Analysis of fertility in somatic hybrids of Nicotiana rustica and N. tabacum and progeny over two sexual generations

An interspecific Nicotiana hybrid as a useful and cost-effective platform for production of animal vaccines

The use of transgenic plants to produce novel products has great biotechnological potential as the relatively inexpensive inputs of light, water, and nutrients are utilised in return for potentially valuable bioactive metabolites, diagnostic proteins and vaccines. Extensive research is ongoing in this area internationally with the aim of producing plant-made vaccines of importance for both animals and humans. Vaccine purification is generally regarded as being integral to the preparation of safe and effective vaccines for use in humans. However, the use of crude plant extracts for animal immunisation may enable plant-made vaccines to become a cost-effective and efficacious approach to safely immunise large numbers of farm animals against diseases such as avian influenza. Since the technology associated with genetic transformation and large-scale propagation is very well established in Nicotiana, the genus has attributes well-suited for the production of plant-made vaccines. However the presence of potentially toxic alkaloids in Nicotiana extracts impedes their use as crude vaccine preparations. In the current study we describe a Nicotiana tabacum and N. glauca hybrid that expresses the HA glycoprotein of influenza A in its leaves but does not synthesize alkaloids. We demonstrate that injection with crude leaf extracts from these interspecific hybrid plants is a safe and effective approach for immunising mice. Moreover, this antigen-producing alkaloid-free, transgenic interspecific hybrid is vigorous, with a high capacity for vegetative shoot regeneration after harvesting. These plants are easily propagated by vegetative cuttings and have the added benefit of not producing viable pollen, thus reducing potential problems associated with bio-containment. Hence, these Nicotiana hybrids provide an advantageous production platform for partially purified, plant-made vaccines which may be particularly well suited for use in veterinary immunization programs.

Random chloroplast segregation and frequent mtDNA rearrangements in fertile somatic hybrids between Nicotiana tabacum L. and N. glutinosa L

Patterns of organelle inheritance were examined among fertile somatic hybrids between allotetraploid Nicotiana tabacum L. (2n=4x=48) and a diploid wild relative N. glutinosa L. (2n=2x=24). Seventy somatic hybrids resistant to methotrexate and kanamycin were recovered following fusion of leaf mesophyll protoplasts of transgenic methotrexate-resistant N. tabacum and kanamycin-resistant N. glutinosa. Evidence for hybridization of nuclear genomes was obtained by analysis of glutamate oxaloacetate transaminase and peroxidase isoenzymes and by restriction fragment length polymorphism (RFLP) analysis using a heterologous nuclear ribosomal DNA probe. Analysis of chloroplast genomes in a population of 41 hybrids revealed a random segregation of chloroplasts since 25 possessed N. glutinosa chloroplasts and 16 possessed N. tabacum chloroplasts. This contrasts with the markedly non-random segregation of plastids in N. tabacum (+)N. rustica and N. tabacum (+) N. debneyi somatic hybrids which we described previously and which were recovered using the same conditions for fusion and selection. The organization of the mitochondrial DNA (mtDNA) in 40 individuals was examined by RFLP analysis with a heterologous cytochrome B gene. Thirty-eight somatic hybrids possessed mitochondrial genomes which were rearranged with respect to the parental genomes, two carried mtDNA similar to N. tabacum, while none had mtDNA identical to N. glutinosa. The somatic hybrids were self-fertile and fertile in backcrosses with the tobacco parent.

Non-random chloroplast segregation inNicotiana tabacum (+)N. rustica somatic hybrids selected by dual nuclear-encoded resistance

Nicotiana tabacum (+)N. rustica interspecific somatic hybrids were produced by fusion of leaf mesophyll protoplasts of transgenic methotrexate-resistantNicotiana tabacum L. with leaf mesophyll protoplasts of transgenic kanamycin-resistantN. rustica L. Somatic hybrids were selected on the basis of resistance to both methotrexate and kanamycin. Evidence for nuclear hybridization was obtained for 21 hybrids by restriction-fragment-length-polymorphism (RFLP) analysis using a heterologous wheat nuclear ribosomal-DNA (rDNA) probe and by analysis of glutamate-oxaloacetate transaminase (GOT) isoenzymes. Chloroplasts segregated non-randomly as 20 of the somatic hybrids possessedN. rustica chloroplasts and only one hadN. tabacum chloroplasts. Patterns of mitochondrial inheritance were examined by hybridization of a heterologous wheat cytochrome oxidase subunit II (coxII) gene with genomic DNA of the somatic hybrids. Four somatic hybrids with hybridization patterns similar toN. rustica and 17 with hybridization patterns consistent with mitochondrial DNA (mtDNA) rearrangement or recombination were obtained. None of the somatic hybrids had patterns ofcoxll hybridization identical withN. tabacum. Male-fertility levels in the hybrids ranged from undetectable to 87% and only nine hybrids produced a limited amount of viable seed. There was no apparent correlation between the patterns of organelle inheritance in the somatic hybrids and the relative degree of fertility.

A selection method for the synthesis of triploid hybrids by fusion of microspore protoplasts (n) with somatic cell protoplasts (2n)

Microspore protoplasts (n) isolated at the tetrad stage from plants of Nicotiana tabacum Km(+) (2n=4x=48) were fused with somatic cell protoplasts (2n) of WT N. rustica (2n=4x=48) to produce triploid plants. A total of 21.2×10(6) microspore protoplasts were fused with 11.2×10(6) somatic cell protoplasts using the high pH/Ca(+ +) method. Microspore protoplasts did not divide and WT N. rustica protoplasts stopped dividing when the protoplast-derived colonies were transferred to a selection medium containing kanamycin. A total of 104 actively growing green colonies were recovered on the selection medium. Ninety-six of these colonies were tested for their hybrid nature by PAGE of peroxidases and were found to contain bands characteristic of both parents. Hybrid nature of the plants regenerated from some of the selected colonies was confirmed by IEF of leaf esterases, by NPT II activity assay and by hybridizing total DNAs restricted with EcoR I to a cloned 18s rDNA fragment. Root tip squashes of six of the hybrid plants revealed chromosome numbers ranging from 58-72. From chromosomal and biochemical analysis, it can be concluded that the procedure of fusing microspore protoplasts (n) of species A carrying a dominant selection marker with WT somatic cell protoplasts (2n) of species B can be a convenient selection method for the synthesis of triploid plants. The significance of triploids lies in their subsequent use for transferring alien chromosomes and genes of species A to species B.