release of pollen grains from tetrads, transgenic pollen displayed an increasingly flake-like structure, which gradually rounded up during the maturation process. We further investigated whether STS expression may have resulted in an impaired flavonol or sporopollenin formation. A specific flavonol aglycone stain was used to demonstrate that significant amounts of these substances were produced only in late stages of normal pollen development, therefore excluding a diminished flavonol aglycone production as a reason for pollen ablation. A detailed analysis of the exine layer by transmission electron microscopy revealed minor structural changes in the exine layer of ablated pollen, and pyrolysis-gas chromatography-mass spectroscopy indicated that the biochemistry of sporopollenin production was unaffected. The promoter-STS construct may be useful for the ablation of pollen formation in coniferous gymnosperms and male sterility may potentially be viewed as a prerequisite for the commercial use of transgenic conifers.

Characterization and spatial distribution of ectomycorrhizas colonizing aspen clones released in an experimental field

Ectomycorrhizas (EM) from aspen clones released on an experimental field were characterized by morphotyping, restriction analysis and internal transcribed spacer (ITS) sequencing. In addition, their community structure and spatial distribution was analyzed. Among the 23 observed morphotypes, six mycobionts dominated, forming roughly 90% of all ectomycorrhizas: Cenococcum geophilum, Laccaria sp., Phialocephala fortinii, two different Thelephoraceae, and one member of the Pezizales. The three most common morphotypes had an even spatial distribution, reflecting the high degree of homogeneity of the experimental field. The distribution of three other morphotypes was correlated with the distances to the spruce forest and deciduous trees bordering the experimental field. These two patterns allowed two invasion strategies of ectomycorrhizal fungi (EMF) to be recognized, the success of which depends on adaptation of the EMF to local ecological conditions.

Identification of a mitochondrial external NADPH dehydrogenase by overexpression in transgenic Nicotiana sylvestris

The plant respiratory chain contains a complex setup of non-energy conserving NAD(P)H dehydrogenases, the physiological consequences of which are highly unclear. An expression construct for the potato (Solanum tuberosum L., cv. Desiree) ndb1 gene, a homologue of bacterial and fungal type II NAD(P)H dehydrogenases, was introduced into Nicotiana sylvestris. Transgenic lines with high transcript and protein levels for St-NDB1 had up to threefold increased activity of external NADPH dehydrogenase in isolated mitochondria as compared to the wild type (WT). In two lines, the external NADPH dehydrogenase activity was instead 10-fold decreased, indicating that the corresponding N. sylvestris gene had been suppressed. Activities of external and internal rotenone-insensitive NADH dehydrogenases were unchanged in the transgenic lines. The results demonstrate that the St-ndb1 encodes an external dehydrogenase specific for NADPH and dependent on calcium for activity. Transgenic lines overexpressing St-ndb1 had specifically increased protein levels for alternative oxidase and uncoupling protein, as compared to the WT and one co-suppressing line. This indicates cross-talk in the expressional control, or metabolic conditions influencing it, for the different categories of energy-dissipating proteins that bypass oxidative phosphorylation. The potential effects of external NADPH oxidation on other cellular processes are discussed.

Transgene introgression from genetically modified crops to their wild relatives

Transgenes engineered into annual crops could be unintentionally introduced into the genomes of their free-living wild relatives. The fear is that these transgenes might persist in the environment and have negative ecological consequences. Are some crops or transgenic traits of more concern than others? Are there natural genetic barriers to minimize gene escape? Can the genetic transformation process be exploited to produce new barriers to gene flow? Questions abound, but luckily so do answers.

FLP/FRT-mediated restoration of normal phenotypes and clonal sectors formation in rolC transgenic tobacco

Site-specific recombination systems have been shown to excise transgene DNA sequences positioned between their cognate target sites, and thus be used to generate clonal sectors in transgenic plants. Here we characterized clonal sectors derived from genetic reversion of rolC (A. rhizogenes)--induced vegetative and reproductive phenotypes, mediated by FLP recombinase from S. cerevisiae, in tobacco. The constitutive expression of rolC induces pleiotropic effects including reduced apical dominance and plant height, lanceolate and pale green leaves and small, male-sterile flowers. Two transgenic male-sterile tobacco lines (N. tabacum, Samsun NN) expressing a 35sP-rolC gene construct flanked by two FRT (FLP recombinase target) sites, were cross-pollinated with pollen from a constitutive 35sP-FLP expressing line. Three main phenotypes were generated in result of recombinase-mediated excision of the 35sP-rolC locus in the F1 (FLP x FRT-35sP-rolC-FRT) hybrid progenies: (a) restoration of male fertility, associated with reversion to normal leaf phenotypes prior to flower bud formation, (b) development of normal and fertile lateral shoot sectors on the background of rolC-type plants, (c) restoration of partially fertile flowers, associated with display of peripheral normal leaf sectors surrounding rolC-type inner-leaf tissues, consistent with periclinal chimeras. These results, supported by DNA molecular analysis, indicate that site-specific recombination might be used as a relatively efficient tool for generation of transgenic periclinal chimeric plants.

Induction of male sterility in plants by metabolic engineering of the carbohydrate supply

Extracellular invertase mediates phloem unloading via an apoplastic pathway. The gene encoding isoenzyme Nin88 from tobacco was cloned and shown to be characterized by a specific spatial and temporal expression pattern. Tissue-specific antisense repression of Nin88 under control of the corresponding promoter in tobacco results in a block during early stages of pollen development, thus, causing male sterility. This result demonstrates a critical role of extracellular invertase in pollen development and strongly supports the essential function of extracellular sucrose cleavage for supplying carbohydrates to sink tissues via the apoplast. The specific interference with phloem unloading, the sugar status, and metabolic signaling during pollen formation will be a potentially valuable approach to induce male sterility in various crop species for hybrid seed production.

The restoration of fertility in male sterile tobacco demonstrates that transgene silencing can be mediated by T-DNA that has no DNA homology to the silenced transgene

Male sterile tobacco plants expressing a pathogenesis-related (PR) beta-1,3-glucanase gene driven by the Arabidopsis thaliana A3 or A9 tapetum-specific promoter, were partially restored to fertility by retransformation with a range of pA9-driven sense and antisense PR glucanase fragments. The restored plants exhibited improved seed set. PR glucanase protein was undetectable in the anthers of these plants and there was an associated increase in microsporocyte callose, the structural target of the A3 and A9-driven PR glucanase. This phenotype was not solely dependent on interactions between sense and antisense PR glucanase transcripts since a pA9-driven restorer was also capable of down regulating a pA3-GUS construct in the absence of extensive promoter, coding region, or terminator sequence homology. Since the A3 and A9 promoters have similar temporal and spatial expression patterns, it is possible that trans-acting factors common to both promoters become limiting in the PR glucanase double transformants resulting in improved levels of fertility. An alternative hypothesis is that additional sequences present in both the silencing and target T-DNAs can mediate the silencing of adjacent non-homologous transgenes.

Gene silencing in transgenic tobacco hybrids: frequency of the event and visualization of somatic inactivation pattern

We have investigated the stability of the expression of different T-DNA-borne genes in hybrid tobacco lines. These lines were constructed to rescue rolC-induced male sterility in kanamycin-resistant P35s-rolC transgenic tobacco plants by expression of rolC antisense genes. Using five different tester lines, a total of 158 hybrids was obtained. We observed inactivation of transgene expression in 20% of the F1 progeny and in 35% of the backcrossed F2 progeny, as indicated by the loss of kanamycin resistance. In 3% of all crosses complete loss of antibiotic resistance was noted, while in most affected hybrid progeny only part of the population became kanamycin sensitive. Single genes could be selectively inactivated on T-DNAs harboring several genes. Gene inactivation was not restricted to one of the two T-DNAs examined. Somatic silencing, visualized by a cell-specific 35SGUSINT marker gene, occurred in a random fashion or exhibited an inherited specific pattern. The type of somatic silencing pattern observed indicated developmental control of the process. Two phenotypic classes could be distinguished with respect to frequency and timing of the inactivation process. Rapid gene inactivation, occurring within a few weeks after germination of hybrid seedlings, was characterized by complete methylation of restriction sites in the promoter of the silenced gene, resetting of gene expression during meiosis, heredity of the developmentally controlled program of gene silencing in subsequent generations, and rapid reactivation of gene expression after genetic separation of the different T-DNAs. In contrast, a slow type of gene inactivation was of a more stochastic nature and was recognized only in hybrids of the backcrossed F2 generation. In this case the degree of promoter methylation, which could extend beyond the T-DNA borders, was not correlated with the reduction in steady-state poly(A)+ mRNA levels, the silenced state was transmitted through meiosis and reactivation lasted several generations. The implications of the observations for our understanding of the gene inactivation process are discussed.

Constitutive or light-regulated expression of the rolC gene in transgenic potato plants has different effects on yield attributes and tuber carbohydrate composition

Tetraploid potato clones, transgenic for the rolC gene of Agrobacterium rhizogenes under control of the light-inducible ribulose bisphosphate carboxylase small subunit promoter (rbcS-rolC), were compared, with respect to yield attributes and tuber carbohydrates, with transformed and untransformed controls and with 35S-rolC transgenic potato plants. In rbcS-rolC plants, the expression of the rolC gene was located mainly in leaves, while in 35S-rolC plant transcripts were detected as well in shoots and roots. Phenotypically, rbcS-rolC transgenic plants were found to be slightly reduced in plant size with a few more tillers than control plants. Photosynthetic rate and chlorophyll content were significantly lower in all rolC transgenic plants irrespective of the type of construct used. Tuber yield was not significantly different between controls and rbcS-rolC transgenic plants, but was reduced in the 35S-rolC transformants. Sucrose level was unchanged in all rolC clones investigated, whereas fructose content was significantly enhanced in 35S-rolC transformants, but not in the plants expressing the rolC gene in aerial plant parts only. In both types of rolC transgenic plants, glucose content was lower than in controls, resulting in a significant reduction of reducing sugar in tubers. The results suggest a hormonal influence on the carbohydrate composition of potato tubers.