Agrobacterium tumefaciens-mediated high frequency genetic transformation of an Indian cowpea (Vigna unguiculata L. Walp.) cultivar and transmission of transgenes into progeny

Recalcitrance to transformation, a hindrance for genome editing of legumes

Plant genome editing, a recently discovered method for targeted mutagenesis, has emerged as a promising tool for crop improvement and gene function research. Many genome-edited plants, such as rice, wheat, and tomato, have emerged over the last decade. As the preliminary steps in the procedure for genome editing involve genetic transformation, amenability to genome editing depends on the efficiency of genetic engineering. Hence, there are numerous reports on the aforementioned crops because they are transformed with relative ease. Legume crops are rich in protein and, thus, are a favored source of plant proteins for the human diet in most countries. However, legume cultivation often succumbs to various biotic/abiotic threats, thereby leading to high yield loss. Furthermore, certain legumes like peanuts possess allergens, and these need to be eliminated as these deprive many people from gaining the benefits of such crops. Further genetic variations are limited in certain legumes. Genome editing has the potential to offer solutions to not only combat biotic/abiotic stress but also generate desirable knock-outs and genetic variants. However, excluding soybean, alfalfa, and Lotus japonicus, reports obtained on genome editing of other legume crops are less. This is because, excluding the aforementioned three legume crops, the transformation efficiency of most legumes is found to be very low. Obtaining a higher number of genome-edited events is desirable as it offers the option to genotypically/phenotypically select the best candidate, without the baggage of off-target mutations. Eliminating the barriers to genetic engineering would directly help in increasing genome-editing rates. Thus, this review aims to compare various legumes for their transformation, editing, and regeneration efficiencies and discusses various solutions available for increasing transformation and genome-editing rates in legumes.

Using plant growth-promoting microorganisms (PGPMs) to improve plant development under in vitro culture conditions

The use of beneficial microorganisms improves the performance of in vitro - cultured plants through the improvement of plant nutrition, the biological control of microbial pathogens or the production of phytohormones that promote plant growth and development. Plant in vitro culture techniques are highly useful to obtain significant amounts of true-to-type and disease-free plant materials. One of these techniques is clonal micropropagation which consists on the establishment of shoot tip cultures, shoot multiplication, in vitro rooting and acclimatization to ex vitro conditions. However, in some cases, the existence of recalcitrant genotypes, with a compromised multiplication and rooting ability, or the difficulties to overcome the overgrowth of endophytic contaminations might seriously limit its efficiency. In this sense, the establishment of beneficial interactions between plants and plant growth-promoting microorganisms (PGPMs) under in vitro culture conditions might represent a valuable approach to efficiently solve those restrictions. During the last years, significant evidence reporting the use of beneficial microorganisms to improve the yield of in vitro multiplication or rooting as well as their acclimatization to greenhouse or soil conditions have been provided. Most of these positive effects are strongly linked to the ability of these microorganisms to provide in vitro plants with nutrients such as nitrogen or phosphorous, to produce plant growth regulators, to control the growth of pathogens or to mitigate stress conditions. The culture of A. thaliana under aseptic conditions has provided high-quality knowledge on the root development signaling pathways, involving hormones, triggered in the presence of PGPMs. Overall, the present article offers a brief overview of the use of microorganisms to improve in vitro plant performance during the in vitro micropropagation stages, as well as the main mechanisms of plant growth promotion associated with these microorganisms.

Tissue Culture- and Selection-Independent Agrobacterium tumefaciens-Mediated Transformation of a Recalcitrant Grain Legume, Cowpea (Vigna unguiculata L. Walp)

A simple and generally fast Agrobacterium-mediated transformation system with no tissue culture and selection steps has been developed for the first time in a recalcitrant food legume, cowpea. The approach involves wounding of 1-day-old germinated seeds with a needle or sonication either alone or in combination of vacuum infiltration with A. tumefaciens EH105 (pCAMBIA2301) carrying a β-glucuronidase (GUS) gene (uidA) and a neomycin phosphotransferase (nptII) gene for stable transformation. Sonicated and vacuum infiltrated seedlings showed the highest transient GUS activity in 90% of the explants. The sprouted co-cultured seeds directly established in soil and without selection were allowed to develop into plants which on maturity produced T₀ seeds. The presence of the alien genes, nptII and uidA in T₀ plants and their integration into the genome of T₁ plants were confirmed by polymerase chain reaction (PCR) and Southern blot analyses, respectively. The transgenes were inherited in the subsequent T₂ generation in a Mendelian fashion and their expression was confirmed by semi-quantitative PCR. The transformation frequency of 1.90% was obtained with sonication followed by vacuum infiltration with Agrobacterium. This approach provides favorable circumstances for the rapid meristem transformation and likely makes translational research ease in an important recalcitrant food legume, cowpea.

Developing a rapid and highly efficient cowpea regeneration, transformation and genome editing system using embryonic axis explants

Cowpea (Vigna unguiculata (L.) Walp.) is one of the most important legume crops planted worldwide, but despite decades of effort, cowpea transformation is still challenging due to inefficient Agrobacterium-mediated transfer DNA delivery, transgenic selection and in vitro shoot regeneration. Here, we report a highly efficient transformation system using embryonic axis explants isolated from imbibed mature seeds. We found that removal of the shoot apical meristem from the explants stimulated direct multiple shoot organogenesis from the cotyledonary node tissue. The application of a previously reported ternary transformation vector system provided efficient Agrobacterium-mediated gene delivery, while the utilization of spcN as selectable marker enabled more robust transgenic selection, plant recovery and transgenic plant generation without escapes and chimera formation. Transgenic cowpea plantlets developed exclusively from the cotyledonary nodes at frequencies of 4% to 37% across a wide range of cowpea genotypes. CRISPR/Cas-mediated gene editing was successfully demonstrated. The transformation principles established here could also be applied to other legumes to increase transformation efficiencies.

Developing a rapid and highly efficient cowpea regeneration, transformation and genome editing system using embryonic axis explants

Cowpea (Vigna unguiculata (L.) Walp.) is one of the most important legume crops planted worldwide, but despite decades of effort, cowpea transformation is still challenging due to inefficient Agrobacterium-mediated transfer DNA delivery, transgenic selection and in vitro shoot regeneration. Here, we report a highly efficient transformation system using embryonic axis explants isolated from imbibed mature seeds. We found that removal of the shoot apical meristem from the explants stimulated direct multiple shoot organogenesis from the cotyledonary node tissue. The application of a previously reported ternary transformation vector system provided efficient Agrobacterium-mediated gene delivery, while the utilization of spcN as selectable marker enabled more robust transgenic selection, plant recovery and transgenic plant generation without escapes and chimera formation. Transgenic cowpea plantlets developed exclusively from the cotyledonary nodes at frequencies of 4% to 37% across a wide range of cowpea genotypes. CRISPR/Cas-mediated gene editing was successfully demonstrated. The transformation principles established here could also be applied to other legumes to increase transformation efficiencies.

Transgenic cowpea plants expressing Bacillus thuringiensis Cry2Aa insecticidal protein imparts resistance to Maruca vitrata legume pod borer

Fertile independent transgenic cowpea lines expressing the BtCry2Aa toxin with increased resistance to the most devastating lepidopteran insect pest, Maruca pod borer has been developed for the first time. Cowpea is a staple legume important for food and nutritional security in sub-Saharan Africa and Asia, where its production is limited by the key pest, legume pod borer (Maruca vitrata). Cowpea varieties resistant to M. vitrata are not known, hence, development of Maruca pod borer resistance cowpea through genetic engineering is a promising approach to improve its production. In the present study, transgenic cowpea plants expressing Bacillus thuringiensis Cry2Aa insecticidal protein were developed for the first time using Agrobacterium tumefaciens-mediated transformation of cotyledonary explants. T₀ plants recovered from Agrobacterium cocultured explants on medium containing 120 mgl^-1 of kanamycin were identified on the basis of the presence of transgenes by PCR, their integration into genome by Southern hybridization and expression of their transcripts by semi quantitative PCR (sqRT-PCR) and quantitative Real-time-PCR (qRT-PCR) and protein by Western blot analysis. The transformation efficiency obtained was 3.47% with 11 independent T₀ transgenic lines. The bioefficacy of Cry2Aa protein expressed in randomly selected four T₀ plant's leaves and pods was evaluated by feeding Maruca pod borer demonstrated a significant lower damage and a high level of Maruca mortality (more than 90%) for all these Bt lines. The inheritance of transgenes from T₀ to T₁ progeny plants was demonstrated by PCR analysis. The transgenic plants generated in this study can be used in cowpea breeding program for durable and sustainable legume pod borer resistance.

Efficient Plant Production of Recombinant NS1 Protein for Diagnosis of Dengue

Dengue fever is endemic in more than 120 countries, which account for 3.9 billion people at risk of infection worldwide. The absence of a vaccine with effective protection against the four serotypes of this virus makes differential molecular diagnosis the key step for the correct treatment of the disease. Rapid and efficient diagnosis prevents progression to a more severe stage of this disease. Currently, the limiting factor in the manufacture of dengue (DENV) diagnostic kits is the lack of large-scale production of the non-structural 1 (NS1) protein (antigen) to be used in the capture of antibodies from the blood serum of infected patients. In this work, we use plant biotechnology and genetic engineering as tools for the study of protein production for research and commercial purposes. Gene transfer, integration and expression in plants is a valid strategy for obtaining large-scale and low-cost heterologous protein production. The authors produced NS1 protein of the dengue virus serotype 2 (NS1DENV2) in the Arabidopsis thaliana plant. Transgenic plants obtained by genetic transformation expressed the recombinant protein that was purified and characterized for diagnostic use. The yield was 203 μg of the recombinant protein per gram of fresh leaf. By in situ immunolocalization, transgenic protein was observed within the plant tissue, located in aggregates bodies. These antigens showed high sensitivity and specificity to both IgM (84.29% and 91.43%, respectively) and IgG (83.08% and 87.69%, respectively). The study goes a step further to validate the use of plants as a strategy for obtaining large-scale and efficient protein production to be used in dengue virus diagnostic tests.

Genome Editing in Cowpea Vigna unguiculata Using CRISPR-Cas9

Cowpea (Vigna unguiculata) is widely cultivated across the world. Due to its symbiotic nitrogen fixation capability and many agronomically important traits, such as tolerance to low rainfall and low fertilization requirements, as well as its high nutrition and health benefits, cowpea is an important legume crop, especially in many semi-arid countries. However, research in Vigna unguiculata is dramatically hampered by the lack of mutant resources and efficient tools for gene inactivation in vivo. In this study, we used clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated protein 9 (Cas9). We applied the CRISPR/Cas9-mediated genome editing technology to efficiently disrupt the representative symbiotic nitrogen fixation (SNF) gene in Vigna unguiculata. Our customized guide RNAs (gRNAs) targeting symbiosis receptor-like kinase (SYMRK) achieved ~67% mutagenic efficiency in hairy-root-transformed plants, and nodule formation was completely blocked in the mutants with both alleles disrupted. Various types of mutations were observed near the PAM region of the respective gRNA. These results demonstrate the applicability of the CRISPR/Cas9 system in Vigna unguiculata, and therefore should significantly stimulate functional genomics analyses of many important agronomical traits in this unique crop legume.

An Improved Transformation System for Cowpea (Vigna unguiculata L. Walp) via Sonication and a Kanamycin-Geneticin Selection Regime

An improved cowpea transformation method utilizing Agrobacterium-mediated gene delivery to explants derived from the cotyledonary nodes of imbibed cowpea seed is described. The explants were regenerated following a sonication procedure and a stringent selection comprising alternating regimes of kanamycin and geneticin. The method was reproducible and led to the recovery of independent fertile transgenic plants in the greenhouse at a level of about one per cent of starting explants. A transgene encoding an insecticidal protein from Bacillus thuringiensis was used to demonstrate the efficacy of the system.

The Vigna unguiculata Gene Expression Atlas (VuGEA) from de novo assembly and quantification of RNA-seq data provides insights into seed maturation mechanisms

Legume research and cultivar development are important for sustainable food production, especially of high-protein seed. Thanks to the development of deep-sequencing technologies, crop species have been taken to the front line, even without completion of their genome sequences. Black-eyed pea (Vigna unguiculata) is a legume species widely grown in semi-arid regions, which has high potential to provide stable seed protein production in a broad range of environments, including drought conditions. The black-eyed pea reference genotype has been used to generate a gene expression atlas of the major plant tissues (i.e. leaf, root, stem, flower, pod and seed), with a developmental time series for pods and seeds. From these various organs, 27 cDNA libraries were generated and sequenced, resulting in more than one billion reads. Following filtering, these reads were de novo assembled into 36 529 transcript sequences that were annotated and quantified across the different tissues. A set of 24 866 unique transcript sequences, called Unigenes, was identified. All the information related to transcript identification, annotation and quantification were stored into a gene expression atlas webserver (http://vugea.noble.org), providing a user-friendly interface and necessary tools to analyse transcript expression in black-eyed pea organs and to compare data with other legume species. Using this gene expression atlas, we inferred details of molecular processes that are active during seed development, and identified key putative regulators of seed maturation. Additionally, we found evidence for conservation of regulatory mechanisms involving miRNA in plant tissues subjected to drought and seeds undergoing desiccation.

Development of an efficient in vitro plant regeneration system amenable to Agrobacterium- mediated transformation of a recalcitrant grain legume blackgram (Vigna mungo L. Hepper)

An efficient, rapid and direct multiple shoot regeneration system amenable to Agrobacterium-mediated transformation from primary leaf with intact petiole of blackgram (Vigna mungo) is established for the first time. The effect of the explant type and its age, type and concentration of cytokinin and auxin either alone or in combination and genotype on multiple shoot regeneration efficiency and frequency was optimized. The primary leaf explants with petiole excised from 4-day-old seedlings directly developed multiple shoots (an average of 10 shoots/ explant) from the cut ends of the petiole in 95 % of the cultures on MSB (MS salts and B5 vitamins) medium containing 1.0 μM 6-benzylaminopurine. Elongated (2-3 cm) shoots were rooted on MSB medium with 2.5 μM indole-butyric acid and resulted plantlets were hardened and established in soil, where they resumed growth and reached maturity with normal seed set. The regenerated plants were morphologically similar to seed-raised plants and required 8 weeks time from initiation of culture to establish them in soil. The regeneration competent cells present at the cut ends of petiole are fully exposed and are, thus, easily accessible to Agrobacterium, making this plant regeneration protocol amenable for the production of transgenic plants. The protocol was further successfully used to develop fertile transgenic plants of blackgram using Agrobacterium tumefaciens strain EHA 105 carrying a binary vector pCAMBIA2301 that contains a neomycin phosphotransferase gene (nptII) and a β-glucuronidase (GUS) gene (uidA) interrupted with an intron. The presence and integration of transgenes in putative T0 plants were confirmed by polymerase chain reaction (PCR) and Southern blot hybridization, respectively. The transgenes were inherited in Mendelian fashion in T1 progeny and a transformation frequency of 1.3 % was obtained. This protocol can be effectively used for transferring new traits in blackgram and other legumes for their quantitative and qualitative improvements.

Tropical food legumes: virus diseases of economic importance and their control

Diverse array of food legume crops (Fabaceae: Papilionoideae) have been adopted worldwide for their protein-rich seed. Choice of legumes and their importance vary in different parts of the world. The economically important legumes are severely affected by a range of virus diseases causing significant economic losses due to reduction in grain production, poor quality seed, and costs incurred in phytosanitation and disease control. The majority of the viruses infecting legumes are vectored by insects, and several of them are also seed transmitted, thus assuming importance in the quarantine and in the epidemiology. This review is focused on the economically important viruses of soybean, groundnut, common bean, cowpea, pigeonpea, mungbean, urdbean, chickpea, pea, faba bean, and lentil and begomovirus diseases of three minor tropical food legumes (hyacinth bean, horse gram, and lima bean). Aspects included are geographic distribution, impact on crop growth and yields, virus characteristics, diagnosis of causal viruses, disease epidemiology, and options for control. Effectiveness of selection and planting with virus-free seed, phytosanitation, manipulation of crop cultural and agronomic practices, control of virus vectors and host plant resistance, and potential of transgenic resistance for legume virus disease control are discussed.

Modeling evolution of resistance by Maruca vitrata (Lepidoptera: Crambidae) to transgenic insecticidal cowpea in Africa

We created a detailed model of the Maruca vitrata (F.) and cowpea [Vigna unguiculata (L.) Walp] system to study the possible evolution of resistance by the insect to transgenic insecticidal cowpea, which is under development. We focused on population dynamics and genetics in a region of west Africa. We simulated single-toxin and pyramided (two-toxin) cowpea and emphasized conservative, worst-case scenarios in our analysis. The results indicate that as long as a pyramided, transgenic cowpea can be developed, seed saving by farmers and reliance on natural refuge are not major problems for resistance management. Furthermore, it is possible that one or both toxins in the pyramid may not need to be high dose for evolution to be delayed significantly (>20 yr or 80 generations for resistance to become a concern if transgenic cowpea is deployed in areas where M. vitrata is endemic). If efforts are made to deploy transgenic cowpea only into the regions where M. vitrata is not endemic, then there is little to no concern with resistance emerging in the M. vitrata population.

Successful recovery of transgenic cowpea (Vigna unguiculata) using the 6-phosphomannose isomerase gene as the selectable marker

A new method for obtaining transgenic cowpea was developed using positive selection based on the Escherichia coli 6-phosphomannose isomerase gene as the selectable marker and mannose as the selective agent. Only transformed cells were capable of utilizing mannose as a carbon source. Cotyledonary node explants from 4-day-old in vitro-germinated seedlings of cultivar Pusa Komal were inoculated with Agrobacterium tumefaciens strain EHA105 carrying the vector pNOV2819. Regenerating transformed shoots were selected on medium supplemented with a combination of 20 g/l mannose and 5 g/l sucrose as carbon source. The transformed shoots were rooted on medium devoid of mannose. Transformation efficiency based on PCR analysis of individual putative transformed shoots was 3.6%. Southern blot analysis on five randomly chosen PCR-positive plants confirmed the integration of the pmi transgene. Qualitative reverse transcription (qRT-PCR) analysis demonstrated the expression of pmi in T₀ transgenic plants. Chlorophenol red (CPR) assays confirmed the activity of PMI in transgenic plants, and the gene was transmitted to progeny in a Mendelian fashion. The transformation method presented here for cowpea using mannose selection is efficient and reproducible, and could be used to introduce a desirable gene(s) into cowpea for biotic and abiotic stress tolerance.

Improved Agrobacterium-mediated transformation of cowpea via sonication and vacuum infiltration

An improved method of Agrobacterium-mediated transformation of cowpea was developed employing both sonication and vacuum infiltration treatments. 4 day-old cotyledonary nodes were used as explants for co-cultivation with Agrobacterium tumefaciens strain EHA105 harbouring the binary vector pSouv-cry1Ac. Among the different injury treatments, vacuum infiltration and their combination treatments tested, sonication for 20 s followed by vacuum infiltration for 5 min with A. tumefaciens resulted in highest transient GUS expression efficiency (93% explants expressing GUS at regenerating sites). After 3 days of co-cultivation, the explants were cultured in 150 mg/l kanamycin-containing selection medium and putative transformed plants were recovered. The presence, integration and expression of nptII and cry1Ac genes in T0 transgenic plants were confirmed by polymerase chain reaction (PCR), genomic Southern and qualitative reverse transcription (RT)-PCR analysis. Western blot hybridization and enzyme-linked immunosorbent assay (ELISA) detected and demonstrated the accumulation of Cry1Ac protein in transgenic plants. The cry1Ac gene transmitted in a Mendelian fashion. The stable transformation efficiency increased by 88.4% using both sonication-assisted Agrobacterium-mediated transformation (SAAT) and vacuum infiltration than simple Agrobacterium-mediated transformation in cowpea.

Genetic engineering in Cowpea (Vigna unguiculata): history, status and prospects

In the last three decades, a number of attempts have been made to develop reproducible protocols for generating transgenic cowpea that permit the expression of genes of agronomic importance. Pioneer works focused on the development of such systems vis-à-vis an in vitro culture system that would guarantee de novo regeneration of transgenic cowpea arising from cells amenable to one form of gene delivery system or another, but any such system has eluded researchers over the years. Despite this apparent failure, significant progress has been made in generating transgenic cowpea, bringing researchers much nearer to their goal than thirty years ago. Now, various researchers have successfully established transgenic procedures for cowpea with evidence of inherent transgenes of interest, effected by progenies in a Mendelian fashion. New opportunities have thus emerged to optimize existing protocols and devise new strategies to ensure the development of transgenic cowpea with desirable agronomic traits. This review chronicles the important milestones in the last thirty years that have marked the evolution of genetic engineering of cowpea. It also highlights the progress made and describes new strategies that have arisen, culminating in the current status of transgenic technologies for cowpea.

Efficient in vitro propagation from preconditioned embryonic axes of Turkish cowpea (Vigna unguiculata L.) cultivar Akkiz

Cowpea is an important grain legume crop. The study reports an efficient in vitro multiplication and shoots regeneration protocol from preconditioned embryonic axes of the Turkish cowpea cultivar Akkiz. The embryonic axes were preconditioned with 10 mg/l BA on agar solidified MS medium for 5 days. Thereafter they were cultured on MS medium containing 0.25, 0.50, 0.75 and 1.00 mg/l BA with or without 0.10 mg/l NAA. Mean frequency (%) of shoot regeneration, number of shoots per explant and shoot length decreased with each increase in BA concentration used singly. However, a positive increase was recorded in all parameters in the presence of 0.10 mg/l NAA in the regeneration medium. A maximum mean number of 10.33 shoots per explant was recorded on an MS medium containing 1.00 mg/l BA -0.1 mg/l NAA. Regenerated shoots were rooted on an MS medium containing 0.50 mg/l IBA. Rooted plants were acclimatized at room temperature in soil mix contained in pots where they were subjected to an intermittent mistwater spray for 24 h that maintained 90% relative humidity during the first few days which was gradually reduced to 40% for 10 days. All plants flowered and set seeds in a greenhouse after 3 months.

Transgenic cowpea (Vigna unguiculata) seeds expressing a bean alpha-amylase inhibitor 1 confer resistance to storage pests, bruchid beetles

Cowpea is one of the important grain legumes. Storage pests, Callosobruchus maculatus and C. chinensis cause severe damage to the cowpea seeds during storage. We employ a highly efficient Agrobacterium-mediated cowpea transformation method for introduction of the bean (Phaseolus vulgaris) alpha-amylase inhibitor-1 (alphaAI-1) gene into a commercially important Indian cowpea cultivar, Pusa Komal and generated fertile transgenic plants. The use of constitutive expression of additional vir genes in resident pSB1 vector in Agrobacterium strain LBA4404, thiol compounds during cocultivation and a geneticin based selection system resulted in twofold increase in stable transformation frequency. Expression of alphaAI-1 gene under bean phytohemagglutinin promoter results in accumulation of alphaAI-1 in transgenic seeds. The transgenic protein was active as an inhibitor of porcine alpha-amylase in vitro. Transgenic cowpeas expressing alphaAI-1 strongly inhibited the development of C. maculatus and C. chinensis in insect bioassays.

TDZ-induced direct shoot organogenesis and somatic embryogenesis on cotyledonary node explants of lentil (Lens culinaris Medik.)

An efficient and simple procedure for inducing high frequency direct shoot organogenesis and somatic embryogenesis in lentil from cotyledonary node explants (without both the cotyledons) in response to TDZ alone is reported. TDZ at concentration lower than 2.0 μM induced shoot organogenesis whereas at higher concentration (2.5-15 μM) it caused a shift in regeneration from shoot organogenesis to somatic embryogenesis. The cotyledonary node and seedling cultures developed only shoots even at high concentrations of BAP and TDZ, respectively. TDZ at 0.5 and 5.0 μM was found to be optimal for inducing an average of 4-5 shoots per cotyledonary node in 93 % of the cultures and 55 somatic embryos in 68 % of the cultures, respectively. The somatic embryos were germinated when transferred to lower TDZ concentration (0.5-1.0 μM). The shoots were rooted on MS basal medium containing 2.5 μM IBA. The plantlets were obtained within 8 weeks from initiation of culture and were morphologically similar to seed-raised plants. The possible role of stress in thidiazuron induced somatic embryogenesis is discussed.

Biolistic-mediated genetic transformation of cowpea (Vigna unguiculata) and stable Mendelian inheritance of transgenes

We describe a novel system of exploiting the biolistic process to generate stable transgenic cowpea (Vigna unguiculata) plants. The system is based on combining the use of the herbicide imazapyr to select transformed meristematic cells after physical introduction of the mutated ahas gene (coding for a mutated acetohydroxyacid synthase, under control of the ahas 5' regulatory sequence) and a simple tissue culture protocol. The gus gene (under control of the act2 promoter) was used as a reporter gene. The transformation frequency (defined as the total number of putative transgenic plants divided by the total number of embryonic axes bombarded) was 0.90%. Southern analyses showed the presence of both ahas and gus expression cassettes in all primary transgenic plants, and demonstrated one to three integrated copies of the transgenes into the genome. The progenies (first and second generations) of all self-fertilized transgenic lines revealed the presence of the transgenes (gus and ahas) co-segregated in a Mendelian fashion. Western blot analysis revealed that the GUS protein expressed in the transgenic plants had the same mass and isoelectric point as the bacterial native protein. This is the first report of biolistic-mediated cowpea transformation in which fertile transgenic plants transferred the foreign genes to next generations following Mendelian laws.

Gene manipulation of a heavy metal hyperaccumulator species Thlaspi caerulescens L. via Agrobacterium-mediated transformation

Thlaspi caerulescens L. is well known as a Zn/Cd hyperaccumulator. The genetic manipulation of T. caerulescens through transgenic technology can modify plant features for use in phytoremediation. Here, we describe the efficient transformation of T. caerulescens using Agrobacterium tumefaciens strain EHA105 harboring a binary vector pBI121 with the nptII gene as a selectable marker, the gus gene as a reporter and a foreign catalase gene. Based on the optimal concentration of growth regulators, the shoot cluster regeneration system via callus phase provided the basis of the genetic transformation in T. caerulescens. The key variables in transformation were examined, such as co-cultivation period and bacterial suspension density. Optimizing factors for T-DNA delivery resulted in kanamycin-resistant transgenic shoots with transformation efficiency more than 20%, proven by histochemical GUS assay and PCR analysis. Southern analysis of nptII and RT-PCR of catalase gene demonstrated that the foreign genes were integrated in the genome of transformed plantlets. Moreover, the activity of catalase enzyme in transgenic plants was obviously higher than in wild-type plants. This method offers new prospects for the genetic engineering of this important hyperaccumulator species.

Advances in development of transgenic pulse crops

It is three decades since the first transgenic pulse crop has been developed. Todate, genetic transformation has been reported in all the major pulse crops like Vigna species, Cicer arietinum, Cajanus cajan, Phaseolus spp, Lupinus spp, Vicia spp and Pisum sativum, but transgenic pulse crops have not yet been commercially released. Despite the crucial role played by pulse crops in tropical agriculture, transgenic pulse crops have not moved out from laboratories to large farm lands compared to their counterparts - 'cereals' and the closely related leguminous oil crop - 'soybean'. The reason for lack of commercialization of transgenic pulse crops can be attributed to the difficulty in developing transgenics with reproducibility, which in turn is due to lack of competent totipotent cells for transformation, long periods required for developing transgenics and lack of coordinated research efforts by the scientific community and long term funding. With optimization of various factors which influence genetic transformation of pulse crops, it will be possible to develop transgenic plants in this important group of crop species with more precision and reproducibility. A translation of knowledge from information available in genomics and functional genomics in model legumes like Medicago truncatula and Lotus japonicus relating to factors which contribute to enhancing crop yield and ameliorate the negative consequences of biotic and abiotic stress factors may provide novel insights for genetic manipulation to improve the productivity of pulse crops.