Resistance to Passion fruit woodiness virus in Transgenic Passionflower Expressing the Virus Coat Protein Gene

Cotyledon peeling method for passion fruit protoplasts: a versatile cell system for transient gene expression in passion fruit (Passiflora edulis)

Passion fruit (Passiflora edulis) is a perennial evergreen vine that grows mainly in tropical and subtropical regions due to its nutritional, medicinal and ornamental values. However, the molecular biology study of passion fruit is extremely hindered by the lack of an easy and efficient method for transformation. The protoplast transformation system plays a vital role in plant regeneration, gene function analysis and genome editing. Here, we present a new method ('Cotyledon Peeling Method') for simple and efficient passion fruit protoplast isolation using cotyledon as the source tissue. A high yield (2.3 × 107 protoplasts per gram of fresh tissues) and viability (76%) of protoplasts were obtained upon incubation in the enzyme solution [1% (w/v) cellulase R10, 0.25% (w/v) macerozyme R10, 0.4 M mannitol, 10 mM CaCl2, 20 mM KCl, 20 mM MES and 0.1% (w/v) BSA, pH 5.7] for 2 hours. In addition, we achieved high transfection efficiency of 83% via the polyethylene glycol (PEG)-mediated transformation with a green fluorescent protein (GFP)-tagged plasmid upon optimization. The crucial factors affecting transformation efficiency were optimized as follows: 3 μg of plasmid DNA, 5 min transfection time, PEG concentration at 40% and protoplast density of 100 × 104 cells/ml. Furthermore, the established protoplast system was successfully applied for subcellular localization analysis of multiple fluorescent organelle markers and protein-protein interaction study. Taken together, we report a simple and efficient passion fruit protoplast isolation and transformation system, and demonstrate its usage in transient gene expression for the first time in passion fruit. The protoplast system would provide essential support for various passion fruit biology studies, including genome editing, gene function analysis and whole plant regeneration.

Advances in micropropagation, somatic embryogenesis, somatic hybridizations, genetic transformation and cryopreservation for Passiflora improvement

Passion fruit is an essential commercial plant in the tropics and subtropics, which has lately seen a rise in demand for high-quality fruits and large-scale production. Generally, different species of passion fruit (Passiflora sp.) are propagated by sexual reproduction. However, asexual reproduction, such as stem cuttings, grafting, or tissue culture, is also available and advantageous in many instances. Recent research on passion fruit has concentrated on improving and establishing methodologies for embryogenesis, clonal proliferation via (somatic embryos), homozygote regeneration (by anther culture), germplasm preservation (via cryopreservation), and genetic transformation. These developments have resulted in potentially new directions for asexual propagation. Even though effective embryo culture and cryogenics are now available, however the limited frequency of embryogenic callus transformation to ex-vitro seedlings still restricts the substantial clonal replication of passion fruit. Here, in this review the advancement related to biotechnological approaches and the current understanding of Passiflora tissue culture. In vitro culture, organogenesis, cryopreservation, breeding, and productivity of Passiflora will significantly improve with novel propagation approaches, which could be applied to a wider range of germplasm.

Establishment of a Novel and Efficient Agrobacterium-Mediated in Planta Transformation System for Passion Fruit (Passiflora edulis)

Passion fruit (Passiflora edulis) is an important fruit crop with high economic value. Genetic engineering plays an important role in crop improvement with desired traits and gene functional studies. The lack of a simple, efficient, and stable transformation system for passion fruit has greatly limited gene functional studies. In this study, a simple and efficient Agrobacterium-mediated in planta transformation system for passion fruit was established, using Agrobacterium virulent strain EHA105 harboring the binary vectors pCAMBIA1301 and pCAMBIA1302 with GUS and GFP reporter genes. The system requires less time and labor costs than conventional transformation systems, and no additional phytohormones and sterile conditions are required. Regeneration efficiency of 86% and transformation efficiency of 29% were achieved, when the wounds were wrapped with Parafilm and the plants were kept in darkness for 15 days. Approximately 75% of the regenerated plants had a single shoot and 26% multiple shoots. The transformation was confirmed at the DNA and RNA levels as well as by GUS staining and GFP fluorescent measurements. The developed protocol will contribute to the genetic improvement of passion fruit breeding.

A simple and fast Agrobacterium-mediated transformation system for passion fruit KPF4 (Passiflora edulis f. edulis × Passiflora edulis f. flavicarpa)

BACKGROUND

Passion fruit (Passiflora edulis Sims) is an important horticultural crop in the tropics and subtropics, where it has great commercial potential due to high demand for fresh edible fruits and processed juice as well as source of raw materials in cosmetic industries. Genetic engineering shows great potential in passion fruit improvement and can compensate for the limitations of conventional breeding. Despite the success achieved in genetic modification of few passion fruit varieties, transgenic passion fruit production is still difficult for farmer-preferred cultivars. Therefore, it is important to establish a simple and fast Agrobacterium-mediated cell transformation of commercial hybrid passion fruit KPF4 (Passiflora edulis f. edulis × Passiflora edulis f. flavicarpa).

RESULTS

In the present study, we have developed a simple and fast Agrobacterium-mediated transformation system for hybrid passion fruit KPF4 using leaf disc explants. Factors affecting the rate of transient beta (β)-glucuronidase (gusA) expression and consequently transformation efficiency were optimized as follows: Agrobacterium cell density with an OD₆₀₀ of 0.5, 30 min infection time, 3 days of co-cultivation duration and the incorporation of 200 µM acetosyringone into Agrobacterium infection suspension medium. Using the optimized conditions, transgenic plants of KPF4 were produced within 2 months with an average transformation efficiency of 0.67%. The β-glucuronidase (GUS) histochemical staining confirmed the expression and integration of an intron-containing gusA gene into transformed leaf discs and transgenic plant lines of KPF4. The presence of gusA gene in the transgenic plants was confirmed by polymerase chain reaction (PCR). The results confirmed that the gusA gene was efficiently integrated into the passion fruit genome.

CONCLUSIONS

The developed transformation protocol is simple and rapid and could be useful for functional genomic studies and transferring agronomically important traits into passion fruit hybrid KPF4. This study developed a method that can be used to transfer traits such as resistance to viral diseases, low fruit quality and short storage life. To the best of our knowledge, this is the first report on genetic transformation system for commercial passion fruit hybrid KPF4.

Utilization of engineered resistance to viruses in crops of the developing world, with emphasis on sub-Saharan Africa

Viral diseases in crop plants constitute a major obstacle to food security in the developing world. Subsistence crops, including cassava, sweetpotato, potato, banana, papaya, common bean, rice and maize are often infected with RNA and/or DNA viruses that cannot be controlled with pesticides. Hence, healthy planting materials and virus-resistant cultivars are essential for high yields of good quality. However, resistance genes are not available for all viral diseases of crop plants. Therefore, virus resistance engineered in plants using modern biotechnology methods is an important addition to the crop production toolbox.

Perspectives for a Framework to Understand Aril Initiation and Development

A differentiated structure called "aril" has been described in seeds of several plant species during the course of evolution and might be considered as a supernumerary integument. Besides its ecological function in seed dispersal, the structure also represents a relevant character for systematic classification and exhibits important properties that impart agronomic value in certain species. Little is known about the molecular pathways underlying this morphological innovation because it is absent in currently used model species. A remarkable feature of the seeds of Passiflora species is the presence of a conspicuous aril. This genus is known for the ornamental, medicinal, and food values of its species. In view of the molecular resources and tools available for some Passiflora species, we highlight the potential of these species as models for developmental studies of the aril.

Transgenic resistance

Transgenic resistance to plant viruses is an important technology for control of plant virus infection, which has been demonstrated for many model systems, as well as for the most important plant viruses, in terms of the costs of crop losses to disease, and also for many other plant viruses infecting various fruits and vegetables. Different approaches have been used over the last 28 years to confer resistance, to ascertain whether particular genes or RNAs are more efficient at generating resistance, and to take advantage of advances in the biology of RNA interference to generate more efficient and environmentally safer, novel "resistance genes." The approaches used have been based on expression of various viral proteins (mostly capsid protein but also replicase proteins, movement proteins, and to a much lesser extent, other viral proteins), RNAs [sense RNAs (translatable or not), antisense RNAs, satellite RNAs, defective-interfering RNAs, hairpin RNAs, and artificial microRNAs], nonviral genes (nucleases, antiviral inhibitors, and plantibodies), and host-derived resistance genes (dominant resistance genes and recessive resistance genes), and various factors involved in host defense responses. This review examines the above range of approaches used, the viruses that were tested, and the host species that have been examined for resistance, in many cases describing differences in results that were obtained for various systems developed in the last 20 years. We hope this compilation of experiences will aid those who are seeking to use this technology to provide resistance in yet other crops, where nature has not provided such.

Genetic breeding and diversity of the genus Passiflora: progress and perspectives in molecular and genetic studies

Despite the ecological and economic importance of passion fruit (Passiflora spp.), molecular markers have only recently been utilized in genetic studies of this genus. In addition, both basic genetic researches related to population studies and pre-breeding programs of passion fruit remain scarce for most Passiflora species. Considering the number of Passiflora species and the increasing use of these species as a resource for ornamental, medicinal, and food purposes, the aims of this review are the following: (i) to present the current condition of the passion fruit crop; (ii) to quantify the applications and effects of using molecular markers in studies of Passiflora; (iii) to present the contributions of genetic engineering for passion fruit culture; and (iv) to discuss the progress and perspectives of this research. Thus, the present review aims to summarize and discuss the relationship between historical and current progress on the culture, breeding, and molecular genetics of passion fruit.

Somatic embryogenesis of a wild passion fruit species Passiflora cincinnata Masters: histocytological and histochemical evidences

The characterization of cellular changes that occur during somatic embryogenesis is essential for understanding the factors involved in the transition of somatic cells into embryogenically competent cells and determination of cells and/or tissues involved. The present study describes the anatomical and ultrastructural events that lead to the formation of somatic embryos in the model system of the wild passion fruit (Passiflora cincinnata). Mature zygotic embryos were inoculated in Murashige and Skoog induction media supplemented with 2,4-dichlorophenoxyacetic acid and 6-benzyladenine. Zygotic embryo explants at different development stages were collected and processed by conventional methods for studies using light, scanning, and transmission electron microscopy (TEM). Histochemical tests were used to examine the mobilization of reserves. The differentiation of the somatic embryos began in the abaxial side of the cotyledon region. Protuberances were formed from the meristematic proliferation of the epidermal and mesophyll cells. These cells had large nuclei, dense cytoplasm with a predominance of mitochondria, and a few reserve compounds. The protuberances extended throughout the abaxial surface of the cotyledons. The ongoing differentiation of peripheral cells of these structures led to the formation of proembryogenic zones, which, in turn, dedifferentiated into somatic embryos of multicellular origin. In the initial stages of embryogenesis, the epidermal and mesophyll cells showed starch grains and less lipids and protein reserves than the starting explant. These results provide detailed information on anatomical and ultrastructural changes involved in the acquisition of embryogenic competence and embryo differentiation that has been lacking so far in Passiflora.

Genetic Transformation of Passionflower and Evaluation of R1 and R2 Generations for Resistance to Cowpea aphid borne mosaic virus

We report on the production and evaluation of passionflower transgenic lines for resistance to Cowpea aphid borne mosaic virus (CABMV). Genetic transformation was done using Agrobacterium tumefaciens and transgene integration was confirmed by Southern blot analyses, resulting in nine transgenic lines for 'IAC 275' and three for 'IAC 277'. Transgenic lines were clonally propagated and evaluated for resistance to CABMV. After the third inoculation, under higher inoculum pressure, only propagated plants of the transgenic line T16 remained asymptomatic, indicating a high resistance to infection with CABMV. This transgenic line was self-pollinated and the R₁ generation was evaluated together with the R₁ generation of another resistant transgenic line (T2) identified previously. Plants were inoculated with CABMV by means of viruliferous Myzus nicotianae. All 524 T2R₁ plants became infected, whereas 13 of 279 T16R₁ remained asymptomatic after four successive inoculations. A T16R₂ generation was obtained and plants were inoculated with CABMV mechanically or by aphids. After successive inoculations, 118 of 258 plants were symptomless, suggesting that the resistance to CABMV was maintained in the plant genome as the homozygous condition was achieved. Five selected resistant T16R₂ plants which contained the capsid protein gene are being crossed for further analyses.

Transgenic expression of coat protein gene of Rice tungro bacilliform virus in rice reduces the accumulation of viral DNA in inoculated plants

Rice tungro, a devastating disease of rice in south and southeast Asia, is caused by the joint infection of Rice tungro bacilliform virus (RTBV) and Rice tungro spherical virus (RTSV). In order to obtain transgenic resistance against RTBV, indica rice cultivar Pusa Basmati-1 was transformed to express the coat protein (CP) gene of an Indian isolate of RTBV. Rice plants containing the transgene integrated in low copy numbers were obtained, in which the CP was shown to accumulate in the leaf tissue. The progenies representing three independent transformation events were challenged with Indian isolates of RTBV using viruliferous Green leafhoppers, and the viral titers in the inoculated plants were monitored using DNA dot-blot hybridization. As compared to non-transgenic controls, two independent transgenic lines showed significantly low levels of RTBV DNA, especially towards later stages of infection and a concomitant reduction of tungro symptoms.