Development of Selectable Marker-Free Transgenic Rice Plants with Enhanced Seed Tocopherol Content through FLP/FRT-Mediated Spontaneous Auto-Excision

A Prospective Review on Selectable Marker-Free Genome Engineered Rice: Past, Present and Future Scientific Realm

As a staple food crop, rice has gained mainstream attention in genome engineering for its genetic improvement. Genome engineering technologies such as transgenic and genome editing have enabled the significant improvement of target traits in relation to various biotic and abiotic aspects as well as nutrition, for which genetic diversity is lacking. In comparison to conventional breeding, genome engineering techniques are more precise and less time-consuming. However, one of the major issues with biotech rice commercialization is the utilization of selectable marker genes (SMGs) in the vector construct, which when incorporated into the genome are considered to pose risks to human health, the environment, and biodiversity, and thus become a matter of regulation. Various conventional strategies (co-transformation, transposon, recombinase systems, and MAT-vector) have been used in rice to avoid or remove the SMG from the developed events. However, the major limitations of these methods are; time-consuming, leftover cryptic sequences in the genome, and there is variable frequency. In contrast to these methods, CRISPR/Cas9-based marker excision, marker-free targeted gene insertion, programmed self-elimination, and RNP-based delivery enable us to generate marker-free engineered rice plants precisely and in less time. Although the CRISPR/Cas9-based SMG-free approaches are in their early stages, further research and their utilization in rice could help to break the regulatory barrier in its commercialization. In the current review, we have discussed the limitations of traditional methods followed by advanced techniques. We have also proposed a hypothesis, “DNA-free marker-less transformation” to overcome the regulatory barriers posed by SMGs.

Marker-Free Rice (Oryza sativa L. cv. IR 64) Overexpressing PDH45 Gene Confers Salinity Tolerance by Maintaining Photosynthesis and Antioxidant Machinery

Helicases function as key enzymes in salinity stress tolerance, and the role and function of PDH45 (pea DNA helicase 45) in stress tolerance have been reported in different crops with selectable markers, raising public and regulatory concerns. In the present study, we developed five lines of marker-free PDH45-overexpressing transgenic lines of rice (Oryza sativa L. cv. IR64). The overexpression of PDH45 driven by CaMV35S promoter in transgenic rice conferred high salinity (200 mM NaCl) tolerance in the T1 generation. Molecular attributes such as PCR, RT-PCR, and Southern and Western blot analyses confirmed stable integration and expression of the PDH45 gene in the PDH45-overexpressing lines. We observed higher endogenous levels of sugars (glucose and fructose) and hormones (GA, zeatin, and IAA) in the transgenic lines in comparison to control plants (empty vector (VC) and wild type (WT)) under salt treatments. Furthermore, photosynthetic characteristics such as net photosynthetic rate (Pn), stomatal conductance (gs), intercellular CO2 (Ci), and chlorophyll (Chl) content were significantly higher in transgenic lines under salinity stress as compared to control plants. However, the maximum primary photochemical efficiency of PSII, as an estimated from variable to maximum chlorophyll a fluorescence (Fv/Fm), was identical in the transgenics to that in the control plants. The activities of antioxidant enzymes, such as catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR), and guaiacol peroxidase (GPX), were significantly higher in transgenic lines in comparison to control plants, which helped in keeping the oxidative stress burden (MDA and H2O2) lesser on transgenic lines, thus protecting the growth and photosynthetic efficiency of the plants. Overall, the present research reports the development of marker-free PDH45-overexpressing transgenic lines for salt tolerance that can potentially avoid public and biosafety concerns and facilitate the commercialization of genetically engineered crop plants.

Application of the FLP/LoxP-FRT recombination system to switch the eGFP expression in a model prokaryote

In prokaryotes, few studies have applied the flippase (FLP)/P1-flippase recombination target (LoxP-FRT) recombination system to switch gene expression. This study developed a new method for switching gene expression by constructing an FLP/LoxP-FRT site-specific recombination system in Escherichia coli. To this end, we placed the Nos terminator flanked by a pair of LoxP-FRT in front of enhanced green fluorescent protein (eGFP). The Nos terminator was used to block the expression of the eGFP. When a plasmid expressing FLP was available, deletion of the Nos terminator would allow expression of eGFP. The regulatory effect was demonstrated by eGFP expression. The efficiency of the gene switch was calculated as high as 89.67%. The results showed that the FLP/LoxP-FRT recombinase system could be used as a gene switch to regulate gene expression in prokaryotes. This new method for switching gene expression could simplify the gene function analysis in E. coli and other prokaryotes, as well as eukaryotes.

The steroid-inducible pOp6/LhGR gene expression system is fast, sensitive and does not cause plant growth defects in rice (Oryza sativa)

Inducible systems for transgene expression activated by a chemical inducer or an inducer of non-plant origin are desirable tools for both basic plant research and biotechnology. Although, the technology has been widely exploited in dicotyledonous model plants such as Arabidopsis, it has not been optimised for use with the monocotyledonous model species, namely rice. We have adapted the dexamethasone-inducible pOp6/LhGR system for rice and the results indicated that it is fast, sensitive and tightly regulated, with high levels of induction that remain stable over several generations. Most importantly, we have shown that the system does not cause negative growth defects in vitro or in soil grown plants. Interestingly in the process of testing, we found that another steroid, triamcinolone acetonide, is a more potent inducer in rice than dexamethasone. We present serious considerations for the construct design to avoid undesirable effects caused by the system in plants, leakiness and possible silencing, as well as simple steps to maximize translation efficiency of a gene of interest. Finally, we compare the performance of the pOp6/LhGR system with other chemically inducible systems tested in rice in terms of the properties of an ideal inducible system.

An effective method for Agrobacterium tumefaciens-mediated transformation of Jatropha curcas L. using cotyledon explants

Jatropha curcas is one of oilseed crops and has been considered as an energy crop. In the present study, efficient plant regeneration protocol and transformation method were developed for J. curcas. Because the regeneration efficiency of adventitious bud from cotyledon explants of J. curcas induced by traditional methods is low, and it takes a long time to get complete plants. It is necessary to establish a new regeneration system to improve regeneration efficiency. Cotyledon explants were dipped into TDZ solution at different concentrations respectively for various times to obtain higher efficiency of adventitious bud regeneration. This new regeneration method was then applied to genetic transformation of J. curcas. Cotyledon explants were precultured for 1 day after treated with high concentration of Thidiazuron (TDZ) solution (20 mg/L for 40 min), followed by Agrobacterium tumefaciens infection. After co-cultured for 2 days, the explants were placed on the induction hormone-free media for bud regeneration and resistant screening. After 30 days, selected shoot buds were transferred onto elongation medium for 15 days. Young leaf sections of the regenerated shoots were used for PCR (Polymerase chain reaction) detection of the transgenic shoots. The PCR positive shoots were isolated and used for in vitro grafting. The intact plants were obtained within 20 days. GUS (β-Glucosidase) staining and Southern analysis confirmed the transformation events. Briefly, a transformation efficiency of 34.32% was achieved and an intact transgenic plant could be obtained within 65 days.

Selection of ideotype to increase yield potential of GM and non-GM rice cultivars

Using classical breeding, plant breeders envision a plant type whose yield they aim to enhance by selecting for individual traits to create model/ideal plants or ideotypes. To achieve this, those factors restricting yield need to be identified and controlled through the use of new technologies to achieve the desired ideotype. This study aimed to determine the ideotype of seven genetically modified (GM) and non-GM rice (Oryza sativa L.) cultivars. Field experiments were carried out in three isolated regions in the north of Iran under the Iranian bio-safety standard protocol. Four of the GM cultivars carried the cry1Ab gene in the vegetative stage while three non-GM cultivars served as the control. R² values showed that five, six and seven variables in Sari, Amol and Rasht regions accounted for 63 %, 52 % and 74 % of paddy yield variation, respectively. In the same three regions, paddy yield variation due to white heads accounted for 28.38 %, 8.45 % and 3.95 % of the total variation in paddy yield, respectively. The total estimated variation in paddy yield in Sari, Amol and Rasht was 1810.50, 2377.6 and 2176.47 kg ha^-1, respectively. Average data over the three regions indicated that highest loss in paddy yield was observed in non-GM 'Nemat', 'Khazar' and 'Tarom Hashemi'. GM cultivars derived from 'Khazar' showed significantly lower paddy yield loss than the non-GM parent. Dead heart, a condition that occurs in the vegetative stage in which the stem borer larva enters the stem and feeds on the growing shoot, causing the central shoot to dry, as well as white heads, which is a condition in which whole ear heads of adult plants become dry and yield chaffy grains, in all three regions were important variables contributing to paddy yield loss. In the future, producing GM rice resistant to striped stem borer with an active promoter in the reproductive growth stage might allow farmers to reduce a significant part of paddy yield loss resulting from white heads, which is directly negatively correlated with filled spikelets per panicle (R² = -0.57**), in order to achieve an ideotype.

The Current Status and Development of Insect-Resistant Genetically Engineered Poplar in China

Poplar is one of the main afforestation tree species in China, and the use of a single, or only a few, clones with low genetic diversity in poplar plantations has led to increasing problems with insect pests. The use of genetic engineering to cultivate insect-resistant poplar varieties has become a hot topic. Over the past 20 years, there have been remarkable achievements in this area. To date, nearly 22 insect-resistant poplar varieties have been created and approved for small-scale field testing, environmental release, or pilot-scale production. Here, we comprehensively review the development of insect-resistant genetically modified (GM) poplars in China. This review mostly addresses issues surrounding the regulation and commercialization of Bt poplar in China, the various insecticidal genes used, the effects of transgenic poplars on insects, toxic protein expression, multigene transformation, the stability of insect resistance, and biosafety. The efficacy of GM poplars for pest control differed among different transgenic poplar clones, larval instars, and insect species. The Bt protein analysis revealed that the expression level of Cry3A was significantly higher than that of Cry1Ac. Temporal and spatial studies of Bt protein showed that its expression varied with the developmental stage and tissue. The inheritance and expression of the exogenous gene were reviewed in transgenic hybrid poplar progeny lines and grafted sections. Biosafety issues, in terms of transgene stability and the effects on soil microorganisms, natural enemies of insects, and arthropod communities are also discussed.

Enabling Molecular Technologies for Trait Improvement in Wheat

Wheat is the major staple food crop and a source of calories for humans worldwide. A steady increase in the wheat production is essential to meet the demands of an ever-increasing global population and to achieve food security. The large size and structurally intricate genome of polyploid wheat had hindered the genomic analysis. However, with the advent of new genomic technologies such as next generation sequencing has led to genome drafts for bread wheat and its progenitors and has paved the way to design new strategies for crop improvement. Here we provide an overview of the advancements made in wheat genomics together with the available "omics approaches" and bioinformatics resources developed for wheat research. Advances in genomic, transcriptomic, and metabolomic technologies are highlighted as options to circumvent existing bottlenecks in the phenotypic and genomic selection and gene transfer. The contemporary reverse genetics approaches, including the novel genome editing techniques to inform targeted manipulation of a single/multiple genes and strategies for generating marker-free transgenic wheat plants, emphasize potential to revolutionize wheat improvement shortly.

Development of Marker-Free Insect-Resistant Indica Rice by Agrobacterium tumefaciens-Mediated Co-transformation

Agrobacterium-mediated co-transformation is an efficient strategy to generate marker-free transgenic plants. In this study, the vectors pMF-2A^∗ containing a synthetic cry2A^∗ gene driven by maize ubiquitin promoter and pCAMBIA1301 harboring hygromycin phosphotransferase gene (hpt) were introduced into Minghui86 (Oryza sativa L. ssp. indica), an elite indica restorer line. Two independent transformants containing both the cry2A^∗ gene and hpt gene were regenerated. Several homozygous marker-free transgenic progenies were derived from family 2AH2, and three of them were selected for further insect bioassay in the laboratory and field. Insect-resistance assays revealed that all the three transgenic lines were highly resistant to striped stem borer (Chilo suppressalis), yellow stem borer (Tryporyza incertulas) and rice leaf folder (Cnaphalocrocis medinalis). The measurement of Cry2A protein concentration showed that Cry2A protein was stably expressed in leaves and stems of homozygous transgenic lines and their hybrids. The yields of the marker-free homozygous transgenic lines and their hybrids were not significantly different from those of their corresponding controls. Furthermore, the results of flanking sequence isolation showed that the T-DNA in line 8-30 was integrated into the intergenic region of chromosome 2 (between Os02g43680 and Os02g43690). These results indicate that the marker-free transgenic rice has the potential for commercial production.