Plant transformation by particle bombardment of embryogenic pollen

Detection of a biolistic delivery of fluorescent markers and CRISPR/Cas9 to the pollen tube

Biolistic delivery into pollen. In recent years, genome editing techniques, such as the CRISPR/Cas9 system, have been highlighted as a new approach to plant breeding. Agrobacterium-mediated transformation has been widely utilized to generate transgenic plants by introducing plasmid DNA containing CRISPR/Cas9 into plant cells. However, this method has general limitations, such as the limited host range of Agrobacterium and difficulties in tissue culture, including callus induction and regeneration. To avoid these issues, we developed a method to genetically modify germ cells without the need for Agrobacterium-mediated transfection and tissue culture using tobacco as a model. In this study, plasmid DNA containing sequences of Cas9, guide RNA, and fluorescent reporter was introduced into pollen using a biolistic delivery system. Based on the transient expression of fluorescent reporters, the Arabidopsis UBQ10 promoter was found to be the most suitable promoter for driving the expression of the delivered gene in pollen tubes. We also evaluated the delivery efficiency in male germ cells in the pollen by expression of the introduced fluorescent marker. Mutations were detected in the target gene in the genomic DNA extracted from CRISPR/Cas9-introduced pollen tubes, but were not detected in the negative control. Bombarded pollen germinated pollen tubes and delivered their contents into the ovules in vivo. Although it is necessary to improve biolistic delivery efficiency and establish a method for the screening of genome-modified seeds, our findings provide important insights for the detection and production of genome-modified seeds by pollen biolistic delivery.

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.

Enhancement of androgenesis by abiotic stress and other pretreatments in major crop species

Rapid production of doubled haploids (DHs) through androgenesis is an important and promising method for genetic improvement of crop plants. Through androgenesis complete homozygous plants can be produced within a year compared to long inbreeding methods that may take several years and costly. Significant advantage of androgenesis is that it not only speeds up the process to achieve homozygosity, but also increases the selection efficiency. Though success in androgenesis has been achieved in many crop plants, yet there are certain limitations especially, low frequency of embryogenesis and regeneration in few species. In fact in many cereals, induction of embryos and regeneration of green plants is still a hurdle that one needs to overcome to improve the efficiency of androgenesis. Efficient androgenesis is usually induced by the successful application of different stress pretreatment. Since so many stress factors can trigger the reprogramming of microspores and that have been co-related to change the ultrastuctural changes of cells to embryos and finally haploid plants. It has been shown that certain pretreatment such as (i) physical stresses as cold, heat shock, starvation, drought stress, osmotic pressure, gamma irradiation, oxidative stress, reduced atmospheric pressure, and (ii) chemical treatments such as colchicine, heavy metal, ABA, CGA, AEC, Azetidine, 2-NHA, either individual or combined effect of more than one stress factors may positively influence androgenetic efficiency. This review highlights the recent and past work on uses of various abiotic stresses and pretreatments and their impact on enhancing the efficiency of androgenesis on some major crop species for the development of doubled haploid plants.

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.

Embryogenesis and plant regeneration of hot pepper (Capsicum annuum L.) through isolated microspore culture

We report high frequencies of embryo production and plant regeneration through isolated microspore culture of hot pepper (Capsicum annuum L.). Microspores cultured in modified NLN medium (NLNS) divided and developed to embryos. Globular and heart-shaped embryos were observed from 3 weeks after the beginning of culture, and many embryos reached the cotyledonary stage after 4 weeks of culture. These cotyledonary embryos developed to plantlets after transfer to solid B5 basal medium. We also optimized conditions for embryo production by varying the pretreatment media, the carbon sources, and culture densities. Heat shock treatment in sucrose-starvation medium was more effective than in B5 medium. Direct comparisons of sucrose and maltose as carbon sources clearly demonstrated the superiority of sucrose compared to maltose, with the highest frequency of embryo production being obtained in 9% (w/v) sucrose. Microspore plating density was critical for efficient embryonic induction and development, with an optimal plating density of 8 x 10(4)-10 x 10(4)/ml. Under our optimized culture conditions, we obtained over 54 embryos, and an average of 5.5 cotyledonary embryos when 10 x 10(4) microspores were grown on an individual plate.

The resurgence of haploids in higher plants

The life cycle of plants proceeds via alternating generations of sporophytes and gametophytes. The dominant and most obvious life form of higher plants is the free-living sporophyte. The sporophyte is the product of fertilization of male and female gametes and contains a set of chromosomes from each parent; its genomic constitution is 2n. Chromosome reduction at meiosis means cells of the gametophytes carry half the sporophytic complement of chromosomes (n). Plant haploid research began with the discovery that sporophytes can be produced in higher plants carrying the gametic chromosome number (n instead of 2n) and that their chromosome number can subsequently be doubled up by colchicine treatment. Recent technological innovations, greater understanding of underlying control mechanisms and an expansion of end-user applications has brought about a resurgence of interest in haploids in higher plants.

Methotrexate is a new selectable marker for tobacco immature pollen transformation

We report here a new selectable marker for tobacco immature pollen transformation based on the expression of dihydrofolate reductase (dhfr) gene which confers resistance to methotrexate (Mtx). Two immature pollen transformation approaches, i.e., male germ line transformation and particle bombardment of embryogenic mid-bicellular pollen have been used for the production of stable transgenic tobacco plants. In the first method, two methotrexate-resistant plants were selected from a total of 7161 seeds recovered after transformation experiments. In the second method, four methotrexate-resistant plants were obtained from 29 bombardments using 3.7 x 10(5) pollen grains per bombardment. Southern analysis confirmed the transgenic nature of T0 and T1 candidate transgenic plants, and a genetic analysis showed that the transgenes are transmitted to subsequent generations.

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.

The production of transgenic Scots pine (Pinus sylvestris L.) via the application of transformed pollen in controlled crossings

The study demonstrates the production of a transgenic Scots pine (Pinus sylvestris L.) seedling through the application of transformed pollen in controlled crossings. The pollen lots were transformed by particle bombardment, resulting in transient transformation frequencies varying from 15 to 49% of the germinated pollen grains, and bombarded pollen was used to pollinate megasporangiate strobili. Progeny was screened by histochemical, GUS assays, and selected seedlings were further analysed by PCR. PCR amplification revealed the presence of both the nptII and gusA genes in one seedling (23/237). Results were confirmed by Southern blot analysis. The morphology and growth of this transgenic seedling was normal. Although the transformation frequency of recovered plants was very low (1/14999), the present protocol suggests that production of transgenic Scots pine is possible without the use of any tissue culture methods or the involvement of marker genes, for selection of transformants.

A cytological study of pretreatments used to improve isolated microspore cultures of wheat (Triticum aestivum L.) cv. Chris

Isolated microspores of wheat can be induced in vitro to switch their development from the gametophytic pollen pathway to a sporophytic pathway, resulting in embryoid or callus formation. The influence of cold or mannitol pretreatment on karyokinesis and cytokinesis in isolated microspore culture responses were investigated. Anthers were pretreated in mannitol for 7 d at 28°C; spikes at 4°C for 28 d. Microspores often completed the 1st mitotic nuclear division during pretreatment while cytokinesis was delayed. During mannitol pretreatments, the 1st mitotic nuclear division was mostly symmetrical while only asymmetric 1st nuclear divisions were seen during or after cold pretreatment. Following the symmetrical division, the two similar nuclei often appeared to fuse to form a diploid nucleus. Subsequently, these nuclei underwent rapid nuclear divisions to form multinucleate, and later, multicellular structures in induction medium. Cold pretreatments also induced muticellular structures but frequencies were lower than after mannitol. A novel pretreatment of spikes, combining 0.4 M mannitol solution at 4°C for 4 d, delayed the 1st nuclear division, keeping all microspores in a haploid uni-nucleate stage and resulted in higher induction frequencies. The proportion of embryos larger than 2 mm that developed into green plants was as high as 70% when transferred to regeneration media. Ninety-five percent of the plantlets transferred from culture to soil survived. The improved pretreatment enhanced the potential of isolated microspore culture in wheat for plant breeding by producing large numbers of plants and for gene transformation by maintaining a uniform population of haploid uni-nucleate stage microspores as targets.Key words: wheat, pretreatment, karyokinesis, embryogenesis, microspore, cold, mannitol.

Biolistic transformation of haploid isolated microspores of barley (Hordeum vulgare L.)

Transgenic barley plants were produced by the direct delivery of plasmid DNA into isolated microspores of barley cv. Igri using high velocity microprojectiles. The plasmid pAHC25 contained the uidA and bar genes, each under the control of a maize Ubi1 promoter. Bombarded microspores were cultured and selected on solid medium containing varying concentrations (2–5 mg/L) of the Basta herbicide active agent bialaphos. The effectiveness of selection with bialaphos depended on its interaction with the medium component glutamine. Six transgenic plants (R0) were obtained, and the presence of the uidA and bar genes and their integration into nuclear DNA in transformed R0 plants were confirmed by PCR and Southern blot analysis. Phosphinothricin acetyltransferase activity was observed in all six R0 transgenic plants, whereas none showed β-glucuronidase (GUS) activity in histochemical GUS assays. Two of the six R0 plants were haploid and sterile; one of them was trisomic and partially sterile; the remainder were diploid, but one of them was also sterile. Inheritance of the transgenes in progeny of three seed-producing transgenic plants was investigated. Southern blot analysis of genomic DNA from R1 plants showed that the introduced bar and uidA genes were hemizygous and stably cotransmitted to the R1 progeny derived from self-pollination. Analysis of Basta resistance and the integration of the bar gene by PCR analysis in R1 plants indicated that the bar gene was being inherited and expressed as a single dominant trait. Fluorescent in situ hybridization was performed on chromosomes of the trisomic plant to confirm the presence of transgenes in the genome.Key words: barley, microspore, biolistic transformation, bialaphos, haploid, FISH.