Novel approach in plastid transformation

Plastome engineering in vegetable crops: current status and future prospects

Plastome (plastid genome) engineering has grown up and got smarter for the transgene expression. Plastid transformation has profound benefits over nuclear transformation, includes a higher level of transgene expression, integration via homologous recombination, transgene containment, lack of gene silencing, and position effect. Substantial and fruitful progress has been achieved in plastome engineering of vegetable crops through the use of improved regeneration/selection procedures, plastid transformation vectors with efficient promoters, and 3^/, 5^/regulatory sequences. Plastid transformation technology developed for vegetable crops being used as a platform for the production of industrially important proteins and some of the genes of agronomic importance has been stably integrated and expressed in plastome. Although great progress has been accomplished in the plastid transformation of vegetable crops, still it is restricted to few species because of the unavailability of whole plastome sequencing. In this review, the author focus on the technology, progress, and advancements in plastid transformation of vegetable plants such as lettuce, tomato, potato, cabbage, cauliflower, eggplant, carrot, soybean, and bitter melon are reviewed. The conclusions, future prospects, and expansion of plastid transformation technology to other vegetable crops for genetic improvement and production of edible vaccines are proposed.

The myth of interconnected plastids and related phenomena

Studies spread over nearly two and a half centuries have identified the primary plastid in autotrophic algae and plants as a pleomorphic, multifunctional organelle comprising of a double-membrane envelope enclosing an organization of internal membranes submerged in a watery stroma. All plastid units have been observed extending and retracting thin stroma-filled tubules named stromules sporadically. Observations on living plant cells often convey the impression that stromules connect two or more independent plastids with each other. When photo-bleaching techniques were used to suggest that macromolecules such as the green fluorescent protein could flow between already interconnected plastids, for many people this impression changed to conviction. However, it was noticed only recently that the concept of protein flow between plastids rests solely on the words "interconnected plastids" for which details have never been provided. We have critically reviewed botanical literature dating back to the 1880s for understanding this term and the phenomena that have become associated with it. We find that while meticulously detailed ontogenic studies spanning nearly 150 years have established the plastid as a singular unit organelle, there is no experimental support for the idea that interconnected plastids exist under normal conditions of growth and development. In this review, while we consider several possibilities that might allow a single elongated plastid to be misinterpreted as two or more interconnected plastids, our final conclusion is that the concept of direct protein flow between plastids is based on an unfounded assumption.

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 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.

Cooperation of Endo- and Exoribonucleases in Chloroplast mRNA Turnover

Chloroplasts were acquired by eukaryotic cells through endosymbiosis and have retained their own gene expression machinery. One hallmark of chloroplast gene regulation is the predominance of posttranscriptional control, which is exerted both at the gene-specific and global levels. This review focuses on how chloroplast mRNA stability is regulated, through an examination of poly(A)-dependent and independent pathways. The poly(A)-dependent pathway is catalyzed by polynucleotide phosphorylase (PNPase), which both adds and degrades destabilizing poly(A) tails, whereas RNase II and PNPase may both participate in the poly(A)-independent pathway. Each system is initiated through endonucleolytic cleavages that remove 3' stem-loop structures, which are catalyzed by the related proteins CSP41a and CSP41b and possibly an RNase E-like enzyme. Overall, chloroplasts have retained the prokaryotic endonuclease-exonuclease RNA degradation system despite evolution in the number and character of the enzymes involved. This reflects the presence of the chloroplast within a eukaryotic host and the complex responses that occur to environmental and developmental cues.

Manipulating gene expression for the metabolic engineering of plants

Introducing and expressing foreign genes in plants present many technical challenges that are not encountered with microbial systems. This review addresses the variety of issues that must be considered and the variety of options that are available, in terms of choosing transformation systems and designing recombinant transgenes to ensure appropriate expression in plant cells. Tissue specificity and proper developmental regulation, as well as proper subcellular localization of products, must be dealt with for successful metabolic engineering in plants..

Expression of the native cholera toxin B subunit gene and assembly as functional oligomers in transgenic tobacco chloroplasts

The B subunits of enterotoxigenic Escherichia coli (LTB) and cholera toxin of Vibrio cholerae (CTB) are candidate vaccine antigens. Integration of an unmodified CTB-coding sequence into chloroplast genomes (up to 10,000 copies per cell), resulted in the accumulation of up to 4.1 % of total soluble tobacco leaf protein as functional oligomers (410-fold higher expression levels than that of the unmodified LTB gene expressed via the nuclear genome). However, expression levels reported are an underestimation of actual accumulation of CTB in transgenic chloroplasts, due to aggregation of the oligomeric forms in unboiled samples similar to the aggregation observed for purified bacterial antigen. PCR and Southern blot analyses confirmed stable integration of the CTB gene into the chloroplast genome. Western blot analysis showed that the chloroplast- synthesized CTB assembled into oligomers and were antigenically identical with purified native CTB. Also, binding assays confirmed that chloroplast-synthesized CTB binds to the intestinal membrane GM1-ganglioside receptor, indicating correct folding and disulfide bond formation of CTB pentamers within transgenic chloroplasts. In contrast to stunted nuclear transgenic plants, chloroplast transgenic plants were morphologically indistinguishable from untransformed plants, when CTB was constitutively expressed in chloroplasts. Introduced genes were inherited stably in subsequent generations, as confirmed by PCR and Southern blot analyses. Increased production of an efficient transmucosal carrier molecule and delivery system, like CTB, in transgenic chloroplasts makes plant-based oral vaccines and fusion proteins with CTB needing oral administration commercially feasible. Successful expression of foreign genes in transgenic chromoplasts and availability of marker-free chloroplast transformation techniques augurs well for development of vaccines in edible parts of transgenic plants. Furthermore, since the quaternary structure of many proteins is essential for their function, this investigation demonstrates the potential for other foreign multimeric proteins to be properly expressed and assembled in transgenic chloroplasts.