Generation of Marker- and/or Backbone-Free Transgenic Wheat Plants via Agrobacterium-Mediated Transformation

Enhancing wheat resilience: biotechnological advances in combating heat stress and environmental challenges

Climate change, with its increasing temperatures, is significantly disrupting global agricultural systems, and wheat, a key cereal crop faces severe challenges. Heat stress has emerged as a critical threat, accelerating wheat growth, leading to premature maturation, reduced grain filling, and ultimately lower yields. The situation is exacerbated by more frequent and intense heat waves, particularly in regions already struggling with water scarcity. Maintaining the delicate balance of temperature and water necessary for optimal wheat production is becoming challenging, posing a serious risk to global food security. Therefore, there is an urgent need to develop adaptive strategies with innovations in breeding and transgenic technologies crucial to improving wheat resilience to environmental stresses, especially to combat the growing impacts of heat stress. Modern tools like CRISPR/Cas9, Transcription Activator-Like Effector Nucleases, and Zinc Finger Nucleases have been instrumental in developing wheat varieties with improved traits. However, the future of wheat cultivation requires more than just resistance to a single stressor. As climate change intensifies, there is an urgent need for wheat varieties that can withstand multiple stresses, including heat, drought, and pests. Developing these multi-stress-tolerant cultivars is crucial for ensuring food security in a rapidly changing climate.

Interplays of Cuticle Biosynthesis and Stomatal Development: From Epidermal Adaptation to Crop Improvement

Crop production is limited by environmental stresses such as a water deficit, salinity, and extreme temperature. Lipophilic cuticle and stomatal pore govern plant transpirational water loss and photosynthetic gas exchange and contribute to plant adaptation to stressful environments. Intricate interplays between cuticle biosynthesis and stomatal development are supported by increasing evidence from phenotypic observations. Several mutants, initially identified as being deficient in cuticle development, have exhibited altered phenotypes in terms of stomatal ridges, numbers, patterns, and shapes. Similarly, mutants with abnormal stomatal patterning have shown defective cuticle formation. Recently, signaling components and transcription factors orchestrating cuticle biosynthesis and stomatal formation have been characterized in both model and crop plants. In this review, we summarize the genetic interplay between cuticle biosynthesis and stomata formation. Current strategies and future perspectives on exploiting the intertwined cuticle biosynthesis and stomatal development for crop stress resistance improvement are discussed.

An established protocol for generating transgenic wheat for wheat functional genomics via particle bombardment

Wheat is one of the most important food crops in the world and is considered one of the top targets in crop biotechnology. With the high-quality reference genomes of wheat and its relative species and the recent burst of genomic resources in Triticeae, demands to perform gene functional studies in wheat and genetic improvement have been rapidly increasing, requiring that production of transgenic wheat should become a routine technique. While established for more than 20 years, the particle bombardment-mediated wheat transformation has not become routine yet, with only a handful of labs being proficient in this technique. This could be due to, at least partly, the low transformation efficiency and the technical difficulties. Here, we describe the current version of this method through adaptation and optimization. We report the detailed protocol of producing transgenic wheat by the particle gun, including several critical steps, from the selection of appropriate explants (i.e., immature scutella), the preparation of DNA-coated gold particles, and several established strategies of tissue culture. More importantly, with over 20 years of experience in wheat transformation in our lab, we share the many technical details and recommendations and emphasize that the particle bombardment-mediated approach has fewer limitations in genotype dependency and vector construction when compared with the Agrobacterium-mediated methods. The particle bombardment-mediated method has been successful for over 30 wheat genotypes, from the tetraploid durum wheat to the hexaploid common wheat, from modern elite varieties to landraces. In conclusion, the particle bombardment-mediated wheat transformation has demonstrated its potential and wide applications, and the full set of protocol, experience, and successful reports in many wheat genotypes described here will further its impacts, making it a routine and robust technique in crop research labs worldwide.

Exploring and exploiting cuticle biosynthesis for abiotic and biotic stress tolerance in wheat and barley

Wheat and barley are widely distributed cereal crops whose yields are adversely affected by environmental stresses such as drought, salinity, extreme temperatures, and attacks of pathogens and pests. As the interphase between aerial plant organs and their environments, hydrophobic cuticle largely consists of a cutin matrix impregnated and sealed with cuticular waxes. Increasing evidence supports that the cuticle plays a key role in plant adaptation to abiotic and biotic stresses, which could be harnessed for wheat and barley improvement. In this review, we highlighted recent advances in cuticle biosynthesis and its multifaceted roles in abiotic and biotic stress tolerance of wheat and barley. Current strategies, challenges, and future perspectives on manipulating cuticle biosynthesis for abiotic and biotic stress tolerance in wheat and barley are discussed.

Studies on Pure Mlb® (Multiple Left Border) Technology and Its Impact on Vector Backbone Integration in Transgenic Cassava

Imperfect T-DNA processing is common during Agrobacterium-mediated transformation, which integrates vector backbone sequences into the plant genome. However, regulatory restrictions prevent such transgenic plants from being developed for commercial deployment. The binary vector pCAMBIA2300 was modified by incorporating multiple left border (Mlb^®) repeats and was tested in BY2 cells, tobacco, and cassava plants to address this issue. PCR analyses confirmed a twofold increase in the vector backbone free events in the presence of triple left borders in all three systems tested. Vector backbone read-through past the LB was reduced significantly; however, the inclusion of Mlbs^® did not effectively address the beyond right border read-through. Also, Mlbs^® increased the frequency of single-copy and vector backbone free events (clean events) twice compared to a single LB construct. Here, we briefly narrate the strength and limitations of using Mlb^® technology and reporter genes in reducing the vector backbone transfer in transgenic events.

Condensed Tannins in White Clover (Trifolium repens) Foliar Tissues Expressing the Transcription Factor TaMYB14-1 Bind to Forage Protein and Reduce Ammonia and Methane Emissions in vitro

Grazing ruminants contribute to global climate change through enteric methane and nitrous oxide emissions. However, animal consumption of the plant polyphenolics, proanthocyanidins, or condensed tannins (CTs) can decrease both methane emissions and urine nitrogen levels, leading to reduced nitrous oxide emissions, and concomitantly increase animal health and production. CTs are largely absent in the foliage of important temperate pasture legumes, such as white clover (Trifolium repens), but found in flowers and seed coats. Attempts at enhancing levels of CT expression in white clover leaves by mutagenesis and breeding have not been successful. However, the transformation of white clover with the TaMYB14-1 transcription factor from Trifolium arvense has resulted in the production of CTs in leaves up to 1.2% of dry matter (DM). In this study, two generations of breeding elevated foliar CTs to >2% of DM. The CTs consisted predominantly of prodelphinidins (PD, 75-93%) and procyanidins (PC, 17-25%) and had a mean degree of polymerization (mDP) of approximately 10 flavan-3-ol subunits. In vitro studies showed that foliar CTs were bound to bovine serum albumin and white clover proteins at pH 6.5 and were released at pH 2.-2.5. Using rumen in vitro assays, white clover leaves containing soluble CTs of 1.6-2.4% of DM significantly reduced methane production by 19% (p ≤0.01) and ammonia production by 60% (p ≤ 0.01) relative to non-transformed wild type (WT) controls after 6 h of incubation. These results provide valuable information for further studies using CT expressing white clover leaves for bloat prevention and reduced greenhouse gas emissions in vivo.

Efficient expression and function of a receptor-like kinase in wheat powdery mildew defence require an intron-located MYB binding site

The LRK10-like receptor kinases (LRK10L-RLKs) are ubiquitously present in higher plants, but knowledge of their expression and function is still limited. Here, we report expression and functional analysis of TtdLRK10L-1, a typical LRK10L-RLK in durum wheat (Triticum turgidum L. ssp. durum). The introns of TtdLRK10L-1 contained multiple kinds of predicted cis-elements. To investigate the potential effect of these cis-elements on TtdLRK10L-1 expression and function, two types of transgenic wheat lines were prepared, which expressed a GFP-tagged TtdLRK10L-1 protein (TtdLRK10L-1:GFP) from the cDNA or genomic DNA (gDNA) sequence of TtdLRK10L-1 under the native promoter. TtdLRK10L-1:GFP expression was up-regulated by the powdery mildew pathogen Blumeria graminis f. sp. tritici (Bgt) in both types of transgenic plants, with the scale of the elevation being much stronger in the gDNA lines. Both types of transgenic plants exhibited enhanced resistance to Bgt infection relative to wild type control. Notably, the Bgt defence activated in the gDNA lines was significantly stronger than that in the cDNA lines. Further analysis revealed that a putative MYB transcription factor binding site (MYB-BS, CAGTTA) located in TtdLRK10L-1 intron I was critical for the efficient expression and function of TtdLRK10L-1 in Bgt defence. This MYB-BS could also increase the activity of a superpromoter widely used in ectopic gene expression studies in plants. Together, our results deepen the understanding of the expression and functional characteristics of LRK10L-RLKs. TtdLRK10L-1 is likely useful for further dissecting the molecular processes underlying wheat defence against Bgt and for developing Bgt resistant wheat crops.

Overexpression of Soybean-Derived Lunasin in Wheat and Assessment of Its Anti-Proliferative Activity in Colorectal Cancer HT-29 Cells

Lunasin is a soybean-derived peptide that exhibits anticancer bioactivity in different cancer cells and has been identified in different plants. However, recent studies revealed through molecular and chemical analyses that lunasin was absent in wheat and other cereals. In this study, the soybean-derived lunasin was cloned into pCAMBIA3300 and we transferred the expression vector into wheat via an Agrobacterium-mediated transformation. The identification of transgenic wheat was detected by polymerase chain reaction, Western blot analysis, and ultra-performance liquid chromatography with tandem mass spectrometry. An enzyme-linked immunosorbent assay showed that lunasin content in transgenic wheat L32-3, L32-6, and L33-1 was 308.63, 436.78, and 349.07 µg/g, respectively, while lunasin was not detected in wild-type wheat. Lunasin enrichment from transgenic wheat displayed an increased anti-proliferative activity compared with peptide enrichment from wild-type wheat in HT-29 cells. Moreover, the results of a real-time quantitative polymerase chain reaction showed a significant elevation in p21, Bax, and caspase-3 expression, while Bcl-2 was significantly downregulated. In conclusion, soybean-derived lunasin was successfully expressed in wheat via Agrobacterium-mediated transformation and may exert anti-proliferative activity by regulating the apoptosis pathway in HT-29 cells, which provides an effective approach to compensate for the absence of lunasin in wheat.

An optimized double T-DNA binary vector system for improved production of marker-free transgenic tobacco plants

OBJECTIVES

In the plant transformation process, marker genes play a vital role in identifying transformed cells from non-transformed cells. However, once transgenic plants have been obtained, the presence of marker genes may provoke public concern about environmental or biosafety issues. In our previous study, a double T-DNA vector system has been developed to obtain marker-free transgenic plants, but the T-DNA left border (LB) and right border (RB) of the vector showed an RB-LB-RB-LB pattern and led to high linkage integration between the selectable marker gene (SMG) and the gene of interest (GOI). To improve this double T-DNA vector system, we inverted the first T-DNA direction such that a LB-RB-RB-LB pattern resulted to avoid transcriptional read-through at the LB and the subsequent linkage transfer of the SMG and GOI.

RESULTS

We separately inserted the green fluorescent protein (GFP) gene as the GOI and the neomycin phosphotransferase II (NPTII) gene as the SMG in both optimized and original vectors and carried out Agrobacterium-mediated tobacco transformation. Statistical analysis revealed that the linkage frequency was 25.6% in T0 plants transformed with the optimized vector, which is a 42.1% decrease compared with that of the original vector (44.2%). The frequency of obtaining marker-free transgenic plants was 66.7% in T1 plants transformed with the optimized vector, showing a 33.4% increase compared with that of the original vector (50.0%).

CONCLUSION

Our results demonstrate that the optimized double T-DNA binary vector system is a more effective, economical and time-saving approach for obtaining marker-free transgenic plants.

Principles, Applications, and Biosafety of Plant Genome Editing Using CRISPR-Cas9

The terms genome engineering, genome editing, and gene editing, refer to modifications (insertions, deletions, substitutions) in the genome of a living organism. The most widely used approach to genome editing nowadays is based on Clustered Regularly Interspaced Short Palindromic Repeats and associated protein 9 (CRISPR-Cas9). In prokaryotes, CRISPR-Cas9 is an adaptive immune system that naturally protects cells from DNA virus infections. CRISPR-Cas9 has been modified to create a versatile genome editing technology that has a wide diversity of applications in medicine, agriculture, and basic studies of gene functions. CRISPR-Cas9 has been used in a growing number of monocot and dicot plant species to enhance yield, quality, and nutritional value, to introduce or enhance tolerance to biotic and abiotic stresses, among other applications. Although biosafety concerns remain, genome editing is a promising technology with potential to contribute to food production for the benefit of the growing human population. Here, we review the principles, current advances and applications of CRISPR-Cas9-based gene editing in crop improvement. We also address biosafety concerns and show that humans have been exposed to Cas9 protein homologues long before the use of CRISPR-Cas9 in genome editing.

Development and characterization of marker-free and transgene insertion site-defined transgenic wheat with improved grain storability and fatty acid content

Development of marker-free and transgene insertion site-defined (MFTID) transgenic plants is essential for safe application of transgenic crops. However, MFTID plants have not been reported for wheat (Triticum aestivum). Here, we prepared a RNAi cassette for suppressing lipoxygenase (LOX) gene expression in wheat grains using a double right border T-DNA vector. The resultant construct was introduced into wheat genome via Agrobacterium-mediated transformation, with four homozygous marker-free transgenic lines (namely GLRW-1, -3, -5 and -8) developed. Aided by the newly published wheat genome sequence, the T-DNA insertion sites in GLRW-3 and GLRW-8 were elucidated at base-pair resolution. While the T-DNA in GLRW-3 inserted in an intergenic region, that of GLRW-8 inactivated an endogenous gene, which was thus excluded from further analysis. Compared to wild -type (WT) control, GLRW-1, -3 and -5 showed decreased LOX gene expression, lower LOX activity and less lipid peroxidation in the grains; they also exhibited significantly higher germination rates and better seedling growth after artificial ageing treatment. Interestingly, the three GLRW lines also had substantially increased contents of several fatty acids (e.g., linoleic acid and linolenic acid) in their grain and flour samples than WT control. Collectively, our data suggest that suppression of grain LOX activity can be employed to improve the storability and fatty acid content of wheat seeds and that the MFTID line GLRW-3 is likely of commercial value. Our approach may also be useful for developing the MFTID transgenic lines of other crops with enhanced grain storability and fatty acid content.

An efficient and reproducible Agrobacterium-mediated transformation method for hexaploid wheat (Triticum aestivum L.)

BACKGROUND

Despite wheat being a worldwide staple, it is still considered the most difficult to transform out of the main cereal crops. Therefore, for the wheat research community, a freely available and effective wheat transformation system is still greatly needed.

RESULTS

We have developed and optimised a reproducible Agrobacterium-mediated transformation system for the spring wheat cv 'Fielder' that yields transformation efficiencies of up to 25%. We report on some of the important factors that influence transformation efficiencies. In particular, these include donor plant health, stage of the donor material, pre-treatment by centrifugation, vector type and selection cassette. Transgene copy number data for independent plants regenerated from the same original immature embryo suggests that multiple transgenic events arise from single immature embryos, therefore, actual efficiencies might be even higher than those reported.

CONCLUSION

We reported here a high-throughput, highly efficient and repeatable transformation system for wheat and this system has been used successfully to introduce genes of interest, for RNAi, over-expression and for CRISPR-Cas9 based genome editing.

Development of wheat genotypes expressing a glutamine-specific endoprotease from barley and a prolyl endopeptidase from Flavobacterium meningosepticum or Pyrococcus furiosus as a potential remedy to celiac disease

Ubiquitous nature of prolamin proteins dubbed gluten from wheat and allied cereals imposes a major challenge in the treatment of celiac disease, an autoimmune disorder with no known treatment other than abstinence diet. Administration of hydrolytic glutenases as food supplement is an alternative to deliver the therapeutic agents directly to the small intestine, where sensitization of immune system and downstream reactions take place. The aim of the present research was to evaluate the capacity of wheat grain to express and store hydrolytic enzymes capable of gluten detoxification. For this purpose, wheat scutellar calli were biolistically transformed to generate plants expressing a combination of glutenase genes for prolamin detoxification. Digestion of prolamins with barley endoprotease B2 (EP-HvB2) combined with Flavobacterium meningosepticum prolyl endopeptidase (PE-FmPep) or Pyrococcus furiosus prolyl endopeptidase (PE-PfuPep) significantly reduced (up to 67%) the amount of the indigestible gluten peptides of all prolamin families tested. Seven of the 168 generated lines showed inheritance of transgene to the T₂ generation. Reversed phase high-performance liquid chromatography of gluten extracts under simulated gastrointestinal conditions allowed the identification of five T₂ lines that contained significantly reduced amounts of immunogenic, celiac disease-provoking gliadin peptides. These findings were complemented by the R5 ELISA test results where up to 72% reduction was observed in the content of immunogenic peptides. The developed wheat genotypes open new horizons for treating celiac disease by an intraluminal enzyme therapy without compromising their agronomical performance.

An Insight into T-DNA Integration Events in Medicago sativa

The molecular mechanisms of transferred DNA (T-DNA) integration into the plant genome are still not completely understood. A large number of integration events have been analyzed in different species, shedding light on the molecular mechanisms involved, and on the frequent transfer of vector sequences outside the T-DNA borders, the so-called vector backbone (VB) sequences. In this work, we characterized 46 transgenic alfalfa (Medicago sativa L.) plants (events), generated in previous works, for the presence of VB tracts, and sequenced several T-DNA/genomic DNA (gDNA) junctions. We observed that about 29% of the transgenic events contained VB sequences, within the range reported in other species. Sequence analysis of the T-DNA/gDNA junctions evidenced larger deletions at LBs compared to RBs and insertions probably originated by different integration mechanisms. Overall, our findings in alfalfa are consistent with those in other plant species. This work extends the knowledge on the molecular events of T-DNA integration and can help to design better transformation protocols for alfalfa.

CRISPR-Cas9 Targeted Mutagenesis Leads to Simultaneous Modification of Different Homoeologous Gene Copies in Polyploid Oilseed Rape (Brassica napus)

In polyploid species, altering a trait by random mutagenesis is highly inefficient due to gene redundancy. We have stably transformed tetraploid oilseed rape (Brassica napus) with a CRISPR-Cas9 construct targeting two ALCATRAZ (ALC) homoeologs. ALC is involved in valve margin development and, thus, contributes to seed shattering from mature fruits. Knocking out ALC would increase shatter resistance to avoid seed loss during mechanical harvest. We obtained a transgenic T1 plant with four alc mutant alleles by the use of a single target sequence. All mutations were stably inherited to the T2 progeny. The T2 generation was devoid of any wild-type alleles, proving that the underlying T1 was a nonchimeric double heterozygote. T-DNA and ALC loci were not linked, as indicated by random segregation in the T2 generation. Hence, we could select double mutants lacking the T-DNA already in the first offspring generation. However, whole-genome sequencing data revealed at least five independent insertions of vector backbone sequences. We did not detect any off-target effects in two genome regions homologous to the target sequence. The simultaneous alteration of multiple homoeologs by CRISPR-Cas9 mutagenesis without any background mutations will offer new opportunities for using mutant genotypes in rapeseed breeding.

Generation of marker-free transgenic hexaploid wheat via an Agrobacterium-mediated co-transformation strategy in commercial Chinese wheat varieties

Genotype specificity is a big problem lagging the development of efficient hexaploid wheat transformation system. Increasingly, the biosecurity of genetically modified organisms is garnering public attention, so the generation of marker-free transgenic plants is very important to the eventual potential commercial release of transgenic wheat. In this study, 15 commercial Chinese hexaploid wheat varieties were successfully transformed via an Agrobacterium-mediated method, with efficiency of up to 37.7%, as confirmed by the use of Quickstix strips, histochemical staining, PCR analysis and Southern blotting. Of particular interest, marker-free transgenic wheat plants from various commercial Chinese varieties and their F₁ hybrids were successfully obtained for the first time, with a frequency of 4.3%, using a plasmid harbouring two independent T-DNA regions. The average co-integration frequency of the gus and the bar genes located on the two independent T-DNA regions was 49.0% in T₀ plants. We further found that the efficiency of generating marker-free plants was related to the number of bar gene copies integrated in the genome. Marker-free transgenic wheat plants were identified in the progeny of three transgenic lines that had only one or two bar gene copies. Moreover, silencing of the bar gene was detected in 30.7% of T₁ positive plants, but the gus gene was never found to be silenced in T₁ plants. Bisulphite genomic sequencing suggested that DNA methylation in the 35S promoter of the bar gene regulatory region might be the main reason for bar gene silencing in the transgenic plants.