Engineering cytoplasmic male sterility via the chloroplast genome by expression of {beta}-ketothiolase

Recent trends and advances in chloroplast engineering and transformation methods

Chloroplast transformation technology has become a powerful platform for generating plants that express foreign proteins of pharmaceutical and agricultural importance at high levels. Chloroplasts are often chosen as attractive targets for the introduction of new agronomic traits because they have their own genome and protein synthesis machinery. Certain valuable traits have been genetically engineered into plastid genomes to improve crop yield, nutritional quality, resistance to abiotic and biotic stresses, and the production of industrial enzymes and therapeutic proteins. Synthetic biology approaches aim to play an important role in expressing multiple genes through plastid engineering, without the risk of pleiotropic effects in transplastomic plants. Despite many promising laboratory-level successes, no transplastomic crop has been commercialized to date. This technology is mostly confined to model species in academic laboratories and needs to be expanded to other agronomically important crop species to capitalize on its significant commercial potential. However, in recent years, some transplastomic lines are progressing in field trials, offering hope that they will pass regulatory approval and enter the marketplace. This review provides a comprehensive summary of new and emerging technologies employed for plastid transformation and discusses key synthetic biology elements that are necessary for the construction of modern transformation vectors. It also focuses on various novel insights and challenges to overcome in chloroplast transformation.

Advances in plant male sterility for hybrid seed production: an overview of conditional nuclear male sterile lines and biotechnology-based male sterile systems

Male sterility forms the foundation of hybrid seed production technology in field crops. A variety of genetically controlled male sterility/fertility systems starting with cytoplasmic male sterility (CMS), genic male sterility (GMS) including conditional male sterility and transgenic-based male sterility have been developed and deployed for heterosis breeding over the past century. Here we review environment-sensitive genic male sterility (EGMS) and biotechnology-based male sterility systems and describe the underlying molecular mechanisms. Advances in crop genomics and discovery of a large number of nuclear genes governing anther/pollen development, which are shared across species, are helping design diverse types of male sterile lines suitable for different crop species and situations. In particular, gene editing offers quick and easy route to develop novel male sterility systems for hybrid seed production. We discuss the advantages and challenges of biotechnology-based male sterility systems and present alternative strategies to address concerns of transgenics. Finally, we propose development of functional male sterility systems based on pollen competition as the future area that holds great promise for heterosis breeding.

Cytoplasmic male sterility-based hybrids: mechanistic insights

MAIN CONCLUSION

A comprehensive understanding of the nucleocytoplasmic interactions that occur between genes related to the restoration of fertility and cytoplasmic male sterility (CMS) provides insight into the development of hybrids of important crop species. Modern biotechnological techniques allow this to be achieved in an efficient and quick manner. Heterosis is paramount for increasing the yield and quality of a crop. The development of hybrids for achieving heterosis has been well-studied and proven to be robust and efficient. Cytoplasmic male sterility (CMS) has been explored extensively in the production of hybrids. The underlying mechanisms of CMS include the role of cytotoxic proteins, PCD of tapetal cells, and improper RNA editing of restoration factors. On the other hand, the restoration of fertility is caused by the presence of restorer-of-fertility (Rf) genes or restorer genes, which inhibit the effects of sterility-causing genes. The interaction between mitochondria and the nuclear genome is crucial for several regulatory pathways, as observed in the CMS-Rf system and occurs at the genomic, transcriptional, post-transcriptional, translational, and post-translational levels. These CMS-Rf mechanisms have been validated in several crop systems. This review aims to summarize the nucleo-mitochondrial interaction mechanism of the CMS-Rf system. It also sheds light on biotechnological interventions, such as genetic engineering and genome editing, to achieve CMS-based hybrids.

Cytoplasmic male sterility and abortive seed traits generated through mitochondrial genome editing coupled with allotopic expression of atp1 in tobacco

Allotopic expression is the term given for the deliberate relocation of gene function from an organellar genome to the nuclear genome. We hypothesized that the allotopic expression of an essential mitochondrial gene using a promoter that expressed efficiently in all cell types except those responsible for male reproduction would yield a cytoplasmic male sterility (CMS) phenotype once the endogenous mitochondrial gene was inactivated via genome editing. To test this, we repurposed the mitochondrially encoded atp1 gene of tobacco to function in the nucleus under the transcriptional control of a CaMV 35S promoter (construct 35S:nATP1), a promoter that has been shown to be minimally expressed in early stages of anther development. The endogenous atp1 gene was eliminated (Δatp1) from 35S:nATP1 tobacco plants using custom-designed meganucleases directed to the mitochondria. Vegetative growth of most 35S:nATP1/Δatp1 plants appeared normal, but upon flowering produced malformed anthers that failed to shed pollen. When 35S:nATP1/Δatp1 plants were cross-pollinated, ovary/capsule development appeared normal, but the vast majority of the resultant seeds were small, largely hollow and failed to germinate, a phenotype akin to the seedless trait known as stenospermocarpy. Characterization of the mitochondrial genomes from three independent Δatp1 events suggested that spontaneous recombination over regions of microhomology and substoichiometric shifting were the mechanisms responsible for atp1 elimination and genome rearrangement in response to exposure to the atp1-targeting meganucleases. Should the results reported here in tobacco prove to be translatable to other crop species, then multiple applications of allotopic expression of an essential mitochondrial gene followed by its elimination through genome editing can be envisaged. Depending on the promoter(s) used to drive the allotopic gene, this technology may have potential application in the areas of: (1) CMS trait development for use in hybrid seed production; (2) seedless fruit production; and (3) transgene containment.

Male sterility in plants: an overview of advancements from natural CMS to genetically manipulated systems for hybrid seed production

KEY MESSAGE

The male sterility system in plants has traditionally been utilized for hybrid seed production. In last three decades, genetic manipulation for male sterility has revolutionized this area of research related to hybrid seed production technology. Here, we have surveyed some of the natural cytoplasmic male sterility (CMS) systems that existed/ were developed in different crop plants for developing male sterility-fertility restoration systems used in hybrid seed production and highlighted some of the recent biotechnological advancements in the development of genetically engineered systems that occurred in this area. We have indicated the possible future directions toward the development of engineered male sterility systems. Cytoplasmic male sterility (CMS) is an important trait that is naturally prevalent in many plant species, which has been used in the development of hybrid varieties. This is associated with the use of appropriate genes for fertility restoration provided by the restorer line that restores fertility on the corresponding CMS line. The development of hybrids based on a CMS system has been demonstrated in several different crops. However, there are examples of species, which do not have usable cytoplasmic male sterility and fertility restoration systems (Cytoplasmic Genetic Male Sterility Systems-CGMS) for hybrid variety development. In such plants, it is necessary to develop usable male sterile lines through genetic engineering with the use of heterologous expression of suitable genes that control the development of male gametophyte and fertile male gamete formation. They can also be developed through gene editing using the recently developed CRISPR-Cas technology to knock out suitable genes that are responsible for the development of male gametes. The present review aims at providing an insight into the development of various technologies for successful production of hybrid varieties and is intended to provide only essential information on male sterility systems starting from naturally occurring ones to the genetically engineered systems obtained through different means.

An analysis of codon utilization patterns in the chloroplast genomes of three species of Coffea

BACKGROUND

The chloroplast genome of plants is known for its small size and low mutation and recombination rates, making it a valuable tool in plant phylogeny, molecular evolution, and population genetics studies. Codon usage bias, an important evolutionary feature, provides insights into species evolution, gene function, and the expression of exogenous genes. Coffee, a key crop in the global tropical agricultural economy, trade, and daily life, warrants investigation into its codon usage bias to guide future research, including the selection of efficient heterologous expression systems for coffee genetic transformation.

RESULTS

Analysis of the codon utilization patterns in the chloroplast genomes of three Coffea species revealed a high degree of similarity among them. All three species exhibited similar base compositions, with high A/T content and low G/C content and a preference for A/T-ending codons. Among the 30 high-frequency codons identified, 96.67% had A/T endings. Fourteen codons were identified as ideal. Multiple mechanisms, including natural selection, were found to influence the codon usage patterns in the three coffee species, as indicated by ENc-GC3s mapping, PR2 analysis, and neutral analysis. Nicotiana tabacum and Saccharomyces cerevisiae have potential value as the heterologous expression host for three species of coffee genes.

CONCLUSION

This study highlights the remarkable similarity in codon usage patterns among the three coffee genomes, primarily driven by natural selection. Understanding the gene expression characteristics of coffee and elucidating the laws governing its genetic evolution are facilitated by investigating the codon preferences in these species. The findings can enhance the efficacy of exogenous gene expression and serve as a basis for future studies on coffee evolution.

Cell-penetrating peptide for targeted macromolecule delivery into plant chloroplasts

Reports on chloroplast-targeted protein delivery using cell-penetrating peptides are scarce. In this study, a novel peptide-based macromolecule delivery strategy targeting chloroplasts was successfully developed in wheat mesophyll protoplasts. A peptide derived from the signal sequence of the chloroplast-targeted protein of ferredoxin-thioredoxin reductase catalytic chain of Spinacia oleracea with UniProtKB Id-P41348 exhibits properties of cellular internalization. DNase I was efficiently delivered into the chloroplast using 10 μM cTP with an efficiency of more than 90%. This cell-penetrating peptide-mediated approach offers various advantages over the existing chloroplast targeting methods, such as non-invasiveness, biocompatibility, low-toxicity, and target-specific delivery. The present study shows that peptide-based strategies hold tremendous potential in the field of chloroplast biotechnology. KEY POINTS: • Screening of database of chloroplast targeting peptides in order to develop an efficient cell-penetrating peptide termed as cTP. • cTP efficiently crosses the cell barrier and demonstrated chloroplast-localization. • cTP can be incorporated as a promising strategy for delivering macromolecules for crop improvement.

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.

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.

Male Sterility is linked to the Flavonoid Biosynthesis Pathways in Prunus mira

Sterility plays an important role in plant adaptation and evolution and has contributed to the development of high yielding crop hybrids. We used the widely targeted metabolomics profiling to survey the metabolites and biological pathways associated with male sterility in Prunus mira by comparing flowers from fertile and sterile trees. Male sterile flowers displayed abnormal stamen, uncolored anthers, and distorted and shrunken pollen grains with an apparent lack of turgidity. We report 566 metabolites in six flower samples and 140 differentially accumulated metabolites (DAMs) between both flower types. Most of the DAMs belong to the phenyl propanoid biosynthesis pathway, particularly flavonoid, flavone and flavonol biosynthesis pathways, implying that alterations in these key pathways link to male sterility in P. mira. The known link between low levels of flavonoid metabolites, weak expression levels of several structural genes from the phenyl propanoid biosynthesis pathway and hyper accumulation of reactive oxygen species were highlighted for understanding the underlying mechanism leading to the abnormal or aborted pollen grains observed in the sterile flowers. Data on the molecular mechanism of male sterility in Prunus mira will facilitate further in-depth investigations on this important agronomic and ecological trait.

Vaccine and antibody production in plants: developments and computational tools

Plants as bioreactors have been widely used to express efficient vaccine antigens against viral, bacterial and protozoan infections. To date, many different plant-based expression systems have been analyzed, with a growing preference for transient expression systems. Antibody expression in diverse plant species for therapeutic applications is well known, and this review provides an overview of various aspects of plant-based biopharmaceutical production. Here, we highlight conventional and gene expression technologies in plants along with some illustrative examples. In addition, the portfolio of products that are being produced and how they relate to the success of this field are discussed. Stable and transient gene expression in plants, agrofiltration and virus infection vectors are also reviewed. Further, the present report draws attention to antibody epitope prediction using computational tools, one of the crucial steps of vaccine design. Finally, regulatory issues, biosafety and public perception of this technology are also discussed.

Application of Cre-lox gene switch to limit the Cry expression in rice green tissues

The presence of genetically modified (GM) protein in the endosperm is important information for the public when considering the biological safety of transgenic rice. To limit the expression of GM proteins to rice green tissues, we developed a modified Cre-lox gene switch using two cassettes named KEY and LOCK. KEY contains a nuclear-localized Cre recombinase driven by the green-tissue-specific promoter rbcS. LOCK contains a Nos terminator (NosT), which is used to block the expression of the gene of interest (GOI), bounded by two loxP sites. When KEY and LOCK are pyramided into hybrid rice, a complete gene switch system is formed. The Cre recombinase from KEY excises loxP-NosT in LOCK and unlocks the GOI in green tissues but keeps it locked in the endosperm. This regulatory effect was demonstrated by eYFP and Bt expression assays. The presence of eYFP and Cre were confirmed in the leaf, sheath, stem, and glume but not in the root, anther or seed of the gene-switch-controlled eYFP hybrids. Meanwhile, gene switch-controlled Bt hybrid rice not only confined the expression of Bt protein to the green tissues but also showed high resistance to striped stem borers and leaffolders.

Molecular Approaches for Manipulating Male Sterility and Strategies for Fertility Restoration in Plants

Usable pollination control systems have proven to be effective system for the development of hybrid crop varieties, which are important for optimal performance over varied environments and years. They also act as a biocontainment to check horizontal transgene flow. In the last two decades, many genetic manipulations involving genes controlling the production of cytotoxic products, conditional male sterility, altering metabolic processes, post-transcriptional gene silencing, RNA editing and chloroplast engineering methods have been used to develop a proper pollination control system. In this review article, we outline the approaches used for generating male sterile plants using an effective pollination control system to highlight the recent progress that occurred in this area. Furthermore, we propose possible future directions for biotechnological improvements that will allow the farmers to buy hybrid seed once for many generations in a cost-effective manner.

Recent achievements obtained by chloroplast transformation

Chloroplasts play a great role for sustained wellbeing of life on the planet. They have the power and raw materials that can be used as sophisticated biological factories. They are rich in energy as they have lots of pigment-protein complexes capable of collecting sunlight, in sugar produced by photosynthesis and in minerals imported from the plant cell. Chloroplast genome transformation offers multiple advantages over nuclear genome which among others, include: integration of the transgene via homologus recombination that enables to eliminate gene silencing and position effect, higher level of transgene expression resulting into higher accumulations of foreign proteins, and significant reduction in environmental dispersion of the transgene due to maternal inheritance which helps to minimize the major critic of plant genetic engineering. Chloroplast genetic engineering has made fruit full progresses in the development of plants resistance to various stresses, phytoremediation of toxic metals, and production of vaccine antigens, biopharmaceuticals, biofuels, biomaterials and industrial enzymes. Although successful results have been achieved, there are still difficulties impeding full potential exploitation and expansion of chloroplast transformation technology to economical plants. These include, lack of species specific regulatory sequences, problem of selection and shoot regeneration, and massive expression of foreign genes resulting in phenotypic alterations of transplastomic plants. The aim of this review is to critically recapitulate the latest development of chloroplast transformation with special focus on the different traits of economic interest.

Sucrose supplementation suppressed the growth inhibition in polyhydroxyalkanoate-producing plants

Polyhydroxyalkanoate (PHA) is a thermoplastic polymer with several advantageous properties, including biomass origin, biocompatibility, and biodegradability. PHA is synthesized in transgenic plants harboring 3 enzymatic genes: phaA, phaB, and phaC (collectively referred to as phaABC). PHA-producing plants exhibit severe growth inhibition that leads to extremely low PHA accumulation when these enzymes are localized in the cytosol. This growth inhibition could be attributed to the deleterious effects of the PHA biosynthetic pathway on endogenous essential metabolites or to PHA cytotoxicity itself. We performed precise morphological observations of phaABC-overexpressing Arabidopsis (ABC-ox), which displayed typical growth inhibition. On growth medium without sucrose, ABC-ox exhibited a pale green phenotype, dwarfism, including small cotyledons and true leaves, and short roots. ABC-ox partially recovered from this growth inhibition when the growth medium was supplemented with 1% sucrose. This recovery was reversed after ABC-ox grown on 1% sucrose medium was transferred to soil. ABC-ox grown on 1% sucrose medium not only demonstrated recovery from growth inhibition but were also the only examined plants with PHA accumulation, suggesting that growth inhibition was not caused by PHA cytotoxicity but rather by a lack of essential metabolites.

Challenges and perspectives in commercializing plastid transformation technology

Plastid transformation has emerged as an alternative platform to generate transgenic plants. Attractive features of this technology include specific integration of transgenes-either individually or as operons-into the plastid genome through homologous recombination, the potential for high-level protein expression, and transgene containment because of the maternal inheritance of plastids. Several issues associated with nuclear transformation such as gene silencing, variable gene expression due to the Mendelian laws of inheritance, and epigenetic regulation have not been observed in the plastid genome. Plastid transformation has been successfully used for the production of therapeutics, vaccines, antigens, and commercial enzymes, and for engineering various agronomic traits including resistance to biotic and abiotic stresses. However, these demonstrations have usually focused on model systems such as tobacco, and the technology per se has not yet reached the market. Technical factors limiting this technology include the lack of efficient protocols for the transformation of cereals, poor transgene expression in non-green plastids, a limited number of selection markers, and the lengthy procedures required to recover fully segregated plants. This article discusses the technology of transforming the plastid genome, the positive and negative features compared with nuclear transformation, and the current challenges that need to be addressed for successful commercialization.

Complete chloroplast genome sequences of Solanum commersonii and its application to chloroplast genotype in somatic hybrids with Solanum tuberosum

Chloroplast genome of Solanum commersonii and S olanum tuberosum were completely sequenced, and Indel markers were successfully applied to distinguish chlorotypes demonstrating the chloroplast genome was randomly distributed during protoplast fusion. Somatic hybridization has been widely employed for the introgression of resistance to several diseases from wild Solanum species to overcome sexual barriers in potato breeding. Solanum commersonii is a major resource used as a parent line in somatic hybridization to improve bacterial wilt resistance in interspecies transfer to cultivated potato (S. tuberosum). Here, we sequenced the complete chloroplast genomes of Lz3.2 (S. commersonii) and S. tuberosum (PT56), which were used to develop fusion products, then compared them with those of five members of the Solanaceae family, S. tuberosum, Capsicum annum, S. lycopersicum, S. bulbocastanum and S. nigrum and Coffea arabica as an out-group. We then developed Indel markers for application in chloroplast genotyping. The complete chloroplast genome of Lz3.2 is composed of 155,525 bp, which is larger than the PT56 genome with 155,296 bp. Gene content, order and orientation of the S. commersonii chloroplast genome were highly conserved with those of other Solanaceae species, and the phylogenetic tree revealed that S. commersonii is located within the same node of S. tuberosum. However, sequence alignment revealed nine Indels between S. commersonii and S. tuberosum in their chloroplast genomes, allowing two Indel markers to be developed. The markers could distinguish the two species and were successfully applied to chloroplast genotyping (chlorotype) in somatic hybrids and their progenies. The results obtained in this study confirmed the random distribution of the chloroplast genome during protoplast fusion and its maternal inheritance and can be applied to select proper plastid genotypes in potato breeding program.

Chloroplast genomes: diversity, evolution, and applications in genetic engineering

Chloroplasts play a crucial role in sustaining life on earth. The availability of over 800 sequenced chloroplast genomes from a variety of land plants has enhanced our understanding of chloroplast biology, intracellular gene transfer, conservation, diversity, and the genetic basis by which chloroplast transgenes can be engineered to enhance plant agronomic traits or to produce high-value agricultural or biomedical products. In this review, we discuss the impact of chloroplast genome sequences on understanding the origins of economically important cultivated species and changes that have taken place during domestication. We also discuss the potential biotechnological applications of chloroplast genomes.

Complete chloroplast genome sequence of a major economic species, Ziziphus jujuba (Rhamnaceae)

Ziziphus jujuba is an important woody plant with high economic and medicinal value. Here, we analyzed and characterized the complete chloroplast (cp) genome of Z. jujuba, the first member of the Rhamnaceae family for which the chloroplast genome sequence has been reported. We also built a web browser for navigating the cp genome of Z. jujuba ( http://bio.njfu.edu.cn/gb2/gbrowse/Ziziphus_jujuba_cp/ ). Sequence analysis showed that this cp genome is 161,466 bp long and has a typical quadripartite structure of large (LSC, 89,120 bp) and small (SSC, 19,348 bp) single-copy regions separated by a pair of inverted repeats (IRs, 26,499 bp). The sequence contained 112 unique genes, including 78 protein-coding genes, 30 transfer RNAs, and four ribosomal RNAs. The genome structure, gene order, GC content, and codon usage are similar to other typical angiosperm cp genomes. A total of 38 tandem repeats, two forward repeats, and three palindromic repeats were detected in the Z. jujuba cp genome. Simple sequence repeat (SSR) analysis revealed that most SSRs were AT-rich. The homopolymer regions in the cp genome of Z. jujuba were verified and manually corrected by Sanger sequencing. One-third of mononucleotide repeats were found to be erroneously sequenced by the 454 pyrosequencing, which resulted in sequences of 1-4 bases shorter than that by the Sanger sequencing. Analyzing the cp genome of Z. jujuba revealed that the IR contraction and expansion events resulted in ycf1 and rps19 pseudogenes. A phylogenetic analysis based on 64 protein-coding genes showed that Z. jujuba was closely related to members of the Elaeagnaceae family, which will be helpful for phylogenetic studies of other Rosales species. The complete cp genome sequence of Z. jujuba will facilitate population, phylogenetic, and cp genetic engineering studies of this economic plant.

Peculiarities and impacts of expression of bacterial cyanophycin synthetases in plants

Cyanophycin (CP) can be successfully produced in plants by the ectopic expression of the CphA synthetase from Thermosynechococcus elongatus BP-1 (Berg et al. 2000), yielding up to 6.8 % of dry weight (DW) in tobacco leaf tissue and 7.5 % in potato tubers (Huehns et al. 2008, 2009). Though, high amounts of the polymer lead to phenotypical abnormalities in both crops. The extension of abnormalities and the maximum amount of CP tolerated depend on the compartment that CP production is localized at the tissue/crop in which CP was produced (Huehns et al. 2008, 2009; Neumann et al. 2005). It cannot be ascribed to a depletion of arginine, lysine, or aspartate, the substrates for CP synthesis.

Assessing the value of imperfect biocontainment nationally: rapeseed in the United Kingdom as an exemplar

Paternal biocontainment methods (PBMs) act by preventing pollen-mediated transgene flow. They are compromised by transgene escape via the crop-maternal line. We therefore assess the efficacy of PBMs for transgenic rapeseed (Brassica napus) biocontainment across the United Kingdom by estimating crop-maternal hybridization with its two progenitor species. We used remote sensing, field surveys, agricultural statistics, and meta-analysis to determine the extent of sympatry between the crop and populations of riparian and weedy B. rapa and B. oleracea. We then estimated the incidence of crop-maternal hybridization across all settings to predict the efficacy of PBMs. Evidence of crop chloroplast capture by the progenitors was expanded to a national scale, revealing that crop-maternal gene flow occurs at widely variable rates and is dependent on both the recipient and setting. We use these data to explore the value that this kind of biocontainment can bring to genetic modification (GM) risk management in terms of reducing the impact that hybrids have on the environment rather than preventing or reducing hybrid abundance per se.

Expression of a pathogen-induced cysteine protease (AdCP) in tapetum results in male sterility in transgenic tobacco

Usable male sterility systems have immense potential in developing hybrid varieties in crop plants, which can also be used as a biological safety containment to prevent horizontal transgene flow. Barnase-Barstar system developed earlier was the first approach to engineer male sterility in plants. In an analogous situation, we have evolved a system of inducing pollen abortion and male sterility in transgenic tobacco by expressing a plant gene coding for a protein with known developmental function in contrast to the Barnase-Barstar system, which deploys genes of prokaryotic origin, i.e., from Bacillus amyloliquefaciens. We have used a plant pathogen-induced gene, cysteine protease for inducing male sterility. This gene was identified in the wild peanut, Arachis diogoi differentially expressed when it was challenged with the late leaf spot pathogen, Phaeoisariopsis personata. Arachis diogoi cysteine protease (AdCP) was expressed under the strong tapetum-specific promoter (TA29) and tobacco transformants were generated. Morphological and histological analysis of AdCP transgenic plants showed ablated tapetum and complete pollen abortion in three transgenic lines. Furthermore, transcript analysis displayed the expression of cysteine protease in these male sterile lines and the expression of the protein was identified in western blot analysis using its polyclonal antibody raised in the rabbit system.

Hybrid breeding in wheat: technologies to improve hybrid wheat seed production

Global food security demands the development and delivery of new technologies to increase and secure cereal production on finite arable land without increasing water and fertilizer use. There are several options for boosting wheat yields, but most offer only small yield increases. Wheat is an inbred plant, and hybrids hold the potential to deliver a major lift in yield and will open a wide range of new breeding opportunities. A series of technological advances are needed as a base for hybrid wheat programmes. These start with major changes in floral development and architecture to separate the sexes and force outcrossing. Male sterility provides the best method to block self-fertilization, and modifying the flower structure will enhance pollen access. The recent explosion in genomic resources and technologies provides new opportunities to overcome these limitations. This review outlines the problems with existing hybrid wheat breeding systems and explores molecular-based technologies that could improve the hybrid production system to reduce hybrid seed production costs, a prerequisite for a commercial hybrid wheat system.

Transgene-induced pleiotropic effects in transplastomic plants

Since the first demonstration of stable transgene integration in the plastid genome (plastome) of higher plants, plastid transformation has been used for a wide range of purposes, including basic studies as well as biotechnological applications, showing that transplastomic plants are an effective system to produce recombinant proteins. Compared to nuclear transformation, the main advantages of this technology are the high and stable production level of proteins as well as the natural containment of transgenes. To date, more than 100 transgenes have been successfully expressed in plant chloroplasts. In some cases, however, unintended pleiotropic effects on plant growth and physiology were shown in transplastomic plants. In this paper, we review such effects and discuss some of the technologies developed to overcome them.

Efficient auto-excision of a selectable marker gene from transgenic citrus by combining the Cre/loxP system and ipt selection

KEY MESSAGE

A highly efficient Cre-mediated deletion system, offering a good alternative for producing marker-free transgenic plants that will relieve public concerns regarding GMOs, was first developed in citrus. The presence of marker genes in genetically modified crops raises public concerns regarding their safety. The removal of marker genes can prevent the risk of their flow into the environment and hasten the public's acceptance of transgenic products. In this study, a new construct based on the Cre/loxP site-recombination system was designed to delete marker genes from transgenic citrus. In the construct, the selectable marker gene isopentenyltransferase gene (ipt) from Agrobacterium tumefaciens and the Cre recombinase gene were flanked by two loxP recognition sites in the direct orientation. The green fluorescent protein (gfp) reporter gene for monitoring the transformation of foreign genes was located outside of the loxP sequences. Transformation and deletion efficiencies of the vector were investigated using nopaline synthase gene (NosP) and CaMV 35S promoters to drive expression of Cre. Analysis of GFP activity showed that 28.1 and 13.6 % transformation efficiencies could be obtained by NosP- and CaMV 35S-driven deletions, respectively. Molecular analysis demonstrated that 100 % deletion efficiency was observed in the transgenic plants. The complete excision of the marker gene was found in all deletion events driven by NosP and in 81.8 % of deletion events driven by CaMV 35S. The results showed that Cre/loxP-mediated excision was highly efficient and precise in citrus. This approach provides a reliable strategy for auto-deletion of selectable marker genes from transgenic citrus to produce marker-free transgenic plants.

Gene, protein, and network of male sterility in rice

Rice is one of the most important model crop plants whose heterosis has been well-exploited in commercial hybrid seed production via a variety of types of male-sterile lines. Hybrid rice cultivation area is steadily expanding around the world, especially in Southern Asia. Characterization of genes and proteins related to male sterility aims to understand how and why the male sterility occurs, and which proteins are the key players for microspores abortion. Recently, a series of genes and proteins related to cytoplasmic male sterility (CMS), photoperiod-sensitive male sterility, self-incompatibility, and other types of microspores deterioration have been characterized through genetics or proteomics. Especially the latter, offers us a powerful and high throughput approach to discern the novel proteins involving in male-sterile pathways which may help us to breed artificial male-sterile system. This represents an alternative tool to meet the critical challenge of further development of hybrid rice. In this paper, we reviewed the recent developments in our understanding of male sterility in rice hybrid production across gene, protein, and integrated network levels, and also, present a perspective on the engineering of male-sterile lines for hybrid rice production.

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.

Expression of the affinity tags, glutathione-S-transferase and maltose-binding protein, in tobacco chloroplasts

Chloroplast transformation offers an exciting platform for the safe, inexpensive and large-scale production of recombinant proteins in plants. An important advantage for the isolation of proteins produced in the chloroplast would be the use of affinity tags for rapid purification by affinity chromatography. To date, only His-tags have been used. In this study, we have tested the feasibility of expressing two additional affinity tags: glutathione-S-transferase (GST) and a His-tagged derivative of the maltose-binding protein (His₆-MBP). By using the chloroplast 16S rRNA promoter and 5' untranslated region of phage T7 gene 10, GST and His₆-MBP were expressed in homoplastomic tobacco plants at approximately 7% and 37% of total soluble protein, respectively. GST could be purified by one-step-affinity purification using a glutathione column. Much better recoveries were obtained for His₆-MBP by using a twin-affinity purification procedure involving first immobilised nickel followed by binding to amylose. Interestingly, expression of GST led to cytoplasmic male sterility. Overall, our work expands the tools available for purifying recombinant proteins from the chloroplast.

Plastid expression of a double-pentameric vaccine candidate containing human papillomavirus-16 L1 antigen fused with LTB as adjuvant: transplastomic plants show pleiotropic phenotypes

Human papillomavirus (HPV) causes cervical cancer in women worldwide, which is currently prevented by vaccines based on virus-like particles (VLPs). However, these vaccines have certain limitations in their availability to developing countries, largely due to elevated costs. Concerning the highest burden of disease in resource-poor countries, development of an improved mucosal and cost-effective vaccine is a necessity. As an alternative to VLPs, capsomeres have been shown to be highly immunogenic and can be used as vaccine candidate. Furthermore, coupling of an adjuvant like Escherichia coli heat-labile enterotoxin subunit B (LTB) to an antigen can increase its immunogenicity and reduce the costs related to separate co-administration of adjuvants. Our study demonstrates the expression of two pentameric proteins: the modified HPV-16 L1 (L1_2xCysM) and LTB as a fusion protein in tobacco chloroplasts. Homoplasmy of the transplastomic plants was confirmed by Southern blotting. Western blot analysis showed that the LTB-L1 fusion protein was properly expressed in the plastids and the recombinant protein was estimated to accumulate up to 2% of total soluble protein. Proper folding and display of conformational epitopes for both LTB and L1 in the fusion protein was confirmed by GM1-ganglioside binding assay and antigen capture ELISA, respectively. However, all transplastomic lines showed chlorosis, male sterility and growth retardation, which persisted in the ensuing four generations studied. Nevertheless, plants reached maturity and produced seeds by pollination with wild-type plants. Taken together, these results pave the way for the possible development of a low-cost adjuvant-coupled vaccine with potentially improved immunogenicity against cervical cancer.

Plastid biotechnology for crop production: present status and future perspectives

The world population is expected to reach an estimated 9.2 billion by 2050. Therefore, food production globally has to increase by 70% in order to feed the world, while total arable land, which has reached its maximal utilization, may even decrease. Moreover, climate change adds yet another challenge to global food security. In order to feed the world in 2050, biotechnological advances in modern agriculture are essential. Plant genetic engineering, which has created a new wave of global crop production after the first green revolution, will continue to play an important role in modern agriculture to meet these challenges. Plastid genetic engineering, with several unique advantages including transgene containment, has made significant progress in the last two decades in various biotechnology applications including development of crops with high levels of resistance to insects, bacterial, fungal and viral diseases, different types of herbicides, drought, salt and cold tolerance, cytoplasmic male sterility, metabolic engineering, phytoremediation of toxic metals and production of many vaccine antigens, biopharmaceuticals and biofuels. However, useful traits should be engineered via chloroplast genomes of several major crops. This review provides insight into the current state of the art of plastid engineering in relation to agricultural production, especially for engineering agronomic traits. Understanding the bottleneck of this technology and challenges for improvement of major crops in a changing climate are discussed.

Contained and high-level production of recombinant protein in plant chloroplasts using a temporary immersion bioreactor

Chloroplast transformation is a promising approach for the commercial production of recombinant proteins in plants. However, gene containment still remains an issue for the large-scale cultivation of transplastomic plants in the field. Here, we have evaluated the potential of using tobacco transplastomic cell suspensions for the fully contained production of a modified form of the green fluorescent protein (GFP+) and, a vaccine antigen, fragment C of tetanus toxin (TetC). Expression of these proteins in cell suspension cultures (and calli) was much less than in leaves, reaching 0.5%-1.5% of total soluble protein (TSP), but still produced 2.4-7.2 mg/L of liquid culture. Much better expression levels were achieved with a novel protein production platform in which transgenic cell suspension cultures were placed in a temporary immersion bioreactor in the presence of Thidiazuron to initiate shoot formation. GFP+ yield reached 660 mg/L of bioreactor (33% TSP), and TetC accumulated to about 95 mg/L (8% TSP). This new production platform, combining the rapid generation of transplastomic cell suspension cultures and the use of temporary immersion bioreactors, is a promising route for the fully contained low-cost production of recombinant proteins in chloroplasts.

Metallothionein expression in chloroplasts enhances mercury accumulation and phytoremediation capability

Genetic engineering to enhance mercury phytoremediation has been accomplished by expression of the merAB genes that protects the cell by converting Hg[II] into Hg[0] which volatilizes from the cell. A drawback of this approach is that toxic Hg is released back into the environment. A better phytoremediation strategy would be to accumulate mercury inside plants for subsequent retrieval. We report here the development of a transplastomic approach to express the mouse metallothionein gene (mt1) and accumulate mercury in high concentrations within plant cells. Real-time PCR analysis showed that up to 1284 copies of the mt1 gene were found per cell when compared with 1326 copies of the 16S rrn gene, thereby attaining homoplasmy. Past studies in chloroplast transformation used qualitative Southern blots to evaluate indirectly transgene copy number, whereas we used real-time PCR for the first time to establish homoplasmy and estimate transgene copy number and transcript levels. The mt1 transcript levels were very high with 183,000 copies per ng of RNA or 41% the abundance of the 16S rrn transcripts. The transplastomic lines were resistant up to 20 μm mercury and maintained high chlorophyll content and biomass. Although the transgenic plants accumulated high concentrations of mercury in all tissues, leaves accumulated up to 106 ng, indicating active phytoremediation and translocation of mercury. Such accumulation of mercury in plant tissues facilitates proper disposal or recycling. This study reports, for the first time, the use of metallothioneins in plants for mercury phytoremediation. Chloroplast genetic engineering approach is useful to express metal-scavenging proteins for phytoremediation.

Chloroplast-derived vaccines against human diseases: achievements, challenges and scopes

Infectious diseases represent a continuously growing menace that has severe impact on health of the people worldwide, particularly in the developing countries. Therefore, novel prevention and treatment strategies are urgently needed to reduce the rate of these diseases in humans. For this reason, different options can be considered for the production of affordable vaccines. Plants have been proved as an alternative expression system for various compounds of biological importance. Particularly, plastid genetic engineering can be potentially used as a tool for cost-effective vaccine production. Antigenic proteins from different viruses and bacteria have been expressed in plastids. Initial immunological studies of chloroplast-derived vaccines have yielded promising results in animal models. However, because of certain limitations, these vaccines face many challenges on production and application level. Adaptations to the novel approaches are needed, which comprise codon usage and choice of proven expression cassettes for the optimal yield of expressed proteins, use of inducible systems, marker gene removal, selection of specific antigens with high immunogenicity and development of tissue culture systems for edible crops to prove the concept of low-cost edible vaccines. As various aspects of plant-based vaccines have been discussed in recent reviews, here we will focus on certain aspects of chloroplast transformation related to vaccine production against human diseases.

An efficient and rapid transgenic pollen screening and detection method using flow cytometry

Assaying for transgenic pollen, a major vector of transgene flow, provides valuable information and essential data for the study of gene flow and assessing the effectiveness of transgene containment. Most studies have employed microscopic screening methods or progeny analyses to estimate the frequency of transgenic pollen. However, these methods are time-consuming and laborious when large numbers of pollen grains must be analyzed to look for rare transgenic pollen grains. Thus, there is an urgent need for the development of a simple, rapid, and high throughput analysis method for transgenic pollen analysis. In this study, our objective was to determine the accuracy of using flow cytometry technology for transgenic pollen quantification in practical application where transgenic pollen is not frequent. A suspension of non-transgenic tobacco pollen was spiked with a known amount of verified transgenic tobacco pollen synthesizing low or high amounts of green fluorescent protein (GFP). The flow cytometric method detected approximately 75% and 100% of pollen grains synthesizing low and high amounts of GFP, respectively. The method is rapid, as it is able to count 5000 pollen grains per minute-long run. Our data indicate that this flow cytometric method is useful to study gene flow and assessment of transgene containment.

Transplastomic expression of a modified human papillomavirus L1 protein leading to the assembly of capsomeres in tobacco: a step towards cost-effective second-generation vaccines

Certain types of human papillomaviruses (HPV) are causatively associated with cervical carcinoma, the second most common cancer in women worldwide. Due to limitations in the availability of currently used virus-like particle (VLP)-based vaccines against HPV to women of developing countries, where most cases of cervical cancer occur, the development of a cost-effective second-generation vaccine is a necessity. Capsomeres have recently been demonstrated to be highly immunogenic and to have a number of advantages as a potential cost-effective alternative to VLP-based HPV vaccines. We have expressed a mutated HPV-16 L1 (L1_2xCysM) gene that retained the ability to assemble L1 protein to capsomeres in tobacco chloroplasts. The recombinant protein yielded up to 1.5% of total soluble protein. The assembly of capsomeres was examined and verified by cesium chloride density gradient centrifugation and sucrose sedimentation analysis. An antigen capture enzyme-linked immunosorbent assay confirmed the formation of capsomeres by using a conformation-specific monoclonal antibody which recognized the conformational epitopes. Transplastomic tobacco plants exhibited normal growth and morphology, but all such lines showed male sterility in the T₀, T₁ and T₂ generations. Taken together, these results indicate the possibility of producing a low-cost capsomere-based vaccine by plastids.

High levels of bioplastic are produced in fertile transplastomic tobacco plants engineered with a synthetic operon for the production of polyhydroxybutyrate

An optimized genetic construct for plastid transformation of tobacco (Nicotiana tabacum) for the production of the renewable, biodegradable plastic polyhydroxybutyrate (PHB) was designed using an operon extension strategy. Bacterial genes encoding the PHB pathway enzymes were selected for use in this construct based on their similarity to the codon usage and GC content of the tobacco plastome. Regulatory elements with limited homology to the host plastome yet known to yield high levels of plastidial recombinant protein production were used to enhance the expression of the transgenes. A partial transcriptional unit, containing genes of the PHB pathway and a selectable marker gene encoding spectinomycin resistance, was flanked at the 5' end by the host plant's psbA coding sequence and at the 3' end by the host plant's 3' psbA untranslated region. This design allowed insertion of the transgenes into the plastome as an extension of the psbA operon, rendering the addition of a promoter to drive the expression of the transgenes unnecessary. Transformation of the optimized construct into tobacco and subsequent spectinomycin selection of transgenic plants yielded T0 plants that were capable of producing up to 18.8% dry weight PHB in samples of leaf tissue. These plants were fertile and produced viable seed. T1 plants producing up to 17.3% dry weight PHB in samples of leaf tissue and 8.8% dry weight PHB in the total biomass of the plant were also isolated.

[Progress in the study of producing reversible male sterile line by genetic engineering]

Plant male sterility is a kind of resource of heterosis, which has important value in production. It may be derived from natural mutations, artificial mutations, distant hybridizations, and now through cell engineering and genetic engineering. This paper reviews the progress of strategies in production of plant male sterile lines and their corresponding fertile lines via genetic engineering approach. All strategies can be grouped into "single component strategy" and "two-component strategy". "Single component strategy" produces conditional (reversible) male sterile line, whose fertility can be switched under given condition. Conditional male sterile line has two roles, which are CMS and maintainer line for breeding in practice; "two-component strategy" takes advantage of gene interaction and parental hybridization to generate male sterile line. Otherwise, it develops sterile line and restorer line respectively for three-line hybrid system for seed production through gene interaction. This paper discusses the advantages and disadvantages of gene engineering approaches of "single component strategy" and "two-component strategy" for developing male sterile line and corresponding restorer line, as well as the current status and perspective of these approaches in practice.

Transgene introgression in crop relatives: molecular evidence and mitigation strategies

Incorporation of crop genes into wild and weedy relative populations (i.e. introgression) has long been of interest to ecologists and weed scientists. Potential negative outcomes that result from crop transgene introgression (e.g. extinction of native wild relative populations; invasive spread by wild or weedy hosts) have not been documented, and few examples of transgene introgression exist. However, molecular evidence of introgression from non-transgenic crops to their relatives continues to emerge, even for crops deemed low-risk candidates for transgene introgression. We posit that transgene introgression monitoring and mitigation strategies are warranted in cases in which transgenes are predicted to confer selective advantages and disadvantages to recipient hosts. The utility and consequences of such strategies are examined, and future directions provided.

Evaluating biological containment strategies for pollen-mediated gene flow

Several biological containment methods have been developed to reduce pollen dispersal; many of them only have a proof of concept in a model plant species. This review focuses on biological containment measures which were tested for their long-term efficiency at the greenhouse or field scale level, i.e. plastid transformation, transgene excission, cleistogamy and cytoplasmic male sterility (CMS). Pollen-mediated gene transfer in transplastomic tobacco could occur at very low frequencies if the predominant mode of inheritance is maternal. Transgene excision from tobacco pollen can be made highly efficient by coexpression of two recombinases. For cleistogamous oilseed rape it was shown that some flowers were partially open depending on genotypes, environment and recording dates. Reports on the stability of CMS in maize and sunflower indicated that there is a high variability for different genotypes under different environmental conditions and over successive years. But for both crop types some stable lines could be selected. These data demonstrate that the biological containment methods discussed are very promising for reducing gene flow but that no single containment strategy provides 100% reduction. However, the necessary efficiency of biological containment methods depends on the level of containment required. The containment level may need to be higher for safety purposes (e.g. production of special plant-made pharmaceuticals), while much lower containment levels may already be sufficient to reach coexistence goals. It is concluded that where pollen-mediated gene flow must be prevented altogether, combinations of complementary containment systems will be required.

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.

A novel herbicide-inducible male sterility system

BACKGROUND

Heterosis is a phenomenon that first-generation offspring perform better than their parents. Conventional breeding methods have their shortcomings. It would be optimal to construct inducible male sterile plants.

RESULTS

We developed a novel system for creating male sterile transgenic plants by downregulating the specific expression of the glyphosate tolerance CP4 EPSPS gene in male reproductive tissues. Transcriptional repression was achieved by manipulating DNA binding proteins with their specific corresponding sites. We transferred the CP4 EPSPS gene driven by a modified CaMV 35S promoter with three tetracycline operator copies in the vicinity of the TATA box and tetracycline repressor gene under the control of an anther-specific promoter Osg6B to Arabidopsis thaliana. As a result, we successfully obtained controllable transgenic plants: the whole plant could tolerate exposure of glyphosate but the male tissue was sensitive.

CONCLUSION

The novel inducible male sterility system is applied and easy to handle, so it might be applicable to a wide range of crop plants.

Cytoplasmic male sterility-regulated novel microRNAs from maize

In higher plants, microRNA (miRNA) is involved in regulation of developmental processes, including sexual organ development. Seven novel miRNA families with one known miRNA were isolated by constructing a small RNA library from a mixture of anther from a cytoplasmic male sterile line and its maintainer. Two miRNAs are conserved in plant species. A total of 18 potential targets were identified for the eight miRNA families, including 15 proteins annotated with function and three unknown proteins. The known proteins include several proteins relevant to cell structure and stress response, transcription factors, and enzymes associated with metabolic and signaling pathways, playing important roles in microspore development. Quantitative real-time PCR assay revealed different expression patterns of the miRNAs between the cytoplasmic male sterile line and its maintainer. Each of the miRNAs tended to be down-regulated after the tetrad stage in a fertile line. However, most of the miRNAs in the cytoplasmic male sterile line were shown to be up-regulated from the tetrad to mononuclear stage, displaying special expression patterns differing from the ones in fertile line. We conclude that additional inactive miRNA pathways are essential during pollen development for a fertile line to ensure male fertility. Contrarily, miRNAs are up-regulated during the period from the tetrad to mononuclear stage, which contributes to pollen abortion for a cytoplasmic male sterile line.

Overexpression of a natural chloroplast-encoded antisense RNA in tobacco destabilizes 5S rRNA and retards plant growth

BACKGROUND

The roles of non-coding RNAs in regulating gene expression have been extensively studied in both prokaryotes and eukaryotes, however few reports exist as to their roles in organellar gene regulation. Evidence for accumulation of natural antisense RNAs (asRNAs) in chloroplasts comes from the expressed sequence tag database and cDNA libraries, while functional data have been largely obtained from artificial asRNAs. In this study, we used Nicotiana tabacum to investigate the effect on sense strand transcripts of overexpressing a natural chloroplast asRNA, AS5, which is complementary to the region which encodes the 5S rRNA and tRNAArg.

RESULTS

AS5-overexpressing (AS5ox) plants obtained by chloroplast transformation exhibited slower growth and slightly pale green leaves. Analysis of AS5 transcripts revealed four distinct species in wild-type (WT) and AS5ox plants, and additional AS5ox-specific products. Of the corresponding sense strand transcripts, tRNAArg overaccumulated several-fold in transgenic plants whereas 5S rRNA was unaffected. However, run-on transcription showed that the 5S-trnR region was transcribed four-fold more in the AS5ox plants compared to WT, indicating that overexpression of AS5 was associated with decreased stability of 5S rRNA. In addition, polysome analysis of the transformants showed less 5S rRNA and rbcL mRNA associated with ribosomes.

CONCLUSIONS

Our results suggest that AS5 can modulate 5S rRNA levels, giving it the potential to affect Chloroplast translation and plant growth. More globally, overexpression of asRNAs via chloroplast transformation may be a useful strategy for defining their functions.

Anther-specific carbohydrate supply and restoration of metabolically engineered male sterility

Male-sterile plants are used in hybrid breeding as well as for gene confinement for genetically modified plants in field trials and agricultural production. Apart from naturally occurring mutations leading to male sterility, biotechnology has added new possibilities for obtaining male-sterile plants, although so far only one system is used in practical breeding due to limitations in propagating male-sterile plants without segregations in the next generation or insufficient restoration of fertility when fruits or seeds are to be harvested from the hybrid varieties. Here a novel mechanism of restoration for male sterility is presented that has been achieved by interference with extracellular invertase activity, which is normally specifically expressed in the anthers to supply the developing microspores with carbohydrates. Microspores are symplastically isolated in the locular space of the anthers, and thus an unloading pathway of assimilates via the apoplasmic space is mandatory for proper development of pollen. Antisense repression of the anther-specific cell wall invertase or interference with invertase activity by expressing a proteinacious inhibitor under the control of the anther-specific invertase promoter results in a block during early stages of pollen development, thus causing male sterility without having any pleiotropic effects. Restoration of fertility was successfully achieved by substituting the down-regulated endogenous plant invertase activity by a yeast invertase fused to the N-terminal portion of potato-derived vacuolar protein proteinase II (PiII-ScSuc2), under control of the orthologous anther-specific invertase promoter Nin88 from tobacco. The chimeric fusion PiII-ScSuc2 is known to be N-glycosylated and efficiently secreted from plant cells, leading to its apoplastic location. Furthermore, the Nin88::PiII-ScSuc2 fusion does not show effects on pollen development in the wild-type background. Thus, such plants can be used as paternal parents of a hybrid variety, thereby the introgression of Nin88::PiII-ScSuc2 to the hybrid is obtained and fertility is restored. In order to broaden the applicability of this male sterility/restoration system to other plant species, a phylogenic analysis of plant invertases(beta-fructofuranosidases) and related genes of different species was carried out. This reveals a specific clustering of the cell wall invertases with anther-specific expression for dicotyl species and another cluster for monocotyl plants. Thus, in both groups of plants, there seems to be a kind of co-evolution, but no recent common ancestor of these members of the gene family. These findings provide a helpful orientation to classify corresponding candidate genes in further plant species, in addition to the species analysed so far (Arabidopsis, tobacco, tomato, potato, carrots, rice, and wheat).

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.

Plant physiological adaptations to the massive foreign protein synthesis occurring in recombinant chloroplasts

Genetically engineered chloroplasts have an extraordinary capacity to accumulate recombinant proteins. We have investigated in tobacco (Nicotiana tabacum) the possible consequences of such additional products on several parameters of plant development and composition. Plastid transformants were analyzed that express abundantly either bacterial enzymes, alkaline phosphatase (PhoA-S and PhoA-L) and 4-hydroxyphenyl pyruvate dioxygenase (HPPD), or a green fluorescent protein (GFP). In leaves, the HPPD and GFP recombinant proteins are the major polypeptides and accumulate to higher levels than Rubisco. Nevertheless, these engineered metabolic sinks do not cause a measurable difference in growth rate or photosynthetic parameters. The total amino acid content of transgenic leaves is also not significantly affected, showing that plant cells have a limited protein biosynthetic capacity. Recombinant products are made at the expense of resident proteins. Rubisco, which constitutes the major leaf amino acid store, is the most clearly and strongly down-regulated plant protein. This reduction is even more dramatic under conditions of limited nitrogen supply, whereas recombinant proteins accumulate to even higher relative levels. These changes are regulated posttranscriptionally since transcript levels of resident plastid genes are not affected. Our results show that plants are able to produce massive amounts of recombinant proteins in chloroplasts without profound metabolic perturbation and that Rubisco, acting as a nitrogen buffer, is a key player in maintaining homeostasis and limiting pleiotropic effects.

Genetic engineering to enhance mercury phytoremediation

Most phytoremediation studies utilize merA or merB genes to modify plants via the nuclear or chloroplast genome, expressing organomercurial lyase and/or mercuric ion reductase in the cytoplasm, endoplasmic reticulum or within plastids. Several plant species including Arabidopsis, tobacco, poplar, rice, Eastern cottonwood, peanut, salt marsh grass and Chlorella have been transformed with these genes. Transgenic plants grew exceedingly well in soil contaminated with organic (approximately 400 microM PMA) or inorganic mercury (approximately 500 microM HgCl(2)), accumulating Hg in roots surpassing the concentration in soil (approximately 2000 microg/g). However, none of these plants were tested in the field to demonstrate real potential of this approach. Availability of metal transporters, translocators, chelators and the ability to express membrane proteins could further enhance mercury phytoremediation capabilities.