Improved regeneration and transformation protocols for three strawberry cultivars

Overexpression of Vitis GRF4-GIF1 improves regeneration efficiency in diploid Fragaria vesca Hawaii 4

BACKGROUND

Plant breeding played a very important role in transforming strawberries from being a niche crop with a small geographical footprint into an economically important crop grown across the planet. But even modern marker assisted breeding takes a considerable amount of time, over multiple plant generations, to produce a plant with desirable traits. As a quicker alternative, plants with desirable traits can be raised through tissue culture by doing precise genetic manipulations. Overexpression of morphogenic regulators previously known for meristem development, the transcription factors Growth-Regulating Factors (GRFs) and the GRF-Interacting Factors (GIFs), provided an efficient strategy for easier regeneration and transformation in multiple crops.

RESULTS

We present here a comprehensive protocol for the diploid strawberry Fragaria vesca Hawaii 4 (strawberry) regeneration and transformation under control condition as compared to ectopic expression of different GRF4-GIF1 chimeras from different plant species. We report that ectopic expression of Vitis vinifera VvGRF4-GIF1 provides significantly higher regeneration efficiency during re-transformation over wild-type plants. On the other hand, deregulated expression of miRNA resistant version of VvGRF4-GIF1 or Triticum aestivum (wheat) TaGRF4-GIF1 resulted in abnormalities. Transcriptomic analysis between the different chimeric GRF4-GIF1 lines indicate that differential expression of FvExpansin might be responsible for the observed pleiotropic effects. Similarly, cytokinin dehydrogenase/oxygenase and cytokinin responsive response regulators also showed differential expression indicating GRF4-GIF1 pathway playing important role in controlling cytokinin homeostasis.

CONCLUSION

Our data indicate that ectopic expression of Vitis vinifera VvGRF4-GIF1 chimera can provide significant advantage over wild-type plants during strawberry regeneration without producing any pleiotropic effects seen for the miRNA resistant VvGRF4-GIF1 or TaGRF4-GIF1.

Advances in genomics and genome editing for improving strawberry (Fragaria ×ananassa)

The cultivated strawberry, Fragaria ×ananassa, is a recently domesticated fruit species of economic interest worldwide. As such, there is significant interest in continuous varietal improvement. Genomics-assisted improvement, including the use of DNA markers and genomic selection have facilitated significant improvements of numerous key traits during strawberry breeding. CRISPR/Cas-mediated genome editing allows targeted mutations and precision nucleotide substitutions in the target genome, revolutionizing functional genomics and crop improvement. Genome editing is beginning to gain traction in the more challenging polyploid crops, including allo-octoploid strawberry. The release of high-quality reference genomes and comprehensive subgenome-specific genotyping and gene expression profiling data in octoploid strawberry will lead to a surge in trait discovery and modification by using CRISPR/Cas. Genome editing has already been successfully applied for modification of several strawberry genes, including anthocyanin content, fruit firmness and tolerance to post-harvest disease. However, reports on many other important breeding characteristics associated with fruit quality and production are still lacking, indicating a need for streamlined genome editing approaches and tools in Fragaria ×ananassa. In this review, we present an overview of the latest advancements in knowledge and breeding efforts involving CRISPR/Cas genome editing for the enhancement of strawberry varieties. Furthermore, we explore potential applications of this technology for improving other Rosaceous plant species.

Recalcitrance to transformation, a hindrance for genome editing of legumes

Plant genome editing, a recently discovered method for targeted mutagenesis, has emerged as a promising tool for crop improvement and gene function research. Many genome-edited plants, such as rice, wheat, and tomato, have emerged over the last decade. As the preliminary steps in the procedure for genome editing involve genetic transformation, amenability to genome editing depends on the efficiency of genetic engineering. Hence, there are numerous reports on the aforementioned crops because they are transformed with relative ease. Legume crops are rich in protein and, thus, are a favored source of plant proteins for the human diet in most countries. However, legume cultivation often succumbs to various biotic/abiotic threats, thereby leading to high yield loss. Furthermore, certain legumes like peanuts possess allergens, and these need to be eliminated as these deprive many people from gaining the benefits of such crops. Further genetic variations are limited in certain legumes. Genome editing has the potential to offer solutions to not only combat biotic/abiotic stress but also generate desirable knock-outs and genetic variants. However, excluding soybean, alfalfa, and Lotus japonicus, reports obtained on genome editing of other legume crops are less. This is because, excluding the aforementioned three legume crops, the transformation efficiency of most legumes is found to be very low. Obtaining a higher number of genome-edited events is desirable as it offers the option to genotypically/phenotypically select the best candidate, without the baggage of off-target mutations. Eliminating the barriers to genetic engineering would directly help in increasing genome-editing rates. Thus, this review aims to compare various legumes for their transformation, editing, and regeneration efficiencies and discusses various solutions available for increasing transformation and genome-editing rates in legumes.

Virus-induced gene silencing and virus-induced flowering in strawberry (Fragaria × ananassa) using apple latent spherical virus vectors

Apple latent spherical virus (ALSV) vector is a convenient alternative to genetic transformation in horticultural plants, especially in species recalcitrant to genetic transformation. ALSV, an RNA virus, can infect a wide variety of plant species including major horticultural plants without inducing symptoms. Here, methodologies were developed for infection of ALSV vectors to strawberry seedlings and plantlets cultured in vitro. A seed-propagated F₁ hybrid strawberry cultivar 'Yotsuboshi' was aseptically grown on half-strength Murashige-Skoog medium for 1 month and true leaves were inoculated with an ALSV RNA preparation by particle bombardment. ALSV vector infection rates varied from 58 to 100% according to the insertion sequences, in 'Yotsuboshi' seedlings. Plantlets ('Dover') propagated in vitro could also be infected with ALSV vector at a similar infection rate. For virus-induced gene silencing (VIGS), we prepared an ALSV vector carrying a 201 nucleotide segment of the strawberry phytoene desaturase gene. 'Yotsuboshi' and 'Dover' plants infected by this vector generated completely white leaves at fifth or sixth true leaves and above. For virus-induced flowering (VIF), we used an ALSV vector expressing the Arabidopsis thaliana flowering locus T gene. Strawberry seedlings infected by this vector started to flower from about 2 months post inoculation and bore fruits with viable seeds. The ALSV vector was no longer detected in any of the seedlings from early-flowered strawberries. Thus, the ALSV vector may be beneficial for examination of gene functions by VIGS in strawberry, and VIF using ALSV vector constitutes an effective new plant breeding technique for the promotion of cross-breeding in strawberry.

Defense response enhancement in strawberry via elicitors

In this study, cell-suspension culture of strawberry (Fragaria × ananassa), cultivars Camarosa, and Sweet Charlie has been established. Embryogenic callus was induced by incubating the in vitro juvenile leaf explants on medium, containing 2-mg/l picloram at dark. Suspension culture was initiated from 4-week-old embryogenic calli in the liquid MS medium with 1-mg/l 2,4-D and 2-mg/l picloram. Suspension culture was maintained by sub-culturing each 3 weeks into a fresh medium. At week 9 after third sub-cultures, torpedo and cotyledonary embryo stages were observed. Embryos were then developed into shoots on medium 1 mg/l of each BA and IBA. Obtained shoots were successfully rooted on 1-mg/ml GA3, 0.5-mg/ml BA, and 1-mg/ml IBA. To enhance the resistance availability in strawberry plants, elicitation was applied by adding the JA and SA elicitors to the suspension culture with two doses (0.5 and 1 mM) individually and in combination, in addition to the fungal homogenate of Macrophomina phasiolena at concentration of 10⁶ spor/ml. The fawrky-1-Camarosa gene, which has defense-related function, was detected in the different elicited strawberry tissues and isolated via RT-PCR. The isolated gene was submitted to GenBank with accession number (KX096885).