Positive, negative and marker-free strategies for transgenic plant selection

Current progress and challenges in crop genetic transformation

Plant transformation remains the most sought-after technology for functional genomics and crop genetic improvement, especially for introducing specific new traits and to modify or recombine already existing traits. Along with many other agricultural technologies, the global production of genetically engineered crops has steadily grown since they were first introduced 25 years ago. Since the first transfer of DNA into plant cells using Agrobacterium tumefaciens, different transformation methods have enabled rapid advances in molecular breeding approaches to bring crop varieties with novel traits to the market that would be difficult or not possible to achieve with conventional breeding methods. Today, transformation to produce genetically engineered crops is the fastest and most widely adopted technology in agriculture. The rapidly increasing number of sequenced plant genomes and information from functional genomics data to understand gene function, together with novel gene cloning and tissue culture methods, is further accelerating crop improvement and trait development. These advances are welcome and needed to make crops more resilient to climate change and to secure their yield for feeding the increasing human population. Despite the success, transformation remains a bottleneck because many plant species and crop genotypes are recalcitrant to established tissue culture and regeneration conditions, or they show poor transformability. Improvements are possible using morphogenetic transcriptional regulators, but their broader applicability remains to be tested. Advances in genome editing techniques and direct, non-tissue culture-based transformation methods offer alternative approaches to enhance varietal development in other recalcitrant crops. Here, we review recent developments in plant transformation and regeneration, and discuss opportunities for new breeding technologies in agriculture.

The use of phosphomannose isomerase selection system for Agrobacterium-mediated transformation of tobacco and flax aimed for phytoremediation

A plant selection system based on the phosphomannose isomerase gene (pmi) as a selectable marker is often used to avoid selection using antibiotic resistance. Nevertheless, pmi gene is endogenous in several plant species and therefore difficult to use in such cases. Here we evaluated and compared Agrobacterium-mediated transformation of Linum usitatissimum breeding line AGT-952 (without endogenous pmi gene) and Nicotiana tabacum var. WSC-38 (with endogenous pmi gene). Transformation was evaluated for vectors bearing transgenes that have the potential to be involved in improved phytoremediation of contaminated environment. Tobacco regenerants selection resulted in 6.8% transformation efficiency when using a medium supplemented with 30 g/L mannose with stepwise decrease of the sucrose concentration. Similar transformation efficiency (5.3%) was achieved in transformation of flax. Relatively low selection efficiency was achieved (12.5% and 34.8%, respectively). The final detection of efficient pmi selection was conducted using PCR and the non-endogenous genes; pmi transgene for flax and todC2 transgene for tobacco plants.

Effects of Chemical, Organic and Bio-Fertilizers on Some Secondary Metabolites in the Leaves of Bell Pepper ( Capsicum annuum ) and Their Impact on Life Table Parameters of Myzus persicae (Hemiptera: Aphididae)

The green peach aphid, Myzus persicae (Sulzer), is a polyphagous and a holocyclic aphid that causes severe damage on hundreds of host plants in both fields and greenhouses. In this research, the effects of Zinc sulfate spray and amending the soil with 30% vermicompost, Bacillus subtilis , Pseudomonas fluorescens , Glomus intraradices , G. intraradices  ×  B. subtilis , and G. intraradices  ×  P. fluorescens compared with no fertilizer treatments were investigated on secondary metabolites in the leaves of bell pepper and life table parameters of M. persicae . Total phenol contents in the plant leaves varied significantly among different fertilizer treatments. The highest (72.28 mg/ml) value was observed on 30% vermicompost. Life table parameters of M. persicae were significantly affected by different fertilizer treatments. The net reproductive rate ( R 0 ) of M. persicae fed on plants treated with different fertilizer treatments varied from 4.38 to 21.93 female offspring, with the lowest and highest values on 30% vermicompost and Zinc sulfate, respectively. The lowest and the highest intrinsic rate of increase ( r m ) were also observed on 30% vermicompost and Zinc sulfate (0.111 and 0.321 female per female per day, respectively). The longest mean generation time ( T ) was recorded on 30% vermicompost (13.41 d), and the shortest generation time was observed on Zinc sulfate (9.61 d). Results of this study revealed that amending the soil with 30% vermicompost significantly affected the life table parameters of M. persicae . Thus, it was concluded that amending the soil of bell pepper with 30% vermicompost can provide an environmentally safe and efficient control of this aphid.

Development of selectable marker free, insect resistant, transgenic mustard (Brassica juncea) plants using Cre/lox mediated recombination

Background

Antibiotic/ herbicide resistant marker genes have been proven to be very useful in plant transformation for the initial selection of desired transgenic events. However, presence of these genes in the genetically modified crops may render the crop less acceptable to the consumers. Among several different approaches, the effectiveness of Cre/lox mediated recombination strategy for selectable marker gene (SMG) elimination has previously been demonstrated by different groups in several plants including Brassica. In the present study exploiting Cre/lox mediated recombination strategy, attempt has been made for selectable marker gene elimination from Allium sativum leaf agglutinin (ASAL) expressing Brassica plants with hemipteran insect resistant phenotype.

Results

Allium sativum leaf agglutinin (ASAL) linked with lox flanked hygromycin resistant (hpt) gene was introduced in mustard. Cre recombinase gene cassette was also integrated in separate event. A Cre/lox mediated recombination using crossing strategy was adopted to remove the hpt gene from the subsequent generation of selected hybrid events. Reciprocal crosses were made between T₁ASAL-lox-hpt-lox and cre-bar plants. Marker gene elimination was confirmed in the resulting F₁ hybrid progenies by PCR analysis, using hpt, cre and ASAL specific primers followed by Southern hybridization. In marker free plants, expression of ASAL was also confirmed by western blotting and ELISA analysis. Retention of functionality of expressed ASAL was investigated by agglutination assay using rabbit erythrocytes. Expressed ASAL was also found to be thermo-sensitive. In planta insect bioassay on F₁ hybrid progenies exhibited detrimental effect on the performance of devastating target pest, Lipaphis erysimi. The F₁ hybrid hpt negative, ASAL positive plants were allowed to self- fertilize to obtain F₂ progeny plants. In some of these plants cre gene was found to be segregated out of the ASAL gene by genetic segregation yielding completely marker free plants.

Conclusions

The present study establishes the efficient expression of the newly introduced insect resistant ASAL gene even after Cre/lox mediated recombination resulting in elimination of selectable marker gene.

Aromatic Compound-Dependent Staphylococcus aureus Is Safe in a Nasal Colonization Leukopenic Murine Model

Staphylococcus aureus nasal carriage is a risk factor for individuals suffering from trauma, surgical procedures, invasive devices, and/or decreased immunity. Recently, we demonstrated that artificial nasal colonization with an attenuated S. aureus mutant reduced by bacterial interference with the colonization of pathogenic strains of S. aureus. This could be an optional tool to diminish the rate of S. aureus infections in hospitalized patients. The aim of this study was to construct a safe ΔaroA mutant of S. aureus and to discriminate it from nasal colonizing and osteomyelitis S. aureus isolates by SmaI pulsed-field gel electrophoresis (PFGE) typing. The ΔaroA mutant, named RD17, exhibited an LD(50) (3.2 × 10(6) colony-forming unit (CFU)) significantly higher than that of the parental strain (2.2 × 10(3) CFU). The colony number of the RD17 mutants recovered from nares of leukopenic mice was similar to that observed in the animals of the control group. Therefore, the ΔaroA mutant was demonstrated to be safe due to maintaining low growth levels in the nares regardless of immune status of the animals. PFGE typing allowed the unequivocal identification of the S. aureus and differentiation of aroA mutants in nasal colonizing and osteomyelitis isolates. This information could be important to discriminate endogenous infections from laboratory strains of S. aureus.

An efficient method for the production of marker-free transgenic plants of peanut (Arachis hypogaea L.)

Recombinant genes conferring resistance to antibiotics or herbicides are widely used as selectable markers in plant transformation for selecting the primary transgenic events. However, these become redundant once the transgenic plants have been developed and identified. Although, there is no evidence that the selectable marker genes are unsafe for consumers and the environment, it would be desirable if the marker genes can be eliminated from the final transgenic events. The availability of efficient transformation methods can enable the possibility of developing transgenic events that are devoid of the marker gene/s upfront. Taking advantage of the high and consistent transformation potential of peanut, we report a technique for developing its transgenics without the use of any selectable marker gene. Marker-free binary vectors harboring either the phytoene synthase gene from maize (Zmpsy1) or the chitinase gene from rice (Rchit) were constructed and used for Agrobacterium tumefaciens-mediated transformation of peanut. The putative transgenic events growing in vitro were initially identified by PCR and further confirmed for gene integration and expression by dot blots assays, Southern blots, and RT-PCR where they showed a transformation frequency of over 75%. This system is simple, efficient, rapid, and does not require the complex segregation steps and analysis for selection of the transgenic events. This approach for generation of marker-free transgenic plants minimizes the risk of introducing unwanted genetic changes, allows stacking of multiple genes and can be applicable to other plant species that have high shoot regeneration efficiencies.

Development of a phosphomannose isomerase-based Agrobacterium-mediated transformation system for chickpea (Cicer arietinum L.)

To develop an alternative genetic transformation system that is not dependent on an antibiotic selection strategy, the phosphomannose isomerase gene (pmi) system was evaluated for producing transgenic plants of chickpea (Cicer arietinum L.). A shoot morphogenesis protocol based on the thidiazuron (TDZ)-induced shoot morphogenesis system was combined with Agrobacterium-mediated transformation of the pmi gene and selection of transgenic plants on mannose. Embryo axis explants of chickpea cv. C-235 were grown on a TDZ-supplemented medium for shoot proliferation. Embryo axis explants from which the first and second flush of shoots were removed were transformed using Agrobacterium carrying the pmi gene, and emerging shoots were allowed to regenerate on a zeatin-supplemented medium with an initial selection pressure of 20 g l(-1) mannose. Rooting was induced in the selected shoots on an indole-3-butyric acid (IBA)-supplemented medium with a selection pressure of 15 g l(-1) mannose. PCR with marker gene-specific primers and chlorophenol red (CPR) assay of the shoots indicated that shoots had been transformed. RT-PCR and Southern analysis of selected regenerated plants further confirmed integration of the transgene into the chickpea genome. These positive results suggest that the pmi/mannose selection system can be used to produce transgenic plants of chickpea that are free from antibiotic resistance marker genes.

Marker-free transgenic corn plant production through co-bombardment

The use of particle gun for the production of marker-free plants is scant in published literature. Perhaps this is a reflection of the widely held notion that the events generated through bombardment tend to have multiple copies of transgenes, usually integrated at a single locus, features which precludes segregating away the selectable marker gene. However, our previous studies have shown that single-copy integrants are obtained at a high frequency if limited quantity of DNA is used for bombardment. Also, the concatemerized insertion of transgenes has been demonstrated to be greatly reduced if "cassette DNA" is employed in place of whole plasmid DNA for bombardment. Based on the above findings, in the present study the feasibility of co-bombardment was evaluated for the production of marker-free plants in corn, employing a combination of limited quantity DNA and cassette DNA approaches for bombardment. Transgenic events were generated after co-bombardment of a selectable marker cassette containing the nptII gene (2.5 ng per shot) and a GUS gene cassette (15 ng per shot). Among these events single-copy integrants for nptII gene occurred at an average frequency of 68% within which the co-expression frequency of GUS and nptII genes ranged from 41% to 80%. Marker-free corn plants could be identified from the progeny of 28 out of the 103 R0 co-expressing events screened. The results demonstrate that by using cassette DNA and low quantities of DNA for bombardment, marker-free plants are produced at efficiencies comparable to that of Agrobacterium-based co-transformation methods.

Biosafety considerations for selectable and scorable markers used in cassava (Manihot esculentaCrantz) biotechnology

Cassava is an important subsistence crop grown only in the tropics, and represents a major source of calories for many people in developing countries. Improvements in the areas of resistance to insects and viral diseases, enhanced nutritional qualities, reduced cyanogenic content and modified starch characteristics are urgently needed. Traditional breeding is hampered by the nature of the crop, which has a high degree of heterozygosity, irregular flowering, and poor seed set. Biotechnology has the potential to enhance crop improvement efforts, and genetic engineering techniques for cassava have thus been developed over the past decade. Selectable and scorable markers are critical to efficient transformation technology, and must be evaluated for biosafety, as well as efficiency and cost-effectiveness. In order to facilitate research planning and regulatory submission, the literature on biosafety aspects of the selectable and scorable markers currently used in cassava biotechnology is surveyed. The source, mode of action and current use of each marker gene is described. The potential for toxicity, allergenicity, pleiotropic effects, horizontal gene transfer, and the impact of these on food or feed safety and environmental safety is evaluated. Based on extensive information, the selectable marker genes nptII, hpt, bar/pat, and manA, and the scorable marker gene uidA, all have little risk in terms of biosafety. These appear to represent the safest options for use in cassava biotechnology available at this time.