High-efficiency genetic transformation of maize by a mixture of pollen and exogenous DNA

CRISPR/Cas9-gene editing approaches in plant breeding

CRISPR/Cas9 gene editing system is recently developed robust genome editing technology for accelerating plant breeding. Various modifications of this editing system have been established for adaptability in plant varieties as well as for its improved efficiency and portability. This review provides an in-depth look at the various strategies for synthesizing gRNAs for efficient delivery in plant cells, including chemical synthesis and in vitro transcription. It also covers traditional analytical tools and emerging developments in detection methods to analyze CRISPR/Cas9 mediated mutation in plant breeding. Additionally, the review outlines the various analytical tools which are used to detect and analyze CRISPR/Cas9 mediated mutations, such as next-generation sequencing, restriction enzyme analysis, and southern blotting. Finally, the review discusses emerging detection methods, including digital PCR and qPCR. Hence, CRISPR/Cas9 has great potential for transforming agriculture and opening avenues for new advancements in the system for gene editing in plants.

Hidden Pitfalls of Using Onion Pollen in Molecular Research

There is little information on the use of pollen in molecular research, despite the increased interest in genome editing by pollen-mediated transformation. This paper presents an essential toolbox of technical procedures and observations for molecular studies on onion (Allium cepa L.) pollen. PCR is a useful tool as an express method to evaluate editing results before pollination. A direct PCR protocol for pollen suspension has been adapted without needing DNA pre-extraction. We showed that the outer layer of lipids known as pollenkitt is a limiting factor for successful PCR on pollen. A simple pre-washing step of pollen suspension was able to eliminate the pollenkitt and enormously affect the PCR results. Additionally, our pollenkitt study helped us develop a simple and effective pollination method using wetted onion pollen grains. Classical manual pollination usually is conducted by intact pollen without wetting. Most existing methods of the editing system delivery into pollen are carried out in a wet medium with consequent drying before pollination, which adversely affects the viability of pollen. The optimal medium for wet pollination was 12% sucrose water solution. Our method of using wetted pollen grains for pollination might be very beneficial for pollen genetic manipulation.

Efficient and genotype independent maize transformation using pollen transfected by DNA-coated magnetic nanoparticles

Current gene delivery methods for maize are limited to specific genotypes and depend on time-consuming and labor-intensive tissue culture techniques. Here, we report a new method to transfect maize that is culture-free and genotype independent. To enhance efficiency of DNA entry and maintain high pollen viability of 32%-55%, transfection was performed at cool temperature using pollen pretreated to open the germination aperture (40%-55%). Magnetic nanoparticles (MNPs) coated with DNA encoding either red fluorescent protein (RFP), β-glucuronidase gene (GUS), enhanced green fluorescent protein (EGFP) or bialaphos resistance (bar) was delivered into pollen grains, and female florets of maize inbred lines were pollinated. Red fluorescence was detected in 22% transfected pollen grains, and GUS stained 55% embryos at 18 d after pollination. Green fluorescence was detected in both silk filaments and immature kernels. The T1 generation of six inbred lines showed considerable EGFP or GUS transcripts (29%-74%) quantitated by polymerase chain reaction, and 5%-16% of the T1 seedlings showed immunologically active EGFP or GUS protein. Moreover, 1.41% of the bar transfected T1 plants were glufosinate resistant, and heritable bar gene was integrated into the maize genome effectively as verified by DNA hybridization. These results demonstrate that exogenous DNA could be delivered efficiently into elite maize inbred lines recalcitrant to tissue culture-mediated transformation and expressed normally through our genotype-independent pollen transfection system.

Maize transformation: history, progress, and perspectives

Maize functional genomics research and genetic improvement strategies have been greatly accelerated and refined through the development and utilization of genetic transformation systems. Maize transformation is a composite technology based on decades' efforts in optimizing multiple factors involving microbiology and physical/biochemical DNA delivery, as well as cellular and molecular biology. This review provides a historical reflection on the development of maize transformation technology including the early failures and successful milestones. It also provides a current perspective on the understanding of tissue culture responses and their impact on plant regeneration, the pros and cons of different DNA delivery methods, the identification of a palette of selectable/screenable markers, and most recently the development of growth-stimulating or morphogenic genes to improve efficiencies and extend the range of transformable genotypes. Steady research progress in these interdependent components has been punctuated by benchmark reports celebrating the progress in maize transformation, which invariably relied on a large volume of supporting research that contributed to each step and to the current state of the art. The recent explosive use of CRISPR/Cas9-mediated genome editing has heightened the demand for higher transformation efficiencies, especially for important inbreds, to support increasingly sophisticated and complicated genomic modifications, in a manner that is widely accessible. These trends place an urgent demand on taking maize transformation to the next level, presaging a new generation of improvements on the horizon. Once realized, we anticipate a near-future where readily accessible, genotype-independent maize transformation, together with advanced genomics, genome editing, and accelerated breeding, will contribute to world agriculture and global food security.

The present and potential future methods for delivering CRISPR/Cas9 components in plants

BACKGROUND

CRISPR/Cas9 genome editing technology is a DNA manipulation tool for trait improvement. This technology has been demonstrated and successfully applied to edit the genome in various species of plants. The delivery of CRISPR/Cas9 components within rigid plant cells is very crucial for high editing efficiency. Here, we insight the strengths and weaknesses of each method of delivery.

MAIN TEXT

The mutation efficiency of genome editing may vary and affected by different factors. Out of various factors, the delivery of CRISPR/Cas9 components into cells and genome is vital. The way of delivery defines whether the edited plant is transgenic or transgene-free. In many countries, the transgenic approach of improvement is a significant limitation in the regulatory approval of genetically modified crops. Gene editing provides an opportunity for generating transgene-free edited genome of the plant. Nevertheless, the mode of delivery of the CRISPR/Cas9 component is of crucial importance for genome modification in plants. Different delivery methods such as Agrobacterium-mediated, bombardment or biolistic method, floral-dip, and PEG-mediated protoplast are frequently applied to crops for efficient genome editing.

CONCLUSION

We have reviewed different delivery methods with prons and cons for genome editing in plants. A novel nanoparticle and pollen magnetofection-mediated delivery systems which would be very useful in the near future. Further, the factors affecting editing efficiency, such as the promoter, transformation method, and selection pressure, are discussed in the present review.

A Short History and Perspectives on Plant Genetic Transformation

Plant genetic transformation is an important technological advancement in modern science, which has not only facilitated gaining fundamental insights into plant biology but also started a new era in crop improvement and commercial farming. However, for many crop plants, efficient transformation and regeneration still remain a challenge even after more than 30 years of technical developments in this field. Recently, FokI endonuclease-based genome editing applications in plants offered an exciting avenue for augmenting crop productivity but it is mainly dependent on efficient genetic transformation and regeneration, which is a major roadblock for implementing genome editing technology in plants. In this chapter, we have outlined the major historical developments in plant genetic transformation for developing biotech crops. Overall, this field needs innovations in plant tissue culture methods for simplification of operational steps for enhancing the transformation efficiency. Similarly, discovering genes controlling developmental reprogramming and homologous recombination need considerable attention, followed by understanding their role in enhancing genetic transformation efficiency in plants. Further, there is an urgent need for exploring new and low-cost universal delivery systems for DNA/RNA and protein into plants. The advancements in synthetic biology, novel vector systems for precision genome editing and gene integration could potentially bring revolution in crop-genetic potential enhancement for a sustainable future. Therefore, efficient plant transformation system standardization across species holds the key for translating advances in plant molecular biology to crop improvement.

Pollen magnetofection for genetic modification with magnetic nanoparticles as gene carriers

Genetic modification plays a vital role in breeding new crops with excellent traits. Almost all the current genetic modification methods require regeneration from tissue culture, involving complicated, long and laborious processes. In particular, many crop species such as cotton are difficult to regenerate. Here, we report a novel transformation platform technology, pollen magnetofection, to directly produce transgenic seeds without regeneration. In this system, exogenous DNA loaded with magnetic nanoparticles was delivered into pollen in the presence of a magnetic field. Through pollination with magnetofected pollen, transgenic plants were successfully generated from transformed seeds. Exogenous DNA was successfully integrated into the genome, effectively expressed and stably inherited in the offspring. Our system is culture-free and genotype independent. In addition, it is simple, fast and capable of multi-gene transformation. We envision that pollen magnetofection can transform almost all crops, greatly facilitating breeding processes of new varieties of transgenic crops.

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.

Expression of Foreign Genes Demonstrates the Effectiveness of Pollen-Mediated Transformation in Zea mays

Plant genetic transformation has arguably been the core of plant improvement in recent decades. Efforts have been made to develop in planta transformation systems due to the limitations present in the tissue-culture-based methods. Herein, we report an improved in planta transformation system, and provide the evidence of reporter gene expression in pollen tube, embryos and stable transgenicity of the plants following pollen-mediated plant transformation with optimized sonication treatment of pollen. The results showed that the aeration at 4°C treatment of pollen grains in sucrose prior to sonication significantly improved the pollen viability leading to improved kernel set and transformation efficiency. Scanning electron microscopy observation revealed that the removal of operculum covering pollen pore by ultrasonication might be one of the reasons for the pollen grains to become competent for transformation. Evidences have shown that the eGfp gene was expressed in the pollen tube and embryos, and the Cry1Ac gene was detected in the subsequent T₁ and T₂ progenies, suggesting the successful transfer of the foreign genes to the recipient plants. The Southern blot analysis of Cry1Ac gene in T₂ progenies and PCR-identified Apr gene segregation in T₂ seedlings confirmed the stable inheritance of the transgene. The outcome illustrated that the pollen-mediated genetic transformation system can be widely applied in the plant improvement programs with apparent advantages over tissue-culture-based transformation methods.

Pollen grains as a target for introduction of foreign genes into plants: an assessment

Introduction of foreign genes and development of transgenic plants have become an integral part of crop improvement programmes in the last decade. However, most of the present day plant transformation protocols require long periods for development of transgenic plants and need skilled personnel. Development of alternate, simple and rapid transformation protocols for development of transgenic plants can overcome the constraints of in vitro culture, regeneration and associated problems. Pollen grains, due to their abundance and ease with which they can be handled are ideal targets for introduction of foreign genes into the germ line. However, progress in introduction of transgenes into pollen grains and their subsequent use in fertilization leading to development of transgenic plants are limited. With the recent progress made in understanding of pollen development along with reports of successful pollen-mediated transformation in important crop plants, it should be possible to extend this simple method of transformation to other crop plants. The review deals with development of pollen grains as a target for introduction of genes with special emphasis on recent developments.

Gene transfer in cereals

Until recently, gene transfer in plants was achieved only by sexual hybridization. Now, in addition, plant genetic manipulation, with the use of both recombinant DNA and protoplast fusion technology, is being applied to an increasing range of plants. The soil bacterium Agrobacterium tumefaciens, with its associated plasmid, is used as a vector for introducing DNA into the genomes of dicotyledonous plants, but it has not proved suitable for cereals. Instead, the direct uptake of plasmid DNA into cereal protoplasts is being used for the transformation of cells in rice, wheat, and maize. Transformation efficiencies, in some cases, are becoming comparable to those obtained in dicotyledons with Agrobacterium. In rice it is now possible to regenerate efficiently whole plants from protoplasts, and this capability may soon be extended to the other cereals. By means of direct interaction of cereal protoplasts with plasmids, coupled with improved procedures for the regeneration of plants from their protoplasts, gene transfer in the cereals is becoming established at the frontiers of recombinant DNA technology.

The myth of plant transformation

Technology development is innovative to many aspects of basic and applied plant transgenic science. Plant genetic engineering has opened new avenues to modify crops, and provided new solutions to solve specific needs. Development of procedures in cell biology to regenerate plants from single cells or organized tissue, and the discovery of novel techniques to transfer genes to plant cells provided the prerequisite for the practical use of genetic engineering in crop modification and improvement. Plant transformation technology has become an adaptable platform for cultivar improvement as well as for studying gene function in plants. This success represents the climax of years of efforts in tissue culture improvement, in transformation techniques and in genetic engineering. Plant transformation vectors and methodologies have been improved to increase the efficiency of transformation and to achieve stable expression of transgenes in plants. This review provides a comprehensive discussion of important issues related to plant transformation as well as advances made in transformation techniques during three decades.

A history of plant biotechnology: from the Cell Theory of Schleiden and Schwann to biotech crops

Plant biotechnology is founded on the principles of cellular totipotency and genetic transformation, which can be traced back to the Cell Theory of Matthias Jakob Schleiden and Theodor Schwann, and the discovery of genetic transformation in bacteria by Frederick Griffith, respectively. On the 25th anniversary of the genetic transformation of plants, this review provides a historical account of the evolution of the theoretical concepts and experimental strategies that led to the production and commercialization of biotech (transformed or transgenic) plants expressing many useful genes, and emphasizes the beneficial effects of plant biotechnology on food security, human health, the environment, and conservation of biodiversity. In so doing, it celebrates and pays tribute to the contributions of scores of scientists who laid the foundation of modern plant biotechnology by their bold and unconventional thinking and experimentation. It highlights also the many important lessons to be learnt from the fascinating history of plant biotechnology, the significance of history in science teaching and research, and warns against the danger of the growing trends of ignoring history and historical illiteracy.

Recovery of transgenic plants by pollen-mediated transformation in Brassica juncea

The aroA-M1 encoding the mutant of 5-enolpyruvyl-shikimate-3-phosphate synthase (EPSPS) was introduced into the Brassica juncea genome by sonication-assisted, pollen-mediated transformation. The plasmid DNA and collected pollen grains were mixed in 0.3 mol/L sucrose solution and treated with mild ultrasonication. The treated pollen was then pollinated onto the oilseed stigmas after the stamens were removed artificially. Putative transgenic plants were obtained by screening germinating seeds on a medium containing glyphosate. Southern blot analysis of glyphosate-resistant plants indicated that the aroA-M1 gene had been integrated into the oilseed genome. Western blot analysis further confirmed that the EPSPS coded by aroA-M1 gene was expressed in transgenic plants. The transgenic plants exhibited increased resistance to glyphosate compared to untransformed plants. Some of those transgenic plants had considerably high resistance to glyphosate. The genetic analysis of T1 progeny further confirmed that the inheritance of the introduced genes followed the Mendelian rules. The results indicated that foreign genes can be transferred by pollen-mediated transformation combined with mild ultrasonication.

Molecular genetic improvement of cereals: transgenic wheat (Triticum aestivum L.)

Only modest progress has been made in the molecular genetic improvement of wheat following the production of the first transgenic plants in 1992, made possible by the development of efficient, long-term regenerable embryogenic cultures derived from immature embryos and use of the biolistics method for the direct delivery of DNA into regenerable cells. Transgenic lines expressing genes that confer resistance to environmentally friendly non-selective herbicides, and pests and pathogens have been produced, in addition to lines with improved bread-making and nutritional qualities; some of these are ready for commercial production. Reduction of losses caused by weeds, pests and pathogens in such plants not only indirectly increases available arable land and fresh water supplies, but also conserves energy and natural resources. Nevertheless, the work carried out thus far can be considered only the beginning, as many difficult tasks lie ahead and much remains to be done. The challenge now is to produce higher-yielding varieties that are more nutritious, and are resistant or tolerant to a wide variety of biotic as well as abiotic stresses (especially drought, salinity, heavy metal toxicity) that currently cause substantial losses in productivity. How well we will meet this challenge for wheat, and indeed for other cereal and non-cereal crops, will depend largely on establishing collaborative partnerships between breeders, molecular biologists, biotechnologists and industry, and on how effectively they make use of the knowledge and insights gained from basic studies in plant biology and genetics, the sequencing of plant/cereal genomes, the discovery of synteny in cereals, and the availability of DNA-based markers and increasingly detailed chromosomal maps.

Transfection of germinating barley seed electrophoretically with exogenous DNA

A method is described for transfection (genetic transformation) of barley caryopsis electrophoretically with DNA. β-Glucuronidase activity was detected after the electrophoretic transfection with plasmid pBI221 DNA carrying the cauliflower mosaic virus promotor and bacterial β-glucuronidase coding sequence. Electrophoretic transfection is evidently effective with pieces of callus and seeds of many plants.