Inheritance and expression of chimeric genes in the progeny of transgenic maize plants

A key to totipotency: Wuschel-like homeobox 2a unlocks embryogenic culture response in maize (Zea mays L.)

The ability of plant somatic cells to dedifferentiate, form somatic embryos and regenerate whole plants in vitro has been harnessed for both clonal propagation and as a key component of plant genetic engineering systems. Embryogenic culture response is significantly limited, however, by plant genotype in most species. This impedes advancements in both plant transformation-based functional genomics research and crop improvement efforts. We utilized natural variation among maize inbred lines to genetically map somatic embryo generation potential in tissue culture and identify candidate genes underlying totipotency. Using a series of maize lines derived from crosses involving the culturable parent A188 and the non-responsive parent B73, we identified a region on chromosome 3 associated with embryogenic culture response and focused on three candidate genes within the region based on genetic position and expression pattern. Two candidate genes showed no effect when ectopically expressed in B73, but the gene Wox2a was found to induce somatic embryogenesis and embryogenic callus proliferation. Transgenic B73 cells with strong constitutive expression of the B73 and A188 coding sequences of Wox2a were found to produce somatic embryos at similar frequencies, demonstrating that sufficient expression of either allele could rescue the embryogenic culture phenotype. Transgenic B73 plants were regenerated from the somatic embryos without chemical selection and no pleiotropic effects were observed in the Wox2a overexpression lines in the regenerated T0 plants or in the two independent events which produced T1 progeny. In addition to linking natural variation in tissue culture response to Wox2a, our data support the utility of Wox2a in enabling transformation of recalcitrant genotypes.

Sulfadiazine and phosphinothricin selection systems optimised for the transformation of tobacco BY-2 cells

We extended the applicability of the BY-2 cell line as a model by introducing two new selection systems. Our protocol provides guidelines for optimising Basta selection in other recalcitrant models. Tobacco BY-2 cell line is the most commonly used cytological model in plant research. It is uniform, can be simply treated by chemicals, synchronised and easily transformed. However, only a few selection systems are available that complicate advanced studies using multiple stacked transgenes and extensive gene editing. In our work, we adopted for BY-2 cell line two other selection systems: sulfadiazine and phosphinothricin (PPT, an active ingredient of Basta herbicide). We show that sulfadiazine can be used in a wide range of concentrations. It is suitable for co-transformation and subsequent double selection with kanamycin or hygromycin, which are standardly used for BY-2 transformation. We also have domesticated the sulfadiazine resistance for the user-friendly GoldenBraid cloning system. Compared to sulfadiazine, establishing selection on phosphinothricin was considerably more challenging. It did not work in any concentration of PPT with standardly cultured cells. Since the selection is based on blocking glutamine synthetase and consequent ammonium toxicity and deficiency of assimilated nitrogen, we tried to manipulate nitrogen availability. We found that the PPT selection reliably works only with nitrogen-starved cells with reduced nitrate reserves that are selected on a medium without ammonium nitrate. Both these adjustments prevent the release of large amounts of ammonium, which can toxify the entire culture in the case of standardly cultured cells. Since high nitrogen reserves can be a common feature of in vitro cultures grown on MS media, nitrogen starvation could be a key step in establishing phosphinothricin resistance in other plant models.

Commercial scale genetic transformation of mature seed embryo explants in maize

A novel, efficient maize genetic transformation system was developed using Agrobacterium-mediated transformation of embryo explants from mature seeds. Seeds from field grown plants were sterilized and crushed to isolate embryo explants consisting of the coleoptile, leaf primordia, and shoot apical meristem which were then purified from the ground seed bulk preparation. The infection of relevant tissues of seed embryo explants (SEEs) by Agrobacterium was improved by the centrifugation of the explants. Transgenic plants were obtained by multiple bud induction on high cytokinin media, followed by plant regeneration on hormone-free medium. Three different selectable markers (cp4 epsps, aadA, and nptII) were successfully used for producing transgenic plants. Stable integration of transgenes in the maize genome was demonstrated by molecular analyses and germline transmission of the inserted transgenes to the next generation was confirmed by pollen segregation and progeny analysis. Phenotypic evidence for chimeric transgenic tissue was frequently observed in initial experiments but was significantly reduced by including a second bud induction step with optimized cytokinin concentration. Additional improvements, including culturing explants at an elevated temperature during bud induction led to the development of a revolutionary system for efficient transgenic plant production and genome editing. To our knowledge, this is the first report of successful transgenic plant regeneration through Agrobacterium-mediated transformation of maize mature SEEs. This system starts with mature seed that can be produced in large volumes and the SEEs explants are storable. It has significant advantages in terms of scalability and flexibility over methods that rely on immature explants.

Morphogenic Regulators and Their Application in Improving Plant Transformation

Generation of plant lines with transgene or edited gene variants is the desired outcome of transformation technology. Conventional DNA-based plant transformation methods are the most commonly used technology but these approaches are limited to a small number of plant species with efficient transformation systems. The ideal transformation technologies are those that allow biotechnology applications across wide genetic background, especially within elite germplasm of major crop species. This chapter will briefly review key regulatory genes involved in plant morphogenesis with a focus on in vitro somatic embryogenesis and their application in improving plant transformation.

Genetic Transformation in Peach (Prunus persica L.): Challenges and Ways Forward

Almost 30 years have passed since the first publication reporting regeneration of transformed peach plants. Nevertheless, the general applicability of genetic transformation of this species has not yet been established. Many strategies have been tested in order to obtain an efficient peach transformation system. Despite the amount of time and the efforts invested, the lack of success has significantly limited the utility of peach as a model genetic system for trees, despite its relatively short generation time; small, high-quality genome; and well-studied genetic resources. Additionally, the absence of efficient genetic transformation protocols precludes the application of many biotechnological tools in peach breeding programs. In this review, we provide an overview of research on regeneration and genetic transformation in this species and summarize novel strategies and procedures aimed at producing transgenic peaches. Promising future approaches to develop a robust peach transformation system are discussed, focusing on the main bottlenecks to success including the low efficiency of A. tumefaciens-mediated transformation, the low level of correspondence between cells competent for transformation and those that have regenerative competence, and the high rate of chimerism in the few shoots that are produced following transformation.

Co-culturing on dry filter paper significantly increased the efficiency of Agrobacterium-mediated transformations of maize immature embryos

In the Agrobacterium tumefaciens-mediated transformations of maize immature embryos (IEs), the common co-culturing media used are MS or N6-based (MC). Here, we used a novel co-culturing method in which maize 'Qi319' IEs inoculated with Agrobacterium-harboring target vector were placed on dry filter paper (DC) in a petri dish. To compare the effects of the DC and MC co-culturing methods on transformation efficiency, we designed three experiments: (1) A. tumefaciens strain AGL1 independently carrying two plasmids, pXQD12 and pXQD70; (2) two A. tumefaciens strains, AGL1 and EHA105, carrying pXQD12; and (3) strains AGL1 and EHA105 each independently inoculated with pXQD12 and pXQD70 for different infiltration periods, 5, 10, 15, 20 and 25 min. We used A. tumefaciens to inoculate IEs derived from maize ears 9-15 d after pollination, and then IEs were placed in petri dishes for co-culturing. The DC treatment significantly increased the percentage of IEs expressing green fluorescence protein (%GFP), indicating positive transformants. DC-treated IEs had ~ 3 to 4 times the %GFP compared with MC-treated IEs at 8 d after inoculation (3 d co-culture and 5 d restoration). The average regeneration frequency (%GFP positive regenerated calli of infected IEs) and stable transformation frequency (%GFP positive T₀ plants of infected IEs) significantly increased with the DC treatment. Thus, the DC method may be used to develop a more efficient Agrobacterium-mediated transformation method for maize IEs.

Characterization of Agronomy, Grain Physicochemical Quality, and Nutritional Property of High-Lysine 35R Transgenic Rice with Simultaneous Modification of Lysine Biosynthesis and Catabolism

Lysine is the first limiting essential amino acid in rice. We previously constructed a series of transgenic rice lines to enhance lysine biosynthesis (35S), down-regulate its catabolism (Ri), or simultaneously achieve both metabolic effects (35R). In this study, nine transgenic lines, three from each group, were selected for both field and animal feeding trials. The results showed that the transgene(s) caused no obvious effects on field performance and main agronomic traits. Mature seeds of transgenic line 35R-17 contained 48-60-fold more free lysine than in wild type and had slightly lower apparent amylose content and softer gel consistency. Moreover, a 35-day feeding experiment showed that the body weight gain, food efficiency, and protein efficiency ratio of rats fed the 35R-17 transgenic rice diet were improved when compared with those fed wild-type rice diet. These data will be useful for further evaluation and potential commercialization of 35R high-lysine transgenic rice.

Advances in Maize Transformation Technologies and Development of Transgenic Maize

Maize is the principal grain crop of the world. It is also the crop where genetic engineering has been employed to a great extent to improve its various traits. The ability to transform maize is a crucial step for application of gene technology in maize improvement. There have been constant improvements in the maize transformation technologies over past several years. The choice of genotype and the explant material to initiate transformation and the different types of media to be used in various stages of tissue culture can have significant impact on the outcomes of the transformation efforts. Various methods of gene transfer, like the particle bombardment, protoplast transformation, Agrobacterium-mediated, in planta transformation, etc., have been tried and improved over years. Similarly, various selection systems for retrieval of the transformants have been attempted. The commercial success of maize transformation and transgenic development is unmatched by any other crop so far. Maize transformation with newer gene editing technologies is opening up a fresh dimension in transformation protocols and work-flows. This review captures the various past and recent facets in improvement in maize transformation technologies and attempts to present a comprehensive updated picture of the current state of the art in this area.

Advances in Agrobacterium tumefaciens-mediated genetic transformation of graminaceous crops

Steady increase in global population poses several challenges to plant science research, including demand for increased crop productivity, grain yield, nutritional quality and improved tolerance to different environmental factors. Transgene-based approaches are promising to address these challenges by transferring potential candidate genes to host organisms through different strategies. Agrobacterium-mediated gene transfer is one such strategy which is well known for enabling efficient gene transfer in both monocot and dicots. Due to its versatility, this technique underwent several advancements including development of improved in vitro plant regeneration system, co-cultivation and selection methods, and use of hyper-virulent strains of Agrobacterium tumefaciens harbouring super-binary vectors. The efficiency of this method has also been enhanced by the use of acetosyringone to induce the activity of vir genes, silver nitrate to reduce the Agrobacterium-induced necrosis and cysteine to avoid callus browning during co-cultivation. In the last two decades, extensive efforts have been invested towards achieving efficient Agrobacterium-mediated transformation in cereals. Though high-efficiency transformation systems have been developed for rice and maize, comparatively lesser progress has been reported in other graminaceous crops. In this context, the present review discusses the progress made in Agrobacterium-mediated transformation system in rice, maize, wheat, barley, sorghum, sugarcane, Brachypodium, millets, bioenergy and forage and turf grasses. In addition, it also provides an overview of the genes that have been recently transferred to these graminaceous crops using Agrobacterium, bottlenecks in this technique and future possibilities for crop improvement.

Agrobacterium-mediated in planta genetic transformation of sugarcane setts

An efficient, reproducible, and genotype-independent in planta transformation has been developed for sugarcane using setts as explant. Traditional Agrobacterium-mediated genetic transformation and in vitro regeneration of sugarcane is a complex and time-consuming process. Development of an efficient Agrobacterium-mediated transformation protocol, which can produce a large number of transgenic plants in short duration is advantageous. Hence, in the present investigation, we developed a tissue culture-independent in planta genetic transformation system for sugarcane using setts collected from 6-month-old sugarcane plants. The sugarcane setts (nodal cuttings) were infected with three Agrobacterium tumefaciens strains harbouring pCAMBIA 1301-bar plasmid, and the transformants were selected against BASTA(®). Several parameters influencing the in planta transformation such as A. tumefaciens strains, acetosyringone, sonication and exposure to vacuum pressure, have been evaluated. The putatively transformed sugarcane plants were screened by GUS histochemical assay. Sugarcane setts were pricked and sonicated for 6 min and vacuum infiltered for 2 min at 500 mmHg in A. tumefaciens C58C1 suspension containing 100 µM acetosyringone, 0.1 % Silwett L-77 showed the highest transformation efficiency of 29.6 % (with var. Co 62175). The three-stage selection process completely eliminated the chimeric transgenic sugarcane plants. Among the five sugarcane varieties evaluated using the standardized protocol, var. Co 6907 showed the maximum transformation efficiency (32.6 %). The in planta transformation protocol described here is applicable to transfer the economically important genes into different varieties of sugarcane in relatively short time.

YeATS - a tool suite for analyzing RNA-seq derived transcriptome identifies a highly transcribed putative extensin in heartwood/sapwood transition zone in black walnut

The transcriptome provides a functional footprint of the genome by enumerating the molecular components of cells and tissues. The field of transcript discovery has been revolutionized through high-throughput mRNA sequencing (RNA-seq). Here, we present a methodology that replicates and improves existing methodologies, and implements a workflow for error estimation and correction followed by genome annotation and transcript abundance estimation for RNA-seq derived transcriptome sequences (YeATS - Yet Another Tool Suite for analyzing RNA-seq derived transcriptome). A unique feature of YeATS is the upfront determination of the errors in the sequencing or transcript assembly process by analyzing open reading frames of transcripts. YeATS identifies transcripts that have not been merged, result in broken open reading frames or contain long repeats as erroneous transcripts. We present the YeATS workflow using a representative sample of the transcriptome from the tissue at the heartwood/sapwood transition zone in black walnut. A novel feature of the transcriptome that emerged from our analysis was the identification of a highly abundant transcript that had no known homologous genes (GenBank accession: KT023102). The amino acid composition of the longest open reading frame of this gene classifies this as a putative extensin. Also, we corroborated the transcriptional abundance of proline-rich proteins, dehydrins, senescence-associated proteins, and the DNAJ family of chaperone proteins. Thus, YeATS presents a workflow for analyzing RNA-seq data with several innovative features that differentiate it from existing software.

YeATS - a tool suite for analyzing RNA-seq derived transcriptome identifies a highly transcribed putative extensin in heartwood/sapwood transition zone in black walnut

The transcriptome provides a functional footprint of the genome by enumerating the molecular components of cells and tissues. The field of transcript discovery has been revolutionized through high-throughput mRNA sequencing (RNA-seq). Here, we present a methodology that replicates and improves existing methodologies, and implements a workflow for error estimation and correction followed by genome annotation and transcript abundance estimation for RNA-seq derived transcriptome sequences (YeATS - Yet Another Tool Suite for analyzing RNA-seq derived transcriptome). A unique feature of YeATS is the upfront determination of the errors in the sequencing or transcript assembly process by analyzing open reading frames of transcripts. YeATS identifies transcripts that have not been merged, result in broken open reading frames or contain long repeats as erroneous transcripts. We present the YeATS workflow using a representative sample of the transcriptome from the tissue at the heartwood/sapwood transition zone in black walnut. A novel feature of the transcriptome that emerged from our analysis was the identification of a highly abundant transcript that had no known homologous genes (GenBank accession: KT023102). The amino acid composition of the longest open reading frame of this gene classifies this as a putative extensin. Also, we corroborated the transcriptional abundance of proline-rich proteins, dehydrins, senescence-associated proteins, and the DNAJ family of chaperone proteins. Thus, YeATS presents a workflow for analyzing RNA-seq data with several innovative features that differentiate it from existing software.

Effect of Agrobacterium strain and plasmid copy number on transformation frequency, event quality and usable event quality in an elite maize cultivar

Improving Agrobacterium -mediated transformation frequency and event quality by increasing binary plasmid copy number and appropriate strain selection is reported in an elite maize cultivar. Agrobacterium-mediated maize transformation is a well-established method for gene testing and for introducing useful traits in a commercial biotech product pipeline. To develop a highly efficient maize transformation system, we investigated the effect of two Agrobacterium tumefaciens strains and three different binary plasmid origins of replication (ORI) on transformation frequency, vector backbone insertion, single copy event frequency (percentage of events which are single copy for all transgenes), quality event frequency (percentage of single copy events with no vector backbone insertions among all events generated; QE) and usable event quality frequency (transformation frequency times QE frequency; UE) in an elite maize cultivar PHR03. Agrobacterium strain AGL0 gave a higher transformation frequency, but a reduced QE frequency than LBA4404 due to a higher number of vector backbone insertions. Higher binary plasmid copy number positively correlated with transformation frequency and usable event recovery. The above findings can be exploited to develop high-throughput transformation protocols, improve the quality of transgenic events in maize and other plants.

Agrobacterium-mediated high-frequency transformation of an elite commercial maize (Zea mays L.) inbred line

An improved Agrobacterium -mediated transformation protocol is described for a recalcitrant commercial maize elite inbred with optimized media modifications and AGL1. These improvements can be applied to other commercial inbreds. This study describes a significantly improved Agrobacterium-mediated transformation protocol in a recalcitrant commercial maize elite inbred, PHR03, using optimal co-cultivation, resting and selection media. The use of green regenerative tissue medium components, high copper and 6-benzylaminopurine, in resting and selection media dramatically increased the transformation frequency. The use of glucose in resting medium further increased transformation frequency by improving the tissue induction rate, tissue survival and tissue proliferation from immature embryos. Consequently, an optimal combination of glucose, copper and cytokinin in the co-cultivation, resting and selection media resulted in significant improvement from 2.6 % up to tenfold at the T0 plant level using Agrobacterium strain LBA4404 in transformation of PHR03. Furthermore, we evaluated four different Agrobacterium strains, LBA4404, AGL1, EHA105, and GV3101 for transformation frequency and event quality. AGL1 had the highest transformation frequency with up to 57.1 % at the T0 plant level. However, AGL1 resulted in lower quality events (defined as single copy for transgenes without Agrobacterium T-DNA backbone) when compared to LBA4404 (30.1 vs 25.6 %). We propose that these improvements can be applied to other recalcitrant commercial maize inbreds.

Maize transformation technology development for commercial event generation

Maize is an important food and feed crop in many countries. It is also one of the most important target crops for the application of biotechnology. Currently, there are more biotech traits available on the market in maize than in any other crop. Generation of transgenic events is a crucial step in the development of biotech traits. For commercial applications, a high throughput transformation system producing a large number of high quality events in an elite genetic background is highly desirable. There has been tremendous progress in Agrobacterium-mediated maize transformation since the publication of the Ishida et al. (1996) paper and the technology has been widely adopted for transgenic event production by many labs around the world. We will review general efforts in establishing efficient maize transformation technologies useful for transgenic event production in trait research and development. The review will also discuss transformation systems used for generating commercial maize trait events currently on the market. As the number of traits is increasing steadily and two or more modes of action are used to control key pests, new tools are needed to efficiently transform vectors containing multiple trait genes. We will review general guidelines for assembling binary vectors for commercial transformation. Approaches to increase transformation efficiency and gene expression of large gene stack vectors will be discussed. Finally, recent studies of targeted genome modification and transgene insertion using different site-directed nuclease technologies will be reviewed.

Agrobacterium tumefaciens-mediated in planta seed transformation strategy in sugarcane

An efficient, reproducible and genotype-independent in planta transformation has been standardized for sugarcane using seed as explant. Transgenic sugarcane production through Agrobacterium infection followed by in vitro regeneration is a time-consuming process and highly genotype dependent. To obtain more number of transformed sugarcane plants in a relatively short duration, sugarcane seeds were infected with Agrobacterium tumefaciens EHA 105 harboring pCAMBIA 1304-bar and transformed plants were successfully established without undergoing in vitro regeneration. Various factors affecting sugarcane seed transformation were optimized, including pre-culture duration, acetosyringone concentration, surfactants, co-cultivation, sonication and vacuum infiltration duration. The transformed sugarcane plants were selected against BASTA(®) and screened by GUS and GFP visual assay, PCR and Southern hybridization. Among the different combinations and concentrations tested, when 12-h pre-cultured seeds were sonicated for 10 min and 3 min vacuum infiltered in 100 µM acetosyringone and 0.1 % Silwett L-77 containing Agrobacterium suspension and co-cultivated for 72-h showed highest transformation efficiency. The amenability of the standardized protocol was tested on five genotypes. It was found that all the tested genotypes responded favorably, though CoC671 proved to be the best responding cultivar with 45.4 % transformation efficiency. The developed protocol is cost-effective, efficient and genotype independent without involvement of any tissue culture procedure and can generate a relatively large number of transgenic plants in approximately 2 months.

selection of transgenic sugarcane callus utilizing a plant gene encoding a mutant form of acetolactate synthase

Selection genes are routinely used in plant genetic transformation protocols to ensure the survival of transformed cells by limiting the regeneration of non-transgenic cells. In order to find alternatives to the use of antibiotics as selection agents, we followed a targeted approach utilizing a plant gene, encoding a mutant form of the enzyme acetolactate synthase, to convey resistance to herbicides. The sensitivity of sugarcane callus (Saccharum spp. hybrids, cv. NCo310) to a number of herbicides from the sulfonylurea and imidazolinone classes was tested. Callus growth was most affected by sulfonylurea herbicides, particularly 3.6 μg/l chlorsulfuron. Herbicide-resistant transgenic sugarcane plants containing mutant forms of a tobacco acetolactate synthase (als) gene were obtained following biolistic transformation. Post-bombardment, putative transgenic callus was selectively proliferated on MS medium containing 3 mg/l 2,4-dichlorophenoxyacetic acid (2,4-D), 20 g/l sucrose, 0.5 g/l casein, and 3.6 μg/l chlorsulfuron. Plant regeneration and rooting was done on MS medium lacking 2,4-D under similar selection conditions. Thirty vigorously growing putative transgenic plants were successfully ex vitro-acclimatized and established under glasshouse conditions. Glasshouse spraying of putative transgenic plants with 100 mg/l chlorsulfuron dramatically decreased the amount of non-transgenic plants that had escaped the in vitro selection regime. PCR analysis showed that six surviving plants were als-positive and that five of these expressed the mutant als gene. This report is the first to describe a selection system for sugarcane transformation that uses a selectable marker gene of plant origin targeted by a sulfonylurea herbicide.

Genetic transformation using maize immature zygotic embryos

Epidermal and subepidermal cells in the abaxial, basal region of the maize (Zea mays L.) immature zygotic embryo (IZE) scutellum can be induced by exogenous auxin to proliferate and undergo somatic embryogenesis. Successful genetic transformation of IZEs depends not only on optimizing transformation parameters for these totipotent cells, but also on achieving high embryogenic callus induction frequency (ECIF) in a population of targeted explants. In maize, ECIF is strongly influenced by genotype, the tissue culture media used, and the interaction of these two factors. Altering tissue culture media components to increase ECIF and/or transformation frequency (TF) has been one approach used to expand the range of maize genotypes amenable to genetic transformation using the IZE. This chapter outlines such an approach--an Agrobacterium-mediated transformation protocol is used for direct-targeting IZEs of the hybrid Hi Type II and inbred B104 lines. Two different media regimes are used for successful culture and transformation of two distinct genotypes.

Agrobacterium-mediated maize transformation: immature embryos versus callus

Transformation with Agrobacterium tumefaciens is the preferred method for delivery of transgenes into a wide range of plant species including maize. Optimized protocols for the Agrobacterium-mediated transformation of freshly isolated immature embryos and embryogenic Type I callus derived from plant seedlings are described. These protocols are suitable for the transformation of a wide variety of corn genotypes including commercial inbred lines. Agrobacterium harboring a binary vector containing the neomycin phosphotransferase (nptII) or the glyphosate resistant 5-enolpyruvylshikimate-3-phosphate (EPSPS) as selectable marker genes and also the green fluorescence protein gene (gfp) have been used. GFP is a visual screening marker which allows tracking of transformation during different selection and regeneration steps. The described protocols provide double digit transformation frequencies and can be routinely used for the production of a large numbers of transgenic plants.

Silicon carbide whisker-mediated embryogenic callus transformation of cotton (Gossypium hirsutum L.) and regeneration of salt tolerant plants

A silicon carbide whisker-mediated gene transfer system with recovery of fertile and stable transformants was developed for cotton (Gossypium hirsutum L.) cv. Coker-312. Two-month-old hypocotyl-derived embryogenic/non-embryogenic calli at different days after subculture were treated with silicon carbide whiskers for 2 min in order to deliver pGreen0029 encoding GUS gene and pRG229 AVP1 gene, encoding Arabidopsis vacuolar pyrophosphatase, having neomycin phosphotransferaseII (nptII) genes as plant-selectable markers. Three crucial transformation parameters, i.e., callus type, days after subculture and selection marker concentration for transformation of cotton calli were evaluated for optimum efficiency of cotton embryogenic callus transformation giving upto 94% transformation efficiency. Within six weeks, emergence of kanamycin-resistant (kmr) callus colonies was noted on selection medium. GUS and Southern blot analysis showed expression of intact and multiple transgene copies in the transformed tissues. Kanamycin wiping of leaves from T1, T2, and T3 progeny plants revealed that transgenes were inherited in a Mendelian fashion. Salt treatment of T1 AVP1 transgenic cotton plants showed significant enhancement in salt tolerance as compared to control plants. Thus far, this is first viable physical procedure after particle bombardment available for cotton that successfully can be used to generate fertile cotton transformants.

Induction of a polyubiquitin gene promoter by dehydration stresses in transformed rice cells

The expression of the maize polyubiquitin gene promoter UBI1 in rice cells has been used to study the involvement of ubiquitin in cell protection responses to dehydration caused by osmotic, saline or freezing stress. The effect of these stresses on UBI1 activity was investigated by the use of stably transformed rice calli (UBI1:GUS), as well as by transient expression experiments performed with cell lines with high or low tolerance to each type of stress. The theoretical analysis of the UBI1 promoter shows several putative stress-regulated boxes that could account for the stress-related UBI1 induction pattern described in this work. We suggest that the study of the differential UBI1 promoter-driven expression in rice cell lines with different level of tolerance to stress might be useful to elucidate complex signal transduction pathways in response to dehydration stresses in monocots.

Viability and bar expression are negatively correlated in Oregon Wolfe Barley Dominant hybrids

The expression level of bar, which encodes phosphinothricin acetyltransferase (PAT), was correlated with the inviability of barley hybrids between 20 Golden Promise-derived transgenic lines (Ds-bar lines) and a specialized genetic marker stock, Oregon Wolfe Barley Dominant (OWBD). Each Ds-bar line was homozygous for a modified maize Ds element that encoded bar and that had been delivered via transposition to a unique location. All Ds-bar lines were viable and morphologically similar. Only four of the 20 hybrid populations were viable. The remaining populations died prior to producing seed. Phenotypic, enzyme-linked immunosorbent assay and quantitative reverse transcriptase-polymerase chain reaction analyses of these lines, and of lines from unrelated transformation events that also expressed bar, showed that viability was negatively correlated with bar expression. Analysis of crosses of a high-bar-expressing line with the OWB mapping population showed that the sensitivity of OWBD to PAT segregated as a single locus on chromosome 6HL. No sensitivity to PAT could be detected in several other lines and cultivars. OWBD has been shown to be genetically divergent from other germplasm groups within cultivated barley; therefore, the observed sensitivity may be peculiar to OWBD and thus would not impact generally on the utility of bar as a selectable marker or source of herbicide resistance in barley. Nevertheless, these results demonstrate the extent of allelic variability present in Hordeum vulgare, and suggest an additional variable for consideration when devising protocols for the transformation of Hordeum cultivars or landraces that are not known to be tolerant to PAT.

A maize resistance gene functions against bacterial streak disease in rice

Although cereal crops all belong to the grass family (Poacea), most of their diseases are specific to a particular species. Thus, a given cereal species is typically resistant to diseases of other grasses, and this nonhost resistance is generally stable. To determine the feasibility of transferring nonhost resistance genes (R genes) between distantly related grasses to control specific diseases, we identified a maize R gene that recognizes a rice pathogen, Xanthomonas oryzae pv. oryzicola, which causes bacterial streak disease. Bacterial streak is an important disease of rice in Asia, and no simply inherited sources of resistance have been identified in rice. Although X. o. pv. oryzicola does not cause disease on maize, we identified a maize gene, Rxo1, that conditions a resistance reaction to a diverse collection of pathogen strains. Surprisingly, Rxo1 also controls resistance to the unrelated pathogen Burkholderia andropogonis, which causes bacterial stripe of sorghum and maize. The same gene thus controls resistance reactions to both pathogens and nonpathogens of maize. Rxo1 has a nucleotide-binding site-leucine-rich repeat structure, similar to many previously identified R genes. Most importantly, Rxo1 functions after transfer as a transgene to rice, demonstrating the feasibility of nonhost R gene transfer between cereals and providing a valuable tool for controlling bacterial streak disease.

Mutant acetolactate synthase gene is an efficient in vitro selectable marker for the genetic transformation of Brassica juncea (oilseed mustard)

We report in this study, the successful deployment of a double mutant acetolactate synthase gene (ALSdm, containing Pro 197 to Ser and Ser 653 to Asn substitutions) as an efficient in vitro selection marker for the development of transgenic plants in Brassica juncea (oilseed mustard). The ALS enzyme is inhibited by two categories of herbicides, sulfonylureas (e.g. chlorsulfuron) and imidazolinones (e.g. imazethapyr), while the mutant forms are resistant to the same. Three different selection agents (kanamycin, chlorsulfuron and imazethapyr) were tested for in vitro selection efficiency in two B. juncea cultivars, RLM198 and Varuna. For both the cultivars, higher transformation frequencies were obtained using chlorsulfuron (3.8 +/- 0.6% and 4.6 +/- 0.9% for RLM198 and Varuna, respectively) and imazethapyr (10.2 +/- 0.7% for RLM198 and 7.8 +/- 1.2% for Varuna) as compared to that obtained on kanamycin (3.1 +/- 0.2% and 2.8 +/- 0.5% for RLM198 and Varuna, respectively). Additionally, transformation frequencies were higher on imazethapyr than on chlorsulfuron for both the cultivars indicating that imidazolinones are better selective agents than sulfonylureas for the selection of mustard transgenics.

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.

Factors affecting the genetic engineering of plants by microprojectile bombardment

Since its development in the mid-1980s, microprojectile bombardment has been widely employed as a method for direct gene transfer into a wide range of plants, including the previously difficult-to-transform monocotyledonous species. Although the numerous instruments available for microprojectile-mediated gene delivery and their applications have been widely discussed, less attention has been paid to the critical factors which affect the efficiency of this method of gene delivery. In this review we do not wish to describe the array of devices used for microprojectile delivery or their uses which have already been definitively described, but instead wish to report on research developments investigating the factors which affect microprojectile-mediated transformation of plants.

Foreign gene delivery into monocotyledonous species

Monocotyledonous plants are generally more recalcitrant to genetic transformation than dicotyledonous species. The absence of reliable Agrobacterium-mediated transformation methods and the difficulties associated with the culture of monocotyledonous tissues in vitro are mainly responsible for this situation. Until recently, the genetic transformation of monocotyledons was essentially performed by direct transfer of DNA into regenerable protoplasts or intact cells cultured in vitro, via polyethylene glycol treatment, electroporation or particle bombardment. Since 1990, the use of particle gun technology has revolutionized the genetic engineering of monocotyledonous species, allowing transformation to be more independent of the in vitro culture requirements. Today, at least one genotype of each major monocotyledonous crop species, including cereals, can be genetically transformed.

Development of protoporphyrinogen oxidase as an efficient selection marker for Agrobacterium tumefaciens-mediated transformation of maize

In this article, we report the isolation of plant protoporphyrinogen oxidase (PPO) genes and the isolation of herbicide-tolerant mutants. Subsequently, an Arabidopsis double mutant (Y426M + S305L) was used to develop a selectable marker system for Agrobacterium tumefaciens-mediated transformation of maize (Zea mays) and to obtain multiple events tolerant to the PPO family of herbicides. Maize transformants were produced via butafenacil selection using a flexible light regime to increase selection pressure. Butafenacil selection per se did not change transgene copy number distribution relative to other selectable marker systems, but the most tolerant events identified in the greenhouse were more likely to contain multiple copies of the introduced mutant PPO gene. To date, more than 2,500 independent transgenic maize events have been produced using butafenacil selection. The high frequency of A. tumefaciens-mediated transformation via PPO selection enabled us to obtain single-copy transgenic maize lines tolerant to field levels of butafenacil.

Integration, expression and inheritance of transgenes in hexaploid oat (Avena sativa L.)

Two oat varieties, Melys (spring variety) and Bulwark (winter variety) were transformed by particle bombardment of primary embryogenic callus using either a ubi-bar-ubi-gus co-integration vector or co-transformed (Melys) with a ubi-bar plasmid together with one of three plasmids containing the beta-glucuronidase (gus) gene under the control of either a rice actin promoter, a CaMV35S promoter or a wheat high molecular weight glutenin promoter. Morphologically normal and fertile transgenic plants were regenerated following callus selection with glufosinate ammonium. Evidence for the integration and functioning of the selectable (bar) and reporter (gus) genes in T0 and T1 plants was confirmed by PCR, Southern hybridisation, fluorescence in situ hybridisation (FISH), histochemical assays, and by progeny analysis. Transformation rates varied from 0.2 to 5.0 lines/plate of callus bombarded, with co-transformation frequencies of 83 to 100%, and co-expression frequencies of 60 to 100%. Copy numbers for the bar and gus gene varied from 3 to 17 and from 2 to 20 respectively. Cell and tissue specific expression of the gus gene was evident from the different promoters, with the HMW glutenin promoter showing endosperm specific expression in T1 seed. No expression of the gus gene under the CaMV35S promoter was detected in any tissues. Progeny analysis provided evidence of Mendelian inheritance of the introduced genes suggesting either one or two unlinked integration sites. This was confirmed by fluorescence in situ hybridisation to chromosome spread preparations. No segregation of the gus gene from the bar gene was observed in any of the progeny derived from co-transformation.

Microparticle bombardment as a tool in plant science and agricultural biotechnology

Microparticle bombardment technology has evolved as a method for delivering exogenous nucleic acids into plant cells and is a commonly employed technique in plant science. Desired genetic material is precipitated onto micron-sized metal particles and placed within one of a variety of devices designed to accelerate these "microcarriers" to velocities required to penetrate the plant cell wall. In this manner, transgenes can be delivered into the cell's genome or plastome. Since the late 1980s microparticle bombardment has become a powerful tool for the study of gene expression and production of stably transformed tissues and whole transgenic plants for experimental purposes and agricultural applications. This paper reviews development and application of the technology, including the protocols and mechanical systems employed as delivery systems, and the types of plant cells and culture systems employed to generate effective "targets" for receiving the incoming genetic material. Current understanding of how the exogenous DNA becomes integrated into the plant's native genetic background are assessed as are methods for improving the efficiency of this process. Pros and cons of particle bombardment technologies compared to alternative direct gene transfer methods and Agrobacterium based transformation systems are discussed.

High efficiency transgene segregation in co-transformed maize plants using an Agrobacterium tumefaciens 2 T-DNA binary system

For regulatory issues and research purposes it would be desirable to have the ability to segregate transgenes in co-transformed maize. We have developed a highly efficient system to segregate transgenes in maize that was co-transformed using an Agrobacterium tumefaciens 2 T-DNA binary system. Three vector treatments were compared in this study; (1) a 2 T-DNA vector, where the selectable marker gene bar (confers resistance to bialaphos) and the beta-glucuronidase (GUS) reporter gene are on two separate T-DNA's contained on a single binary vector; (2) a mixed strain treatment, where bar and GUS are contained on single T-DNA vectors in two separate Agrobacterium strains; (3) and a single T-DNA binary vector containing both bar and GUS as control treatment. Bialaphos resistant calli were generated from 52 to 59% of inoculated immature embryos depending on treatment. A total of 93.4% of the bialaphos selected calli from the 2 T-DNA vector treatment exhibited GUS activity compared to 11.7% for the mixed strain treatment and 98.2% for the cis control vector treatment. For the 2 T-DNA vector treatment, 86.7% of the bialaphos resistant/GUS active calli produced R0 plants exhibiting both transgenic phenotypes compared to 10% for the mixed strain treatment and 99% for the single T-DNA control vector treatment. A total of 87 Liberty herbicide (contains bialaphos as the active ingredient) resistant/GUS active R0 events from the 2 T-DNA binary vector treatment were evaluated for phenotypic segregation of these traits in the R1 generation. Of these R0 events, 71.4% exhibited segregation of Liberty resistance and GUS activity in the R1 generation. A total of 64.4% of the R0 2 T-DNA vector events produced Liberty sensitive/GUS active (indicating selectable-marker-free) R1 progeny. A high frequency of phenotypic segregation was also observed using the mixed strain approach, but a low frequency of calli producing R0 plants displaying both transgenic phenotypes makes this method less efficient. Molecular analyses were then used to confirm that the observed segregation of R1 phenotypes were highly correlated to genetic segregation of the bar and GUS genes. A high efficiency system to segregate transgenes in co-transformed maize plants has now been demonstrated.

Resistance to wheat streak mosaic virus in transgenic wheat engineered with the viral coat protein gene

Wheat (Triticum aestivum) plants were stably transformed with the coat protein (CP) gene of wheat streak mosaic virus (WSMV) by the biolistic method. Eleven independently transformed plant lines were obtained and five were analyzed for gene expression and resistance to WSMV. One line showed high resistance to inoculations of two WSMV strains. This line had milder symptoms and lower virus titer than control plants after inoculation. After infection, new growth did not show symptoms. The observed resistance was similar to the 'recovery' type resistance described previously using WSMV NIb transgene and in other systems. This line looked morphologically normal but had an unusually high transgene copy number (approximately 90 copies per 2C homozygous genome). Northern hybridization analysis indicated a high level of degraded CP mRNA expression. However, no coat protein expression was detected.

An efficient method for dispersing Ds elements in the barley genome as a tool for determining gene function

To devise a method for function-based gene isolation and characterization in barley, we created a plasmid containing the maize Activator (Ac) transposase (AcTPase) gene and a negative selection gene, codA, and a plasmid containing Dissociation (Ds) inverted-repeat ends surrounding the selectable herbicide resistance gene, bar. These plasmids were used to stably transform barley (Hordeum vulgare). In vitro assays, utilizing a Ds-interrupted uidA reporter gene, were used to demonstrate high-frequency excisions of Ds when the uidA construct was introduced transiently into stably transformed, AcTPase-expressing plant tissue. Crosses were made between stably transformed plants expressing functional transposase under the transcriptional control of either the putative AcTPase promoter or the promoter and first intron from the maize ubiquitin (Ubi1) gene, and plants containing Ds-Ubi-bar. In F(1) plants from these crosses, low somatic and germinal transposition frequencies were observed; however, in F(2) progeny derived from individual selfed F(1) plants, up to 47% of the plants showed evidence of Ds transposition. Further analyses of F(3) plants showed that approximately 75% of the transposed Ds elements reinserted into linked locations and 25% into unlinked locations. Transposed Ds elements in plants lacking the AcTPase transposase gene could be reactivated by reintroducing the transposase gene through classical genetic crossing, making this system functional for targeted gene tagging and studies of gene function. During the analysis of F(3) plants we observed two mutant phenotypes in which the transposed Ds elements co-segregate with the new phenotype, suggesting the additional utility of such a system for tagging genes.

Field evaluation of seed production, shattering, and dormancy in hybrid populations of transgenic rice (Oryza sativa) and the weed, red rice (Oryza sativa)

The genetic and agronomic consequences of transferring glufosinate (Liberty) herbicide resistance from transgenic rice (Oryza sativa L.) lines to the noxious weed red rice (Oryza sativa L.) were evaluated under field conditions. Replicated field trials in Louisiana (LA) and Arkansas (AR) were conducted in 1997 to evaluate ten vegetative and reproductive traits of eight F(2) populations produced from controlled crosses of two transgenic, glufosinate-resistant rice lines and four red rice biotypes. Plant vigor and plant density at both locations were similar among populations derived from either transgenic or non-transgenic parents. Significant differences in plant height and maturity were observed among LA populations produced from transgenic lines when compared to corresponding populations developed from non-transgenic material. However, values for these traits were not greater than those detected in the red rice biotypes. Seed dormancy and seed production were not significantly different at either location among transgenic and non-transgenic populations. Dominant Mendelian segregation of glufosinate resistance was detected in 40% of the populations evaluated. Results of this study indicated that those populations segregating for glufosinate resistance responded in a location-specific manner with respect to life history and fecundity traits.

The maize streak virus coat protein transcription unit exhibits tissue-specific expression in transgenic rice

Maize streak geminivirus (MSV) is a single-stranded DNA virus that infects cereals and other grasses. A promoter region incorporating the MSV large intergenic region and movement protein gene sequence was ligated to the gus (beta-glucuronidase) reporter gene which replaced the virus coat protein (CP) gene. The CP promoter activity was analysed in transgenic rice plants (Oryza sativa L.) and was compared with that obtained in plants transformed with the gus gene downstream of the cauliflower mosaic virus (CaMV) 35S promoter. The MSV CP promoter activity varied in the five plant lines tested, but was always less than that of the CaMV promoter. Histochemistry showed that the MSV CP promoter was active in cells of regenerating callus but in regenerated plants it provided an expression pattern restricted to the vascular tissues of the root, stem, leaf and floral organs. Expression was highest in phloem-associated tissues of the vegetative organs and was absent from the tip and elongation region of seedling roots. Thus, the MSV CP promoter shows a degree of developmental regulation and can be used to confer tissue-specific expression in transgenic rice plants.

Improved biomass productivity and water use efficiency under water deficit conditions in transgenic wheat constitutively expressing the barley HVA1 gene

The ABA-responsive barley gene HVA1, a member of group 3 late embryogenesis abundant (LEA) protein genes, was introduced into spring wheat (Triticum aestivum L.) cv. Hi-Line using the biolistic bombardment method. High levels of expression of the HVA1 gene, regulated by the maize ubi1 promoter, were observed in leaves and roots of independent transgenic wheat plants and were inherited by offspring generations. T(3) progenies of four selected transgenic wheat lines were tested under greenhouse conditions for tolerance of soil water deficit. Potted plants were grown under moderate water deficit and well-watered conditions, respectively. Two homozygous and one heterozygous transgenic lines expressing the HVA1 gene had significantly (P<0.01) higher water use efficiency values, 0.66-0.68 g kg(-1), as compared to 0.57 and 0.53 g kg(-1), respectively, for the non-expressing transgenic and non-transgenic controls under moderate water deficit conditions. The two homozygous transgenic plant lines also had significantly greater total dry mass, root fresh and dry weights, and shoot dry weight compared to the two controls under soil water deficit conditions. Results of this study indicate that growth characteristics were improved in transgenic wheat plants constitutively expressing the barley HVA1 gene in response to soil water deficit.

Engineering chloroplasts: an alternative site for foreign genes, proteins, reactions and products

Plant genetic engineering via the nucleus is a mature technology that has been used very productively for research and commercial biotechnology. By contrast, the ability to introduce foreign genes at specific locations on a chloroplast's chromosome has been acquired relatively recently. Certain limitations of nuclear genome transformation methods might be overcome by the site-specific introduction of genes into plastid chromosomes. In addition, plastids, mitochondria and other subcellular organelles might provide more favorable environments than the nuclear-cytoplasmic compartment for certain biochemical reactions and for accumulating large amounts of some gene and enzyme products.

Expression of murine adenosine deaminase (ADA) in transgenic maize

A murine adenosine deaminase (ADA) gene, driven by the maize ubi-1 promoter and intron region, was transformed into embryogenic maize callus, along with a bar and gusA gene-containing plasmid, using microparticle bombardment. Selection in the presence of either the herbicide Basta or the adenosine analogue 2'-deoxyadenosine resulted in transgenic cultures that expressed GUS and accumulated a 41-kD protein that immunoprecipated with an ADA-specific polyclonal antibody. ADA enzyme activity was observed in extracts from transgenic callus as well as regenerated plants and progeny. Culltures expressing ADA grew in the presence of 200 mg/l 2'-deoxyadenosine, a concentration which completely inhibited the growth of non-transgenic cultures. ADA activity appeared to segregate in progency of regenerated plants as a single, dominant Mendelian trait. These results suggest that ADA, in combination with adenosine analogue selection, represents a potentially viable selectable marker system for transgenic maize production.

Transgenic DNA integrated into the oat genome is frequently interspersed by host DNA

Integration of transgenic DNA into the plant genome was investigated in 13 transgenic oat (Avena sativa L.) lines produced using microprojectile bombardment with one or two cotransformed plasmids. In all transformation events, the transgenic DNA integrated into the plant genome consisted of intact transgene copies that were accompanied by multiple, rearranged, and/or truncated transgene fragments. All fragments of transgenic DNA cosegregated, indicating that they were integrated at single gene loci. Analysis of the structure of the transgenic loci indicated that the transgenic DNA was interspersed by the host genomic DNA. The number of insertions of transgenic DNA within the transgene loci varied from 2 to 12 among the 13 lines. Restriction endonucleases that do not cleave the introduced plasmids produced restriction fragments ranging from 3.6 to about 60 kb in length hybridizing to a probe comprising the introduced plasmids. Although the size of the interspersing host DNA within the transgene locus is unknown, the sizes of the transgene-hybridizing restriction fragments indicated that the entire transgene locus must be at least from 35-280 kb. The observation that all transgenic lines analyzed exhibited genomic interspersion of multiple clustered transgenes suggests a predominating integration mechanism. We propose that transgene integration at multiple clustered DNA replication forks could account for the observed interspersion of transgenic DNA with host genomic DNA within transgenic loci.

Reduced fusarium ear rot and symptomless infection in kernels of maize genetically engineered for European corn borer resistance

ABSTRACT Field experiments were conducted in 1994, 1995, and 1996 to evaluate the incidence and severity of Fusarium ear rot and the incidence of symp-tomless Fusarium infection in kernels of maize hybrids genetically engineered with Bacillus thuringiensis genes encoding for the delta-endotoxin CryIA(b). Treatments included manual infestation with European corn borer (ECB) larvae and insecticide applications to limit ECB activity to specific maize growth stages or mimic standard ECB control practices. Fusarium symptoms and infection were affected by the specific cryIA(b) transformation used in each hybrid that determines tissue-specific expression of CryIA(b). In hybrids expressing CryIA(b) in kernels, incidence and severity of Fusarium ear rot and incidence of symptomless kernel infection were reduced compared with near-isogenic hybrids lacking cryIA(b) genes. In plants that were manually infested with ECB, ear rot incidence was reduced by 87, 58, and 68%; severity was reduced by 96, 54, and 64%; and incidence of kernel infection by Fusarium species was reduced by 17, 38, and 38% in 1994, 1995, and 1996, respectively. Results were similar in treatments that were not manually infested, but differences between transgenic and nontransgenic hybrids were smaller. Most kernel infection was due to F. moniliforme, F. proliferatum, and F. subglutinans (section Liseola) collectively, and it was within this group that transgenic hybrids exhibited reduced infection. Expression of CryIA(b) in plant tissues other than kernels did not consistently affect Fusarium symptoms or infection. Disease incidence was positively correlated with ECB damage to kernels. Insecticide applications also reduced Fusarium symptoms and infection when applied to nontransgenic plants.

PLANT TRANSFORMATION: Problems and Strategies for Practical Application

Plant transformation is now a core research tool in plant biology and a practical tool for cultivar improvement. There are verified methods for stable introduction of novel genes into the nuclear genomes of over 120 diverse plant species. This review examines the criteria to verify plant transformation; the biological and practical requirements for transformation systems; the integration of tissue culture, gene transfer, selection, and transgene expression strategies to achieve transformation in recalcitrant species; and other constraints to plant transformation including regulatory environment, public perceptions, intellectual property, and economics. Because the costs of screening populations showing diverse genetic changes can far exceed the costs of transformation, it is important to distinguish absolute and useful transformation efficiencies. The major technical challenge facing plant transformation biology is the development of methods and constructs to produce a high proportion of plants showing predictable transgene expression without collateral genetic damage. This will require answers to a series of biological and technical questions, some of which are defined.