Transgenic papaya: development, release, impact and challenges

Quality trait improvement in horticultural crops: OMICS and modern biotechnological approaches

Horticultural crops are an essential part of food and nutritional security. Moreover, these form an integral part of the agricultural economy and have enormous economic potential. They are a rich source of nutrients that are beneficial to human health. Plant breeding of horticultural crops has focussed primarily on increasing the productivity and related traits of these crops. However, fruit and vegetable quality is paramount to their perishability, marketability, and consumer acceptance. The improved nutritional value is beneficial to underprivileged and undernourished communities. Due to a declining genetic base, conventional plant breeding does not contribute much to quality improvement as the existing natural allelic variations and crossing barriers between cultivated and wild species limit it. Over the past two decades, 'omics' and modern biotechnological approaches have made it possible to decode the complex genomes of crop plants, assign functions to the otherwise many unknown genes, and develop genome-wide DNA markers. Genetic engineering has enabled the validation of these genes and the introduction of crucial agronomic traits influencing various quality parameters directly or indirectly. This review discusses the significant advances in the quality improvement of horticultural crops, including shelf life, aroma, browning, nutritional value, colour, and many other related traits.

Spray-induced gene silencing to identify powdery mildew gene targets and processes for powdery mildew control

Spray-induced gene silencing (SIGS) is an emerging tool for crop pest protection. It utilizes exogenously applied double-stranded RNA to specifically reduce pest target gene expression using endogenous RNA interference machinery. In this study, SIGS methods were developed and optimized for powdery mildew fungi, which are widespread obligate biotrophic fungi that infect agricultural crops, using the known azole-fungicide target cytochrome P450 51 (CYP51) in the Golovinomyces orontii-Arabidopsis thaliana pathosystem. Additional screening resulted in the identification of conserved gene targets and processes important to powdery mildew proliferation: apoptosis-antagonizing transcription factor in essential cellular metabolism and stress response; lipid catabolism genes lipase a, lipase 1, and acetyl-CoA oxidase in energy production; and genes involved in manipulation of the plant host via abscisic acid metabolism (9-cis-epoxycarotenoid dioxygenase, xanthoxin dehydrogenase, and a putative abscisic acid G-protein coupled receptor) and secretion of the effector protein, effector candidate 2. Powdery mildew is the dominant disease impacting grapes and extensive powdery mildew resistance to applied fungicides has been reported. We therefore developed SIGS for the Erysiphe necator-Vitis vinifera system and tested six successful targets identified using the G. orontii-A. thaliana system. For all targets tested, a similar reduction in powdery mildew disease was observed between systems. This indicates screening of broadly conserved targets in the G. orontii-A. thaliana pathosystem identifies targets and processes for the successful control of other powdery mildew fungi. The efficacy of SIGS on powdery mildew fungi makes SIGS an exciting prospect for commercial powdery mildew control.

Plant Viruses of Agricultural Importance: Current and Future Perspectives of Virus Disease Management Strategies

Plant viruses cause significant losses in agricultural crops worldwide, affecting the yield and quality of agricultural products. The emergence of novel viruses or variants through genetic evolution and spillover from reservoir host species, changes in agricultural practices, mixed infections with disease synergism, and impacts from global warming pose continuous challenges for the management of epidemics resulting from emerging plant virus diseases. This review describes some of the most devastating virus diseases plus select virus diseases with regional importance in agriculturally important crops that have caused significant yield losses. The lack of curative measures for plant virus infections prompts the use of risk-reducing measures for managing plant virus diseases. These measures include exclusion, avoidance, and eradication techniques, along with vector management practices. The use of sensitive, high throughput, and user-friendly diagnostic methods is crucial for defining preventive and management strategies against plant viruses. The advent of next-generation sequencing technologies has great potential for detecting unknown viruses in quarantine samples. The deployment of genetic resistance in crop plants is an effective and desirable method of managing virus diseases. Several dominant and recessive resistance genes have been used to manage virus diseases in crops. Recently, RNA-based technologies such as dsRNA- and siRNA-based RNA interference, microRNA, and CRISPR/Cas9 provide transgenic and nontransgenic approaches for developing virus-resistant crop plants. Importantly, the topical application of dsRNA, hairpin RNA, and artificial microRNA and trans-active siRNA molecules on plants has the potential to develop GMO-free virus disease management methods. However, the long-term efficacy and acceptance of these new technologies, especially transgenic methods, remain to be established.

Small RNA-based plant protection against diseases

Plant diseases cause significant decreases in yield and quality of crops and consequently pose a very substantial threat to food security. In the continuous search for environmentally friendly crop protection, exploitation of RNA interferance machinery is showing promising results. It is well established that small RNAs (sRNAs) including microRNA (miRNA) and small interfering RNA (siRNA) are involved in the regulation of gene expression via both transcriptional and post-transcriptional RNA silencing. sRNAs from host plants can enter into pathogen cells during invasion and silence pathogen genes. This process has been exploited through Host-Induced Gene Silencing (HIGS), in which plant transgenes that produce sRNAs are engineered to silence pest and pathogen genes. Similarly, exogenously applied sRNAs can enter pest and pathogen cells, either directly or via the hosts, and silence target genes. This process has been exploited in Spray-Induced Gene Silencing (SIGS). Here, we focus on the role of sRNAs and review how they have recently been used against various plant pathogens through HIGS or SIGS-based methods and discuss advantages and drawbacks of these approaches.

The Investigation of Phenylalanine, Glucosinolate, Benzylisothiocyanate (BITC) and Cyanogenic Glucoside of Papaya Fruits (Carica papaya L. cv. ‘Tainung No. 2’) under Different Development Stages between Seasons and Their Correlation with Bitter Taste

Papaya fruit is one of economic crops in Taiwan, mostly eaten as table fruits. In some Asian countries, unripe papaya fruit is eaten as salad and this led to trends in Taiwan as well. However, unripe papaya fruit may taste bitter during cool seasons. Glucosinolate and cyanogenic glucoside are among the substances that cause bitter taste in many plants, which can also be found in papaya. However, there is still no report about the relationship between seasons and bitter taste in papaya fruits. Thus, the purpose of this study is to investigate the glucosinolate biosynthesis and its correlation between bitterness intensity during cool and warm seasons. The bitterness intensity was highest at the young fruit stage and decreased as it developed. In addition, the bitterness intensity in cool season fruits is higher than in warm season fruits. Cyanogenic glucoside and BITC content showed negative correlation with bitterness intensity (r = −0.54 ***; −0.46 ***). Phenylalanine showed positive correlation with bitterness intensity (r = 0.35 ***), but its content did not reach the bitterness threshold concentration, which suggested that phenylalanine only acts as cyanogenic glucoside and glucosinolate precusors. Glucosinolate content showed positive correlation with bitterness intensity at different developmental stages (r = 0.805 ***). However, the correlation value in different lines/cultivars decreased (0.44 ***), suggesting that glucosinolate was not the only substance that caused bitter taste in immature papaya fruits.

Strategies for Efficient RNAi-Based Gene Silencing of Viral Genes for Disease Resistance in Plants

RNA interference (RNAi) is an evolutionarily conserved gene silencing mechanism in eukaryotes including fungi, plants, and animals. In plants, gene silencing regulates gene expression, provides genome stability, and protect against invading viruses. During plant virus interaction, viral genome derived siRNAs (vsiRNA) are produced to mediate gene silencing of viral genes to prevent virus multiplication. After the discovery of RNAi phenomenon in eukaryotes, it is used as a powerful tool to engineer plant viral disease resistance against both RNA and DNA viruses. Despite several successful reports on employing RNA silencing methods to engineer plant for viral disease resistance, only a few of them have reached the commercial stage owing to lack of complete protection against the intended virus. Based on the knowledge accumulated over the years on genetic engineering for viral disease resistance, there is scope for effective viral disease control through careful design of RNAi gene construct. The selection of target viral gene(s) for developing the hairpin RNAi (hp-RNAi) construct is very critical for effective protection against the viral disease. Different approaches and bioinformatics tools which can be employed for effective target selection are discussed. The selection of suitable target regions for RNAi vector construction can help to achieve a high level of transgenic virus resistance.

Development and Adoption of Genetically Engineered Plants for Virus Resistance: Advances, Opportunities and Challenges

Plant viruses cause yield losses to crops of agronomic and economic significance and are a challenge to the achievement of global food security. Although conventional plant breeding has played an important role in managing plant viral diseases, it will unlikely meet the challenges posed by the frequent emergence of novel and more virulent viral species or viral strains. Hence there is an urgent need to seek alternative strategies of virus control that can be more readily deployed to contain viral diseases. The discovery in the late 1980s that viral genes can be introduced into plants to engineer resistance to the cognate virus provided a new avenue for virus disease control. Subsequent advances in genomics and biotechnology have led to the refinement and expansion of genetic engineering (GE) strategies in crop improvement. Importantly, many of the drawbacks of conventional breeding, such as long lead times, inability or difficulty to cross fertilize, loss of desirable plant traits, are overcome by GE. Unfortunately, public skepticism towards genetically modified (GM) crops and other factors have dampened the early promise of GE efforts. These concerns are principally about the possible negative effects of transgenes to humans and animals, as well as to the environment. However, with regards to engineering for virus resistance, these risks are overstated given that most virus resistance engineering strategies involve transfer of viral genes or genomic segments to plants. These viral genomes are found in infected plant cells and have not been associated with any adverse effects in humans or animals. Thus, integrating antiviral genes of virus origin into plant genomes is hardly unnatural as suggested by GM crop skeptics. Moreover, advances in deep sequencing have resulted in the sequencing of large numbers of plant genomes and the revelation of widespread endogenization of viral genomes into plant genomes. This has raised the possibility that viral genome endogenization is part of an antiviral defense mechanism deployed by the plant during its evolutionary past. Thus, GM crops engineered for viral resistance would likely be acceptable to the public if regulatory policies were product-based (the North America regulatory model), as opposed to process-based. This review discusses some of the benefits to be gained from adopting GE for virus resistance, as well as the challenges that must be overcome to leverage this technology. Furthermore, regulatory policies impacting virus-resistant GM crops and some success cases of virus-resistant GM crops approved so far for cultivation are discussed.

Research Advances in Potyviruses: From the Laboratory Bench to the Field

Potyviruses (viruses in the genus Potyvirus, family Potyviridae) constitute the largest group of known plant-infecting RNA viruses and include many agriculturally important viruses that cause devastating epidemics and significant yield losses in many crops worldwide. Several potyviruses are recognized as the most economically important viral pathogens. Therefore, potyviruses are more studied than other groups of plant viruses. In the past decade, a large amount of knowledge has been generated to better understand potyviruses and their infection process. In this review, we list the top 10 economically important potyviruses and present a brief profile of each. We highlight recent exciting findings on the novel genome expression strategy and the biological functions of potyviral proteins and discuss recent advances in molecular plant-potyvirus interactions, particularly regarding the coevolutionary arms race. Finally, we summarize current disease control strategies, with a focus on biotechnology-based genetic resistance, and point out future research directions.

Fruit crops in the era of genome editing: closing the regulatory gap

The conventional breeding of fruits and fruit trees has led to the improvement of consumer-driven traits such as fruit size, yield, nutritional properties, aroma and taste, as well as the introduction of agronomic properties such as disease resistance. However, even with the assistance of modern molecular approaches such as marker-assisted selection, the improvement of fruit varieties by conventional breeding takes considerable time and effort. The advent of genetic engineering led to the rapid development of new varieties by allowing the direct introduction of genes into elite lines. In this review article, we discuss three such case studies: the Arctic^® apple, the Pinkglow pineapple and the SunUp/Rainbow papaya. We consider these events in the light of global regulations for the commercialization of genetically modified organisms (GMOs), focusing on the differences between product-related systems (the USA/Canada comparative safety assessment) and process-related systems (the EU "precautionary principle" model). More recently, genome editing has provided an efficient way to introduce precise mutations in plants, including fruits and fruit trees, replicating conventional breeding outcomes without the extensive backcrossing and selection typically necessary to introgress new traits. Some jurisdictions have reacted by amending the regulations governing GMOs to provide exemptions for crops that would be indistinguishable from conventional varieties based on product comparison. This has revealed the deficiencies of current process-related regulatory frameworks, particularly in the EU, which now stands against the rest of the world as a unique example of inflexible and dogmatic governance based on political expediency and activism rather than rigorous scientific evidence.

Genome editing in fruit, ornamental, and industrial crops

The advent of genome editing has opened new avenues for targeted trait enhancement in fruit, ornamental, industrial, and all specialty crops. In particular, CRISPR-based editing systems, derived from bacterial immune systems, have quickly become routinely used tools for research groups across the world seeking to edit plant genomes with a greater level of precision, higher efficiency, reduced off-target effects, and overall ease-of-use compared to ZFNs and TALENs. CRISPR systems have been applied successfully to a number of horticultural and industrial crops to enhance fruit ripening, increase stress tolerance, modify plant architecture, control the timing of flower development, and enhance the accumulation of desired metabolites, among other commercially-important traits. As editing technologies continue to advance, so too does the ability to generate improved crop varieties with non-transgenic modifications; in some crops, direct transgene-free edits have already been achieved, while in others, T-DNAs have successfully been segregated out through crossing. In addition to the potential to produce non-transgenic edited crops, and thereby circumvent regulatory impediments to the release of new, improved crop varieties, targeted gene editing can speed up trait improvement in crops with long juvenile phases, reducing inputs resulting in faster market introduction to the market. While many challenges remain regarding optimization of genome editing in ornamental, fruit, and industrial crops, the ongoing discovery of novel nucleases with niche specialties for engineering applications may form the basis for additional and potentially crop-specific editing strategies.

RNA interference approaches for plant disease control

Plant diseases caused by a variety of pathogens can have severe effects on crop plants and even plants in natural ecosystems. Despite many effective conventional approaches to control plant diseases, new, efficacious, environmentally sound and cost-effective approaches are needed, particularly with our increasing human population and the effects on crop production and plant health caused by climate change. RNA interference (RNAi) is a gene regulation and antiviral response mechanism in eukaryotes; transgenic and non transgenic plant-based RNAi approaches have shown great effectiveness and potential to target specific plant pathogens and help control plant diseases, especially when no alternatives are available. Here we discuss ways in which RNAi has been used against different plant pathogens, and some new potential applications for plant disease control.

Engineering plant virus resistance: from RNA silencing to genome editing strategies

Viral diseases severely affect crop yield and quality, thereby threatening global food security. Genetic improvement of plant virus resistance is essential for sustainable agriculture. In the last decades, several modern technologies were applied in plant antiviral engineering. Here we summarized breakthroughs of the two major antiviral strategies, RNA silencing and genome editing. RNA silencing strategy has been used in antiviral breeding for more than thirty years, and many crops engineered to stably express small RNAs targeting various viruses have been approved for commercial release. Genome editing technology has emerged in the past decade, especially CRISPR/Cas, which provides new methods for genetic improvement of plant virus resistance and accelerates resistance breeding. Finally, we discuss the potential of these technologies for breeding crops, and the challenges and solutions they may face in the future.

Risk-Only Assessment of Genetically Engineered Crops Is Risky

The risks of not considering benefits in risk assessment are often overlooked. Risks are also often evaluated without consideration of the broader context. We discuss these two concepts in relation to genetically engineered (GE) crops. The health, environmental, and economic risks and benefits of GE crops are exemplified and presented in the context of modern agriculture. Misattribution of unique risks to GE crops are discussed. It is concluded that the scale of modern agriculture is its distinguishing characteristic and that the greater knowledge around GE crops allows for a more thorough characterization of risk. By considering the benefits and risks in the context of modern agriculture, society will be better served and benefits will be less likely to be forgone.

Phenotypic Expression and Stability in a Large-Scale Field Study of Genetically Engineered Poplars Containing Sexual Containment Transgenes

Genetic engineering (GE) has the potential to help meet demand for forest products and ecological services. However, high research and development costs, market restrictions, and regulatory obstacles to performing field tests have severely limited the extent and duration of field research. There is a notable paucity of field studies of flowering GE trees due to the time frame required and regulatory constraints. Here we summarize our findings from field testing over 3,300 GE poplar trees and 948 transformation events in a single, 3.6 hectare field trial for seven growing seasons; this trial appears to be the largest field-based scientific study of GE forest trees in the world. The goal was to assess a diversity of approaches for obtaining bisexual sterility by modifying RNA expression or protein function of floral regulatory genes, including LEAFY, AGAMOUS, APETALA1, SHORT VEGETATIVE PHASE, and FLOWERING LOCUS T. Two female and one male clone were transformed with up to 23 different genetic constructs designed to obtain sterile flowers or delay onset of flowering. To prevent gene flow by pollen and facilitate regulatory approval, the test genotypes chosen were incompatible with native poplars in the area. We monitored tree survival, growth, floral onset, floral abundance, pollen production, seed formation and seed viability. Tree survival was above 95%, and variation in site conditions generally had a larger impact on vegetative performance and onset of flowering than did genetic constructs. Floral traits, when modified, were stable over three to five flowering seasons, and we successfully identified RNAi or overexpression constructs that either postponed floral onset or led to sterile flowers. There was an absence of detectable somaclonal variation; no trees were identified that showed vegetative or floral modifications that did not appear to be related to the transgene added. Surveys for seedling and sucker establishment both within and around the plantation identified small numbers of vegetative shoots (root sprouts) but no seedlings, indicative of a lack of establishment of trees via seeds in the area. Overall, this long term study showed that GE containment traits can be obtained which are effective, stable, and not associated with vegetative abnormalities or somaclonal variation.

Signaling Mechanisms Underlying Resistance Responses: What Have We Learned, and How Is It Being Applied?

Plants have evolved highly specific mechanisms to resist pathogens including preformed barriers and the induction of elaborate signaling pathways. Induced signaling requires recognition of the pathogen either via conserved pathogen-derived factors or specific pathogen-encoded proteins called effectors. Recognition of these factors by host encoded receptor proteins can result in the elicitation of different tiers of resistance at the site of pathogen infection. In addition, plants induce a type of systemic immunity which is effective at the whole plant level and protects against a broad spectrum of pathogens. Advances in our understanding of pathogen-recognition mechanisms, identification of the underlying molecular components, and their significant conservation across diverse plant species has enabled the development of novel strategies to combat plant diseases. This review discusses key advances in plant defense signaling that have been adapted or have the potential to be adapted for plant protection against microbial diseases.

A molecular insight into papaya leaf curl-a severe viral disease

Papaya leaf curl disease (PaLCuD) caused by papaya leaf curl virus (PaLCuV) not only affects yield but also plant growth and fruit size and quality of papaya and is one of the most damaging and economically important disease. Management of PaLCuV is a challenging task due to diversity of viral strains, the alternate hosts, and the genomic complexities of the viruses. Several management strategies currently used by plant virologists to broadly control or eliminate the viruses have been discussed. In the absence of such strategies in the case of PaLCuV at present, the few available options to control the disease include methods like removal of affected plants from the field, insecticide treatments against the insect vector (Bemisia tabaci), and gene-specific control through transgenic constructs. This review presents the current understanding of papaya leaf curl disease, genomic components including satellite DNA associated with the virus, wide host and vector range, and management of the disease and suggests possible generic resistance strategies.

Use of RNAi technology to develop a PRSV-resistant transgenic papaya

Papaya ringspot virus (PRSV) seriously limits papaya (Carica papaya L.) production in tropical and subtropical areas throughout the world. Coat protein (CP)- transgenic papaya lines resistant to PRSV isolates in the sequence-homology-dependent manner have been developed in the U.S.A. and Taiwan. A previous investigation revealed that genetic divergence among Hainan isolates of PRSV has allowed the virus to overcome the CP-mediated transgenic resistance. In this study, we designed a comprehensive RNAi strategy targeting the conserved domain of the PRSV CP gene to develop a broader-spectrum transgenic resistance to the Hainan PRSV isolates. We used an optimized particle-bombardment transformation system to produce RNAi-CP-transgenic papaya lines. Southern blot analysis and Droplet Digital PCR revealed that line 474 contained a single transgene insert. Challenging this line with different viruses (PRSV I, II and III subgroup) under greenhouse conditions validated the transgenic resistance of line 474 to the Hainan isolates. Northern blot analysis detected the siRNAs products in virus-free transgenic papaya tissue culture seedlings. The siRNAs also accumulated in PRSV infected transgenic papaya lines. Our results indicated that this transgenic papaya line has a useful application against PRSV in the major growing area of Hainan, China.

New Biotechnological Tools for the Genetic Improvement of Major Woody Fruit Species

The improvement of woody fruit species by traditional plant breeding techniques has several limitations mainly caused by their high degree of heterozygosity, the length of their juvenile phase and auto-incompatibility. The development of new biotechnological tools (NBTs), such as RNA interference (RNAi), trans-grafting, cisgenesis/intragenesis, and genome editing tools, like zinc-finger and CRISPR/Cas9, has introduced the possibility of more precise and faster genetic modifications of plants. This aspect is of particular importance for the introduction or modification of specific traits in woody fruit species while maintaining unchanged general characteristics of a selected cultivar. Moreover, some of these new tools give the possibility to obtain transgene-free modified fruit tree genomes, which should increase consumer's acceptance. Over the decades biotechnological tools have undergone rapid development and there is a continuous addition of new and valuable techniques for plant breeders. This makes it possible to create desirable woody fruit varieties in a fast and more efficient way to meet the demand for sustainable agricultural productivity. Although, NBTs have a common goal i.e., precise, fast, and efficient crop improvement, individually they are markedly different in approach and characteristics from each other. In this review we describe in detail their mechanisms and applications for the improvement of fruit trees and consider the relationship between these biotechnological tools and the EU biosafety regulations applied to the plants and products obtained through these techniques.

Prospects for engineering and improvement of cross-protective virus strains

Mild strain cross-protection is currently an important method for the production of high quality plant products; despite challenge from severe virus isolates the initial protecting strain precludes symptom development. The mechanism of cross-protection is not yet resolved as RNA silencing does not sufficiently explain the phenomenon. Six requirements have been put forward to ensure long-lasting protection. We propose two additional requirements for effective and durable mild strain cross-protection; mild strains based on knowledge of the mechanism and consideration of impacts to consumers. Future research on predicting phenotype from genotype and understanding virus-plant and virus-vector interactions will enable improvement of cross-protective strains. Shared international databases of whole ecosystem interactions across a wide range of virus patho- and symbiotic-systems will form the basis for making step-change advances towards our collective ability to engineer and improve mild strain cross-protection.

Small RNA Based Genetic Engineering for Plant Viral Resistance: Application in Crop Protection

Small RNAs regulate a large set of gene expression in all plants and constitute a natural immunity against viruses. Small RNA based genetic engineering (SRGE) technology had been explored for crop protection against viruses for nearly 30 years. Viral resistance has been developed in diverse crops with SRGE technology and a few viral resistant crops have been approved for commercial release. In this review we summarized the efforts generating viral resistance with SRGE in different crops, analyzed the evolution of the technology, its efficacy in different crops for different viruses and its application status in different crops. The challenge and potential solution for application of SRGE in crop protection are also discussed.

Vector-Borne Bacterial Plant Pathogens: Interactions with Hemipteran Insects and Plants

Hemipteran insects are devastating pests of crops due to their wide host range, rapid reproduction, and ability to transmit numerous plant-infecting pathogens as vectors. While the field of plant-virus-vector interactions has flourished in recent years, plant-bacteria-vector interactions remain poorly understood. Leafhoppers and psyllids are by far the most important vectors of bacterial pathogens, yet there are still significant gaps in our understanding of their feeding behavior, salivary secretions, and plant responses as compared to important viral vectors, such as whiteflies and aphids. Even with an incomplete understanding of plant-bacteria-vector interactions, some common themes have emerged: (1) all known vector-borne bacteria share the ability to propagate in the plant and insect host; (2) particular hemipteran families appear to be incapable of transmitting vector-borne bacteria; (3) all known vector-borne bacteria have highly reduced genomes and coding capacity, resulting in host-dependence; and (4) vector-borne bacteria encode proteins that are essential for colonization of specific hosts, though only a few types of proteins have been investigated. Here, we review the current knowledge on important vector-borne bacterial pathogens, including Xylella fastidiosa, Spiroplasma spp., Liberibacter spp., and 'Candidatus Phytoplasma spp.'. We then highlight recent approaches used in the study of vector-borne bacteria. Finally, we discuss the application of this knowledge for control and future directions that will need to be addressed in the field of vector-plant-bacteria interactions.

The wayward Hawaiian boy returns home

This chapter represents a travelog of my life and career and the philosophical points I acquired along the way. I was born on a sugar plantation on the island of Hawaii and early on had a stuttering problem. I attended the Kamehameha Schools and received my BS and MS degrees from the University of Hawaii and my Ph.D. from the University of California at Davis. I link my life and career to various principles and events, some of which are: the importance of positioning oneself; going for the big enchilada; music, the international language; the red zone of biotechnology; the human side of biotechnology; the transgenic papaya story; and my leadership time at USDA in Hawaii. The guiding light throughout my career were the words from Drs. Eduardo Trujillo and Robert Shepherd, respectively, "Dennis, don't just be a test tube scientist, do something to help people" and "Now tell me, what have you really accomplished?"

Emerging strategies for RNA interference (RNAi) applications in insects

RNA interference (RNAi) in insects is a gene regulatory process that also plays a vital role in the maintenance and in the regulation of host defenses against invading viruses. Small RNAs determine the specificity of the RNAi through precise recognition of their targets. These small RNAs in insects comprise small interfering RNAs (siRNAs), micro RNAs (miRNAs) and Piwi interacting RNAs (piRNAs) of various lengths. In this review, we have explored different forms of the RNAi inducers that are presently in use, and their applications for an effective and efficient fundamental and practical RNAi research with insects. Further, we reviewed trends in next generation sequencing (NGS) technologies and their importance for insect RNAi, including the identification of novel insect targets as well as insect viruses. Here we also describe a rapidly emerging trend of using plant viruses to deliver the RNAi inducer molecules into insects for an efficient RNAi response.

Virus resistance in orchids

Orchid plants, Phalaenopsis and Dendrobium in particular, are commercially valuable ornamental plants sold worldwide. Unfortunately, orchid plants are highly susceptible to viral infection by Cymbidium mosaic virus (CymMV) and Odotoglossum ringspot virus (ORSV), posing a major threat and serious economic loss to the orchid industry worldwide. A major challenge is to generate an effective method to overcome plant viral infection. With the development of optimized orchid transformation biotechnological techniques and the establishment of concepts of pathogen-derived resistance (PDR), the generation of plants resistant to viral infection has been achieved. The PDR concept involves introducing genes that is(are) derived from the virus into the host plant to induce RNA- or protein-mediated resistance. We here review the fundamental mechanism of the PDR concept, and illustrate its application in protecting against viral infection of orchid plants.

Trans-specific gene silencing of acetyl-CoA carboxylase in a root-parasitic plant

Parasitic species of the family Orobanchaceae are devastating agricultural pests in many parts of the world. The control of weedy Orobanchaceae spp. is challenging, particularly due to the highly coordinated life cycles of the parasite and host plants. Although host genetic resistance often provides the foundation of plant pathogen management, few genes that confer resistance to root parasites have been identified and incorporated into crop species. Members of the family Orobanchaceae acquire water, nutrients, macromolecules, and oligonucleotides from host plants through haustoria that connect parasite and host plant roots. We are evaluating a resistance strategy based on using interfering RNA (RNAi) that is made in the host but inhibitory in the parasite as a parasite-derived oligonucleotide toxin. Sequences from the cytosolic acetyl-CoA carboxylase (ACCase) gene from Triphysaria versicolor were cloned in hairpin conformation and introduced into Medicago truncatula roots by Agrobacterium rhizogenes transformation. Transgenic roots were recovered for four of five ACCase constructions and infected with T. versicolor against parasitic weeds. In all cases, Triphysaria root viability was reduced up to 80% when parasitizing a host root bearing the hairpin ACCase. Triphysaria root growth was recovered by exogenous application of malonate. Reverse-transcriptase polymerase chain reaction (RT-PCR) showed that ACCase transcript levels were dramatically decreased in Triphysaria spp. parasitizing transgenic Medicago roots. Northern blot analysis identified a 21-nucleotide, ACCase-specific RNA in transgenic M. truncatula and in T. versicolor attached to them. One hairpin ACCase construction was lethal to Medicago spp. unless grown in media supplemented with malonate. Quantitative RT-PCR showed that the Medicago ACCase was inhibited by the Triphysaria ACCase RNAi. This work shows that ACCase is an effective target for inactivation in parasitic plants by trans-specific gene silencing.

Opportunities and constraints to biotechnological applications in the Caribbean: transgenic papayas in Jamaica and Venezuela

In this opinion article, we briefly review the status of crop biotechnology research-with emphasis on the development of GM crops-in Jamaica and Venezuela. We focus on the transgenic papayas developed for both countries, and examine the factors hindering not only the development and application of this biotechnological commodity for the improvement of agricultural productivity, but also on the challenges influencing societal acceptance of the technology.

The impact of transgenic papaya (TPY10-4) fruit supplementation on immune responses in ovalbumin-sensitised mice

BACKGROUND

A transgenic papaya line (TPY10-4) that is resistant to both papaya ringspot virus (PRSV) and papaya leaf distortion mosaic virus (PLDMV) has been developed in Taiwan. This study investigated the immunomodulatory properties of transgenic TPY10-4 and its native (TCK) papaya fruits using an ovalbumin (OVA)-sensitised mouse model. Both green and ripe papaya fruits at low (0.2 g powder kg(-1) body weight (BW)) and high (1.6 g powder kg(-1) BW) doses were administered to experimental mice by intragastric gavage for 5 weeks. Changes in serum total immunoglobulin A (IgA), IgE, IgG and IgM levels, OVA-specific IgE, IgG1 and IgG2a titres and Th1/Th2 cytokine secretions using splenocytes were determined.

RESULTS

Transgenic TPY10-4 or native TCK papaya fruit supplementation did not significantly affect body, visceral organ and relative tissue weights, total IgE antibody levels, OVA-specific IgE and IgG1 antibody titres or OVA-stimulated interferon-γ (IFN-γ), interleukin-2 (IL-2), IL-4, IL-5 and IL-10 secretions using splenocytes. However, transgenic papaya fruits markedly increased serum total IgM levels.

CONCLUSION

This study suggests that transgenic TPY10-4 papaya fruits do not increase the allergenic potential of OVA by oral administration but may have a protective immunity via increasing the serum total IgM level.

The private sector's role in public sector genetically engineered crop projects

There is widespread interest within academia to work on public good genetically engineered (GE) projects to the benefit of the poor, especially to use GE-technology to contribute to food security. Not a single product from this work has reached the market. The major cause is GE-regulation, which prevents use of the technology for public good beyond proof-of-concept (Potrykus, I. (2010) Lessons from the Humanitarian Golden Rice project: Regulation prevents development of public good GE-products (these Proceedings)). There is, however, another key problem responsible for the lack of deployment of public good GE-plants: the public sector is incompetent and disinterested for work beyond proof-of-concept, and has neither capability nor funding to develop GE-plant products and introduce them to growers and consumers. The private sector has the expertise for both and in the right circumstances can be ready to support the public sector in public good enterprises. Public-private-partnerships are the best solution so far, to advance exploitation of GE-technology to the benefit of the poor. Public-private-partnerships are viable, however, only, if there is mutual interest from the private sector and initiative and funding from the public sector.

Cross-protection: a century of mystery

Cross-protection is a phenomenon in which infection of a plant with a mild virus or viroid strain protects it from disease resulting from a subsequent encounter with a severe strain of the same virus or viroid. In this chapter, we review the history of cross-protection with regard to the development of ideas concerning its likely mechanisms, including RNA silencing and exclusion, and its influence on the early development of genetically engineered virus resistance. We also examine examples of the practical use of cross-protection in averting crop losses due to viruses, as well as the use of satellite RNAs to ameliorate the impact of virus-induced diseases. We also discuss the potential of cross-protection to contribute in future to the maintenance of crop health in the face of emerging virus diseases and related threats to agricultural production.

Papaya ringspot virus-P: characteristics, pathogenicity, sequence variability and control

TAXONOMY

Papaya ringspot virus (PRSV) is an aphid-transmitted plant virus belonging to the genus Potyvirus, family Potyviridae, with a positive sense RNA genome. PRSV isolates belong to either one of two major strains, P or W. The P strains infect both papaya and cucurbits whereas the W strains infect only cucurbits.

GEOGRAPHICAL DISTRIBUTION

PRSV-P is found in all major papaya-growing areas.

PHYSICAL PROPERTIES

Virions are filamentous, non-enveloped and flexuous measuring 760-800 x 12 nm. Virus particles contain 94.5% protein and 5.5% nucleic acid. The protein component consists of the virus coat protein (CP), which has a molecular weight of about 36 kDa as estimated by Western blot analysis. Density of the sedimenting component in purified PRSV preparations is 1.32 g/cm(3) in CsCl.

GENOME

The PRSV genome consists of a unipartite linear single-stranded positive sense RNA of 10 326 nucleotides with a 5' terminus, genome-linked protein, VPg.

TRANSMISSION

The virus is naturally transmitted via aphids in a non-persistent manner. Both the CP and helper component (HC-Pro) are required for vector transmission. This virus can also be transmitted mechanically, and is typically not seed-transmitted.

HOSTS

PRSV has a limited number of hosts belonging to the families Caricaceae, Chenopodiaceae and Cucurbitaceae. Propagation hosts are: Carica papaya, Cucurbita pepo and Cucumis metuliferus cv. accession 2459. Local lesion assay hosts are: Chenopodium quinoa and Chenopodium amaranticolor.

CONTROL

Two transgenic papaya varieties, Rainbow and SunUp, with engineered resistance to PRSV have been commercially grown in Hawaii since 1998. Besides transgenic resistance, tolerant varieties, cross-protection and other cultural practices such as isolation and rogueing of infected plants are used to manage the disease.

VIRUS CODE

00.057.0.01.045.

VIRUS ACCESSION NUMBER

57010045.

USEFUL LINK

http://www.ncbi.nlm.nih.gov/ICTVdb/ICTVdB/57010045.htm.

Recent advances in the development of transgenic papaya technology

Papaya with resistance to papaya ringspot virus (PRSV) is the first genetically modified tree and fruit crop and also the first transgenic crop developed by a public institution that has been commercialized. This chapter reviews the different transformation systems used for papaya and recent advances in the use of transgenic technology to introduce important quality and horticultural traits in papaya. These include the development of the following traits in papaya: resistance to PRSV, mites and Phytophthora, delayed ripening trait or long shelf life by inhibiting ethylene production or reducing loss of firmness, and tolerance or resistance to herbicide and aluminum toxicity. The use of papaya to produce vaccine against tuberculosis and cysticercosis, an infectious animal disease, has also been explored. Because of the economic importance of papaya, there are several collaborative and independent efforts to develop PRSV transgenic papaya technology in 14 countries. This chapter further reviews the strategies and constraints in the adoption of the technology and biosafety to the environment and food safety. Constraints to adoption include public perception, strict and expensive regulatory procedures and intellectual property issues.

Safety of virus-resistant transgenic plants two decades after their introduction: lessons from realistic field risk assessment studies

Potential safety issues have been raised with the development and release of virus-resistant transgenic plants. This review focuses on safety assessment with a special emphasis on crops that have been commercialized or extensively tested in the field such as squash, papaya, plum, grape, and sugar beet. We discuss topics commonly perceived to be of concern to the environment and to human health--heteroencapsidation, recombination, synergism, gene flow, impact on nontarget organisms, and food safety in terms of allergenicity. The wealth of field observations and experimental data is critically evaluated to draw inferences on the most relevant issues. We also express inside views on the safety and benefits of virus-resistant transgenic plants, and recommend realistic risk assessment approaches to assist their timely deregulation and release.