Plastid transformants of tomato selected using mutations affecting ribosome structure

Plastid Transformation in Tomato: A Vegetable Crop and Model Species

Tomato (Solanum lycopersicum L.), a member of the nightshade family (Solanaceae), is one of the most important vegetable crops and has long been an important model species in plant biology. Plastid biology in tomato is especially interesting due to the chloroplast-to-chromoplast conversion occurring during fruit ripening. Moreover, as tomato represents a major food crop with a fleshy fruit that can be eaten raw, the development of a plastid transformation protocol for tomato was of particular interest to plant biotechnologists. Recent methodological improvements have made tomato plastid transformation more efficient, and facilitated applications in metabolic engineering and molecular farming. This chapter describes the basic methods involved in the generation and analysis of tomato plants with transgenic chloroplast genomes and summarizes recent applications of tomato plastid transformation in plant biotechnology.

Nicotiana tabacum: An Update on PEG-Mediated Plastid Transformation

Stable plastid transformation in Nicotiana tabacum has been achieved by using two different methods, the biolistic method, using a particle gun, and the polyethylene glycol (PEG)-mediated transformation. PEG-mediated plastid transformation involves the treatment of isolated protoplasts (plant cells without cell wall) with PEG in the presence of DNA. We have previously shown that in Nicotiana tabacum both methods are equally efficient. The PEG-mediated transformation efficiencies range between 20 and 50 plastid transformants per experiment (106 viable treated protoplasts). One advantage of the PEG method is that no expensive equipment such as a particle gun is required. The only crucial points are the handling and the cultivation of protoplasts. Furthermore, markers for the selection of transformed plastids are required. One of the most often used selection markers is the aadA gene which encodes for spectinomycin and streptomycin resistance. Here we describe a simplified and inexpensive protocol for the transformation of plastids in Nicotiana tabacum using an optimized protoplast culture protocol. PEG-mediated plastid transformation has the potential to be developed into a high-throughput, automated pipeline.

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.

Construction of Plastid Expression Vector and Development of Genetic Transformation System for the Seaweed Pyropia yezoensis

Pyropia yezoensis, belonging to the Rhodophyta, is an economically important seaweed. In this study, we developed a high-efficiency plastid transformation platform for P. yezoensis. In the plastid transformation vector, psbA UTR of P. yezoensis, including the promoter and 3' UTR, was used to express foreign genes. The integration site was a transcriptionally active intergenic region between the rrsB and trnI genes, located in the inverted repeat regions of the plastid genome. The CAT and eGFP genes were integrated into the plastid genome at this site. The expression of CAT in the transformants confers resistance to chloramphenicol through the action of chloramphenicol acetyltransferase, which inactivates the drug, thereby allowing the plant to grow well under selective pressure. The eGFP fluorescence signal was also observed in transformed cells and the transformants. The average survival rate of treated cells was estimated to be approximately 4.2‰ (4 transplastomic colonies per 1000 gametophyte cells). Multiple-PCR analyses confirmed that the CAT and eGFP genes were successfully integrated in the site between rrsB and trnI. Western blot also showed eGFP expression in the cells of transformants. Thus, this study presents the first convenient plastid gene expression system for P. yezoensis and provides an important platform for studying gene function in P. yezoensis.

Engineering plastid genomes: methods, tools, and applications in basic research and biotechnology

The small bacterial-type genome of the plastid (chloroplast) can be engineered by genetic transformation, generating cells and plants with transgenic plastid genomes, also referred to as transplastomic plants. The transformation process relies on homologous recombination, thereby facilitating the site-specific alteration of endogenous plastid genes as well as the precisely targeted insertion of foreign genes into the plastid DNA. The technology has been used extensively to analyze chloroplast gene functions and study plastid gene expression at all levels in vivo. Over the years, a large toolbox has been assembled that is now nearly comparable to the techniques available for plant nuclear transformation and that has enabled new applications of transplastomic technology in basic and applied research. This review describes the state of the art in engineering the plastid genomes of algae and land plants (Embryophyta). It provides an overview of the existing tools for plastid genome engineering, discusses current technological limitations, and highlights selected applications that demonstrate the immense potential of chloroplast transformation in several key areas of plant biotechnology.

Plastid genomics in horticultural species: importance and applications for plant population genetics, evolution, and biotechnology

During the evolution of the eukaryotic cell, plastids, and mitochondria arose from an endosymbiotic process, which determined the presence of three genetic compartments into the incipient plant cell. After that, these three genetic materials from host and symbiont suffered several rearrangements, bringing on a complex interaction between nuclear and organellar gene products. Nowadays, plastids harbor a small genome with ∼130 genes in a 100-220 kb sequence in higher plants. Plastid genes are mostly highly conserved between plant species, being useful for phylogenetic analysis in higher taxa. However, intergenic spacers have a relatively higher mutation rate and are important markers to phylogeographical and plant population genetics analyses. The predominant uniparental inheritance of plastids is like a highly desirable feature for phylogeny studies. Moreover, the gene content and genome rearrangements are efficient tools to capture and understand evolutionary events between different plant species. Currently, genetic engineering of the plastid genome (plastome) offers a number of attractive advantages as high-level of foreign protein expression, marker gene excision, gene expression in operon and transgene containment because of maternal inheritance of plastid genome in most crops. Therefore, plastid genome can be used for adding new characteristics related to synthesis of metabolic compounds, biopharmaceutical, and tolerance to biotic and abiotic stresses. Here, we describe the importance and applications of plastid genome as tools for genetic and evolutionary studies, and plastid transformation focusing on increasing the performance of horticultural species in the field.

Plastid transformation in lettuce (Lactuca sativa L.) by polyethylene glycol treatment of protoplasts

A detailed protocol for PEG-mediated plastid transformation of Lactuca sativa cv. Flora, using leaf protoplasts, is described. Successful plastid transformation using protoplasts requires a large number of viable cells, high plating densities, and an efficient regeneration system. Transformation was achieved using a vector that targets genes to the trnI/trnA intergenic region of the lettuce plastid genome. The aadA gene, encoding an adenylyltransferase enzyme that confers spectinomycin resistance, was used as a selectable marker. With the current method, the expected transformation frequency is 1-2 spectinomycin-resistant cell lines per 10(6) viable protoplasts. Fertile, diploid, homoplasmic, plastid-transformed lines were obtained. Transmission of the plastid-encoded transgene to the T1 generation was demonstrated.

Plastid transformation in tomato

Tomato (Solanum lycopersicum) is one of the most important vegetable crops and has long been an important model species in plant biology. Plastid biology in tomato is especially interesting due to the chloroplast-to-chromoplast conversion occurring during fruit ripening. Moreover, as tomato represents a major food crop with an edible fruit that can be eaten raw, the development of a plastid transformation protocol for tomato was of particular interest to plant biotechnology. Recent methodological improvements have made tomato plastid transformation more efficient and facilitated applications in metabolic engineering and molecular farming. This article describes the basic methods involved in the generation and analysis of tomato plants with transgenic chloroplast genomes and summarizes current applications of tomato plastid transformation.

Nicotiana tabacum: PEG-mediated plastid transformation

Stable plastid transformation in Nicotiana tabacum has been achieved by using two different methods, the biolistic method, using a particle gun, and the polyethylene glycol (PEG)-mediated transformation. PEG-mediated plastid transformation involves the treatment of isolated protoplasts (plant cells without cell wall) with PEG in the presence of DNA. We have previously shown that in Nicotiana tabacum both methods are equally efficient. The PEG-mediated transformation efficiencies range between 20 and 50 plastid transformants per experiment (10(6) viable treated protoplasts). One advantage of the PEG method is that no expensive equipment such as a particle gun is required. The only crucial points are the handling and the cultivation of protoplasts. Furthermore, markers for the selection of transformed chloroplasts are required. One of the most often used selection markers is the aadA gene which encodes for spectinomycin and streptomycin resistance. Here we describe a simplified and inexpensive protocol for the transformation of chloroplasts in Nicotiana tabacum using an optimized protoplast culture protocol.

Plastid genetic engineering in Solanaceae

Plastid genetic engineering has come of age, becoming today an attractive alternative approach for the expression of foreign genes, as it offers several advantages over nuclear transformants. Significant progress has been made in plastid genetic engineering in tobacco and other Solanaceae plants, through the use of improved regeneration procedures and transformation vectors with efficient promoters and untranslated regions. Many genes encoding for industrially important proteins and vaccines, as well as genes conferring important agronomic traits, have been stably integrated and expressed in the plastid genome. Despite these advances, it remains a challenge to achieve marked levels of plastid transgene expression in non-green tissues. In this review, we summarize the basic requirements of plastid genetic engineering and discuss the current status, limitations, and the potential of plastid transformation for expanding future studies relating to Solanaceae plants.

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.

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.

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.

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.

Transplastomic tobacco plants expressing a fatty acid desaturase gene exhibit altered fatty acid profiles and improved cold tolerance

The possibility of altering the unsaturation level of fatty acids in plant lipids by genetic transformation has implications for the stress tolerance of higher plants as well as for their nutritional value and industrial utilisation. While the integration and expression of transgenes in the plastome has several potential advantages over nuclear transformation, very few attempts have been made to manipulate fatty acid biosynthesis using plastid transformation. We produced transplastomic tobacco plants that express a Delta(9) desaturase gene from either the wild potato species Solanum commersonii or the cyanobacterium Anacystis nidulans, using PEG-mediated DNA uptake by protoplasts. Incorporation of chloroplast antibiotic-insensitive point mutations in the transforming DNA was used to select transformants. The presence of the transcript and the Delta(9) desaturase protein in transplastomic plants was confirmed by northern and western blot analyses. In comparison with control plants, transplastomic plants showed altered fatty acid profiles and an increase in their unsaturation level both in leaves and seeds. The two transgenes produced comparable results. The results obtained demonstrate the feasibility of using plastid transformation to engineer lipid metabolic pathways in both vegetative and reproductive tissues and suggest an increase of cold tolerance in transplastomic plants showing altered leaf fatty acid profiles. This is the first example of transplastomic plants expressing an agronomically relevant gene produced with the "binding-type" vectors, which do not contain a heterologous marker gene. In fact, the transplastomic plants expressing the S. commersonii gene contain only plant-derived sequences, a clear attraction from a public acceptability perspective.

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.

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.

New vectors and marker excision systems mark progress in engineering the plastid genome of higher plants

The transformation of the plastid genome, until recently restricted to tobacco, is now being extended to a rapidly growing list of crops. This perspective provides an overview of emerging trends of technology development in the field with a focus on vector design and marker excision systems. The new tools will facilitate engineering of the photosynthetic machinery and enable novel agricultural and industrial applications.