Plastid transformation in Lesquerella fendleri, an oilseed Brassicacea

Exogenous methyl jasmonate treatment induced the transcriptional responses and accumulation of volatile terpenoids in Oenanthe javanica (Blume) DC

Water dropwort is favored by consumers for its unique flavor and medicinal value. Terpenoids were identified as the main volatile compounds related to its flavor. In this study, water dropwort was treated with different concentrations of exogenous methyl jasmonate (MeJA). The contents of volatile terpenoids were determined under various MeJA treatments. The results indicated that 0.1 mM of MeJA most effectively promoted the biosynthesis of flavor-related terpenoids in water dropwort. Terpinolene accounted the highest proportion among terpene compounds in water dropwort. The contents of jasmonates in water dropwort were also increased after exogenous MeJA treatments. Transcriptome analysis indicated that DEGs involved in the terpenoid biosynthesis pathway were upregulated. The TPS family was identified from water dropwort, and the expression levels of Oj0473630, Oj0287510 and Oj0240400 genes in TPS-b subfamily were consistent with the changes of terpene contents under MeJA treatments. Oj0473630 was cloned from the water dropwort and designated as OjTPS3, which is predicted to be related to the biosynthesis of terpinolene in water dropwort. Subcellular localization indicated that OjTPS3 protein was localized in chloroplast. Protein purification and enzyme activity of OjTPS3 protein were conducted. The results showed that the purified OjTPS3 protein catalyzed the biosynthesis of terpinolene by using geranyl diphosphate (GPP) as substrate in vitro. This study will facilitate to further understand the molecular mechanism of terpenoid biosynthesis and provide a strategy to improve the flavor of water dropwort.

Prospects of chloroplast metabolic engineering for developing nutrient-dense food crops

Addressing nutritional deficiencies in food crops through biofortification is a sustainable approach to tackling malnutrition. Biofortification is continuously being attempted through conventional breeding as well as through various plant biotechnological interventions, ranging from molecular breeding to genetic engineering and genome editing for enriching crops with various health-promoting metabolites. Genetic engineering is used for the rational incorporation of desired nutritional traits in food crops and predominantly operates through nuclear and chloroplast genome engineering. In the recent past, chloroplast engineering has been deployed as a strategic tool to develop model plants with enhanced nutritional traits due to the various advantages it offers over nuclear genome engineering. However, this approach needs to be extended for the nutritional enhancement of major food crops. Further, this platform could be combined with strategies, such as synthetic biology, chloroplast editing, nanoparticle-mediated rapid chloroplast transformation, and horizontal gene transfer through grafting for targeting endogenous metabolic pathways for overproducing native nutraceuticals, production of biopharmaceuticals, and biosynthesis of designer nutritional compounds. This review focuses on exploring various features of chloroplast genome engineering for nutritional enhancement of food crops by enhancing the levels of existing metabolites, restoring the metabolites lost during crop domestication, and introducing novel metabolites and phytonutrients needed for a healthy daily diet.

Plastid Transformation: How Does it Work? Can it Be Applied to Crops? What Can it Offer?

In recent years, plant genetic engineering has advanced agriculture in terms of crop improvement, stress and disease resistance, and pharmaceutical biosynthesis. Cells from land plants and algae contain three organelles that harbor DNA: the nucleus, plastid, and mitochondria. Although the most common approach for many plant species is the introduction of foreign DNA into the nucleus (nuclear transformation) via Agrobacterium- or biolistics-mediated delivery of transgenes, plastid transformation offers an alternative means for plant transformation. Since there are many copies of the chloroplast genome in each cell, higher levels of protein accumulation can often be achieved from transgenes inserted in the chloroplast genome compared to the nuclear genome. Chloroplasts are therefore becoming attractive hosts for the introduction of new agronomic traits, as well as for the biosynthesis of high-value pharmaceuticals, biomaterials and industrial enzymes. This review provides a comprehensive historical and biological perspective on plastid transformation, with a focus on current and emerging approaches such as the use of single-walled carbon nanotubes (SWNTs) as DNA delivery vehicles, overexpressing morphogenic regulators to enhance regeneration ability, applying genome editing techniques to accelerate double-stranded break formation, and reconsidering protoplasts as a viable material for plastid genome engineering, even in transformation-recalcitrant species.

Recombinant Protein Production in Microalgae: Emerging Trends

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In recent years, microalgae have emerged as an alternative platform for large-scale production of recombinant proteins for different commercial applications. As a production platform, it has several advantages, including rapid growth, easily scale up and ability to grow with or without the external carbon source. Genetic transformation of several species has been established. Of these, Chlamydomonas reinhardtii has become significantly attractive for its potential to express foreign proteins inexpensively. All its three genomes – nuclear, mitochondrial and chloroplastic – have been sequenced. As a result, a wealth of information about its genetic machinery, protein expression mechanism (transcription, translation and post-translational modifications) is available. Over the years, various molecular tools have been developed for the manipulation of all these genomes. Various studies show that the transformation of the chloroplast genome has several advantages over nuclear transformation from the biopharming point of view. According to a recent survey, over 100 recombinant proteins have been expressed in algal chloroplasts. However, the expression levels achieved in the algal chloroplast genome are generally lower compared to the chloroplasts of higher plants. Work is therefore needed to make the algal chloroplast transformation commercially competitive. In this review, we discuss some examples from the algal research, which could play their role in making algal chloroplast commercially successful.

Investigating the In Vitro Regeneration Potential of Commercial Cultivars of Brassica

In vitro regeneration is a pre-requisite for developing transgenic plants through tissue culture-based genetic engineering approaches. Huge variations among different genotypes of the genus Brassica necessitate the identification of a set of regeneration conditions for a genotype, which can be reliably used in transformation experiments. In this study, we evaluated the morphogenesis potential of four commercial cultivars (Faisal canola, Punjab canola, Aari canola, Nifa Gold) and one model, Westar, from four different explants namely cotyledons, hypocotyls, petioles and roots on three different Brassica regeneration protocols, BRP-I, -II and -III. The regeneration efficiency was observed in the range of 6-73%, 4-79.3%, 0-50.6%, and 0-42.6% from cotyledons, petioles, hypocotyls and roots, respectively, whereas, the regeneration response in terms of average shoots per explant was found to be 0.76-10.9, 0.2-3.2, 0-3.4 and 0-2.7 from these explants. Of the commercial varieties tested, almost all varieties showed poorer regeneration than Westar except Aari canola. In comparison to Westar, its regeneration frequency from cotyledons was up to 7.5-fold higher on BRP-I, while it produced up to 21.9-fold more shoots per explant. Our data show that the explant has strong influence on the regeneration response, ranging from 24% to 92%. While the growth of commercial cultivars was least affected by the regeneration conditions provided, the effect on Westar was twice that of the commercial cultivars. After determining the optimal explant type and regeneration conditions, we also determined the minimum kanamycin concentration levels required to selectively inhibit the growth of untransformed cells for these cultivars. Regenerated shoots of Aari canola could be successfully grown to maturity within 16-18 weeks, with no altered phenotype noted and normal seed yields obtained. Therefore, the commercial variety, Aari canola, could be a good candidate for future genetic transformation studies.

Construction of a species-specific vector for improved plastid transformation efficiency in Capsicum annuum L

In the present study, we focused on designing a species-specific chloroplast vector for Capsicum annuum L. and finding out its transformation efficiency compared to a heterologous vector. The plastid transformation vector (CaIA) was designed to target homologous regions trnA and trnI of IR region. A selectable marker gene aadA, whose expression is controlled by psbA promoter and terminator, was cloned between two flanking regions. A heterologous vector pRB95, which targets trnfM and trnG of LSC region along with aadA driven by rrn promoter and psbA terminator, was also used for developing plastid transformation in Capsicum. Cotyledonary explants were bombarded with stabilized biolistic parameters: 900 psi pressure and 9 cm flight distance, and optimized regeneration protocol (0.7 mg/L TDZ + 0.2 mg/L IAA) was used to obtain transplastomic lines on selection medium (300 mg/L spectinomycin). The aadA integration and homoplasmy were confirmed by obtaining 1.2 and 3.7 kb amplicons in CaIA transformants and subsequently verified by Southern blotting, whereas in pRB95 transformants, integration was confirmed by PCR with 1.45 kb and 255 bp amplicons corresponding to aadA integration and flanks, respectively. The transformation efficiencies attained with two plastid vectors were found to be 20%, i.e., 10 transplastomic lines in 50 bombarded plates, with CaIA and 2%, i.e., 1 transplastomic line in 50 bombarded plates, with heterologous pRB95, respectively.

Construction of chloroplast transformation vector and its functional evaluation in Momordica charantia L

Chloroplast transformation vectors require an expression cassette flanked by homologous plastid sequences to drive plastome recombination. The rrn16-rrn23 plastome region was selected and using this region, a new species-specific plastid transformation vector CuIA was developed with pKS+II as a backbone by inserting the rrn16-trnI and trnA-rrn23 sequences from Cucumis sativus L. An independent expression cassette with aadA gene encoding aminoglycoside 3'-adenylyltransferase with psbA controlling elements is added into the trnI-trnA intergenic region that confers resistance to spectinomycin. An efficient plastid transformation in bitter melon (Momordica charantia L.) was achieved by bombardment of petiole segments. The frequency of transplastomic plants yielded using standardized biolistic parameters with CuIA vector was two per 15 bombarded plates, each containing 20 petiole explants. Integration of aadA gene was verified by PCR analysis in transplastomes. Transplastomic technology developed may be a novel approach for high level expression of pharmaceutical traits.

Stable plastid transformation in Scoparia dulcis L

In the present investigation we report stable plastid transformation in Scoparia dulcis L., a versatile medicinal herb via particle gun method. The vector KNTc, harbouring aadA as a selectable marker and egfp as a reporter gene which were under the control of synthetic promoter pNG1014a, targets inverted repeats, trnR/trnN of the plastid genome. By use of this heterologous vector, recovery of transplastomic lines with suitable selection protocol have been successfully established with overall efficiency of two transgenic lines for 25 bombarded leaf explants. PCR and Southern blot analysis demonstrated stable integration of foreign gene into the target sequences. The results represent a significant advancement of the plastid transformation technology in medicinal plants, which relevantly implements a change over in enhancing and regulating of certain metabolic pathways.

Low frequency paternal transmission of plastid genes in Brassicaceae

Plastid-encoded genes are maternally inherited in most plant species. Transgenes located on the plastid genome are thus within a natural confinement system, preventing their distribution via pollen. However, a low-frequency leakage of plastids via pollen seems to be universal in plants. Here we report that a very low-level paternal inheritance in Arabidopsis thaliana occurs under field conditions. As pollen donor an Arabidopsis accession (Ler-Ely) was used, which carried a plastid-localized atrazine resistance due to a point mutation in the psbA gene. The frequency of pollen transmission into F1 plants, based on their ability to express the atrazine resistance was 1.9 × 10(-5). We extended our analysis to another cruciferous species, the world-wide cultivated crop Brassica napus. First, we isolated a fertile and stable plastid transformant (T36) in a commercial cultivar of B. napus (cv Drakkar). In T36 the aadA and the bar genes were integrated in the inverted repeat region of the B. napus plastid DNA following particle bombardment of hypocotyl segments. Southern blot analysis confirmed transgene integration and homoplasmy of plastid DNA. Line T36 expressed Basta resistance from the inserted bar gene and this trait was used to estimate the frequency of pollen transmission into F1 plants. A frequency of <2.6 × 10(-5) was determined in the greenhouse. Taken together, our data show a very low rate of paternal plastid transmission in Brassicacea. Moreover, the establishment of plastid transformation in B. napus facilitates a safe use of this important crop plant for plant biotechnology.

Targeted metabolomics of Physaria fendleri, an industrial crop producing hydroxy fatty acids

Physaria fendleri (syn. Lesquerella) is a Brassicaceae producing lesquerolic acid, a highly valued hydroxy fatty acid that could be used for several industrial applications, such as cosmetics, lubricating greases, paints, plastics and biofuels. Free of toxins, Physaria oil is an attractive alternative to imported castor (Ricinus communis) oil, and is hence on the verge of commercialization. Gas chromatography-mass spectrometry analysis of fatty acid methyl esters revealed that lesquerolic acid was synthesized and accumulated in the embryos, reaching 60% (w/w) of the total fatty acids. The sequential extraction and characterization of biomass compounds revealed that Physaria embryo metabolism switched from protein to fatty acid biosynthesis between 18 and 24 days post-anthesis (DPA). In order to unravel the metabolic pathways involved in fatty acid synthesis, a targeted metabolomics study was conducted on Physaria embryos at different stages of development. For this purpose, two novel high-throughput liquid chromatography-tandem mass spectrometry methods were developed and validated to quantify sugars, sugar alcohols and amino acids. Specificity was achieved using multiple reaction monitoring, and the limits of quantification were in the pmole-fmole range. The comparative metabolomic study underlined that: (i) the majority of the metabolites accumulate in Physaria embryos between 18 and 27 DPA; (ii) the oxidative pentose phosphate pathway, glycolysis, the tricarboxilic acid cycle and the anaplerotic pathway drain a substantial amount of carbon; and (iii) ribulose-1,5-bisphosphate is present, which specifically indicates that the Calvin cycle is occurring. The importance and the relevance of these findings regarding fatty acid synthesis were discussed.

Expression of the truncated tissue plasminogen activator (K2S) gene in tobacco chloroplast

As because the plant plastid genome is highly polyploid, the transformation of chloroplasts permits the introduction of thousands of copies of foreign genes per plant cell and generates extraordinarily high levels of recombinant protein. Human tissue-type plasminogen activator is one of the most important pharmaceutical proteins involved in the breakdown of blood clots in brain and heart blood vessels. We report the introduction and expression of the truncated human tissue plasminogen activator (K2S) gene in tobacco chloroplasts. The K2S-containing vector pKCZK2S was successfully transferred to tobacco plastomes using the biolistic delivery procedure. Transplastomic plants were selected on RMOP medium containing spectinomycin (500 mg/l). In order to achieve homoplasmy, several rounds of selection and regeneration were performed. The presence, site-specific integration, homoplasmy, expression and activity assay of the transgene were confirmed in the transplastomic plants by PCR, Southern-blot, RT-PCR, SDS-PAGE, ELISA, Dot-blot, Western-blot and zymography analysis. Our results show that the tissue plasminogen activator (K2S form) protein to be expressed in tobacco chloroplasts in active form.

Spectinomycin resistance mutations in the rrn16 gene are new plastid markers in Medicago sativa

We report here the isolation of spectinomycin-resistant mutants in cultured cells of Medicago sativa line RegenSY-T2. Spectinomycin induces bleaching of cultured alfalfa cells due to inhibition of protein synthesis on the prokaryotic type 70S plastid ribosomes. Spontaneous mutants resistant to spectinomycin bleaching were identified by their ability to form green shoots on plant regeneration medium containing selective spectinomycin concentrations in the range of 25-50 mg/l. Sequencing of the plastid rrn16 gene revealed that spectinomycin resistance is due to mutations in a conserved stem structure of the 16S rRNA. Resistant plants transferred to the greenhouse developed normally and produced spectinomycin-resistant seed progeny. In light of their absence in soybean, a related leguminous plant, the isolation of spectinomycin-resistant mutants in M. sativa was unexpected. The new mutations are useful for the study of plastid inheritance, as demonstrated by detection of predominantly paternal plastid inheritance in the RegenSY-T2 × Szapko57 cross, and can be used as selective markers in plastid transformation vectors to obtain cisgenic plants.

Engineering plastid fatty acid biosynthesis to improve food quality and biofuel production in higher plants

The ability to manipulate plant fatty acid biosynthesis by using new biotechnological approaches has allowed the production of transgenic plants with unusual fatty acid profile and increased oil content. This review focuses on the production of very long chain polyunsaturated fatty acids (VLCPUFAs) and the increase in oil content in plants using molecular biology tools. Evidences suggest that regular consumption of food rich in VLCPUFAs has multiple positive health benefits. Alternative sources of these nutritional fatty acids are found in cold-water fishes. However, fish stocks are in severe decline because of decades of overfishing, and also fish oils can be contaminated by the accumulation of toxic compounds. Recently, there is also an increase in oilseed use for the production of biofuels. This tendency is partly associated with the rapidly rising costs of petroleum, increased concern about the environmental impact of fossil oil and the attractive need to develop renewable sources of fuel. In contrast to this scenario, oil derived from crop plants is normally contaminant free and less environmentally aggressive. Genetic engineering of the plastid genome (plastome) offers a number of attractive advantages, including high-level foreign protein expression, marker-gene excision and transgene containment because of maternal inheritance of plastid genome in most crops. Here, we describe the possibility to improve fatty acid biosynthesis in plastids, production of new fatty acids and increase their content in plants by genetic engineering of plastid fatty acid biosynthesis via plastid transformation.

The chloroplast transformation toolbox: selectable markers and marker removal

Plastid transformation is widely used in basic research and for biotechnological applications. Initially developed in Chlamydomonas and tobacco, it is now feasible in a broad range of species. Selection of transgenic lines where all copies of the polyploid plastid genome are transformed requires efficient markers. A number of traits have been used for selection such as photoautotrophy, resistance to antibiotics and tolerance to herbicides or to other metabolic inhibitors. Restoration of photosynthesis is an effective primary selection method in Chlamydomonas but can only serve as a screening tool in flowering plants. The most successful and widely used markers are derived from bacterial genes that inactivate antibiotics, such as aadA that confers resistance to spectinomycin and streptomycin. For many applications, the presence of a selectable marker that confers antibiotic resistance is not desirable. Efficient marker removal methods are a major attraction of the plastid engineering tool kit. They exploit the homologous recombination and segregation pathways acting on chloroplast genomes and are based on direct repeats, transient co-integration or co-transformation and segregation of trait and marker genes. Foreign site-specific recombinases and their target sites provide an alternative and effective method for removing marker genes from plastids.

Genetic engineering of radish: current achievements and future goals

Radish is a major root crop grown in the Far East and is especially important to some low-income countries where it is consumed on a daily basis. Developments in gene technology systems have helped to accelerate the production of useful germplasms, but progress has been slow, though achieved, via in planta methods and useful traits have been introduced. In the wake of the new Millennium, future goals in terms of improving transformation efficiency and selection of new traits for generating late-flowering radish are described. Furthermore, the techniques available for incorporating pharmaceutical proteins into radish to deliver edible proteins on-site are discussed. Finally, the concerns of releasing transgenic radish to the field in terms of pollen-mediated gene transfer are also reviewed. Such a report identifies key areas of research that is required to allow the crop satisfy the need of poor impoverished countries in the Far East.

Stable chloroplast transformation of immature scutella and inflorescences in wheat (Triticum aestivum L.)

Chloroplast transformation in wheat was achieved by bombardment of scutella from immature embryos and immature inflorescences, respectively. A wheat chloroplast site-specific expression vector, pBAGNRK, was constructed by placing an expression cassette containing neomycin phosphotransferase II (nptII) and green fluorescent protein (gfp) as selection and reporter genes, respectively, in the intergenic spacer between atpB and rbcL of wheat chloroplast genome. Integration of gfp gene in the plastome was identified by polymerase chain reaction (PCR) analysis and Southern blotting using gfp gene as a probe. Expression of GFP protein was examined by western blot. Three positive transformants were obtained and the Southern blot of partial fragment of atpB and rbcL (targeting site) probes verified that one of them was homoplasmic. Stable expression of GFP fluorescence was confirmed by confocal microscopy in the leaf tissues from T(1) progeny seedlings. PCR analysis of gfp gene also confirmed the inheritance of transgene in the T(1) progeny. These results strengthen the feasibility of wheat chloroplast transformation and also give a novel method for the introduction of important agronomic traits in wheat through chloroplast transformation.

Spontaneous spectinomycin resistance mutations detected after biolistic transformation of Daucus carota L

Spectinomycin resistant mutant carrot (Daucus carota L.) callus lines detected in the experiments on biolistic transformation of plastome were analyzed. It has been found that this antibiotic resistance is determined by point nucleotide substitutions at two distinct sites of the chloroplast gene rrn16, coding for 16S rRNA, namely, G1012T, G1012C, and A1138G. The detected mutations are localized to the 16S rRNA region forming helix h34, which contains spectinomycin binding site, and lead to its destabilization by several kilocalories per mole. Comparative analysis of rrn16 gene sequences has demonstrated conservation of the positions of the nucleotide substitutions determining this antibiotic resistance in carrot (D. carota L.), tobacco (Nicotiana tabacum L.), and bladder pod (Lesquerella fendleri L.), as well as in Escherichia coli.

High efficiency plastid transformation in potato and regulation of transgene expression in leaves and tubers by alternative 5' and 3' regulatory sequences

Transformation of potato plastids is limited by low transformation frequencies and low transgene expression in tubers. In order to improve the transformation efficiency, we modified the regeneration procedure and prepared novel vectors containing potato flanking sequences for transgene integration by homologous recombination in the Large Single Copy region of the plastome. Vector delivery was performed by the biolistic approach. By using the improved regeneration procedure and the potato flanking sequences, we regenerated about one shoot every bombardment. This efficiency corresponds to 15-18-fold improvement compared to previous results with potato and is comparable to that usually achieved with tobacco. Further, we tested five promoters and terminators, and four 5'-UTRs, to increase the expression of the gfp transgene in tubers. In leaves, accumulation of GFP to about 4% of total soluble protein (TSP) was obtained with the strong promoter of the rrn operon, a synthetic rbcL-derived 5'-UTR and the bacterial rrnB terminator. GFP protein was detected in tubers of plants transformed with only four constructs out of eleven. Best results (up to approximately 0.02% TSP) were achieved with the rrn promoter and rbcL 5'-UTR construct, described above, and another containing the same terminator, but with the promoter and 5'-UTR from the plastid clpP gene. The results obtained suggest the potential use of clpP as source of novel regulatory sequences in constructs aiming to express transgenes in amyloplasts and other non-green plastids. Furthermore, they represent a significant advancement of the plastid transformation technology in potato, of relevance to its implementation in potato breeding and biotechnology.

Transplastomics in Arabidopsis: progress toward developing an efficient method

Protocols developed for plastome engineering in Nicotiana tabacum rely on biolistic delivery of the transforming DNA to chloroplasts in intact leaf tissue; integration of the foreign DNA into the plastid genome by homologous recombination via flanking plastid DNA (ptDNA) targeting regions; and gradual dilution of non-transformed ptDNA during cultivation in vitro. Plastid transformation in Arabidopsis was obtained by combining the tobacco leaf transformation protocol with Arabidopsis-specific tissue culture and plant regeneration protocols. Because the leaf cells in Arabidopsis are polyploid, this protocol yielded sterile plants. Meristematic cells in a shoot apex or cells of a developing embryo are diploid. Therefore, we developed a regulated embryogenic root culture system that will generate diploid tissue for plastid transformation. This embryogenic culture system is created by steroid-inducible expression of the BABY BOOM transcription factor. Plastid transformation in Arabidopsis will enable the probing of plastid gene function, and the characterization of posttranscriptional mechanisms of gene regulation and the regulatory interactions of plastid and nuclear genes.

Plant plastid engineering

Genetic material in plants is distributed into nucleus, plastids and mitochondria. Plastid has a central role of carrying out photosynthesis in plant cells. Plastid transformation is becoming more popular and an alternative to nuclear gene transformation because of various advantages like high protein levels, the feasibility of expressing multiple proteins from polycistronic mRNAs, and gene containment through the lack of pollen transmission. Recently, much progress in plastid engineering has been made. In addition to model plant tobacco, many transplastomic crop plants have been generated which possess higher resistance to biotic and abiotic stresses and molecular pharming. In this mini review, we will discuss the features of the plastid DNA and advantages of plastid transformation. We will also present some examples of transplastomic plants developed so far through plastid engineering, and the various applications of plastid transformation.

Chloroplasts as expression platforms for plant-produced vaccines

Production of recombinant subunit vaccines from genes incorporated in the plastid genome is advantageous because of the attainable expression level due to high transgene copy number and the absence of gene silencing; biocontainment as a consequence of maternal inheritance of plastids and no transgene presence in the pollen; and expression of multiple transgenes in prokaryotic-like operons. We discuss the core technology of plastid transformation in Chlamydomonas reinhardtii, a unicellular alga, and Nicotiana tabacum (tobacco), a flowering plant species, and demonstrate the utility of the technology for the production of recombinant vaccine antigens.

Advances in chloroplast engineering

The chloroplast is a pivotal organelle in plant cells and eukaryotic algae to carry out photosynthesis, which provides the primary source of the world's food. The expression of foreign genes in chloroplasts offers several advantages over their expression in the nucleus: high-level expression, transgene stacking in operons and a lack of epigenetic interference allowing stable transgene expression. In addition, transgenic chloroplasts are generally not transmitted through pollen grains because of the cytoplasmic localization. In the past two decades, great progress in chloroplast engineering has been made. In this paper, we review and highlight recent studies of chloroplast engineering, including chloroplast transformation procedures, controlled expression of plastid transgenes in plants, the expression of foreign genes for improvement of plant traits, the production of biopharmaceuticals, metabolic pathway engineering in plants, plastid transformation to study RNA editing, and marker gene excision system.

High-level expression of active human alpha1-antitrypsin in transgenic tobacco chloroplasts

We have produced human alpha1-antitrypsin (A1AT), a major therapeutic protein, in genetically engineered tobacco plastids. Four different expression vectors have been evaluated which encode A1AT under the control of various 5' and 3' plastid expression elements. The use of heterologous promoter and terminator sequences derived from the corn and soybean plastid genomes leads to simpler and predictable recombinant genome patterns, avoiding unwanted recombination products between introduced and resident tobacco sequences. High level expression of unglycosylated A1AT, representing up to 2% of total soluble proteins, has been measured in leaves of transgenic tobacco lines. Some heterogeneity in the recombinant A1AT is detected after 2D protein separation, but the chloroplast-made protease inhibitors are fully active and bind to porcine pancreatic elastase.

Selectable tolerance to herbicides by mutated acetolactate synthase genes integrated into the chloroplast genome of tobacco

Strategies employed for the production of genetically modified (GM) crops are premised on (1) the avoidance of gene transfer in the field; (2) the use of genes derived from edible organisms such as plants; (3) preventing the appearance of herbicide-resistant weeds; and (4) maintaining transgenes without obstructing plant cell propagation. To this end, we developed a novel vector system for chloroplast transformation with acetolactate synthase (ALS). ALS catalyzes the first step in the biosynthesis of the branched amino acids, and its enzymatic activity is inhibited by certain classes of herbicides. We generated a series of Arabidopsis (Arabidopsis thaliana) mutated ALS (mALS) genes and introduced constructs with mALS and the aminoglycoside 3'-adenyltransferase gene (aadA) into the tobacco (Nicotiana tabacum) chloroplast genome by particle bombardment. Transplastomic plants were selected using their resistance to spectinomycin. The effects of herbicides on transplastomic mALS activity were examined by a colorimetric assay using the leaves of transplastomic plants. We found that transplastomic G121A, A122V, and P197S plants were specifically tolerant to pyrimidinylcarboxylate, imidazolinon, and sulfonylurea/pyrimidinylcarboxylate herbicides, respectively. Transplastomic plants possessing mALSs were able to grow in the presence of various herbicides, thus affirming the relationship between mALSs and the associated resistance to herbicides. Our results show that mALS genes integrated into the chloroplast genome are useful sustainable markers that function to exclude plants other than those that are GM while maintaining transplastomic crops. This investigation suggests that the resistance management of weeds in the field amid growing GM crops is possible using (1) a series of mALSs that confer specific resistance to herbicides and (2) a strategy that employs herbicide rotation.

Genetic transformation of the sugar beet plastome

It is very important for the application of chloroplast engineering to extend the range of species in which this technology can be achieved. Here, we describe the development of a chloroplast transformation system for the sugar beet (Beta vulgaris L. ssp. vulgaris, Sugar Beet Group) by biolistic bombardment of leaf petioles. Homoplasmic plastid-transformed plants of breeding line Z025 were obtained. Transformation was achieved using a vector that targets genes to the rrn16/rps12 intergenic region of the sugar beet plastome, employing the aadA gene as a selectable marker against spectinomycin and the gfp gene for visual screening of plastid transformants. gfp gene transcription and protein expression were shown in transplastomic plants. Detection of GFP in Comassie blue-stained gels suggested high GFP levels. Microscopy revealed GFP fluorescence within the chloroplasts. Our results demonstrate the feasibility of engineering the sugar beet chloroplast genome; this technology provides new opportunities for the genetic improvement of this crop and for social acceptance of genetically modified sugar beet plants.

Expression of a Bacillus thuringiensis toxin (cry1Ab) gene in cabbage (Brassica oleracea L. var. capitata L.) chloroplasts confers high insecticidal efficacy against Plutella xylostella

Chloroplast genetic engineering is an environmentally friendly approach, where the foreign integrated gene is often expressed at a higher level than nuclear transformation. The cry1Ab gene was successfully transferred into the cabbage chloroplast genome in this study. The aadA and cry1Ab genes were inserted into the pASCC201 vector and driven by the prrn promoter. The cabbage-specific plastid vectors were transferred into the chloroplasts of cabbage via particle gun mediated transformation. Regenerated plantlets were selected by their resistance to spectinomycin and streptomycin. According to antibiotic selection, the regeneration percentage of the two cabbage cultivars was 4-5%. The results of PCR, Southern, Northern hybridization and western analyses indicated that the aadA and cry1Ab genes were not only successfully integrated into the chloroplast genome, but functionally expressed at the mRNA and protein level. Expression of Cry1Ab protein was detected in the range of 4.8-11.1% of total soluble protein in transgenic mature leaves of the two species. Insecticidal effects on Plutella xylostella were also demonstrated in cry1Ab transformed cabbage. The objectives of this study were to establish a gene transformation system for Brassica chloroplasts, and to study the possibility for insect-resistance in dicot vegetables using chloroplast gene transformation.

Stable chloroplast transformation in cabbage (Brassica oleracea L. var. capitata L.) by particle bombardment

The objectives of this research were first to isolate plastid gene sequences from cabbage (Brassica oleracea L. var. capitata L.), and to establish the chloroplast transformation technology of Brassica. A universal transformation vector (pASCC201) for Brassica chloroplast was constructed with trnV-rrn16S (left) and trnI-trnA-rrn23S (right) of the IR(_A) region as a recombination site for the transformed gene. In transforming plasmid pASCC201, a chimeric aadA gene was cloned between the rrn16S and rrn23S plastid gene borders. Expression of aadA confers resistance to spectinomycin and streptomycin antibiotics. The uidA gene was also inserted into the pASCC201 and transferred into the leaf cells of cabbage via particle gun mediated transformation. Regenerated plantlets were selected by 200 mg/l spectinomycin and streptomycin. After antibiotic selection, the regeneration percentage of the two cabbage cultivars was about 2.7-3.3%. The results of PCR testing and Southern blot analysis confirmed that the uidA and aadA genes were present in the chloroplast genome via homologously recombined. Northern blot hybridizations, immunoblotting and GUS histochemical assays indicated that the uidA gene were stable integrated into the chloroplast genome. Foreign protein was accumulated at 3.2-5.2% of the total soluble protein in transgenic mature leaves. These results suggest that the expression of a variety of foreign genes in the chloroplast genome will be a powerful tool for use in future studies.

Improving containment strategies in biopharming

This review examines the challenges of segregating biopharmed crops expressing pharmaceutical or veterinary agents from mainstream crops, particularly those destined for food or feed use. The strategy of using major food crops as production vehicles for the expression of pharmaceutical or veterinary agents is critically analysed in the light of several recent episodes of contamination of the human food chain by non-approved crop varieties. Commercially viable strategies to limit or avoid biopharming intrusion into the human food chain require the more rigorous segregation of food and non-food varieties of the same crop species via a range of either physical or biological methods. Even more secure segregation is possible by the use of non-food crops, non-crop plants or in vitro plant cultures as production platforms for biopharming. Such platforms already under development range from outdoor-grown Nicotiana spp. to glasshouse-grown Arabidopsis, lotus and moss. Amongst the more effective methods for biocontainment are the plastid expression of transgenes, inducible and transient expression systems, and physical containment of plants or cell cultures. In the current atmosphere of heightened concerns over food safety and biosecurity, the future of biopharming may be largely determined by the extent to which the sector is able to maintain public confidence via a more considered approach to containment and security of its plant production systems.

The genetic transformation of plastids

Biolistic delivery of DNA initiated plastid transformation research and still is the most widelyused approach to generate transplastomic lines in both algae and higher plants. The principal designof transformation vectors is similar in both phylogenetic groups. Although important additions tothe list of species transformed in their plastomes have been made in algae and in higher plants, thekey organisms in the area are still the two species, in which stable plastid transformation was initiallysuccessful, i.e., Chlamydomonas reinhardtii and tobacco. Basicresearch into organelle biology has substantially benefited from the homologous recombination-basedcapability to precisely insert at predetermined loci, delete, disrupt, or exchange plastid genomesequences. Successful expression of recombinant proteins, including pharmaceutical proteins, hasbeen demonstrated in Chlamydomonas as well as in higher plants,where some interesting agronomic traits were also engineered through plastid transformation.

Simple and efficient plastid transformation system for the liverwort Marchantia polymorpha L. suspension-culture cells

We have established a simple and efficient plastid transformation system for liverwort, Marchantia polymorpha L., suspension-culture cells, which are homogenous, chloroplast-rich and rapidly growing. Plasmid pCS31 was constructed to integrate an aadA expression cassette for spectinomycin-resistance into the trnI-trnA intergenic region of the liverwort plastid DNA by homologous recombination. Liverwort suspension-culture cells were bombarded with pCS31-coated gold projectiles and selected on a medium containing spectinomycin. Plastid transformants were reproducibly isolated from the obtained spectinomycin-resistant calli. Selection on a sucrose-free medium greatly improved the efficiency of selection of plastid transformants. Homoplasmic plastid transformant lines were established by successive subculturing for 14 weeks or longer on the spectinomycin-containing medium. The plastid transformation system of liverwort suspension-culture cells should facilitate the investigation of the fundamental genetic systems of plastid DNA, such as replication.

Genetic transformation of plastids of different Solanaceae species using tobacco cells as organelle hosts

The plastid genome of angiosperms represents an attractive target for genetic manipulations. However plastid transformation of higher plants, especially of agriculturally valuable crops is an extremely difficult problem. Transformation protocols developed for tobacco 15 years ago failed to produce similar results with more than a handful of other species so far. We have analyzed plastid transformability of remote cytoplasmic hybrids (cybrids) that combine nuclei of tobacco, an easily transformable species, and plastids of some other, recalcitrant Solanaceae species. Here, we demonstrate that the plastids of five species of Solanaceae family, representing two subfamilies and three tribes, can be easily transformed if the plastids of these species are transferred into a cell of a transformable species (tobacco). The results can be considered to be an alternative approach to the development of plastid transformation technologies for recalcitrant species using a transformable intermediary ("clipboard") host.

Competition affects gene flow from oilseed rape (female symbol) to Brassica rapa (male symbol)

Unlike most studies on hybridisation between oilseed rape and Brassica rapa, this study focused on hybridisation with oilseed rape as the maternal parent. This is a key cross because, assuming that plastids are inherited maternally, F(1)-hybrid production with maternal oilseed rape (B. napus) is the only transgene escape route from transplastomic oilseed rape. We investigated such F(1)-hybrid production in winter oilseed rape co-cultivated with weedy B. rapa at three plant densities each with two proportions of the different species. The paternity of the progeny produced on oilseed rape was assessed, and several fitness parameters were determined in oilseed rape mother plants in order to correlate hybridisation and plant competition. At higher density, the vegetative fitness per mother plant decreased significantly, but the density only affected the frequency of F(1)-hybrids significantly (a decrease) in the treatment with equal proportions of each species. As to the proportions, at higher B. napus frequencies, there were fewer F(1)-hybrids per mother plant and a significant increase in most biomass components. Thus, B. rapa was the stronger competitor in its effect on both the vegetative and reproductive fitness in B. napus, and the hybridisation frequency. In conclusion, the relative frequency of the two species was a more influential parameter than the density. Hybridisation with B. napus as the female will be most likely at current field densities of B. napus and when B. rapa is an abundant weed.

Isolation of precise plastid deletion mutants by homology-based excision: a resource for site-directed mutagenesis, multi-gene changes and high-throughput plastid transformation

We describe a simple and efficient homology-based excision method to delete plastid genes. The procedure allows one or more adjacent plastid genes to be deleted without the retention of a marker gene. We used aadA-based transformation to duplicate a 649 bp region of plastid DNA corresponding to the atpB promoter region. Efficient recombination between atpB repeats deletes the intervening foreign genes and 1,984 bp of plastid DNA (co-ordinates 57,424-59,317) containing the rbcL gene. Only five foreign bases are present in DeltarbcL plants illustrating the precision of homology-based excision. Sequence analysis of non-functional rbcL-related sequences in DeltarbcL plants indicated an extra-plastidic origin. Mutant DeltarbcL plants were heterotrophic, pale-green and contained round plastids with reduced amounts of thylakoids. Restoration of autotrophy and leaf pigmentation following aadA-based transformation with the wild-type rbcL gene ruled out mutations in other genes. Excision and re-use of aadA shows that, despite the multiplicity of plastid genomes, homology-based excision ensures complete removal of functional aadA genes. Rescue of the DeltarbcL mutation and autotrophic growth stabilizes transgenic plastids in heteroplasmic transformants following antibiotic withdrawal, enhancing the overall efficiency of plastid transformation. Unlike the available set of homoplasmic knockout mutants in 25 plastid genes, the rbcL deletion mutant isolated here is readily transformed with the efficient aadA marker gene. This improvement in deletion design facilitates advanced studies that require the isolation of double mutants in distant plastid genes and the replacement of the deleted locus with site-directed mutant alleles and is not easily achieved using other methods.

Efficient and stable transformation of Lactuca sativa L. cv. Cisco (lettuce) plastids

Transgenic plastids offer unique advantages in plant biotechnology, including high-level foreign protein expression. However, broad application of plastid genome engineering in biotechnology has been largely hampered by the lack of plastid transformation systems for major crops. Here we describe the development of a plastid transformation system for lettuce, Lactuca sativa L. cv. Cisco. The transforming DNA carries a spectinomycin-resistance gene (aadA) under the control of lettuce chloroplast regulatory expression elements, flanked by two adjacent lettuce plastid genome sequences allowing its targeted insertion between the rbcL and accD genes. On average, we obtained 1 transplastomic lettuce plant per bombardment. We show that lettuce leaf chloroplasts can express transgene-encoded GFP to approximately 36% of the total soluble protein. All transplastomic T0 plants were fertile and the T1 progeny uniformly showed stability of the transgene in the chloroplast genome. This system will open up new possibilities for the efficient production of edible vaccines, pharmaceuticals, and antibodies in plants.

Stable plastid transformation in lettuce (Lactuca sativa L.)

Although plastid transformation in higher plants was first demonstrated in the early 1990s it is only recently that the technology is being extended to a broader range of species. To date, the production of fertile transplastomic plants has been reported for tobacco, tomato, petunia, soybean, cotton and Lesquerella fendleri (Brassicaceae). In this study we demonstrate a polyethylene glycol-mediated plastid transformation system for lettuce that generates fertile, homoplasmic, plastid-transformed lines. Transformation was achieved using a vector that targets genes to the trnA/trnI intergenic region of the lettuce plastid genome employing the aadA gene as a selectable marker against spectinomycin. Spectinomycin resistance and heterologous gene transcription were shown in T(1) plants derived from self-pollinated primary regenerants demonstrating transmission of the plastid-encoded transgene to the first seed generation. Crossing with male sterile wild-type lettuce showed that spectinomycin resistance was not transmitted via pollen. Constructs containing the gfp gene showed plastid-based expression of green fluorescent protein. The lettuce plastid could have potential both as a production and a delivery system for edible human therapeutic proteins.

Plastid transformants of tomato selected using mutations affecting ribosome structure

Tomato plastid transformants were obtained using two vectors containing cloned plastid DNA of either Nicotiana tabacum or Solanum nigrum and including point mutations conferring resistance to spectinomycin and streptomycin. Transformants were recovered after PEG-mediated direct DNA uptake into protoplasts, followed by selection on spectinomycin-containing medium. Sixteen lines contained the point mutation, as confirmed by mapping restriction enzyme sites. One line obtained with each vector was analysed in more detail, in comparison with a spontaneous spectinomycin-resistant mutant. Integration of the cloned Solanum or Nicotiana plastid DNA, by multiple recombination events, into the tomato plastome was confirmed by sequence analysis of the targeted region of plastid DNA in the inverted repeat region. Maternal inheritance of spectinomycin and streptomycin resistances or sensitivity in seedlings also confirmed the transplastomic status of the two transformants. The results demonstrate the efficacy in tomato of a selection strategy which avoids the integration of a dominant bacterial antibiotic resistance gene.

Generation and analysis of soybean plastid transformants expressing Bacillus thuringiensis Cry1Ab protoxin

We describe the generation of fertile and homoplasmic soybean plastid transformants, expressing the Bacillus thuringiensis insecticidal protoxin Cry1Ab. Transgenes were targeted in the intergenic region of Glycine max plastome, between the rps12/7 and trnV genes and selection was carried out using the aadA gene encoding spectinomycin resistance. Molecular analysis confirmed the integration of the cry1Ab and aadA expression cassettes at the expected location in the soybean plastome, and the transmission of the transgenes to the next generation. Western blot analyses showed that the Cry1Ab protoxin is highly expressed in leaves, stems and seeds, but not in roots. Its expression confers strong insecticidal activity to the generated transgenic soybean, as exemplified with velvetbean caterpillar (Anticarsia gemmatalis).

Stable transformation of petunia plastids

Plastid transformation results in stably expressed foreign genes, which for most Angiosperms are largely excluded from sperm cells, thereby greatly reducing the risk of foreign gene spread through pollen. Prior to this work, fertile plastid transformants were restricted to tobacco, tomato and Lesquerella. Application of plastid engineering in the important floriculture industry requires the development of stable plastid transformation in a major ornamental plant species such as Petunia hybrida. Here we describe the successful isolation of fertile and stable plastid transformants in a commercial cultivar of P. hybrida (var. Pink Wave). Plastid targeting regions from tobacco were used to integrate aadA and gusA between the accD and rbcL genes of P. hybrida plastid DNA following particle bombardment of leaves. For three spectinomycin and streptomycin resistant lines, DNA blot analysis confirmed transgene integration into plastid DNA and homoplasmy. Maternal inheritance and homoplasmy resulted in 100% transmission of spectinomycin resistance to progeny after selfing. Plastid transformants expressed the gusA gene uniformly within leaves and to comparable levels in all three lines. Insertion of trait genes in place of gusA coding sequences enables immediate applications of our plastid transformation vector. Establishment of plastid transformation in P. hybrida facilitates a safe and reliable use of this important ornamental plant for research and plant biotechnology.