In vivo Assembly in Escherichia coli of Transformation Vectors for Plastid Genome Engineering

A heat-inducible expression system for external control of gene expression in plastids

Inducible expression systems can overcome the trade-off between high-level transgene expression and its pleiotropic effects on plant growth. In addition, they can facilitate the expression of biochemical pathways that produce toxic metabolites. Although a few inducible expression systems for the control of transgene expression in plastids have been developed, they all depend on chemical inducers and/or nuclear transgenes. Here we report a temperature-inducible expression system for plastids that is based on the bacteriophage λ leftward and rightward promoters (pL/pR) and the temperature-sensitive repressor cI857. We show that the expression of green fluorescent protein (GFP) in plastids can be efficiently repressed by cI857 under normal growth conditions, and becomes induced over time upon exposure to elevated temperatures in a light-dependent process. We further demonstrate that by introducing into plastids an expression system based on the bacteriophage T7 RNA polymerase, the temperature-dependent accumulation of GFP increased further and was ~24 times higher than expression driven by the pL/pR promoter alone, reaching ~0.48% of the total soluble protein. In conclusion, our heat-inducible expression system provides a new tool for the external control of plastid (trans) gene expression that is cost-effective and does not depend on chemical inducers.

Complete protection from Henosepilachna vigintioctopunctata by expressing long double-stranded RNAs in potato plastids

RNA interference (RNAi) has emerged as a powerful technology for pest management. Previously, we have shown that plastid-mediated RNAi (PM-RNAi) can be utilized to control the Colorado potato beetle, an insect pest in the Chrysomelidae family; however, whether this technology is suitable for controlling pests in the Coccinellidae remained unknown. The coccinellid 28-spotted potato ladybird (Henosepilachna vigintioctopunctata; HV) is a serious pest of solanaceous crops. In this study, we identified three efficient target genes (β-Actin, SRP54, and SNAP) for RNAi using in vitro double-stranded RNAs (dsRNAs) fed to HV, and found that dsRNAs targeting β-Actin messenger RNA (dsACT) induced more potent RNAi than those targeting the other two genes. We next generated transplastomic and nuclear transgenic potato (Solanum tuberosum) plants expressing HV dsACT. Long dsACT stably accumulated to up to 0.7% of the total cellular RNA in the transplastomic plants, at least three orders of magnitude higher than in the nuclear transgenic plants. Notably, the transplastomic plants also exhibited a significantly stronger resistance to HV, killing all larvae within 6 d. Our data demonstrate the potential of PM-RNAi as an efficient pest control measure for HV, extending the application range of this technology to Coccinellidae pests.

Plastid Transformation: New Challenges in the Circular Economy Era

In a circular economy era the transition towards renewable and sustainable materials is very urgent. The development of bio-based solutions, that can ensure technological circularity in many priority areas (e.g., agriculture, biotechnology, ecology, green industry, etc.), is very strategic. The agricultural and fishing industry wastes represent important feedstocks that require the development of sustainable and environmentally-friendly industrial processes to produce and recover biofuels, chemicals and bioactive molecules. In this context, the replacement, in industrial processes, of chemicals with enzyme-based catalysts assures great benefits to humans and the environment. In this review, we describe the potentiality of the plastid transformation technology as a sustainable and cheap platform for the production of recombinant industrial enzymes, summarize the current knowledge on the technology, and display examples of cellulolytic enzymes already produced. Further, we illustrate several types of bacterial auxiliary and chitinases/chitin deacetylases enzymes with high biotechnological value that could be manufactured by plastid transformation.

A simple technology for plastid transformation with fragmented DNA

Plastid engineering has several unique advantages such as high expression of transgenes due to high polyploidy of plastid genomes and environmental biosafety because of maternal inheritance of transgenes, and has become a promising tool for molecular farming, metabolic engineering, and genetic improvement. However, there are no standard vectors available for plastid transformation. Moreover, the construction of plastid transformation vectors containing long operons or genes encoding proteins that are toxic to Escherichia coli was tedious or difficult. Here, we developed a simple plastid transformation technology without the need for in vitro vector construction by using multiple linear DNA fragments which share homologous sequences (HSs) at their ends. The strategy is based on homologous recombination between HSs of DNA fragments via endogenous recombination machinery in plastids, which subsequently are integrated into the plastid genome. We found that HSs of 200 bp or longer were sufficient for mediating the integration into the plastid genome with at least similar efficiency to that of plasmid DNA-based plastid transformation. Furthermore, we successfully used this method to introduce a phage lysin-encoding gene and a long operon into a tobacco plastid genome. The establishment of this technology simplifies the plastid transformation procedure and provides a novel solution for expressing proteins, which are either toxic to the cloning host or large operons in plastids, without need of vector cloning.

Application progress of plant-mediated RNAi in pest control

RNA interference (RNAi)-based biopesticides are novel biologic products, developed using RNAi principles. They are engineered to target genes of agricultural diseases, insects, and weeds, interfering with their target gene expression so as to hinder their growth and alleviate their damaging effects on crops. RNAi-based biopesticides are broadly classified into resistant plant-based plant-incorporated protectants (PIPs) and non-plant-incorporated protectants. PIP RNAi-based biopesticides are novel biopesticides that combine the advantages of RNAi and resistant transgenic crops. Such RNAi-based biopesticides are developed through nuclear or plastid transformation to breed resistant plants, i.e., dsRNA-expressing transgenic plants. The dsRNA of target genes is expressed in the plant cell, with pest and disease control being achieved through plant-target organism interactions. Here, we review the action mechanism and strategies of RNAi for pest management, the development of RNAi-based transgenic plant, and the current status and advantages of deploying these products for pest control, as well as the future research directions and problems in production and commercialization. Overall, this study aims to elucidate the current development status of RNAi-based biopesticides and provide guidelines for future research.

Modification of Chloroplast Antioxidant Capacity by Plastid Transformation

As immobile organisms, green plants must be frequently challenged by a broad range of environmental stresses. During these constantly adverse conditions, reactive oxygen species (ROS) levels can rise extremely in plants, leading to cellular dysfunction and cell death presumably due to irreversible protein overoxidation. Once considered merely as deleterious molecules, cells seek to remove them as efficiently as possible. To enhance ROS scavenging capacity, genes encoding antioxidative enzymes can be directly expressed from the genome of plastid (chloroplast), a major compartment for ROS production in photosynthetic organisms. Thus, overexpression of antioxidant enzymes by plastid engineering may provide an alternative to enhance plant's tolerance to stressful conditions specifically related with chloroplast-derived ROS. Here, we describe basic procedures for expressing glutathione reductase, a vital component of ascorbate-glutathione pathway, in tobacco via plastid transformation technology.

Resistance to RNA interference by plant-derived double-stranded RNAs but not plant-derived short interfering RNAs in Helicoverpa armigera

Plant-mediated RNA interference (RNAi) has emerged as a promising technology for pest control through expression of double-stranded RNAs (dsRNAs) targeted against essential insect genes. However, little is known about the underlying molecular mechanisms and whether long dsRNA or short interfering RNAs (siRNAs) are the effective triggers of the RNAi response. Here we generated transplastomic and nuclear transgenic tobacco plants expressing dsRNA against the Helicoverpa armigera ATPaseH gene. We showed that expression of long dsRNA of HaATPaseH was at least three orders of magnitude higher in transplastomic plants than in transgenic plants. HaATPaseH-derived siRNAs are absent from transplastomic plants, while they are abundant in transgenic plants. Feeding transgenic plants to H. armigera larvae reduced gene expression of HaATPaseH and delayed growth. Surprisingly, no effect of transplastomic plants on insect growth was observed, despite efficient dsRNA expression in plastids. Furthermore, we found that dsRNA ingested by H. armigera feeding on transplastomic plants was rapidly degraded in the intestinal fluid. In contrast, siRNAs are relatively stable in the digestive system. These results suggest that plant-derived siRNAs may be more effective triggers of RNAi in Lepidoptera than dsRNAs, which will aid the optimization of the strategies for plant-mediated RNAi to pest control.

Efficient control of western flower thrips by plastid-mediated RNA interference

Plastid-mediated RNA interference (PM-RNAi) has emerged as a promising strategy for pest control. Expression from the plastid genome of stable double-stranded RNAs (dsRNAs) targeted against essential insect genes can effectively control some herbivorous beetles, but little is known about the efficacy of the transplastomic approach in other groups of pest insects, especially nonchewing insects that do not consume large amounts of leaf material. Here we have investigated the susceptibility of the western flower thrip (WFT, Frankliniella occidentalis), a notorious pest in greenhouses and open fields, to PM-RNAi. We show that WFTs ingest chloroplasts and take up plastid-expressed dsRNAs. We generated a series of transplastomic tobacco plants expressing dsRNAs and hairpin RNAs (hpRNAs) targeted against four essential WFT genes. Unexpectedly, we discovered plastid genome instability in transplastomic plants expressing hpRNAs, suggesting that dsRNA cassettes are preferable over hpRNA cassettes when designing PM-RNAi strategies. Feeding studies revealed that, unlike nuclear transgenic plants, transplastomic plants induced a potent RNAi response in WFTs, causing efficient suppression of the targeted genes and high insect mortality. Our study extends the application range of PM-RNAi technology to an important group of nonchewing insects, reveals design principles for the construction of dsRNA-expressing transplastomic plants, and provides an efficient approach to control one of the toughest insect pests in agriculture and horticulture.

Gene introduction approaches in chloroplast transformation and its applications

BACKGROUND

Chloroplast is a type of plastid that is believed to be originated from ancestral cyanobacteria. Chloroplast besides being a major component for photosynthesis, also takes part in another major plant metabolism, making it one of the major components of plants.

MAIN BODY

Chloroplast transformation is an alternative and better genetic engineering approach compared to the nuclear transformation that has been widely applied in plant genetic engineering. Chloroplast transformation has exhibited various positive effects as compared to nuclear transformation. This is a more preferred technique by researchers. To carry out chloroplast transformation, the vector design must be performed, and a selectable marker needs to be incorporated before the chloroplast could uptake the construct. The common way of introducing a gene into the host, which is the chloroplast, involves the biolistic, PEG-mediated, carbon nanotubes carriers, UV-laser microbeam, and Agrobacterium-mediated transformation approaches. Apart from discussing the processes involved in introducing the gene into the chloroplast, this review also focuses on the various applications brought about by chloroplast transformation, particularly in the field of agriculture and environmental science.

CONCLUSION

Chloroplast transformation has shown a lot of advantages and proven to be a better alternative compared to nuclear genome transformation. Further studies must be conducted to uncover new knowledge regarding chloroplast transformation as well as to discover its additional applications in the fields of biotechnology.

Assessment on effects of transplastomic potato plants expressing Colorado potato beetle β-Actin double-stranded RNAs for three non-target pests

RNA interference has been proved as an efficient technology for pest control through the silencing of essential genes of targeted insects. We had previously shown that the expression of double-stranded RNAs (dsRNAs) in plastids of plants offers a great potential for efficiently controlling Colorado potato beetle (CPB, Leptinotarsa decemlineata) (Coleoptera, Chrysomelidae). However, whether these transplastomic plants have an impact on other non-target pests was not investigated. In this study, we evaluated the potential effects of transplastomic plants expression dsRNAs target CPB β-Actin (referred to as ACT plants) on three other potato pests: Myzus persicae (Hemiptera, Aphididae), Henosepilachna vigintioctopunctata (Coleoptera, Coccinellidae), and Spodoptera litura (Lepidoptera, Noctuidae). Although no effects on M. persicae or S. litura were observed by feeding ACT plants, we found that feeding H. vigintioctopunctata with ACT plants can result in its growth retardation and suppressing the gene expression of HvACT, which has 91.7% identity to CPB β-Actin and shared 66 potential 21-mer matches. Taking together, these results indicated that ACT plants had cross-resistance to H. vigintioctopunctata, another coleopteran insect with the highly conserved nucleotide sequence of β-Actin gene. It also provided an opportunity to simultaneously control L. decemlineata and H. vigintioctopunctata by RNAi induced by intermediate dsRNAs with optimized sequences.

Knockdown of Quinolinate Phosphoribosyltransferase Results in Decreased Salicylic Acid-Mediated Pathogen Resistance in Arabidopsis thaliana

Nicotinamide adenine dinucleotide (NAD) is a pivotal coenzyme that has emerged as a central hub linking redox equilibrium and signal transduction in living cells. The homeostasis of NAD is required for plant growth, development, and adaption to environmental stresses. Quinolinate phosphoribosyltransferase (QPRT) is a key enzyme in NAD de novo synthesis pathway. T-DNA-based disruption of QPRT gene is embryo lethal in Arabidopsis thaliana. Therefore, to investigate the function of QPRT in Arabidopsis, we generated transgenic plants with decreased QPRT using the RNA interference approach. While interference of QPRT gene led to an impairment of NAD biosynthesis, the QPRT RNAi plants did not display distinguishable phenotypes under the optimal condition in comparison with wild-type plants. Intriguingly, they exhibited enhanced sensitivity to an avirulent strain of Pseudomonas syringae pv. tomato (Pst-avrRpt2), which was accompanied by a reduction in salicylic acid (SA) accumulation and down-regulation of pathogenesis-related genes expression as compared with the wild type. Moreover, oxidative stress marker genes including GSTU24, OXI1, AOX1 and FER1 were markedly repressed in the QPRT RNAi plants. Taken together, these data emphasized the importance of QPRT in NAD biosynthesis and immunity defense, suggesting that decreased antibacterial immunity through the alteration of NAD status could be attributed to SA- and reactive oxygen species-dependent pathways.

Importation of taxadiene synthase into chloroplast improves taxadiene production in tobacco

MAIN CONCLUSION

Importation of taxadiene synthase into chloroplasts is important for the efficient heterologous production of taxadiene. Taxadiene, the first committed precursor to taxol, is synthesized from geranylgeranyl pyrophosphate (GGPP) by action of taxadiene synthase (TS). Heterologous production of taxadiene could potentially rely on both cytosolic mevalonic acid (MVA) pathway and the plastidic methylerythritol phosphate (MEP) pathway. We suggest the compartmentalized engineering in chloroplast as an efficient approach for taxadiene production. In this study, we directly introduced the TS gene from Taxus brevifolia into the tobacco chloroplast genome and found that the transplastomic plants accumulated a low content of taxadiene, ~ 5.6 μg/g dry weight (DW). Moreover, we tried a combination of MEP and MVA pathways for taxadiene synthesis by nuclear transformation with a truncated version of TS (without encoding a transit peptide) into the transplastomic plants. However, this did not further improve the taxadiene production. In contrast, we found that taxadiene could be produced up to 87.8 μg/g DW in leaves of transgenic plants expressing TS with a chloroplast transit peptide, which was significantly higher than that in leaves of transplastomic plants. Thus, this study highlights the importance of TS importation into chloroplast for production of taxadiene.

Plastid Transformation in Poplar: A Model for Perennial Trees

Poplar (Populus) is an important forest tree and considered model for perennial trees. Here we describe a method for poplar plastid transformation, which involves preparation of explants, vector construction, biolistic bombardment, regeneration and selection of transplastomic poplar plants. The young leaves of 4-week-old poplar plants are used for biolistic bombardment and aadA gene as selectable marker. Homoplasmic transplastomic lines are obtained after regeneration and several rounds of selection with spectinomycin over 10 months. Homoplasmy is further confirmed by Southern blot. The establishment of a plastid transformation system for Populus is likely to make a significant contribution to tree genetic improvement.

Nanotechnology Approaches for Chloroplast Biotechnology Advancements

Photosynthetic organisms are sources of sustainable foods, renewable biofuels, novel biopharmaceuticals, and next-generation biomaterials essential for modern society. Efforts to improve the yield, variety, and sustainability of products dependent on chloroplasts are limited by the need for biotechnological approaches for high-throughput chloroplast transformation, monitoring chloroplast function, and engineering photosynthesis across diverse plant species. The use of nanotechnology has emerged as a novel approach to overcome some of these limitations. Nanotechnology is enabling advances in the targeted delivery of chemicals and genetic elements to chloroplasts, nanosensors for chloroplast biomolecules, and nanotherapeutics for enhancing chloroplast performance. Nanotechnology-mediated delivery of DNA to the chloroplast has the potential to revolutionize chloroplast synthetic biology by allowing transgenes, or even synthesized DNA libraries, to be delivered to a variety of photosynthetic species. Crop yield improvements could be enabled by nanomaterials that enhance photosynthesis, increase tolerance to stresses, and act as nanosensors for biomolecules associated with chloroplast function. Engineering isolated chloroplasts through nanotechnology and synthetic biology approaches are leading to a new generation of plant-based biomaterials able to self-repair using abundant CO₂ and water sources and are powered by renewable sunlight energy. Current knowledge gaps of nanotechnology-enabled approaches for chloroplast biotechnology include precise mechanisms for entry into plant cells and organelles, limited understanding about nanoparticle-based chloroplast transformations, and the translation of lab-based nanotechnology tools to the agricultural field with crop plants. Future research in chloroplast biotechnology mediated by the merging of synthetic biology and nanotechnology approaches can yield tools for precise control and monitoring of chloroplast function in vivo and ex vivo across diverse plant species, allowing increased plant productivity and turning plants into widely available sustainable technologies.

Length-dependent accumulation of double-stranded RNAs in plastids affects RNA interference efficiency in the Colorado potato beetle

Transplastomic potato plants expressing double-stranded RNA (dsRNA) targeted against essential genes of the Colorado potato beetle (CPB) can be lethal to larvae by triggering an RNA interference (RNAi) response. High accumulation levels of dsRNAs in plastids are crucial to confer an efficient RNAi response in the insects. However, whether length and sequence of the dsRNA determine the efficacy of RNAi and/or influence the level of dsRNA accumulation in plastids is not known. We compared the RNAi efficacy of different lengths of dsRNA targeted against the CPB β-Actin gene (ACT) by feeding in vitro-synthesized dsRNAs to larvae. We showed that, while the 60 bp dsRNA induced only a relatively low RNAi response in CPB, dsRNAs of 200 bp and longer caused high mortality and similar larval growth retardation. When the dsRNAs were expressed from the plastid (chloroplast) genome of potato plants, we found that their accumulation were negatively correlated with length. The level of dsRNA accumulation was positively associated with the observed mortality, suppression of larval growth, and suppression of target gene expression. Importantly, transplastomic potato plants expressing the 200 bp dsRNA were better protected from CPB than plants expressing the 297 bp dsRNA, the best-performing line in our previous study. Our results suggest that the length of dsRNAs is an important factor that influences their accumulation in plastids and thus determines the strength of the insecticidal RNAi effect. Our findings will aid the design of optimized dsRNA expression constructs for plant protection by plastid-mediated RNAi.

Plastid-expressed Bacillus thuringiensis (Bt) cry3Bb confers high mortality to a leaf eating beetle in poplar

The Bacillus thuringiensis (Bt) cry3Bb gene was successfully introduced into poplar plastid genome, leading to transplastomic poplar with high mortality to Plagiodera versicolora. Poplar (Populus L.) is one of the main resource of woody industry, but being damaged by insect pests. The feasibility and efficiency of plastid transformation technology for controlling two lepidopteran caterpillars have been demonstrated previously. Here, we introduced B. thuringiensis (Bt) cry3Bb into poplar plastid genome by biolistic bombardment for controlling P. versicolora, a widely distributed forest pest. Chimeric cry3Bb gene is controlled by the tobacco plastid rRNA operon promoter combined with the 5'UTR from gene10 of bacteriophage T7 (NtPrrn:T7g10) and the 3'UTR from the E. coli ribosomal RNA operon rrnB (TrrnB). The integration of transgene and homoplasmy of transplastomic poplar plants was confirmed by Southern blot analysis. Northern blot analysis indicated that cry3Bb was transcribed to both read through and shorter length transcripts in plastid. The transplastomic poplar expressing Cry3Bb insecticidal protein showed the highest accumulation level in young leaves, which reach up to 16.8 μg/g fresh weight, and comparatively low levels in mature and old leaves. Feeding the young leaves from Bt-Cry3Bb plastid lines to P. versicolora caused 100% mortality in the first-instar larvae after only 1 day, in the second-instar larvae after 2 days, and in the third-instar larvae for 3 days. Thus, we report a successful extension of plastid engineering poplar against the chrysomelid beetle.

Enhanced Tolerance to Methyl Viologen-Mediated Oxidative Stress via AtGR2 Expression From Chloroplast Genome

Owing to their sessile life habit, plants are continuously subjected to a broad range of environmental stresses. During periods of (a)biotic stresses, reactive oxygen species (ROS) levels can rise excessively, leading to oxidative stress. Glutathione reductase (GR) plays an important role in scavenging the ROS and maintenance of redox potential of the cell during oxidative stress. To enhance ROS scavenging capacity, and hence stress tolerance, the Arabidopsis thalianaGR2 (AtGR2) gene was expressed from the tobacco plastid (chloroplast) genome, the main source of ROS production in plant photosynthetic tissues, in this study. Leaves of transplastomic tobacco plants had about seven times GR activity and 1.5 times total glutathione levels compared to wild type. These transplastomic tobacco plants showed no discernible phenotype and exhibited more tolerance to methyl viologen-induced oxidative stress than wild-type control plants. The results indicate that introducing AtGR2 in chloroplasts is an efficient approach to increase stress tolerance. This study also provides evidence that increasing antioxidant enzyme via plastid genome engineering is an alternative to enhance plant's tolerance to stressful conditions.

In vivo DNA assembly using common laboratory bacteria: A re-emerging tool to simplify molecular cloning

Molecular cloning is a cornerstone of biomedical, biotechnological, and synthetic biology research. As such, improved cloning methodologies can significantly advance the speed and cost of research projects. Whereas current popular cloning approaches use in vitro assembly of DNA fragments, in vivo cloning offers potential for greater simplification. It is generally assumed that bacterial in vivo cloning requires Escherichia coli strains with enhanced recombination ability; however, this is incorrect. A widely present, bacterial RecA-independent recombination pathway is re-emerging as a powerful tool for molecular cloning and DNA assembly. This poorly understood pathway offers optimal cloning properties (i.e. seamless, directional, and sequence-independent) without requiring in vitro DNA assembly or specialized bacteria, therefore vastly simplifying cloning procedures. Although the use of this pathway to perform DNA assembly was first reported over 25 years ago, it failed to gain popularity, possibly due to both technical and circumstantial reasons. Technical limitations have now been overcome, and recent reports have demonstrated its versatility for DNA manipulation. Here, we summarize the historical trajectory of this approach and collate recent reports to provide a roadmap for its optimal use. Given the simplified protocols and minimal requirements, cloning using in vivo DNA assembly in E. coli has the potential to become widely employed across the molecular biology community.

Bacillus thuringiensis cry1C expression from the plastid genome of poplar leads to high mortality of leaf-eating caterpillars

Plastid transformation technology has several attractive features compared with traditional nuclear transformation technology. However, only a handful of species are able to be successfully transformed. Here, we report an efficient and stable plastid transformation protocol for poplar, an economically important tree species grown worldwide. We transformed the Bacillus thuringiensis cry1C gene into the poplar plastid genome, and homoplasmic transplastomic poplar was obtained after two to three rounds of regeneration under antibiotic selection for 7-12 months. The transplastomic poplar expressing Cry1C insecticidal protein showed the highest accumulation level in young leaves, which reached up to 20.7 μg g-1 fresh weight, and comparatively low levels in mature and old leaves, and hardly detectable levels in non-green tissues, such as phloem, xylem and roots. Transplastomic poplar showed high toxicity to Hyphantria cunea and Lymantria dispar, two notorious forest pests worldwide, without affecting plant growth. These results are the first successful examples of insect-resistant poplar generation by plastid genome engineering and provide a new avenue for future genetic improvement of poplar plants.

Accumulation of glycine betaine in transplastomic potato plants expressing choline oxidase confers improved drought tolerance

Plastid genome engineering is an effective method to generate drought-resistant potato plants accumulating glycine betaine in plastids. Glycine betaine (GB) plays an important role under abiotic stress, and its accumulation in chloroplasts is more effective on stress tolerance than that in cytosol of transgenic plants. Here, we report that the codA gene from Arthrobacter globiformis, which encoded choline oxidase to catalyze the conversion of choline to GB, was successfully introduced into potato (Solanum tuberosum) plastid genome by plastid genetic engineering. Two independent plastid-transformed lines were isolated and confirmed as homoplasmic via Southern-blot analysis, in which the mRNA level of codA was much higher in leaves than in tubers. GB accumulated in similar levels in both leaves and tubers of codA-transplastomic potato plants (referred to as PC plants). The GB content was moderately increased in PC plants, and compartmentation of GB in plastids conferred considerably higher tolerance to drought stress compared to wild-type (WT) plants. Higher levels of relative water content and chlorophyll content under drought stress were detected in the leaves of PC plants compared to WT plants. Moreover, PC plants presented a significantly higher photosynthetic performance as well as antioxidant enzyme activities during drought stress. These results suggested that biosynthesis of GB by chloroplast engineering was an effective method to increase drought tolerance.

The Multiplanetary Future of Plant Synthetic Biology

The interest in human space journeys to distant planets and moons has been re-ignited in recent times and there are ongoing plans for sending the first manned missions to Mars in the near future. In addition to generating oxygen, fixing carbon, and recycling waste and water, plants could play a critical role in producing food and biomass feedstock for the microbial manufacture of materials, chemicals, and medicines in long-term interplanetary outposts. However, because life on Earth evolved under the conditions of the terrestrial biosphere, plants will not perform optimally in different planetary habitats. The construction or transportation of plant growth facilities and the availability of resources, such as sunlight and liquid water, may also be limiting factors, and would thus impose additional challenges to efficient farming in an extraterrestrial destination. Using the framework of the forthcoming human missions to Mars, here we discuss a series of bioengineering endeavors that will enable us to take full advantage of plants in the context of a Martian greenhouse. We also propose a roadmap for research on adapting life to Mars and outline our opinion that synthetic biology efforts towards this goal will contribute to solving some of the main agricultural and industrial challenges here on Earth.