Efficient Genetic Transformation of Rice for CRISPR/Cas9 Mediated Genome-Editing and Stable Overexpression Studies: A Case Study on Rice Lipase 1 and Galactinol Synthase Encoding Genes

Rice is a staple food crop for almost half of the world’s population, especially in the developing countries of Asia and Africa. It is widely grown in different climatic conditions, depending on the quality of the water, soil, and genetic makeup of the rice cultivar. Many (a)biotic stresses severely curtail rice growth and development, with an eventual reduction in crop yield. However, for molecular functional analysis, the availability of an efficient genetic transformation protocol is essential. To ensure food security and safety for the continuously increasing global population, the development of climate-resilient crops is crucial. Here, in this study, the rice transformation protocol has been effectively optimized for the efficient and rapid generation of rice transgenic plants. We also highlighted the critical steps and precautionary measures to be taken while performing the rice transformation. We further assess the efficacy of this protocol by transforming rice with two different transformation constructs for generating galactinol synthase (GolS) overexpression lines and CRISPR/Cas9-mediated edited lines of lipase (Lip) encoding the OsLip1 gene. The putative transformants were subjected to molecular analysis to confirm gene integration/editing, respectively. Collectively, the easy, efficient, and rapid rice transformation protocol used in this present study can be applied as a potential tool for gene(s) function studies in rice and eventually to the rice crop improvement.

Transformation and Overexpression of Primary Cell Wall Synthesis-Related Zinc Finger Gene Gh_A07G1537 to Improve Fiber Length in Cotton

Molecular interventions have helped to explore the genes involved in fiber length, fiber strength, and other quality parameters with improved characteristics, particularly in cotton. The current study is an extension and functional validation of previous findings that Gh_A07G1537 influences fiber length in cotton using a chromosomal segment substitution line MBI7747 through RNA-seq data. The recombinant Gh_A07G1537 derived from the MBI7747 line was over-expressed in CCRI24, a genotype with a low profile of fiber quality parameters. Putative transformants were selected on MS medium containing hygromycin (25mg/ml), acclimatized, and shifted to a greenhouse for further growth and proliferation. Transgene integration was validated through PCR and Southern Blot analysis. Stable integration of the transgene (ΔGh_A07G1537) was validated by tracking its expression in different generations (T0, T1, and T2) of transformed cotton plants. It was found to be 2.97-, 2.86-, and 2.92-folds higher expression in T0, T1, and T2 plants, respectively, of transgenic compared with non-transgenic cotton plants. Fiber quality parameters were also observed to be improved in the engineered cotton line. Genetic modifications of Gh_A07G1537 support the improvement in fiber quality parameters and should be appreciated for the textile industry.

Deciphering the role of fungal calmodulin gene using Host-mediated gene silencing approach in apple

Premature leaf fall caused by Marssonina coronaria is one of the most destructive diseases of apple in India. In this study, host induced gene silencing approach was exploited to develop resistance to this disease in an apple cultivar ‘Red Chief’. Calmodulin gene (CaM) having its role in fungal differentiation, development and pathogenicity was selected as target. hpRNAi construct was prepared from the conserved off target free partial gene sequence of CaM and used for transformation trials. Upto 6% kanamycin resistant shoots were obtained on selective medium having 5–6 mg/l kan after 7 weeks of coculture. In PCR analysis of 13 RNAi putative transformants, 10 lines were found positive with CaM and nptII gene specific primers and six lines showed hybridization signal. Semi qRT-PCR revealed variable levels of transgene expression among RNAi lines which seems to be related to copy number of integrated gene. In vitro detached leaf assay revealed lesion development and disease progression in wild type after 5 dpi but not visible in five CaM RNAi lines. Microscopic examination of infected control leaves showed fully developed, septate mycelium, and conidia along with necrosis of whole tissue while three transformants showed reduced growth and differentiation of fungus and in rest three, hyphal development and necrosis were strongly restricted. We conclude that trafficking of dsRNA/ siRNA from apple plants to pathogen might have triggered the down regulation of fungal CaM gene which confirms that deciphering the role of CaM through HIGS lead to resistance to Marssonina blotch in apple.

An Apical Meristem-Targeted in planta Transformation Method for the Development of Transgenics in Flax (Linum usitatissimum): Optimization and Validation

Efficient regeneration of explants devoid of intrinsic somaclonal variations is a cardinal step in plant tissue culture, thus, a vital component of transgenic technology. However, recalcitrance of economically important crops to tissue culture-based organogenesis ensues a setback in the use of transgenesis in the genetic engineering of crop plants. The present study developed an optimized, genotype-independent, nonconventional tissue culture-independent in planta strategy for the genetic transformation of flax/linseed. This apical meristem-targeted in planta transformation protocol will accelerate value addition in the dual purpose industrially important but recalcitrant fiber crop flax/linseed. The study delineated optimization of Agrobacterium tumefaciens-mediated transformation and stable T-DNA (pCambia2301:GUS:nptII) integration in flax. It established successful use of a stringent soilrite-based screening in the presence of 30 mg/L kanamycin for the identification of putative transformants. The amenability, authenticity, and reproducibility of soilrite-based kanamycin screening were further verified at the molecular level by GUS histochemical analysis of T0 seedlings, GUS and nptII gene-specific PCR, genomic Southern hybridization for stable integration of T-DNA, and expression analysis of transgenes by sqRT-PCR. This method resulted in a screening efficiency of 6.05% in the presence of kanamycin, indicating amenability of in planta flax transformation. The strategy can be a promising tool for the successful development of transgenics in flax.

Engineering Chickpea Variety HC-1 with OsRuvB Gene for Salt Stress Tolerance

Background: Salinity is a major problem worldwide and is increasing day by day. Salt stress causes severe yield losses in crop plants and the damages in chickpea are up to 100%. To overcome these losses, the present study was undertaken to develop transgenic chickpea plants (var. HC-1) carrying OsRuvB gene for salt stress tolerance. Methods: Transgenic chickpea plants harboring OsRuvB gene were developed using Agrobacterium-mediated transformation. T0 putative transgenic chickpea plants were screened for the presence of OsRuvB gene through PCR using gene specific primers. The stable integration and copy number of transgene in transgenic chickpea plants were confirmed through Southern hybridization and qRT-PCR. T1 generation transgenic chickpea plants were screened for the presence of OsRuvB gene using direct PCR (Phire Direct PCR kit). Result: PCR-based screening of putative transformants using gene-specific primers showed a transformation frequency of 17%. Southern blot and real-time PCR analysis revealed stable and single-copy insertion. In T1 generation a total of 74 plants (out of 170) showed the presence of OsRuvB gene. The engineered lines developed in the present investigation can be further undertaken to develop transgenic chickpea plants for salt stress tolerance.

Influence of Selected Antibiotics on the Tomato Regeneration in In Vitro Cultures

Generally, antimicrobial agents are frequently used in micropropagation techniques to obtain free elite clones or after genetic transformation to select putative transformants. Their successful application minimizes bacterial contamination however; they may be phytotoxic and may diversely affect the regeneration ability in plant tissue cultures. The objective of the current study was to estimate the effects of four antibiotics i.e. ampicillin, carbenicillin, cefotaxime and kanamycin on morphogenesis of three Polish tomato cultivars, cultured on MS medium with phytohormones. In this experiment the ability of tomato cotyledon explants to regenerate entire plants via indirect organogenesis was tested. Among four antibiotics tested, kanamycin was most harmful for the explants. This antibiotic, even at low doses (10-20 mg/L), inhibited tomato morphogenesis. On the other hand, the current study revealed significant influence of different concentrations of ampicillin, carbenicillin, cefotaxime on the frequency of bud formation. While the addition of cefotaxime at low concentration (100-200 mg/L) stimulated the bud formation, its increasing concentration adversely affected the organogenesis of tomato. The results clearly pointed out that carbenicillin and ampicillin at low concentrations (100-400 mg/L) were not only non-toxic, but they promoted bud regeneration. The obtained results show the crucial role not only of use of efficient antibiotics, but also of their proper doses in obtaining successful transformation and regeneration of tomato. ********* In press - Online First. Article has been peer reviewed, accepted for publication and published online without pagination. It will receive pagination when the issue will be ready for publishing as a complete number (Volume 47, Issue 3, 2019). The article is searchable and citable by Digital Object Identifier (DOI). DOI link will become active after the article will be included in the complete issue. *********

USING MANNOSE AS A POSITIVE SELECTION OF TRANSFORMED CARICA PAPAYA L. VAR ‘EKSOTIKA’

The main objective of this study is to develop marker-free transgenic papaya plants via positive selection using phosphomannose isomerase (pmi) gene. Phosphomannose isomerase (PMI) is an enzyme that converts mannose-6-phosphate to fructose-6-phosphate, a glycolysis intermediate that supports the growth of plant cells. To establish a marker-free positive selection system using this PMI, the effect of mannose on the growth and development of embryogenic ‘Eksotika’ papaya callus was evaluated. One-month old embryogenic calli were cultured on Murashige and Skoog (MS) medium in which 60 g/L sucrose in the original recipe was replaced with different concentrations of mannose and sucrose. Mannose was supplied as the sole carbon source or in combination with sucrose at 0, 5, 10, 15, 20, 25 or 30 g/L. Embryogenic calli cultured on medium supplemented with a ratio of 0:60 g/L mannose: sucrose was used as a control. The results after six sub-cultures showed that most of the embryogenic calli transferred on media containing only mannose turned brown. Higher concentrations of mannose resulted in higher percentage of brown calli (dead). Mannose at 30 g/L mannose was found to be effective for screening transformed embryogenic calli. Evaluation of papaya transformation efficiency using this positive selection system was pursued using 650 one-month-old embryogenic calli Agrobacterium-transformed with pNOV2819 harboring the pmi gene. Only transformed cells are capable of utilizing mannose as a carbon source to grow. After five months on mannose selection, all 67 putative transformants obtained were PCR-positives for the pmi gene.

Agrobacterium-mediated transformation of chitinase gene from the actinorhizal tree Casuarina equisetifolia in Nicotiana tabacum

Genetic transformation of plants offers the possibility of testing hypotheses on the function of individual genes and enables exploration of transgenes for targeted trait improvement. Cloning of the full-length class I chitinase from the Casuarina equisetifolia (CeChi1) was earlier reported by our team. In the present study, tobacco was used as a model system to functionally evaluate the potential of CeChi1 driven by ubiquitin promoter. The pUH-CeChi1 construct was introduced into tobacco by Agrobacterium-mediated transformation and the putative transformants were confirmed for stable gene integration, transgene expression and recombinant protein production using PCR, RT-qPCR, antifungal assays and in planta analysis. The in vitro antifungal bioassay using the total proteins from leaves of transformed plantlets revealed the characteristic lysis of hyphal tips of pathogenic fungi including Trichosporium vesiculosum, Fusarium oxysporum and Rhizoctonia solani. The in planta bioassay of transformed tobacco showed reduced disease symptoms when compared to untransformed wild plants. The study revealed that the class I chitinase isolated from C. equisetifolia can act as a potential gene resource in future transformation programs for incorporating disease tolerance.

Efficient use of the PMI/mannose selection system in Agrobacterium-mediated transformation of tobacco (Nicotiana tabacum)

A safe and environment friendly selectable marker system using phosphomannose isomerase (pmi) gene was employed for Agrobacterium tumefaciens mediated transformation of tobacco (Nicotinia tabacum L.). Sensitivity of tobacco leaf explants to mannose as selective agent was determined before genetic transformation. Twenty mg/L mannose was chosen for selection to suppress the emergence of untransformed shoots from the explants. Leaf explants from 4-week old seedlings of tobacco cv. Gewone Groene were transformed using Agrobacterium harbouring plasmid pCAMBIA3300-PMI carrying the pmi gene. Regenerating transformed shoots were selected on Murashige & Skoog basal medium supplemented with 1.0 mg/L 6-benzylaminopurine, 0.1 mg/L indole-3-butyric acid, and 20 g/L mannose as carbon source. The mannose-resistant shoots were rooted on a plant growth regulator-free Murashige & Skoog basal medium and the potted transgenic plants were acclimatized successfully in the greenhouse, with a survival rate of 100%. Selection efficiency and transformation efficiency based on PCR analysis of individual putative transformants were achieved as high as 94.56% and 21.75%, respectively, with a lower rate of untransformed plant escapes. Southern blot analysis of selected putative transformed plants confirmed the transgene integration into the tobacco genome. The following reverse transcription PCR analysis showed transcriptional activity of the pmi transgene in all T0 transgenic plants analysed with differences in the level of pmi transcripts, and chlorophenol red assay confirmed the activity of PMI in transgenic plants. The PMI/mannose-based transformation system presented here is efficient and reproducible, and can be used for the mass production of transgenic tobacco plants to produce novel products with industrial and medicinal values.

An Agrobacterium mediated transformation system of guava (Psidium guajava L.) with endochitinase gene

Genetic transformation of guava (Psidium guajava L.) was developed for the first time using in vitro grown shoot tip explant co-cultivated with Agrobacterium tumefaciens strain LBA4404 harbouring binary vector pIIHR-JBMch with endochitinase and nptII genes. The highest transformation efficiency was achieved by wounding explants with tungsten particles (0.5 µm) through particle acceleration system, followed by infection for 45 minutes with A. tumefaciens, grown overnight with 100 µM acetosyringone, corresponding to OD600=0.5 followed by co-cultivation for 72 hours under dark condition on co-cultivation medium (MS+100 µM acetosyringone+100 mg L-1 L-Cystein). Putative transformed explants regenerated shoots on selection medium stressed with 200 mg L-1 kanamycin for 12 weeks. Molecular analysis of putative transformants by PCR confirmed the integration of endochitinase and nptII gene in the plant nuclear genome.