Relocating a gene for herbicide tolerance: A chloroplast gene is converted into a nuclear gene

Does mRNA targeting explain gene retention in chloroplasts?

During their evolution from cyanobacteria, plastids have relinquished most of their genes to the host cell nucleus, but have retained a core set of genes that are transcribed and translated within the organelle. Previous explanations have included incompatible codon or base composition, problems importing certain proteins across the double membrane, or the need for tight regulation in concert with the redox status of the electron transport chain. In this opinion article we propose the 'mRNA targeting hypothesis'. Studies in cyanobacteria suggest that mRNAs encoding core photosynthetic proteins have features that are crucial for membrane targeting and coordination of early steps in complex assembly. We propose that the requirement for intimate involvement of mRNA molecules at the thylakoid surface explains the retention of core photosynthetic genes in chloroplasts.

Dinoflagellate chloroplasts as a model for extreme genome reduction and fragmentation in organelles - The COCOA principle for gene retention

The genomes of peridinin-containing dinoflagellate chloroplasts have a very unusual organisation. These genomes are highly fragmented and greatly reduced, with most of the usual complement of chloroplast genes relocated to the nucleus. Dinoflagellate chloroplasts highlight evolutionary changes that are found to varying extents in a number of other organelle genomes. These include the chloroplast genome of the green alga Boodlea and other Cladophorales, and the mitochondrial genomes of blood-sucking and chewing lice, the parasitic plant Rhopalocnemis phalloides, the red alga Rhodosorus marinus and other members of the Stylonematophyceae, diplonemid flagellates, and some Cnidaria. Consideration of the coding content of the remnant chloroplast genomes indicates that organelles may preferentially retain genes for proteins important in initiating assembly of complexes, and the same is largely true for mitochondria. We propose a new principle, of CO-location for COntrol of Assembly (COCOA), indicating the importance of retaining these genes in the organelle. This adds to, but does not invalidate, the existing hypotheses of the multisubunit completion principle, CO-location for Redox Regulation (CORR) and Control by Epistasy of Synthesis (CES).

Enhancing the light reactions of photosynthesis: Strategies, controversies, and perspectives

Photosynthesis is central to life on Earth, employing sunlight, water, and carbon dioxide to produce chemical energy and oxygen. It is generally accepted that boosting its efficiency offers one promising way to increase crop yields under agronomically realistic conditions. Since the components, structure, and regulatory mechanisms of the light reactions of photosynthesis are well understood, concepts for enhancing the process have been suggested and partially tested. These approaches vary in complexity, from targeting single components to comprehensive redesign of the whole process on the scales from single cells or tissues to whole canopies. Attempts to enhance light utilization per leaf, by decreasing pigmentation, increasing levels of photosynthetic proteins, prolonging the lifespan of the photosynthetic machinery, or massive reconfiguration of the photosynthetic machinery and the incorporation of nanomaterials, are discussed in this review first. Secondly, strategies intended to optimize the acclimation of photosynthesis to changes in the environment are presented, including redesigning mechanisms to dissipate excess excitation energy (e.g., non-photochemical quenching) or reduction power (e.g., flavodiiron proteins). Moreover, schemes for improving acclimation, inspired by natural or laboratory-induced adaptation, are introduced. However, all these endeavors are still in an early exploratory phase and/or have not resulted in the desired outcome, largely because photosynthesis is embedded within large networks of closely interacting cellular and metabolic processes, which can vary among species and even cultivars. This explains why integrated, systems-wide approaches are required to achieve the breakthroughs required for effectively increasing crop yields.

A novel psbA mutation (Phe274-Val) confers resistance to PSII herbicides in wild radish (Raphanus raphanistrum)

BACKGROUND

Wild radish (Raphanus raphanistrum) is a globally important weed of crops. Two atrazine-resistant wild radish populations (R1 and R2), collected from the Western Australia grain belt, were investigated for resistance to photosystem II (PSII) herbicides.

RESULTS

Sequencing of the full-length psbA gene revealed the well-known Ser264-Gly substitution in population R1, whereas population R2 displayed a novel Phe274-Val substitution. Herbicide dose-response studies confirmed that the population with the Ser264-Gly mutation exhibited high-level resistance to atrazine, but super-sensitivity to bromoxynil. Plants possessing the novel Phe274-Val mutation exhibited a modest level of resistance to atrazine, metribuzin and diuron, and were bromoxynil susceptible. Structural modelling of the mutant D1 proteins predicts that the Ser264-Gly mutation endows atrazine resistance by abolishing H-bonds, but confers bromoxynil super-sensitivity by enhancing hydrogen bonding. The Phe274-Val substitution provides resistance to atrazine and diuron by indirectly affecting H-bond formation between the Ser264 residue and the herbicides.

CONCLUSION

The results demonstrate that the Phe274-Val mutation is likely responsible for resistance to PSII-inhibiting triazine and urea herbicides. To our knowledge, this is the first evidence of the psbA Phe274-Val mutation in wild radish conferring resistance to PSII herbicides. © 2018 Society of Chemical Industry.

Assessment of atrazine toxicity to the estuarine phytoplankter, Dunaliella tertiolecta (Chlorophyta), under varying nutrient conditions

Anthropogenic inputs of chemical environmental contaminants are frequently associated with developing harmful algal blooms, but little is known about how estuarine phytoplankton assemblages respond to multiple, co-occurring chemical stressors in chronically disturbed habitats. The goals of this research were to establish a robust protocol for testing the effects of atrazine on estuarine phytoplankton, and then to use that protocol to compare the effects of atrazine exposure with and without nutrient enrichment on a cosmopolitan estuarine/marine alga, Dunaliella tertiolecta (Chlorophyta). Atrazine sensitivity in nutrient-replete media (96-h growth inhibition [Formula: see text]) was 159.16 μg l^-1, but sensitivity was influenced by exposure duration, and inhibitory effects of herbicide on algal growth decreased under imbalanced nutrient regimes and low nitrogen and phosphorus supplies. These findings advance knowledge about how nutrient regimes and herbicides interact to control estuarine phytoplankton population dynamics.

Ectopic expression of Rubisco subunits in maize mesophyll cells does not overcome barriers to cell type-specific accumulation

In maize (Zea mays), Rubisco accumulates in bundle sheath but not mesophyll chloroplasts, but the mechanisms that underlie cell type-specific expression are poorly understood. To explore the coordinated expression of the chloroplast rbcL gene, which encodes the Rubisco large subunit (LS), and the two nuclear RBCS genes, which encode the small subunit (SS), RNA interference was used to reduce RBCS expression. This resulted in Rubisco deficiency and was correlated with translational repression of rbcL. Thus, as in C3 plants, LS synthesis depends on the presence of its assembly partner SS. To test the hypothesis that the previously documented transcriptional repression of RBCS in mesophyll cells is responsible for repressing LS synthesis in mesophyll chloroplasts, a ubiquitin promoter-driven RBCS gene was expressed in both bundle sheath and mesophyll cells. This did not lead to Rubisco accumulation in the mesophyll, suggesting that LS synthesis is impeded even in the presence of ectopic SS expression. To attempt to bypass this putative mechanism, a ubiquitin promoter-driven nuclear version of the rbcL gene was created, encoding an epitope-tagged LS that was expressed in the presence or absence of the Ubi-RBCS construct. Both transgenes were robustly expressed, and the tagged LS was readily incorporated into Rubisco complexes. However, neither immunolocalization nor biochemical approaches revealed significant accumulation of Rubisco in mesophyll cells, suggesting a continuing cell type-specific impairment of its assembly or stability. We conclude that additional cell type-specific factors limit Rubisco expression to bundle sheath chloroplasts.

Role of intercompartmental DNA transfer in producing genetic diversity

In eukaryotic cells, genes are found in three compartments-the nucleus, mitochondria, and plastids-and extensive gene transfer has occurred between them. Most organellar genes in the nucleus migrated there long ago, but transfer is ongoing and ubiquitous. It now generates mostly noncoding nuclear DNA, can also disrupt gene functions, and reshape genes by adding novel exons. Plastid or nuclear sequences have also contributed to the formation of mitochondrial tRNA genes. It is now clear that organelle-to-nucleus DNA transfer involves the escape of DNA molecules from the organelles at times of stress or at certain developmental stages, and their subsequent incorporation at sites of double-stranded breaks in nuclear DNA by nonhomologous recombination. Intercompartmental DNA transfer thus appears to be an inescapable phenomenon that has had a broad impact on eukaryotic evolution, affecting DNA repair, gene and genome evolution, and redirecting proteins to different target compartments.

Herbicide resistance in transgenic plants expressing a bacterial detoxification gene

The herbicide bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) is a photosynthetic (photosystem II) inhibitor in plants. A gene, bxn, encoding a specific nitrilase that converts bromoxynil to its primary metabolite 3,5-dibromo-4-hydroxybenzoic acid, was cloned from the natural soil bacterium Klebsiella ozaenae. For expression in plants, the bxn gene was placed under control of a light-regulated tissue-specific promoter, the ribulose bisphosphate carboxylase small subunit. Transfer of this chimeric gene and expression of a bromoxynil-specific nitrilase in leaves of transgenic tobacco plants conferred resistance to high levels of a commercial formulation of bromoxynil. The results presented indicate a successful approach to obtain herbicide resistance by introducing a novel catabolic detoxification gene in plants.

DNA transfer from organelles to the nucleus: the idiosyncratic genetics of endosymbiosis

In eukaryotes, DNA is exchanged between endosymbiosis-derived compartments (mitochondria and chloroplasts) and the nucleus. Organelle-to-nucleus DNA transfer involves repair of double-stranded breaks by nonhomologous end-joining, and resulted during early organelle evolution in massive relocation of organelle genes to the nucleus. A large fraction of the products of the nuclear genes so acquired are retargeted to their ancestral compartment; many others now function in new subcellular locations. Almost all present-day nuclear transfers of mitochondrial or plastid DNA give rise to noncoding sequences, dubbed nuclear mitochondrial DNAs (NUMTs) and nuclear plastid DNAs (NUPTs). Some of these sequences were recruited as exons, thus introducing new coding sequences into preexisting nuclear genes by a novel mechanism. In organisms derived from secondary or tertiary endosymbiosis, serial gene transfers involving nucleus-to-nucleus migration of DNA have also occurred. Intercompartmental DNA transfer therefore represents a significant driving force for gene and genome evolution, relocating and refashioning genes and contributing to genetic diversity.

Why are plastid genomes retained in non-photosynthetic organisms?

The evolution of the plastid from a photosynthetic bacterial endosymbiont involved a dramatic reduction in the complexity of the plastid genome, with many genes either discarded or transferred to the nucleus of the eukaryotic host. However, this evolutionary process has not gone to completion and a subset of genes remains in all plastids examined to date. The various hypotheses put forward to explain the retention of the plastid genome have tended to focus on the need for photosynthetic organisms to retain a genetic system in the chloroplast, and they fail to explain why heterotrophic plants and algae, and the apicomplexan parasites all retain a genome in their non-photosynthetic plastids. Here we consider two additional explanations: the 'essential tRNAs' hypothesis and the 'transfer-window' hypothesis.

Biodegradation of atrazine in transgenic plants expressing a modified bacterial atrazine chlorohydrolase (atzA) gene

Atrazine is one of the most widely used herbicides in the USA. Atrazine chlorohydrolase (AtzA), the first enzyme in a six-step pathway leading to the mineralization of atrazine in Gram-negative soil bacteria, catalyses the hydrolytic dechlorination and detoxification of atrazine to hydroxyatrazine. In this study, we investigated the potential use of transgenic plants expressing atzA to take up, dechlorinate and detoxify atrazine. Alfalfa, Arabidopsis thaliana and tobacco were transformed with a modified bacterial atzA gene, p-atzA, under the control of the cassava vein mosaic virus promoter. All transgenic plant species actively expressed p-atzA and grew over a wide range of atrazine concentrations. Thin layer chromatography analyses indicated that in planta expression of p-atzA resulted in the production of hydroxyatrazine. Hydroponically grown transgenic tobacco and alfalfa dechlorinated atrazine to hydroxyatrazine in leaves, stems and roots. Moreover, p-atzA was found to be useful as a conditional-positive selection system to isolate alfalfa and Arabidopsis transformants following Agrobacterium-mediated transformation. Our work suggests that the in planta expression of p-atzA may be useful for the development of plants for the phytoremediation of atrazine-contaminated soils and soil water, and as a marker gene to select for the integration of exogenous DNA into the plant genome.

Why genes persist in organelle genomes

Mitochondria and plastids (including chloroplasts) have a small but vital genetic coding capacity, but what are the properties of some genes that dictate that they must remain encoded in organelles?

Mitochondria and plastids (including chloroplasts) have a small but vital genetic coding capacity, but what are the properties of some genes that dictate that they must remain encoded in organelles?

Selectable marker genes in transgenic plants: applications, alternatives and biosafety

Approximately fifty marker genes used for transgenic and transplastomic plant research or crop development have been assessed for efficiency, biosafety, scientific applications and commercialization. Selectable marker genes can be divided into several categories depending on whether they confer positive or negative selection and whether selection is conditional or non-conditional on the presence of external substrates. Positive selectable marker genes are defined as those that promote the growth of transformed tissue whereas negative selectable marker genes result in the death of the transformed tissue. The positive selectable marker genes that are conditional on the use of toxic agents, such as antibiotics, herbicides or drugs were the first to be developed and exploited. More recent developments include positive selectable marker genes that are conditional on non-toxic agents that may be substrates for growth or that induce growth and differentiation of the transformed tissues. Newer strategies include positive selectable marker genes which are not conditional on external substrates but which alter the physiological processes that govern plant development. A valuable companion to the selectable marker genes are the reporter genes, which do not provide a cell with a selective advantage, but which can be used to monitor transgenic events and manually separate transgenic material from non-transformed material. They fall into two categories depending on whether they are conditional or non-conditional on the presence of external substrates. Some reporter genes can be adapted to function as selectable marker genes through the development of novel substrates. Despite the large number of marker genes that exist for plants, only a few marker genes are used for most plant research and crop development. As the production of transgenic plants is labor intensive, expensive and difficult for most species, practical issues govern the choice of selectable marker genes that are used. Many of the genes have specific limitations or have not been sufficiently tested to merit their widespread use. For research, a variety of selection systems are essential as no single selectable marker gene was found to be sufficient for all circumstances. Although, no adverse biosafety effects have been reported for the marker genes that have been adopted for widespread use, biosafety concerns should help direct which markers will be chosen for future crop development. Common sense dictates that marker genes conferring resistance to significant therapeutic antibiotics should not be used. An area of research that is growing rapidly but is still in its infancy is the development of strategies for eliminating selectable marker genes to generate marker-free plants. Among the several technologies described, two have emerged with significant potential. The simplest is the co-transformation of genes of interest with selectable marker genes followed by the segregation of the separate genes through conventional genetics. The more complicated strategy is the use of site-specific recombinases, under the control of inducible promoters, to excise the marker genes and excision machinery from the transgenic plant after selection has been achieved. In this review each of the genes and processes will be examined to assess the alternatives that exist for producing transgenic plants.

Evolution of the chloroplast genome

We discuss the suggestion that differences in the nucleotide composition between plastid and nuclear genomes may provide a selective advantage in the transposition of genes from plastid to nucleus. We show that in the adenine, thymine (AT)-rich genome of Borrelia burgdorferi several genes have an AT-content lower than the average for the genome as a whole. However, genes whose plant homologues have moved from plastid to nucleus are no less AT-rich than genes whose plant homologues have remained in the plastid, indicating that both classes of gene are able to support a high AT-content. We describe the anomalous organization of dinoflagellate plastid genes. These are located on small circles of 2-3 kbp, in contrast to the usual plastid genome organization of a single large circle of 100-200 kbp. Most circles contain a single gene. Some circles contain two genes and some contain none. Dinoflagellate plastids have retained far fewer genes than other plastids. We discuss a similarity between the dinoflagellate minicircles and the bacterial integron system.

Coordination of nuclear and chloroplast gene expression in plant cells

Plastid proteins are encoded in two genomes, one in the nucleus and the other in the organelle. The expression of genes in these two compartments in coordinated during development and in response to environmental parameters such as light. Two converging approaches reveal features of this coordination: the biochemical analysis of proteins involved in gene expression, and the genetic analysis of mutants affected in plastid function or development. Because the majority of proteins implicated in plastid gene expression are encoded in the nucleus, regulatory processes in the nucleus and in the cytoplasm control plastid gene expression, in particular during development. Many nucleus-encoded factors involved in transcriptional and posttranscriptional steps of plastid gene expression have been characterized. We are also beginning to understand whether and how certain developmental or environmental signals perceived in one compartment may be transduced to the other.

On the mode of integration of plastid-encoded components of the cytochrome bf complex into thylakoid membranes

Four distinct integration/translocation routes into/across thylakoid membranes have recently been deduced for nuclear-encoded polypeptides of the photosynthetic membrane. Corresponding information for the plastid-encoded protein complement is lacking. We have investigated this aspect with in-organello assays employing chimeric constructs generated with codoncorrect cassettes for genes of plastid-encoded thylakoid proteins, and appropriate transit peptides from six nuclear genes, representing three targeting classes, as a strategy. The three major plastid-encoded components of the cytochrome b (6)f complex, namely pre-apocytochrome f, (including apocytochrome f, and pre-apocytochrome f lacking the C-terminal transmembrane segment), cytochrome b(6), and subunit IV, which differ in the number of their transmembrane segments, were studied. Import into chloroplasts could be observed in all instances but with relatively low efficiency. Thylakoid integration can occurr post-translationally, but only components with secretory/secretory pathway (SEC)-route-specific epitopes were correctly assembled with the cytochrome complex, or competed with this process. Inhibitor studies were consistent with these findings. Imported cytochrome b(6) and subunit IV operated with uncleaved targeting signals for thylakoid integration. The corresponding determinant for cytochrome f is its signal peptide; its C-terminal hydrophobic segment did not, or did not appreciably, contribute to this process. The N-termini of cytochrome b(6) and subunit IV appear to reside on the same (lumenal) side of the membrane, consistent with the currently favored four-helix model for the cytochrome, but in disagreement with the topography proposed for both components. The impact of the findings for protein routing, including for applied approaches such as compartment-alien transformation, is discussed.

Tobacco (Nicotiana tobaccum) nuclear transgenics with high copy number can express NPTII driven by the chloroplast psbA promoter

A chloroplast expression vector containing the NPTII gene under the control of apsbA promoter (psbA-NPTII) was constructed, and was biolistically delivered into both suspension cells and leaf strips of tobacco (Nicotiana tabaccum). Analyses of subsequently recovered kanamycin-resistant transgenic plants indicate that the psbA-NPTII gene was not located in the chloroplast, but was in the nucleus in very high copy number. This conclusion was based upon results from: (1) Southern hybridization analyses of chloroplast and nuclear DNAs using NPTII, chloroplast-marker, and nuclear-marker probes; (2) pulse-field gel electrophoresis; and (3) kanamycin screening of sexual progenies. This study suggests that the nuclear expression of the NPTII gene may have been associated with many copies of the psbA-NPTII construction. Very high copy number in the nucleus might either allow NPTII expression from the otherwise inadequate psbA promoter, or might increase the chance of recombining with upstream tobacco regulatory sequences.

The N-terminal hydrophobic region of the mature phosphate translocator is sufficient for targeting to the chloroplast inner envelope membrane

To locate the sequence required for directing the phosphate translocator to the chloroplast inner envelope membrane, a series of chimeric proteins constituting parts of the phosphate translocator and the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase, which is normally located in the stroma, has been produced. Reciprocal exchanges of the presequences and mature sequences of the phosphate translocator and the small subunit indicated that the phosphate translocator presequence contains stromal targeting information and that the mature protein is responsible for inner envelope membrane targeting. Chimeric proteins containing the N-terminal 46 amino acid residues of the phosphate translocator were directed to the inner envelope membrane. Subdivision of this region into its composite hydrophilic and hydrophobic regions showed that the hydrophobic region alone, which consists of amino acid residues 24 to 45, was able to direct the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase to the inner envelope membrane.

Substrate- and species-specific processing enzymes for chloroplast precursor proteins

Using different precursors of chloroplast proteins and stromal extracts from both Chlamydomonas reinhardii and pea chloroplasts, we analysed the specificity of stroma-localized processing peptidases. By gel filtration of a stromal extract from isolated Chlamydomonas chloroplasts, fractions could be separated containing enzymic activities for processing the precursors of the small subunit of ribulose-1,5-bisphosphate carboxylase (pSS) and of the protein OEE1 from the photosynthetic water-splitting complex (pOEE1). The enzymes differed not only in molecular size, but also in their sensitivity to inhibitors and in their pH optima. Obviously, in the stroma of Chlamydomonas chloroplasts different peptidases exist for processing of pSS and pOEE1, the latter being converted into an intermediate-sized form, iOEE1, which was found to be further processed to mature OEE1 by a thylakoid-associated protease. To study the species-specificity of the stromal peptidases, stromal extracts from Chlamydomonas and pea chloroplasts were incubated with pSS from either of these organisms. In the heterologous combinations, the precursors were partly hydrolysed, but not to the correct size. In importation assays, pSS from pea (but also the precursor of the ribosomal protein L12 from spinach) could not enter into chloroplasts from Chlamydomonas. In contrast, the algal pSS was imported into chloroplasts from pea, although it was not processed to mature SS. Our results indicate that the importation machinery and the pSS-processing enzymes in higher plants and green algae have different specificities and that in Chlamydomonas several stromal peptidases for different precursor proteins exist.

Relocation of the plastid rbcL gene to the nucleus yields functional ribulose-1,5-bisphosphate carboxylase in tobacco chloroplasts

The conserved plastid localization of rbcL suggests that biosynthesis of the large subunit of ribulose-1,5-bisphosphate carboxylase [Rubisco; 3-phospho-D-glycerate carboxy-lyase (dimerizing), EC 4.1.1.39] in chloroplasts is required to obtain functional enzyme. To examine the validity of this hypothesis, we relocated the plastid rbcL gene to the nucleus. First, we deleted the rbcL gene from the tobacco plastid genome by targeted insertion of a selectable aadA gene encoding spectinomycin resistance. The rbcL coding region was then inserted into an expression cassette and introduced into the nuclear genome of these plants by Agrobacterium-mediated transformation. We report that the nuclear rbcL functionally complements the defective plastids when the Rubisco large subunit is targeted to chloroplasts by a transit peptide. Therefore, the evolutionary process that relocates functional plastid genes to the nucleus has not yet occurred in the case of the rbcL gene. Targeted deletion of plastid genes, combined with their allotopic expression, will provide opportunities for studying the function of plastid enzyme complexes.

A serine-to-threonine substitution in the triazine herbicide-binding protein in potato cells results in atrazine resistance without impairing productivity

A mutation of the psbA gene was identified in photoautotrophic potato (Solanum tuberosum L. cv Superior x U.S. Department of Agriculture line 66-142) cells selected for resistance to 6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine (atrazine). Photoaffinity labeling with 6-azido-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine detected a thylakoid membrane protein with a M(r) of 32,000 in susceptible, but not in resistant, cells. This protein was identified as the secondary quinone acceptor of photosystem II (QB) protein. Atrazine resistance in selected cells was attributable to a mutation from AGT (serine) to ACT (threonine) in codon 264 of the psbA gene that encodes the QB protein. Although the mutant cells exhibited extreme levels of resistance to atrazine, no concomitant reductions in photosynthetic electron transport or cell growth rates compared to the unselected cells were detected. This is in contrast with the losses in productivity observed in atrazine-resistant mutants that contain a glycine-264 alteration.

Male-sterility induction in transgenic tobacco plants with an unedited atp9 mitochondrial gene from wheat

Cytoplasmic male sterility in plants is associated with mitochondrial dysfunction. We have proposed that a nuclear-encoded chimeric peptide formed by mitochondrial sequences when imported into the mitochondria may impair organelle function and induce male sterility in plants. A model developed to test this hypothesis is reported here. Assuming that the editing process in higher plant mitochondria reflects a requirement for producing active proteins, we have used edited and unedited coding sequences of wheat ATP synthase subunit 9 (atp9) fused to the coding sequence of a yeast coxIV transit peptide. Transgenic plants containing unedited atp9 exhibited either fertile, semifertile, or male-sterile phenotypes; controls containing edited atp9 or only the selectable marker gave fertile plants. Pollen fertility ranged from 31% to 75% in fertile plants, 10% to 20% in semifertile plants, and < 2% in male-sterile plants. Genetic and molecular data showed that the chimeric plasmid containing the transgene is inherited as a Mendelian trait. The transgenic protein is imported into the mitochondria. The production and frequency of semifertile or male-sterile transgenic plants conform to the proposed hypothesis.

Strategies for expression of foreign genes in plants. Potential use of engineered viruses

Advances in gene transfer techniques for higher plants have already permitted important achievements towards crop protection and improvement using recombinant DNA technology. Besides plant genetic engineering, the possible use of plant viruses to express foreign genes could be of considerable interest to plant biotechnology. However, insuring containment of engineered viruses for environmental use is an important safety issue that must be addressed.

Sulfonamide resistance gene for plant transformation

The sulfonamide resistance gene from plasmid R46 encodes for a mutated dihydropteroate synthase insensitive to inhibition by sulfonamides. Its coding sequence was fused to the pea ribulose bisphosphate carboxylase/oxygenase transit peptide sequence. Incubation of isolated chloroplasts with the fusion protein synthesised in vitro, showed that the bacterial enzyme was transported to the chloroplast stroma and processed into a mature form. Expression of the gene fusion in transgenic plants resulted in a high level of resistance to sulfonamides. Direct selection of transformed shoots on leaf explants was efficient using sulfonamides as sole selective agents. Transformed shoots rooted normally on sulfonamides at concentrations toxic for untransformed ones. Sulfonamide resistance was transmitted to the progeny of transformed plants as a single Mendelian dominant character. These results demonstrate that this chimeric gene can be used as an efficient and versatile selectable marker for plant transformation.

Transient foreign gene expression in chloroplasts of cultured tobacco cells after biolistic delivery of chloroplast vectors

Expression of chloramphenicol acetyltransferase (cat) by suitable vectors in chloroplasts of cultured tobacco cells, delivered by high-velocity microprojectiles, is reported here. Several chloroplast expression vectors containing bacterial cat genes, placed under the control of either psbA promoter region from pea (pHD series) or rbcL promoter region from maize (pAC series) have been used in this study. In addition, chloroplast expression vectors containing replicon fragments from pea, tobacco, or maize chloroplast DNA have also been tested for efficiency and duration of cat expression in chloroplasts of tobacco cells. Cultured NT1 tobacco cells collected on filter papers were bombarded with tungsten particles coated with pUC118 (negative control), 35S-CAT (nuclear expression vector), pHD312 (repliconless chloroplast expression vector), and pHD407, pACp18, and pACp19 (chloroplast expression vectors with replicon). Sonic extracts of cells bombarded with pUC118 showed no detectable cat activity in the autoradiograms. Nuclear expression of cat reached two-thirds of the maximal 48 hr after bombardment and the maximal at 72 hr. Cells bombarded with chloroplast expression vectors showed a low level of expression until 48 hr of incubation. A dramatic increase in the expression of cat was observed 24 hr after the addition of fresh medium to cultured cells in samples bombarded with pHD407; the repliconless vector pHD312 showed about 50% of this maximal activity. The expression of nuclear cat and the repliconless chloroplast vector decreased after 72 hr, but a high level of chloroplast cat expression was maintained in cells bombarded with pHD407. Organelle-specific expression of cat in appropriate compartments was checked by introducing various plasmid constructions into tobacco protoplasts by electroporation. Although the nuclear expression vector 35S-CAT showed expression of cat, no activity was observed with any chloroplast vectors.

Chloroplast transformation in Chlamydomonas with high velocity microprojectiles

Bombardment of three mutants of the chloroplast atpB gene of Chlamydomonas reinhardtii with high-velocity tungsten microprojectiles that were coated with cloned chloroplast DNA carrying the wild-type gene permanently restored the photosynthetic capacity of the algae. In most transformants of one of the mutants, a fragment with a 2.5-kilobase deletion was restored to normal size by a homologous replacement event; in about 25 percent of the transformants the restored restriction fragment was 50 to 100 base pairs smaller or larger than that of wild type. About one-fourth of the transformants of this mutant contained unintegrated donor plasmid when first examined. This plasmid persisted in four different transformants after 65 cell generations of continuous liquid culture but was lost from all transformants maintained on plates of selective medium. The restored wild-type atpB gene remains in all transformants as an integral part of the chloroplast genome and is expressed and inherited normally.

Recent developments in chloroplast protein transport

Most proteins located in chloroplasts are encoded by nuclear genes, synthesized in the cytoplasm, and transported into the organelle. The study of protein uptake by chloroplasts has greatly expanded over the past few years. The increased activity in this field is due, in part, to the application of recombinant DNA methodology to the analysis of protein translocation. Added interest has also been gained by the realization that the transport mechanisms that mediate protein uptake by chloroplasts, mitochondria and the endoplasmic reticulum display certain characteristics in common. These include amino terminal sequences that target proteins to particular organelles, a transport process that is mechanistically independent from the events of translation, and an ATP-requiring transport step that is thought to involve partial unfolding of the protein to be translocated. In this review we examine recent studies on the binding of precursors to the chloroplast surface, the energy-dependent uptake of proteins into the stroma, and the targeting of proteins to the thylakoid lumen. These aspects of protein transport into chloroplasts are discussed in the context of recent studies on protein uptake by mitochondria.