Transgenic plants for insect pest control: a forward looking scientific perspective

One of the first successes of plant biotechnology has been the creation and commercialisation of transgenic crops exhibiting resistance to major insect pests. First generation products encompassed plants with single insecticidal Bt genes with resistance against major pests of corn and cotton. Modelling studies predicted that usefulness of these resistant plants would be short-lived, as a result of the ability of insects to develop resistance against single insecticidal gene products. However, despite such dire predictions no such collapse has taken place and the acreage of transgenic insect resistance crops has been increasing at a steady rate over the 9 years since the deployment of the first transgenic insect resistant plant. However, in order to assure durability and sustainability of resistance, novel strategies have been contemplated and are being developed. This perspective addresses a number of potentially useful strategies to assure the longevity of second and third generation insect resistant plants.

Recent Progress in Plantibody Technology

Antibodies are an important class of proteins that can be used for the prevention, treatment and diagnosis of many diseases. Consequently, there is an intense and growing demand for recombinant antibodies, placing immense pressure on current production capacity which is based largely on microbial cultures and mammalian cells. Alternative systems for cost effective antibody production would be very welcome, and plants are now gaining widespread acceptance as green bioreactors with advantages in terms of cost, scalability and safety. Several plant-produced antibodies (plantibodies) are undergoing clinical trials and the first commercial approval could be only a few years away. The performance of the first generation of products has been very encouraging so far. In terms of product authenticity, differences in glycosylation between plantibodies and their mammalian counterparts have been defined, and the scientific evaluation of any possible consequences is underway. Ongoing studies are addressing the remaining biochemical constraints, and aim to further improve product yields, homogeneity and authenticity, particularly where the antibody is intended for injection into human patients. A remaining practical challenge is the implementation of large-scale production and processing under good manufacturing practice conditions that are yet to be endorsed by regulatory bodies. The current regulatory uncertainty and the associated costs represent an entry barrier for the pharmaceutical industry. However, the favourable properties of plants are likely to make the plant systems a useful alternative for small, medium and large scale production throughout the development of new antibody-based pharmaceuticals.

Long-term vertical changes of the anterior maxillary teeth adjacent to single implants in young and mature adults. A retrospective study

AIM

To evaluate the effects of the tooth eruption process on the position of teeth adjacent to implant-borne restorations in adult patients compared to patients in their late adolescence.

SUBJECT AND METHODS

The sample included 28 patients divided into two groups. A "young adult" group consisting of 14 patients, aged from 15.5 to 21 years, and a "mature adult" group consisting of 14 patients, aged from 40 to 55 years. All patients presented missing anterior teeth, requiring insertion of 40 implant fixtures (16 central incisors, 12 lateral incisors, 12 canines). The implants were of the Straumann Dental Implant System, clinically and radiologically re-evaluated 1 year or more after the surgical procedure (mean interval=4.2 years). Assessment of the eruption of the adjacent teeth was performed using the implant as a stable point of reference: measurements of the different reference points were compared after implant placement and at follow-up examination.

RESULTS

In the "young adult" group, all patients showed infra-occlusion of the implant-supported crowns: the vertical step measured on radiographs varied between 0.1 and 1.65 mm. In the "mature adult" group, all patients showed a vertical difference between the teeth adjacent to the implant-supported crown and the implant: the measured step ranged from 0.12 to 1.86 mm. No difference was found in the amount of vertical eruption between male and female patients, nor according to localization of the implant.

CONCLUSION

Mature adults can exhibit major vertical steps after anterior restorations with osseointegrated fixtures to the same extent as adolescents or "young adult" individuals with residuous growth potential.

Producing transglutaminases by molecular farming in plants: Minireview article

Transglutaminases have a range of catalytic activities, most of which concern the post-translational modification of proteins. The most important of these activities, both in terms of biology and biotechnology, is the cross-linking of proteins into large supramolecular networks. The widespread use of transglutaminases in research, medicine and industry has increased the demand for an inexpensive, efficient and safe source of recombinant enzymes. We describe initial results concerning the production of a mammalian transglutaminase in transgenic rice plants as a first step towards the large-scale molecular farming of this enzyme.

Transgenic Rice as a Vehicle for the Production of the Industrial Enzyme Transglutaminase

Transglutaminases have a range of catalytic activities, most of which concern the post-translational modification of proteins. The most important of these activities is the cross-linking of proteins into large supramolecular networks. The widespread use of transglutaminases has increased the demand for an inexpensive, efficient and safe source of recombinant enzyme. We explored the use of plant-based systems for the production of this important industrial enzyme. Transgenic rice plants engineered with a rat prostate transglutaminase (rTGp), driven by the strong constitutive maize-1 ubiquitin promoter and its first intron, were shown to express the recombinant enzyme at the mRNA and protein levels. The Ca2+ dependence of the recombinant enzyme was confirmed by the biotin-labelled cadaverine-incorporation assay. In this communication we report the molecular and biochemical characterisation of transgenic plants expressing rTGp and this sets the stage for establishing a bioreactor system for the production of transglutaminases in plants.

Endogenous enzyme activities and polyamine levels in diverse rice cultivars depend on the genetic background and are not affected by the presence of the hygromycin phosphotransferase selectable marker

We used the polyamine biosynthetic pathway and rice as a relevant model to understand the genetic basis of variation in endogenous levels of metabolites and key enzymes involved in the pathway. Wild-type tissues and also tissues containing a commonly used selectable marker gene were employed. We detected a wide variation in levels of arginine decarboxylase activity and in the three polyamines, putrescine, spermidine and spermine, in different tissues and varieties, but this was not dependent on the presence of the selectable marker. A more-extensive profile of enzyme activities (ADC, ODC, SAMDC, DAO and PAO) and polyamine levels in different tissues was generated in two different varieties. Our results indicate that genetic background is important in terms of the basal levels of metabolites and enzyme activity, particularly in situations in which we aim to engineer metabolic pathways that are also encoded by homologous endogenous genes. We did not find any evidence that the presence of a selectable marker in any way influences enzyme activity or metabolite levels.

Native and artificial reticuloplasmins co-accumulate in distinct domains of the endoplasmic reticulum and in post-endoplasmic reticulum compartments

We compared the subcellular distribution of native and artificial reticuloplasmins in endosperm, callus, and leaf tissues of transgenic rice (Oryza sativa) to determine the distribution of these proteins among endoplasmic reticulum (ER) and post-ER compartments. The native reticuloplasmin was calreticulin. The artificial reticuloplasmin was a recombinant single-chain antibody (scFv), expressed with an N-terminal signal peptide and the C-terminal KDEL sequence for retrieval to the ER (scFvT84.66-KDEL). We found that both molecules were distributed in the same manner. In endosperm, each accumulated in ER-derived prolamine protein bodies, but also in glutelin protein storage vacuoles, even though glutelins are known to pass through the Golgi apparatus en route to these organelles. This finding may suggest that similar mechanisms are involved in the sorting of reticuloplasmins and rice seed storage proteins. However, the presence of reticuloplasmins in protein storage vacuoles could also be due to simple dispersal into these compartments during protein storage vacuole biogenesis, before glutelin deposition. In callus and leaf mesophyll cells, both reticuloplasmins accumulated in ribosome-coated vesicles probably derived directly from the rough ER.

Expression of legumin and vicilin genes in pea mutants and the production of legumin in transgenic plants

Pea seeds contain two major storage proteins, legumin and vicilin, in proportions that are genetically and environmentally determined. They are synthesized from at least 40 genes and at least 10 different genetic loci. Mutant alleles at loci involved in starch synthesis, which result in perturbations in starch accumulation, also affect the expression of legumin genes, thereby influencing the legumin: vicilin ratio within the total seed protein. Examples of such alleles include r (starch-branching enzyme) and rb (ADP-glucose pyrophosphorylase), both of which result in a reduction in legumin synthesis; double mutants (rrb) show a particularly severe reduction in the amount of legumin. The effects of such mutations are specific to legumins. The amounts of vicilin are unaffected by mutations at r or rb. One of the consequences of the production of legumin from many genes is structural heterogeneity that is believed to preclude the purification of homogeneous legumin for crystallization and 3D-structure determination. Expression of cloned legumin cDNA in E. coli can result in sequence homogeneity, but E. coli is unable to carry out the normal proteolytic processing of legumin precursors and consequently such material is different from that produced in pea seeds. This paper describes the high-level synthesis, processing and assembly of pea legumin in transgenic wheat seeds, leading to the spontaneous in vitro formation of paracrystalline arrays of legumin, which may be attributed to the fact that the legumin consists of a single type of subunit. Such material might be used as a source of single-sequence, processed and assembled pea legumin for structural investigation.

Over-expression of a cDNA for human ornithine decarboxylase in transgenic rice plants alters the polyamine pool in a tissue-specific manner

We investigated how over-expression of a cDNA for human ornithine decarboxylase (odc) affects the polyamine pools in transgenic rice. We further investigated tissue-specific expression patterns and product accumulation levels of the transgene driven by either constitutive or seed-specific promoters. Our results indicate that: (1) whereas the expression of a heterologous arginine decarboxylase (adc) cDNA in rice resulted in increased putrescine and spermine levels only in seeds, plants engineered to express odc cDNA exhibited significant changes in the levels of all three major polyamines in seeds and also in vegetative tissues (leaves and roots); (2) there was no linear correlation between odc mRNA levels, ODC enzyme activity and polyamine accumulation, suggesting that control of the polyamine pathway in plants is more complex than in mammalian systems; (3) ODC activity and polyamine changes varied in different tissues, indicating that the pathway is regulated in a tissue-specific manner. Our results suggest that ODC rather than ADC is responsible for the regulation of putrescine synthesis in plants.

Tagged Transcriptome Display (TTD) in indica rice using Ac transposition

We have developed a population of transgenic indica rice lines containing the autonomous Activator transposon (Ac) from maize. A transposon excision assay using as the reporter the green fluorescent protein (GFP) gene driven by the ubiquitin promoter was used to monitor Ac excision in various tissues. Our results, based on Ac excision and re-insertion events in 289 independent rice transformants, provide an insight into transposon biology in this heterologous model cereal system. Twenty percent of the transformed calli displayed uniform GFP activity, indicating very early Ac excision, while later excision in another 40% of calli was revealed by mosaic GFP activity. Both phenotypes were confirmed by molecular analysis. Progeny analysis revealed active transposition, with some lines displaying transposition to unlinked positions. Amplification of the Ac copy number was observed in approximately 30% of the lines, thus tagging multiple sites. We developed a "transposon insertion display" procedure, involving the use of CpG methylation-sensitive enzymes and a macroarray-based approach with cDNA as a complex hybridization probe, to selectively detect transposon insertions in transcribed sequences. Sequencing of tagged sites identified by hybridization to leaf cDNA revealed that all the hybridizing tags were homologous to genes or ESTs present in databases. Extrapolation from these data suggests that 12% of all amplified tags display homology to genes or ESTs - five times more than would be expected on the basis of random transpositional insertion. This Tagged Transcriptome Display (TTD) technique, using a population of Ac insertion-tagged lines, therefore represents a convenient approach for the identification of tagged genes involved in specific processes, as revealed by their expression patterns.

Expression of Arabidopsis GAI in transgenic rice represses multiple gibberellin responses

Bioactive gibberellins (GAs) are essential endogenous regulators of plant growth. GA signaling is mediated via GAI, a nuclear member of the GRAS family of plant transcription factors. Previous experiments have suggested that GAI is a GA-derepressible repressor of plant growth. Here we test this hypothesis by examining the effects of the expression of Arabidopsis GAI in transgenic Basmati rice. High-level expression of GAI caused dwarfism and reduced GA responses, and the strength of this effect was correlated with the level of transgene expression. In particular, the expression of GAI abolished the GA-mediated induction of rice aleurone alpha-amylase activity, thus implicating GAI orthologs in the well-characterized cereal aleurone GA response. The GA derepressible repressor model predicts that high-level expression of GAI should confer dwarfism, and these observations are consistent with this prediction.

Pea legumin overexpressed in wheat endosperm assembles into an ordered paracrystalline matrix

Legumin, a major component of pea seed storage vacuoles, is synthesized by a number of paralogous genes. The polypeptides are cleaved posttranslationally and can form mixed hexamers. This heterogeneity hampers structural studies, based on the production of hexamer crystals in vitro. To study a single type of homogenous legumin we produced pea legumin A in transgenic wheat (Triticum aestivum) endosperm where prolamins are predominant and only small amounts of globulins accumulate in separate inclusions. We demonstrated that the legumin precursor was cleaved posttranslationally and we confirmed assembly into 11S hexamers. Legumin was deposited within specific regions of the inclusion bodies. Angular legumin crystals extended from the inclusion bodies into the vacuole, correlating with the high legumin content. This suggests that the high-level production of a single type of legumin polypeptide resulted in the spontaneous formation of crystals in vivo. The use of a heterologous cereal system such as wheat endosperm to produce, isolate, and recrystallize homogenous 11S legume globulins offers exciting possibilities for structural analysis and characterization of these important seed storage proteins.

Transgenic rice plants expressing the ferredoxin-like protein (AP1) from sweet pepper show enhanced resistance to Xanthomonas oryzae pv. oryzae

We used particle bombardment to cotransform mature seed-derived rice callus (Oryza sativa L., ssp. japonica, cv. Eyi 105) with plasmids containing the linked marker genes gusA and hpt, and the ap1 gene encoding an amphipathic protein previously shown to delay the hypersensitive response induced in non-host plants by the pathogen Pseudomonas syringae pv. syringae (Pss). Thirty-two independent lines of transgenic rice plants were regenerated, and 27 of these lines carried all three transgenes as shown by molecular analysis. A bacterial blight inoculation test was carried out on ten lines. In each case, plants carrying the ap1 gene showed enhanced resistance to Xanthomonas oryzae pv. oryzae (Xoo) race 6 at various levels. This suggests the ap1 gene could be a useful candidate for genetic engineering strategies in rice to provide bacterial blight resistance.

Overexpression of the calcium-dependent protein kinase OsCDPK2 in transgenic rice is repressed by light in leaves and disrupts seed development

Independent transgenic rice lines overexpressing the rice CDPK isoform OsCDPK2 were generated by particle bombardment. High levels of OsCDPK2 were detected in leaves removed from etiolated plants, as well as in stems and flowers. However, there was no overexpression in green leaves that had been exposed to light, confirming that OsCDPK2 protein stability was subject to light regulation. The morphological phenotype of transgenic plants producing high levels of recombinant OsCDPK2 was normal until the onset of seed development. Flowers developed normally, producing well-shaped ovaries and stigmas, and mature anthers filled with pollen grains. However, seed formation in these plants was strongly inhibited, with only 3-7% of the flowers producing seeds. Seed development was arrested at an early stage. We discuss these data with respect to the possible requirement for specific CDPK isoforms during rice seed development.

Widely separated multiple transgene integration sites in wheat chromosomes are brought together at interphase

We have investigated the organization of transgenes delivered by particle bombardment into the wheat genome, combining conventional molecular analysis with fluorescence in situ hybridization (FISH) and three-dimensional confocal microscopy. We selected a representative population of transformed wheat lines and carried out molecular and expression analysis. FISH on metaphase chromosomes showed that transgene integration sites were often separated by considerable lengths of genomic DNA (>1 Mbp), or could even be on opposite chromosome arms. Plants showing multiple integration sites on a single chromosome were selected for three-dimensional confocal analysis of interphase nuclei in root and embryo tissue sections. Confocal microscopy revealed that these sites lay in close physical proximity in the interphase nuclei. Our results clearly show that multiple transgenes physically separated by large intervening regions of endogenous DNA at metaphase can be brought together at interphase. This may reflect the original physical organization of the endogenous DNA at the moment of transformation, with DNA strand breaks introduced into several co-localized DNA loops by the intruding gold particles. Alternatively, the transgenes may be brought together after transformation, either by an ectopic homologous pairing mechanism, or by recruitment to a common transcription site.

Constitutive expression of soybean ferritin cDNA in transgenic wheat and rice results in increased iron levels in vegetative tissues but not in seeds

We used particle bombardment to produce transgenic wheat and rice plants expressing recombinant soybean ferritin, a protein that can store large amounts of iron. The cDNA sequence was isolated from soybean by RT-PCR and expressed using the constitutive maize ubiquitin-1 promoter. The presence of ferritin mRNA and protein was confirmed in the vegetative tissues and seeds of transgenic wheat and rice plants by northern and western blot analysis, respectively. The levels of ferritin mRNA were similar in the vegetative tissues of both species, but ferritin protein levels were higher in rice. Both ferritin mRNA and protein levels were lower in wheat and rice seeds. ICAP spectrometry showed that iron levels increased only in vegetative tissues of transgenic plants, and not in the seeds. These data indicate that recombinant ferritin expression under the control of the maize ubiquitin promoter significantly increases iron levels in vegetative tissues, but that the levels of recombinant ferritin in seeds are not sufficient to increase iron levels significantly over those in the seeds of non-transgenic plants.

Simultaneous reduction of the activity of two related enzymes, involved in early steps of the polyamine biosynthetic pathway, by a single antisense cDNA in transgenic rice

Transgenic rice cell lines transformed with a heterologous cDNA derived from the arginine decarboxylase gene of oat, in an antisense orientation, exhibited significant (P < 0.05) down-regulation of the activity of the endogenous arginine and ornithine decarboxylases, compared to wild type and controls transformed only with the selectable marker (hpt). Changes in enzyme activity were reflected in a marked decrease in the level of putrescine (P < 0.001) and spermidine (P < 0.01) but not spermine (P > 0.05) in the majority of cell lines analyzed. In agreement with previous results, we confirmed that cell lines with low levels of polyamines exhibited normal morphogenic responses. In vegetative tissue at the whole plant level no significant variation (P > 0.05) in polyamine levels was observed. However, we measured significant reductions (P < 0.001) in putrescine levels in seeds derived from three out of five plants analyzed in detail. Thus, simultaneous reduction of the activity of the two alternative enzymes in the early steps of the polyamine pathway results in significant reduction in end-product accumulation in the seeds of transgenic plants.

A transgenic rice cell lineage expressing the oat arginine decarboxylase (adc) cDNA constitutively accumulates putrescine in callus and seeds but not in vegetative tissues

We introduced the oat adc cDNA into rice under the control of the constitutive maize ubiquitin 1 promoter. We studied molecularly and biochemically sixteen independent transgenic plant lines. Significant increases in mRNA levels, ADC enzyme activity and polyamines were measured in transgenic callus. These increases were not maintained in vegetative tissue or seeds in regenerated plants, with the exception of one lineage. This particular lineage showed very significant increases in putrescine preferentially in seeds (up to 10 times compared to wild type and controls transformed with the hpt selectable marker alone). We have demonstrated that in cereals such as rice, over-expression of the oat adc cDNA results in increased accumulation of polyamines at different stages of development. We have also demonstrated that strong constitutive promoters, such as the maize ubiquitin 1 promoter, are sufficient to facilitate heritable high-level polyamine accumulation in seed. Our results demonstrate that by screening adequate numbers of independently derived transgenic plants, it is possible to identify those individuals which express a desired phenotype or genotype.

The maize streak virus coat protein transcription unit exhibits tissue-specific expression in transgenic rice

Maize streak geminivirus (MSV) is a single-stranded DNA virus that infects cereals and other grasses. A promoter region incorporating the MSV large intergenic region and movement protein gene sequence was ligated to the gus (beta-glucuronidase) reporter gene which replaced the virus coat protein (CP) gene. The CP promoter activity was analysed in transgenic rice plants (Oryza sativa L.) and was compared with that obtained in plants transformed with the gus gene downstream of the cauliflower mosaic virus (CaMV) 35S promoter. The MSV CP promoter activity varied in the five plant lines tested, but was always less than that of the CaMV promoter. Histochemistry showed that the MSV CP promoter was active in cells of regenerating callus but in regenerated plants it provided an expression pattern restricted to the vascular tissues of the root, stem, leaf and floral organs. Expression was highest in phloem-associated tissues of the vegetative organs and was absent from the tip and elongation region of seedling roots. Thus, the MSV CP promoter shows a degree of developmental regulation and can be used to confer tissue-specific expression in transgenic rice plants.

Transformation of the tropane alkaloid-producing medicinal plant Hyoscyamus muticus by particle bombardment

We report an efficient whole plant transformation system for Hyoscyamus muticus, an important medicinal plant of the Solanaceous family. We developed a system using a plasmid carrying the nptII and gusA genes, which was delivered into leaf explants by particle bombardment. Ten percent of bombarded leaf explants formed kanamycin-resistant callus, from which putative transgenic plants were recovered. The nptII gene conferring kanamycin resistance was found to be incorporated into the genome of all transgenic plants screened. Over 50% of the kanamycin resistant plants showed strong expression of the non-selected gusA gene. The majority of transgenic plants reached maturity, could be self pollinated, and produced fertile seed. A simple and efficient whole plant transformation system for this medicinal plant is an important step in furthering our understanding of tropane alkaloid production in plants.

Rice cell culture as an alternative production system for functional diagnostic and therapeutic antibodies

We investigated the suitability of transformed rice cell lines as a system for the production of therapeutic recombinant antibodies. Expression constructs encoding a single-chain Fv fragment (scFvT84.66, specific for CEA, the carcinoembryonic antigen present on many human tumours) were introduced into rice tissue by particle bombardment. We compared antibody production levels when antibodies were either secreted to the apoplast or retained in the endoplasmic reticulum (ER) using a KDEL retention signal. Production levels were up to 14 times higher when antibodies were retained in the ER. Additionally, we compared construct sencoding different leader peptides (plant codon optimised murine immunoglobulin heavy and light chain leader peptides from mAb24) and carrying alternative 5' untranslated regions (the petunia chalcone synthase gene 5' UTR and the tobacco mosaic virus omega sequence). We observed no significant differences in antibody production levels among cell lines transformed with these constructs. The highest level of antibody production we measured was 3.8 micrograms g-1 callus (fresh weight). Immunological analysis of transgenic rice callus confirmed the presence of functional scFvT84.66. We discuss the potential merits of cell culture for the production of recombinant antibodies and other valuable macromolecules.

Resistance to green leafhopper (Nephotettix virescens) and brown planthopper (Nilaparvata lugens) in transgenic rice expressing snowdrop lectin (Galanthus nivalis agglutinin; GNA)

Transgenic rice plants expressing snowdrop lectin (Galanthus nivalis agglutinin; GNA) were screened for resistance to green leafhopper (Nephotettix virescens; GLH), a major homopteran pest of rice. Survival was reduced by 29% and 53% (P<0.05) respectively, on plants where GNA expression was tissue-specific (phloem and epidermal layer) or constitutive. Similar levels of resistance in GNA-expressing transgenic rice were previously reported for rice brown planthopper (Nilaparvata lugens; BPH). GNA binding to glycoproteins in gut tissues showed that BPH contained more "receptors" than GLH, and that the binding affinity was stronger, particularly in the midgut. Subsequent toxicity of GNA is thus unlikely to be directly related to the amount of lectin bound. GNA was not detected in the honeydew of either insect species when they were fed on GNA-expressing plants, in contrast to results from artificial diet studies. This result suggests that GNA is not being delivered to the insect efficiently. When offered a free choice vs control plants, BPH nymphs tended to avoid plants expressing GNA; avoidance was less pronounced and took longer to develop on plants where GNA expression was tissue-specific, In contrast to BPH, GLH nymphs were attracted to plants expressing GNA, whether constitutively or in a tissue-specific manner.

Cereal crops as viable production and storage systems for pharmaceutical scFv antibodies

This report describes the stable expression of a medically important antibody in the staple cereal crops rice and wheat. We successfully expressed a single-chain Fv antibody (ScFvT84.66) against carcinoembryonic antigen (CEA), a well characterized tumor-associated marker antigen. scFv constructs were engineered for recombinant antibody targeting to the plant cell apoplast and ER. Up to 30 microg/g of functional recombinant antibody was detected in the leaves and seeds of wheat and rice. We confirmed that transgenic dry seeds could be stored for at least five months at room temperature, without significant loss of the amount or activity of scFvT84.66. Our results represent the first transition from model plant expression systems, such as tobacco and Arabidopsis, to widely cultivated cereal crops, such as rice and wheat, for expression of an antibody molecule that has already shown efficacy in clinical applications. Thus, we have established that molecular pharming in cereals can be a viable production system for such high-value pharmaceutical macromolecules. Our findings provide a strong foundation for exploiting alternative uses of cereal crops both in industrialized and developing countries.