Post-transcriptional gene silencing in plum pox virus resistant transgenic European plum containing the plum pox potyvirus coat protein gene

CRISPR/Cas as a Genome-Editing Technique in Fruit Tree Breeding

CRISPR (short for "Clustered Regularly Interspaced Short Palindromic Repeats") is a technology that research scientists use to selectively modify the DNA of living organisms. CRISPR was adapted for use in the laboratory from the naturally occurring genome-editing systems found in bacteria. In this work, we reviewed the methods used to introduce CRISPR/Cas-mediated genome editing into fruit species, as well as the impacts of the application of this technology to activate and knock out target genes in different fruit tree species, including on tree development, yield, fruit quality, and tolerance to biotic and abiotic stresses. The application of this gene-editing technology could allow the development of new generations of fruit crops with improved traits by targeting different genetic segments or even could facilitate the introduction of traits into elite cultivars without changing other traits. However, currently, the scarcity of efficient regeneration and transformation protocols in some species, the fact that many of those procedures are genotype-dependent, and the convenience of segregating the transgenic parts of the CRISPR system represent the main handicaps limiting the potential of genetic editing techniques for fruit trees. Finally, the latest news on the legislation and regulations about the use of plants modified using CRISPR/Cas systems has been also discussed.

Trans-grafting plum pox virus resistance from transgenic plum rootstocks to apricot scions

Introduction

Trans-grafting could be a strategy to transfer virus resistance from a transgenic rootstock to a wild type scion. However contradictory results have been obtained in herbaceous and woody plants. This work was intended to determine if the resistance to sharka could be transferred from transgenic plum rootstocks to wild-type apricot scions grafted onto them.

Methods

To this end, we conducted grafting experiments of wild- type apricots onto plum plants transformed with a construction codifying a hairpin RNA designed to silence the PPV virus and studied if the resistance was transmitted from the rootstock to the scion.

Results

Our data support that the RNA-silencing-based PPV resistance can be transmitted from PPV-resistant plum rootstocks to non-transgenic apricot scions and that its efficiency is augmented after successive growth cycles. PPV resistance conferred by the rootstocks was robust, already occurring within the same growing cycle and maintained in successive evaluation cycles. The RNA silencing mechanism reduces the relative accumulation of the virus progressively eliminating the virus from the wild type scions grafted on the transgenic resistant PPV plants. There was a preferential accumulation of the 24nt siRNAs in the scions grafted onto resistant rootstocks that was not found in the scions grafted on the susceptible rootstock. This matched with a significantly lower relative accumulation of hpRNA in the resistant rootstocks compared with the susceptible or the tolerant ones.

Discussion

Using transgenic rootstocks should mitigate public concerns about transgenes dispersion and eating transgenic food and allow conferring virus resistance to recalcitrant to transformation cultivars or species.

Cauliflower mosaic virus: Virus-host interactions and its uses in biotechnology and medicine

Cauliflower mosaic virus (CaMV) was the first discovered plant virus with genomic DNA that uses reverse transcriptase for replication. The CaMV 35S promoter is a constitutive promoter and thus, an attractive driver of gene expression in plant biotechnology. It is used in most transgenic crops to activate foreign genes which have been artificially inserted into the host plant. In the last century, producing food for the world's population while preserving the environment and human health is the main topic of agriculture. The damage caused by viral diseases has a significant negative economic impact on agriculture, and disease control is based on two strategies: immunization and prevention to contain virus spread, so correct identification of plant viruses is important for disease management. Here, we discuss CaMV from different aspects: taxonomy, structure and genome, host plants and symptoms, transmission and pathogenicity, prevention, control and application in biotechnology as well as in medicine. Also, we calculated the CAI index for three ORFs IV, V, and VI of the CaMV virus in host plants, the results of which can be used in the discussion of gene transfer or antibody production to identify the CaMV.

Recent trends and advances of RNA interference (RNAi) to improve agricultural crops and enhance their resilience to biotic and abiotic stresses

Over the last two decades, significant advances have been made using genetic engineering technology to modify genes from various exotic origins and introduce them into plants to induce favorable traits. RNA interference (RNAi) was discovered earlier as a natural process for controlling the expression of genes across all higher species. It aims to enhance precision and accuracy in pest/pathogen resistance, quality improvement, and manipulating the architecture of plants. However, it existed as a widely used technique recently. RNAi technologies could well be used to down-regulate any genes' expression without disrupting the expression of other genes. The use of RNA interference to silence genes in various organisms has become the preferred method for studying gene functions. The establishment of new approaches and applications for enhancing desirable characters is essential in crops by gene suppression and the refinement of knowledge of endogenous RNAi mechanisms in plants. RNAi technology in recent years has become an important and choicest method for controlling insects, pests, pathogens, and abiotic stresses like drought, salinity, and temperature. Although there are certain drawbacks in efficiency of this technology such as gene candidate selection, stability of trigger molecule, choice of target species and crops. Nevertheless, from past decade several target genes has been identified in numerous crops for their improvement towards biotic and abiotic stresses. The current review is aimed to emphasize the research done on crops under biotic and abiotic stress using RNAi technology. The review also highlights the gene regulatory pathways/gene silencing, RNA interference, RNAi knockdown, RNAi induced biotic and abiotic resistance and advancements in the understanding of RNAi technology and the functionality of various components of the RNAi machinery in crops for their improvement.

A versatile Agrobacterium-based plant transformation system for genetic engineering of diverse citrus cultivars

Developing an efficient transformation system is vital in genetically engineering recalcitrant crops, particularly trees. Here, we outline an Agrobacterium tumefaciens-based stable plant transformation methodology for citrus genetic engineering. The process was optimized to suit the requirements of fourteen citrus varieties by establishing appropriate infection, co-cultivation, selection, and culture media conditions. The procedure includes transforming seedling-derived epicotyl segments with an A. tumefaciens strain, then selecting and regenerating transformed tissues. Transgenic shoots were further identified by a visual reporter (e.g., β-glucuronidase) and confirmed by Northern and Southern blot analysis. Transgene integrations among the transgenic lines ranged between one to four. The methodology can yield transformation efficiencies of up to 11%, and transgenic plants can be recovered as early as six months, depending on the variety. In addition, we show that incorporating A. tumefaciens helper virulence genes (virG and virE), spermidine, and lipoic acid in the resuspension buffer before transformation improved the transformation efficiency of specific recalcitrant cultivars, presumably by enhancing T-DNA integration and alleviating oxidative stress on the explant tissues. In conclusion, the optimized methodology can be utilized to engineer diverse recalcitrant citrus varieties towards trait improvement or functional genetics applications.

Long-Term Efficacy and Safety of RNAi-Mediated Virus Resistance in 'HoneySweet' Plum

Interfering RNA technology has been established as an effective strategy to protect plants against viral infection. Despite this success, interfering RNA (RNAi) has rarely been applied due to the regulatory barriers that confront genetically engineered plants and concerns over possible environmental and health risks posed by non-endogenous small RNAs. 'HoneySweet' was developed as a virus-resistant plum variety that is protected by an RNAi-mediated process against Sharka disease caused by the plum pox virus. 'HoneySweet' has been approved for cultivation in the United States but not in countries where the plum pox virus is endemic. In this study, we evaluated the long-term efficacy of virus resistance in 'HoneySweet,' the nature and stability of its sRNA profile, and the potential health risks of consuming 'HoneySweet' plums. Graft-challenged 'HoneySweet' trees carrying large non-transgenic infected limbs remained virus-free after more than 10 years in the field, and the viral sequences from the non-transgenic infected limbs showed no evidence of adaptation to the RNAi-based resistance. Small RNA profiling revealed that transgene-derived sRNA levels were stable across different environments and, on average, were more than 10 times lower than those present in symptom-less fruits from virus-infected trees. Comprehensive 90-day mouse feeding studies showed no adverse health impacts in mice, and there was no evidence for potential siRNA off-target pathologies predicted by comparisons of the most abundant transgene-derived sRNAs to the mouse genome. Collectively, the data confirmed that RNAi provides a highly effective, stable, and safe strategy to combat virus diseases in crop plants.

Optimized RNA-Silencing Strategies for Rice Ragged Stunt Virus Resistance in Rice

Rice ragged stunt virus (RRSV) is one of the most damaging viruses of the rice culture area in south and far-eastern Asia. To date, no genetic resistance has been identified and only expensive and non-environmentally friendly chemical treatments are deployed to fight this important disease. Non-chemical approaches based on RNA-silencing have been developed as resistance strategies against viruses. Here, we optimized classical miRNA and siRNA-based strategies to obtain efficient and durable resistance to RRSV. miRNA-based strategies are involved in producing artificial miRNA (amiR) targeting viral genomes in plants. Classically, only one amiR is produced from a single construct. We demonstrated for the first time that two amiRs targeting conserved regions of RRSV genomes could be transgenically produced in Nicotiana benthamiana and in rice for a single precursor. Transgenic rice plants producing either one or two amiR were produced. Despite efficient amiR accumulations, miRNA-based strategies with single or double amiRs failed to achieve efficient RRSV resistance in transformed rice plants. This suggests that this strategy may not be adapted to RRSV, which could rapidly evolve to counteract them. Another RNA-silencing-based method for viral resistance concerns producing several viral siRNAs targeting a viral fragment. These viral siRNAs are produced from an inverted repeat construct carrying the targeted viral fragment. Here, we optimized the inverted repeat construct using a chimeric fragment carrying conserved sequences of three different RRSV genes instead of one. Of the three selected homozygous transgenic plants, one failed to accumulate the expected siRNA. The two other ones accumulated siRNAs from either one or three fragments. A strong reduction of RRSV symptoms was observed only in transgenic plants expressing siRNAs. We consequently demonstrated, for the first time, an efficient and environmentally friendly resistance to RRSV in rice based on the siRNA-mediated strategy.

Robust Response to Plum pox virus Infection via Plant Biotechnology

Our goal was to target silencing of the Plum pox virus coat protein (PPV CP) gene independently expressed in plants. Clone C-2 is a transgenic plum expressing CP. We introduced and verified, in planta, the effects of the inverse repeat of CP sequence split by a hairpin (IRSH) that was characterized in the HoneySweet plum. The IRSH construct was driven by two CaMV35S promoter sequences flanking the CP sequence and had been introduced into C1738 plum. To determine if this structure was enough to induce silencing, cross-hybridization was made with the C1738 clone and the CP expressing but PPV-susceptible C2 clone. In total, 4 out of 63 clones were silenced. While introduction of the IRSH is reduced due to the heterozygous character in C1738 plum, the silencing induced by the IRSH PPV CP is robust. Extensive studies, in greenhouse containment, demonstrated that the genetic resource of C1738 clone can silence the CP production. In addition, these were verified through the virus transgene pyramiding in the BO70146 BlueByrd cv. plum that successfully produced resistant BlueByrd BO70146 × C1738 (HybC1738) hybrid plums.

Defining the 'HoneySweet' insertion event utilizing NextGen sequencing and a de novo genome assembly of plum (Prunus domestica)

'HoneySweet' plum (Prunus domestica) is resistant to Plum pox potyvirus, through an RNAi-triggered mechanism. Determining the precise nature of the transgene insertion event has been complicated due to the hexaploid genome of plum. DNA blots previously indicated an unintended hairpin arrangement of the Plum pox potyvirus coat protein gene as well as a multicopy insertion event. To confirm the transgene arrangement of the insertion event, 'HoneySweet' DNA was subjected to whole genome sequencing using Illumina short-read technology. Results indicated two different insertion events, one containing seven partial copies flanked by putative plum DNA sequence and a second with the predicted inverted repeat of the coat protein gene driven by a double 35S promoter on each side, flanked by plum DNA. To determine the locations of the two transgene insertions, a phased plum genome assembly was developed from the commercial plum 'Improved French'. A subset of the scaffolds (2447) that were >10 kb in length and representing, >95% of the genome were annotated and used for alignment against the 'HoneySweet' transgene reads. Four of eight matching scaffolds spanned both insertion sites ranging from 157,704 to 654,883 bp apart, however we were unable to identify which scaffold(s) represented the actual location of the insertion sites due to potential sequence differences between the two plum cultivars. Regardless, there was no evidence of any gene(s) being interrupted as a result of the insertions. Furthermore, RNA-seq data verified that the insertions created no new transcriptional units and no dramatic expression changes of neighboring genes.

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.

Innovative RNAi Strategies and Tactics to Tackle Plum Pox Virus (PPV) Genome in Prunus domestica-Plum

We developed an innovative RNAi concept based on two gene constructs built from the capsid gene (CP) cistron of the Plum pox virus (PPV) genome. First, designated as amiCPRNA, a potential molecule interfering with PPV genome translation and the second one is the ami-siCPRNA to target viral genome translation and PPV RNA replication. Following the previous engineering of these constructs in an experimental herbaceous host, they were introduced into Prunus domestica (plum tree) genome. Previously propagated onto a susceptible rootstock, these clones were graft-inoculated with PPV. After four dormancy cycles, and consistent with our experience of PPV infection, some clones showed a common phenomenon of silencing that can differ between the detailed plant phenotypes. Three different phenotypes were developed by the amisiCPRNA clones. First, the high resistance character shown by the amisiCPRNA plum-7 that was similar to the resistance expressed by HoneySweet plum. Secondly, a recovery reaction was developed by the two other amisiCPRNA plum-3 and plum-4 that differed from the rest, characterized as susceptible clones, among these were the amiCPRNA plums. Having assessed the behavior of these plums versus the herbaceous host accumulating the similar form of RNAi: ami-, si-, and ami-siRNA, challenging assays in perennials consistently reflect the natural context of viral genome targeting.

RNAi-Mediated Resistance Against Viruses in Perennial Fruit Plants

Small RNAs (sRNAs) are 20–30-nucleotide-long, regulatory, noncoding RNAs that induce silencing of target genes at the transcriptional and posttranscriptional levels. They are key components for cellular functions during plant development, hormone signaling, and stress responses. Generated from the cleavage of double-stranded RNAs (dsRNAs) or RNAs with hairpin structures by Dicer-like proteins (DCLs), they are loaded onto Argonaute (AGO) protein complexes to induce gene silencing of their complementary targets by promoting messenger RNA (mRNA) cleavage or degradation, translation inhibition, DNA methylation, and/or histone modifications. This mechanism of regulating RNA activity, collectively referred to as RNA interference (RNAi), which is an evolutionarily conserved process in eukaryotes. Plant RNAi pathways play a fundamental role in plant immunity against viruses and have been exploited via genetic engineering to control disease. Plant viruses of RNA origin that contain double-stranded RNA are targeted by the RNA-silencing machinery to produce virus-derived small RNAs (vsRNAs). Some vsRNAs serve as an effector to repress host immunity by capturing host RNAi pathways. High-throughput sequencing (HTS) strategies have been used to identify endogenous sRNA profiles, the “sRNAome”, and analyze expression in various perennial plants. Therefore, the review examines the current knowledge of sRNAs in perennial plants and fruits, describes the development and implementation of RNA interference (RNAi) in providing resistance against economically important viruses, and explores sRNA targets that are important in regulating a variety of biological processes.

Multilocation comparison of fruit composition for 'HoneySweet', an RNAi based plum pox virus resistant plum

'HoneySweet', a transgenic plum (Prunus domestica) resistant to plum pox virus through RNAi, was deregulated in the U.S. in 2011. The compositional study of 'HoneySweet' fruit was expanded to include locations outside of the US as well as utilizing a wide variety of comparators and different collection years to see the variability possible. The results revealed that plums have a wide variation in composition and that variation among locations was greater than variation among cultivars. This was also the case for different years at one location. The results supported the supposition that the transgene and insertion event had no significant effect on the composition of 'HoneySweet' fruit even under virus pressure, and that it fell in the normal range of composition of commercially grown plums. It also suggested that the effect of environment is as great as that of genetics on the fruit composition of plums.

Agrobacterium-Mediated Transformation of Tree Fruit Crops: Methods, Progress, and Challenges

Genetic engineering based on Agrobacterium-mediated transformation has been a desirable tool to manipulate single or multiple genes of existing genotypes of woody fruit crops, for which conventional breeding is a difficult and lengthy process due to heterozygosity, sexual incompatibility, juvenility, or a lack of natural sources. To date, successful transformation has been reported for many fruit crops. We review the major progress in genetic transformation of these fruit crops made in the past 5 years, emphasizing reproducible transformation protocols as well as the strategies that have been tested in fruit crops. While direct transformation of scion cultivars was mostly used for fruit quality improvement, biotic and abiotic tolerance, and functional gene analysis, transgrafting on genetically modified (GM) rootstocks showed a potential to produce non-GM fruit products. More recently, genome editing technology has demonstrated a potential for gene(s) manipulation of several fruit crops. However, substantial efforts are still needed to produce plants from gene-edited cells, for which tremendous challenge remains in the context of either cell's recalcitrance to regeneration or inefficient gene-editing due to their polyploidy. We propose that effective transient transformation and efficient regeneration are the key for future utilization of genome editing technologies for improvement of fruit crops.

Current achievements and future directions in genetic engineering of European plum (Prunus domestica L.)

In most woody fruit species, transformation and regeneration are difficult. However, European plum (Prunus domestica) has been shown to be amenable to genetic improvement technologies from classical hybridization, to genetic engineering, to rapid cycle crop breeding ('FasTrack' breeding). Since the first report on European plum transformation with marker genes in the early 90 s, numerous manuscripts have been published reporting the generation of new clones with agronomically interesting traits, such as pests, diseases and/or abiotic stress resistance, shorter juvenile period, dwarfing, continuous flowering, etc. This review focuses on the main advances in genetic transformation of European plum achieved to date, and the lines of work that are converting genetic engineering into a contemporary breeding tool for this species.

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.

Environmental Fate of Insecticidal Plant-Incorporated Protectants from Genetically Modified Crops: Knowledge Gaps and Research Opportunities

Plant-incorporated protectants (PIPs) are biopesticides expressed in genetically modified (GM) crops and are typically macromolecular in nature. First-generation insecticidal PIPs were Cry proteins expressed in GM crops containing transgenes from the soil bacterium Bacillus thuringiensis; next-generation double-stranded ribonucleic acid (dsRNA) PIPs have been recently approved. Like conventional synthetic pesticides, the use of either Cry protein or dsRNA PIPs results in their release to receiving environments. However, as opposed to conventional low molecular weight pesticides, the environmental fate of macromolecular PIPs remains less studied and is poorly understood. This Feature highlights the knowledge gaps and challenges that have emerged while investigating the environmental fate of Cry protein PIPs and suggests new avenues to advance the state of the research necessary for the ongoing environmental fate assessment of dsRNA PIPs.

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.

The Impact of "Coat Protein-Mediated Virus Resistance" in Applied Plant Pathology and Basic Research

Worldwide, plant viruses cause serious reductions in marketable crop yield and in some cases even plant death. In most cases, the most effective way to control virus diseases is through genetically controlled resistance. However, developing virus-resistant (VR) crops through traditional breeding can take many years, and in some cases is not even possible. Because of this, the demonstration of the first VR transgenic plants in 1985 generated much attention. This seminal report served as an inflection point for research in both basic and applied plant pathology, the results of which have dramatically changed both basic research and in a few cases, commercial crop production. The typical review article on this topic has focused on only basic or only applied research results stemming from this seminal discovery. This can make it difficult for the reader to appreciate the full impact of research on transgenic virus resistance, and the contributions from fundamental research that led to translational applications of this technology. In this review, we take a global view of this topic highlighting the significant changes to both basic and applied plant pathology research and commercial food production that have accumulated in the last 30 plus years. We present these milestones in the historical context of some of the scientific, economic, and environmental drivers for developing specific VR crops. The intent of this review is to provide a single document that adequately records the significant accomplishments of researchers in both basic and applied plant pathology research on this topic and how they relate to each other. We hope this review therefore serves as both an instructional tool for students new to the topic, as well as a source of conversation and discussion for how the technology of engineered virus resistance could be applied in the future.

An efficient viral vector for functional genomic studies of Prunus fruit trees and its induced resistance to Plum pox virus via silencing of a host factor gene

RNA silencing is a powerful technology for molecular characterization of gene functions in plants. A commonly used approach to the induction of RNA silencing is through genetic transformation. A potent alternative is to use a modified viral vector for virus-induced gene silencing (VIGS) to degrade RNA molecules sharing similar nucleotide sequence. Unfortunately, genomic studies in many allogamous woody perennials such as peach are severely hindered because they have a long juvenile period and are recalcitrant to genetic transformation. Here, we report the development of a viral vector derived from Prunus necrotic ringspot virus (PNRSV), a widespread fruit tree virus that is endemic in all Prunus fruit production countries and regions in the world. We show that the modified PNRSV vector, harbouring the sense-orientated target gene sequence of 100-200 bp in length in genomic RNA3, could efficiently trigger the silencing of a transgene or an endogenous gene in the model plant Nicotiana benthamiana. We further demonstrate that the PNRSV-based vector could be manipulated to silence endogenous genes in peach such as eukaryotic translation initiation factor 4E isoform (eIF(iso)4E), a host factor of many potyviruses including Plum pox virus (PPV). Moreover, the eIF(iso)4E-knocked down peach plants were resistant to PPV. This work opens a potential avenue for the control of virus diseases in perennial trees via viral vector-mediated silencing of host factors, and the PNRSV vector may serve as a powerful molecular tool for functional genomic studies of Prunus fruit trees.

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.

Use of Posttranscription Gene Silencing in Squash to Induce Resistance against the Egyptian Isolate of the Squash Leaf Curl Virus

Squash leaf curl virus (SqLCV) is a bipartite begomovirus affecting squash plants. It is transmitted by whitefly Bemisia tabaci biotype B causing severe leaf curling, vein banding, and molting ending by stunting. In this study full-length genomic clone of SqLCV Egyptian isolated and posttranscriptional gene silencing (PTGS) has been induced to develop virus resistance. The Noubaria SqLCV has more than 95% homology with Jordon, Israel, Lebanon, Palestine, and Cairo isolates. Two genes fragment from SqLCV introduced in sense and antisense orientations using pFGC5049 vector to be expressed as hairpin RNA. The first fragment was 348 bp from replication associated protein gene (Rep). The second fragment was 879 bp representing the full sequence of the movement protein gene (BC1). Using real-time PCR, a silencing record of 97% has been recorded to Rep/TrAP construct; as a result it has prevented the appearance of viral symptoms in most tested plants up to two months after infection, while construct containing the BC1 gene scored a reduction in the accumulation of viral genome expression as appearing in real-time PCR results 4.6-fold giving a silencing of 79%, which had a positive effect on symptoms development in most tested plants.

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.

A current overview of the Papaya meleira virus, an unusual plant virus

Papaya meleira virus (PMeV) is the causal agent of papaya sticky disease, which is characterized by a spontaneous exudation of fluid and aqueous latex from the papaya fruit and leaves. The latex oxidizes after atmospheric exposure, resulting in a sticky feature on the fruit from which the name of the disease originates. PMeV is an isometric virus particle with a double-stranded RNA (dsRNA) genome of approximately 12 Kb. Unusual for a plant virus, PMeV particles are localized on and linked to the polymers present in the latex. The ability of the PMeV to inhabit such a hostile environment demonstrates an intriguing interaction of the virus with the papaya. A hypersensitivity response is triggered against PMeV infection, and there is a reduction in the proteolytic activity of papaya latex during sticky disease. In papaya leaf tissues, stress responsive proteins, mostly calreticulin and proteasome-related proteins, are up regulated and proteins related to metabolism are down-regulated. Additionally, PMeV modifies the transcription of several miRNAs involved in the modulation of genes related to the ubiquitin-proteasome system. Until now, no PMeV resistant papaya genotype has been identified and roguing is the only viral control strategy available. However, a single inoculation of papaya plants with PMeV dsRNA delayed the progress of viral infection.

RNA Interference (RNAi) Induced Gene Silencing: A Promising Approach of Hi-Tech Plant Breeding

RNA interference (RNAi) is a promising gene regulatory approach in functional genomics that has significant impact on crop improvement which permits down-regulation in gene expression with greater precise manner without affecting the expression of other genes. RNAi mechanism is expedited by small molecules of interfering RNA to suppress a gene of interest effectively. RNAi has also been exploited in plants for resistance against pathogens, insect/pest, nematodes, and virus that cause significant economic losses. Keeping beside the significance in the genome integrity maintenance as well as growth and development, RNAi induced gene syntheses are vital in plant stress management. Modifying the genes by the interference of small RNAs is one of the ways through which plants react to the environmental stresses. Hence, investigating the role of small RNAs in regulating gene expression assists the researchers to explore the potentiality of small RNAs in abiotic and biotic stress management. This novel approach opens new avenues for crop improvement by developing disease resistant, abiotic or biotic stress tolerant, and high yielding elite varieties.

Temporal and spatial control of gene expression in horticultural crops

Biotechnology provides plant breeders an additional tool to improve various traits desired by growers and consumers of horticultural crops. It also provides genetic solutions to major problems affecting horticultural crops and can be a means for rapid improvement of a cultivar. With the availability of a number of horticultural genome sequences, it has become relatively easier to utilize these resources to identify DNA sequences for both basic and applied research. Promoters play a key role in plant gene expression and the regulation of gene expression. In recent years, rapid progress has been made on the isolation and evaluation of plant-derived promoters and their use in horticultural crops, as more and more species become amenable to genetic transformation. Our understanding of the tools and techniques of horticultural plant biotechnology has now evolved from a discovery phase to an implementation phase. The availability of a large number of promoters derived from horticultural plants opens up the field for utilization of native sequences and improving crops using precision breeding. In this review, we look at the temporal and spatial control of gene expression in horticultural crops and the usage of a variety of promoters either isolated from horticultural crops or used in horticultural crop improvement.

Plum pox virus and sharka: a model potyvirus and a major disease

TAXONOMIC RELATIONSHIPS

Plum pox virus (PPV) is a member of the genus Potyvirus in the family Potyviridae. PPV diversity is structured into at least eight monophyletic strains.

GEOGRAPHICAL DISTRIBUTION

First discovered in Bulgaria, PPV is nowadays present in most of continental Europe (with an endemic status in many central and southern European countries) and has progressively spread to many countries on other continents.

GENOMIC STRUCTURE

Typical of potyviruses, the PPV genome is a positive-sense single-stranded RNA (ssRNA), with a protein linked to its 5' end and a 3'-terminal poly A tail. It is encapsidated by a single type of capsid protein (CP) in flexuous rod particles and is translated into a large polyprotein which is proteolytically processed in at least 10 final products: P1, HCPro, P3, 6K1, CI, 6K2, VPg, NIapro, NIb and CP. In addition, P3N-PIPO is predicted to be produced by a translational frameshift.

PATHOGENICITY FEATURES

PPV causes sharka, the most damaging viral disease of stone fruit trees. It also infects wild and ornamental Prunus trees and has a large experimental host range in herbaceous species. PPV spreads over long distances by uncontrolled movement of plant material, and many species of aphid transmit the virus locally in a nonpersistent manner.

SOURCES OF RESISTANCE

A few natural sources of resistance to PPV have been found so far in Prunus species, which are being used in classical breeding programmes. Different genetic engineering approaches are being used to generate resistance to PPV, and a transgenic plum, 'HoneySweet', transformed with the viral CP gene, has demonstrated high resistance to PPV in field tests in several countries and has obtained regulatory approval in the USA.

Biosafety considerations of RNAi-mediated virus resistance in fruit-tree cultivars and in rootstock

A major application of RNA interference (RNAi) is envisaged for the production of virus-resistant transgenic plants. For fruit trees, this remains the most, if not the only, viable option for the control of plant viral disease outbreaks in cultivated orchards, due to the difficulties associated with the use of traditional and conventional disease-control measures. The use of RNAi might provide an additional benefit for woody crops if silenced rootstock can efficiently transmit the silencing signal to non-transformed scions, as has already been demonstrated in herbaceous plants. This would provide a great opportunity to produce non-transgenic fruit from transgenic rootstock. In this review, we scrutinise some of the concerns that might arise with the use of RNAi for engineering virus-resistant plants, and we speculate that this virus resistance has fewer biosafety concerns. This is mainly because RNAi-eliciting constructs only express small RNA molecules rather than proteins, and because this technology can be applied using plant rootstock that can confer virus resistance to the scion, leaving the scion untransformed. We discuss the main biosafety concerns related to the release of new types of virus-resistant plants and the risk assessment approaches in the application of existing regulatory systems (in particular, those of the European Union, the USA, and Canada) for the evaluation and approval of RNAi-mediated virus-resistant plants, either as transgenic varieties or as plant virus resistance induced by transgenic rootstock.

Silencing of the host factor eIF(iso)4E gene confers plum pox virus resistance in plum

Plum pox virus (PPV) causes the most economically-devastating viral disease in Prunus species. Unfortunately, few natural resistance genes are available for the control of PPV. Recessive resistance to some potyviruses is associated with mutations of eukaryotic translation initiation factor 4E (eIF4E) or its isoform eIF(iso)4E. In this study, we used an RNA silencing approach to manipulate the expression of eIF4E and eIF(iso)4E towards the development of PPV resistance in Prunus species. The eIF4E and eIF(iso)4E genes were cloned from plum (Prunus domestica L.). The sequence identity between plum eIF4E and eIF(iso)4E coding sequences is 60.4% at the nucleotide level and 52.1% at the amino acid level. Quantitative real-time RT-PCR analysis showed that these two genes have a similar expression pattern in different tissues. Transgenes allowing the production of hairpin RNAs of plum eIF4E or eIF(iso)4E were introduced into plum via Agrobacterium-mediated transformation. Gene expression analysis confirmed specific reduced expression of eIF4E or eIF(iso)4E in the transgenic lines and this was associated with the accumulation of siRNAs. Transgenic plants were challenged with PPV-D strain and resistance was evaluated by measuring the concentration of viral RNA. Eighty-two percent of the eIF(iso)4E silenced transgenic plants were resistant to PPV, while eIF4E silenced transgenic plants did not show PPV resistance. Physical interaction between PPV-VPg and plum eIF(iso)4E was confirmed. In contrast, no PPV-VPg/eIF4E interaction was observed. These results indicate that eIF(iso)4E is involved in PPV infection in plum, and that silencing of eIF(iso)4E expression can lead to PPV resistance in Prunus species.

GUS expression in sweet oranges (Citrus sinensis L. Osbeck) driven by three different phloem-specific promoters

Huanglongbing (HLB) is associated with Candidatus Liberibacter spp., endogenous, sieve tube-restricted bacteria that are transmitted by citrus psyllid insect vectors. Transgenic expression in the phloem of specific genes that might affect Ca. Liberibacter spp. growth and development may be an adequate strategy to improve citrus resistance to HLB. To study specific phloem gene expression in citrus, we developed three different binary vector constructs with expression cassettes bearing the β-glucuronidase (GUS) reporter gene (uidA) under the control of one of the three different promoters: Citrus phloem protein 2 (CsPP2), Arabidopsis thaliana phloem protein 2 (AtPP2), and Arabidopsis thaliana sucrose transporter 2 (AtSUC2). Transgenic lines of 'Hamlin', 'Pera', and 'Valencia' sweet oranges [Citrus sinensis (L.) Osbeck] were produced via Agrobacterium tumefaciens transformation. The epicotyl segments collected from in vitro germinated seedlings were used as explants. The gene nptII, which confers resistance to the antibiotic kanamycin, was used for selection. The transformation efficiency was expressed as the number of GUS-positive shoots over the total number of explants and varied from 1.54 to 6.08 % among the three cultivars and three constructs studied. Several lines of the three sweet orange cultivars analyzed using PCR and Southern blot analysis were genetically transformed with the three constructs evaluated. The histological GUS activity in the leaves indicates that the uidA gene was preferentially expressed in the phloem, which suggests that the use of the three promoters might be adequate for producing HLB-resistant transgenic sweet oranges. The results reported here conclusively demonstrate the preferential expression of GUS in the phloem driven by two heterologous and one homologous gene promoters. Key message The results reported here conclusively demonstrate the preferential expression of GUS in the phloem driven by two heterologous and one homologous gene promoters.

Transgenic banana plants expressing small interfering RNAs targeted against viral replication initiation gene display high-level resistance to banana bunchy top virus infection

The banana aphid-transmitted Banana bunchy top virus (BBTV) is the most destructive viral pathogen of bananas and plantains worldwide. Lack of natural sources of resistance to BBTV has necessitated the exploitation of proven transgenic technologies for obtaining BBTV-resistant banana cultivars. In this study, we have explored the concept of using intron-hairpin-RNA (ihpRNA) transcripts corresponding to viral master replication initiation protein (Rep) to generate BBTV-resistant transgenic banana plants. Two ihpRNA constructs namely ihpRNA-Rep and ihpRNA-ProRep generated using Rep full coding sequence or Rep partial coding sequence together with its 5′ upstream regulatory region, respectively, and castor bean catalase intron were successfully transformed into banana embryogenic cells. ihpRNA-Rep- and ihpRNA-ProRep-derived transgenic banana plants, selected based on preliminary screening for efficient reporter gene expression, were completely resistant to BBTV infection as indicated by the absence of disease symptoms after 6 months of viruliferous aphid inoculation. The resistance to BBTV infection was also evident by the inability to detect cDNAs coding for viral coat protein, movement protein and Rep protein by RT-PCR from inoculated transgenic leaf extracts. Southern analysis of the two groups of transgenics showed that ihpRNA transgene was stably integrated into the banana genome. The detection of small interfering RNAs (siRNAs) derived from the ihpRNA transgene sequence in transformed BBTV-resistant plants positively established RNA interference as the mechanism underlying the observed resistance to BBTV. Efficient screening of optimal transformants in this vegetatively propagated non-segregating fruit crop ensured that all the transgenic plants assayed were resistant to BBTV infection.

Transformation of Mexican lime with an intron-hairpin construct expressing untranslatable versions of the genes coding for the three silencing suppressors of Citrus tristeza virus confers complete resistance to the virus

Citrus tristeza virus (CTV), the causal agent of the most devastating viral disease of citrus, has evolved three silencing suppressor proteins acting at intra- (p23 and p20) and/or intercellular level (p20 and p25) to overcome host antiviral defence. Previously, we showed that Mexican lime transformed with an intron-hairpin construct including part of the gene p23 and the adjacent 3' untranslated region displays partial resistance to CTV, with a fraction of the propagations from some transgenic lines remaining uninfected. Here, we transformed Mexican lime with an intron-hairpin vector carrying full-length, untranslatable versions of the genes p25, p20 and p23 from CTV strain T36 to silence the expression of these critical genes in CTV-infected cells. Three transgenic lines presented complete resistance to viral infection, with all their propagations remaining symptomless and virus-free after graft inoculation with CTV-T36, either in the nontransgenic rootstock or in the transgenic scion. Accumulation of transgene-derived siRNAs was necessary but not sufficient for CTV resistance. Inoculation with a divergent CTV strain led to partially breaking the resistance, thus showing the role of sequence identity in the underlying mechanism. Our results are a step forward to developing transgenic resistance to CTV and also show that targeting simultaneously by RNA interference (RNAi) the three viral silencing suppressors appears critical for this purpose, although the involvement of concurrent RNAi mechanisms cannot be excluded.

Molecular diagnosis of Papaya meleira virus (PMeV) from leaf samples of Carica papaya L. using conventional and real-time RT-PCR

Papaya meleira virus (PMeV) is the causal agent of papaya sticky disease. This study describes two methods for molecular diagnosis of PMeV using conventional and real-time PCR. These methods were shown to be more efficient than current methods of viral detection using extraction of PMeV dsRNA and observation of symptoms in the field. The methods described here were used to evaluate the effect of inoculation of papaya plants with purified PMeV dsRNA on the progress of PMeV infection. A single inoculation with PMeV dsRNA was observed to delay the progress of the virus infection by several weeks. The possibility of vertical transmission of PMeV was also investigated. No evidence was found for PMeV transmission through seeds collected from diseased fruit. The implications of these results for the epidemiology of PMeV and the management of papaya sticky disease are discussed.

RNAi-mediated resistance to Cassava brown streak Uganda virus in transgenic cassava

Cassava brown streak disease (CBSD), caused by Cassava brown streak Uganda virus (CBSUV) and Cassava brown streak virus (CBSV), is of new epidemic importance to cassava (Manihot esculenta Crantz) production in East Africa, and an emerging threat to the crop in Central and West Africa. This study demonstrates that at least one of these two ipomoviruses, CBSUV, can be efficiently controlled using RNA interference (RNAi) technology in cassava. An RNAi construct targeting the near full-length coat protein (FL-CP) of CBSUV was expressed constitutively as a hairpin construct in cassava. Transgenic cassava lines expressing small interfering RNAs (siRNAs) against this sequence showed 100% resistance to CBSUV across replicated graft inoculation experiments. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis showed the presence of CBSUV in leaves and some tuberous roots from challenged controls, but not in the same tissues from transgenic plants. This is the first demonstration of RNAi-mediated resistance to the ipomovirus CBSUV in cassava.

Gene flow in Prunus species in the context of novel trait risk assessment

Prunus species are important commercial fruit (plums, apricot, peach and cherries), nut (almond) and ornamental trees cultivated broadly worldwide. This review compiles information from available literature on Prunus species in regard to gene flow and hybridization within this complex of species. The review serves as a resource for environmental risk assessment related to pollen mediated gene flow and the release of transgenic Prunus. It reveals that Prunus species, especially plums and cherries show high potential for transgene flow. A range of characteristics including; genetic diversity, genetic bridging capacity, inter- and intra-specific genetic compatibility, self sterility (in most species), high frequency of open pollination, insect assisted pollination, perennial nature, complex phenotypic architecture (canopy height, heterogeneous crown, number of flowers produced in an individual plant), tendency to escape from cultivation, and the existence of ornamental and road side Prunus species suggest that there is a tremendous and complicated ability for pollen mediated gene movement among Prunus species. Ploidy differences among Prunus species do not necessarily provide genetic segregation. The characteristics of Prunu s species highlight the complexity of maintaining coexistence between GM and non-GM Prunus if there were commercial production of GM Prunus species. The results of this review suggest that the commercialization of one GM Prunus species can create coexistence issues for commercial non-GM Prunus production. Despite advances in molecular markers and genetic analysis in agroecology, there remains limited information on the ecological diversity, metapopulation nature, population dynamics, and direct measures of gene flow among different subgenera represented in the Prunus genus. Robust environmental impact, biosafety and coexistence assessments for GM Prunus species will require better understanding of the mechanisms of gene flow and hybridization among species within the Prunus species complex.

RNAi-mediated resistance to diverse isolates belonging to two virus species involved in Cassava brown streak disease

Cassava brown streak disease (CBSD) is emerging as one of the most important viral diseases of cassava (Manihot esculenta) and is considered today as the biggest threat to cassava cultivation in East Africa. The disease is caused by isolates of at least two phylogenetically distinct species of single-stranded RNA viruses belonging to the family Potyviridae, genus Ipomovirus. The two species are present predominantly in the coastal lowland [Cassava brown streak virus (CBSV); Tanzania and Mozambique] and highland [Cassava brown streak Uganda virus (CBSUV); Lake Victoria Basin, Uganda, Kenya and Malawi] in East Africa. In this study, we demonstrate that CBSD can be efficiently controlled using RNA interference (RNAi). Three RNAi constructs targeting the highland species were generated, consisting of the full-length (FL; 894 nucleotides), 397-nucleotide N-terminal and 491-nucleotide C-terminal portions of the coat protein (CP) gene of a Ugandan isolate of CBSUV (CBSUV-[UG:Nam:04]), and expressed constitutively in Nicotiana benthamiana. After challenge with CBSUV-[UG:Nam:04], plants homozygous for FL-CP showed the highest resistance, followed by the N-terminal and C-terminal lines with similar resistance. In the case of FL, approximately 85% of the transgenic plant lines produced were completely resistant. Some transgenic lines were also challenged with six distinct isolates representing both species: CBSV and CBSUV. In addition to nearly complete resistance to the homologous virus, two FL plant lines showed 100% resistance and two C-terminal lines expressed 50-100% resistance, whereas the N-terminal lines succumbed to the nonhomologous CBSV isolates. Northern blotting revealed a positive correlation between the level of transgene-specific small interfering RNAs detected in transgenic plants and the level of virus resistance. This is the first demonstration of RNAi-mediated resistance to CBSD and protection across very distant isolates (more than 25% in nucleotide sequence) belonging to two different species: Cassava brown streak virus and Cassava brown streak Uganda virus.

Silencing of Plum pox virus 5'UTR/P1 sequence confers resistance to a wide range of PPV strains

An effective disease-control strategy should protect the host from the major economically important and geographically widespread variants of a pathogen. Plum pox virus (PPV) is the causal agent of sharka, the most devastating viral disease of Prunus species. We have shown previously that the hairpin RNA expression driven by h-UTR/P1, h-P1/HCPro, h-HCPro and h-HCPro/P3 constructs, derived from the PPV-M ISPaVe44 isolate, confers resistance to the homologous virus in Nicotiana benthamiana plants. Since the production of transgenic stone fruits and their evaluation for PPV resistance would take several years, the ISPaVe44-resistant plant lines were used to evaluate which construct would be the best candidate to be transferred to Prunus elite cultivars. To do that, nine PPV isolates of the D, M, Rec, EA and C strains originally collected from five Prunus species in different geographical areas, were typed by sequencing and used to challenge the transgenic N. benthamiana lines; 464 out of 464 virus-inoculated plants of lines h-UTR/P1, h-HCPro and h-HCPro/P3 showed complete and long-lasting resistance to the seven PPV isolates of D, M and Rec strains. Moreover, the h-UTR/P1 plants were also fully resistant to PPV-C and -EA isolates. Our data suggest that the h-UTR/P1 construct is of particular practical interest to obtain stone fruit plants resistant to the sharka disease.

Resistance to Citrus psorosis virus in transgenic sweet orange plants is triggered by coat protein-RNA silencing

The lack of naturally occurring resistance to Citrus psorosis virus (CPsV) has demanded exploitation of a transgenic approach for the development of CPsV-resistant sweet orange plants. Transgenic sweet orange plants producing intron-hairpin RNA transcripts (ihpRNA) corresponding to viral cp, 54K or 24K genes were generated and analyzed at the molecular and phenotypic levels. Two independent CPsV challenge assays demonstrated that expression of ihpRNA derived from the cp gene (ihpCP) provided a high level of virus resistance, while those derived from 54K and 24K genes (ihp54K and ihp24K) provided partial or no resistance. The presence of small interfering RNA molecules (siRNAs) in the ihpCP transgenic sweet orange plants prior to virus challenge, indicated that CPsV resistance was due to pre-activated RNA silencing, but siRNAs accumulation level was not directly correlated to the degree of the triggered virus resistance among the different lines. However, pre-activation of the RNA-silencing machinery and a certain minimum accumulation level of siRNA molecules targeting the viral genome are key factors for creating virus-resistant plants. This is the first report of resistance in citrus plants against a negative-strand RNA virus as CPsV.

Engineering pathogen resistance in crop plants: current trends and future prospects

Transgenic crops are now grown commercially in 25 countries worldwide. Although pathogens represent major constraints for the growth of many crops, only a tiny proportion of these transgenic crops carry disease resistance traits. Nevertheless, transgenic disease-resistant plants represent approximately 10% of the total number of approved field trials in North America, a proportion that has remained constant for 15 years. In this review, we explore the socioeconomic and biological reasons for the paradox that although technically useful solutions now exist for providing transgenic disease resistance, very few new crops have been introduced to the global market. For bacteria and fungi, the majority of transgenic crops in trials express antimicrobial proteins. For viruses, three-quarters of the transgenics express coat protein (CP) genes. There is a notable trend toward more biologically sophisticated solutions involving components of signal transduction pathways regulating plant defenses. For viruses, RNA interference is increasingly being used.

Role of siRNAs and miRNAs in the processes of RNA-mediated gene silencing during viral infections

Phenomenon of RNA-induced gene silencing is a highly conservative mechanism among eukaryotic organisms. Several classes of small RNAs (siRNAs and miRNAs) 21-25 nt in length, which play a significant role in the processes of development of an organism, occurred important components of antiviral defence in animals and plants. This review shortly describes the main stages of gene silencing mechanism, features of antiviral RNA silencing in plants, invertebrates, mammals, ways of suppression of RNA-interference by viruses, as well as possible approaches of utilization of abovementioned phenomenon for struggling against viral infections.

Genetic transformation of sweet orange with the coat protein gene of Citrus psorosis virus and evaluation of resistance against the virus

Citrus psorosis is a serious viral disease affecting citrus trees in many countries. Its causal agent is Citrus psorosis virus (CPsV), the type member of genus Ophiovirus. CPsV infects most important citrus varieties, including oranges, mandarins and grapefruits, as well as hybrids and citrus relatives used as rootstocks. Certification programs have not been sufficient to control the disease and no sources of natural resistance have been found. Pathogen-derived resistance (PDR) can provide an efficient alternative to control viral diseases in their hosts. For this purpose, we have produced 21 independent lines of sweet orange expressing the coat protein gene of CPsV and five of them were challenged with the homologous CPV 4 isolate. Two different viral loads were evaluated to challenge the transgenic plants, but so far, no resistance or tolerance has been found in any line after 1 year of observations. In contrast, after inoculation all lines showed characteristic symptoms of psorosis in the greenhouse. The transgenic lines expressed low and variable amounts of the cp gene and no correlation was found between copy number and transgene expression. One line contained three copies of the cp gene, expressed low amounts of the mRNA and no coat protein. The ORF was cytosine methylated suggesting a PTGS mechanism, although the transformant failed to protect against the viral load used. Possible causes for the failed protection against the CPsV are discussed.

Role of the 25-26 nt siRNA in the resistance of transgenic Prunus domestica graft inoculated with plum pox virus

The reaction of a genetically engineered plum clone (C5) resistant to plum pox virus (PPV) by graft inoculation with the virus was evaluated. The resistance in this clone has been demonstrated to be mediated through post-transcriptional gene silencing (PTGS). A single C5 plant out of 30 plants inoculated with PPV M strain by double chip-budding showed mild diffuse mosaic 'Sharka' symptom at the bottom section of the scion. The upper leaves of this PPV-infected C5 plant remained symptomless and the virus was not detected in them by either DAS-ELISA or RT-PCR. An RNA silencing associated small interfering RNA duplex, siRNA (21-26 nt), was detected in non-inoculated C5 plants and in the portions of inoculated C5 plant in which PPV could not be detected. In the PPV-infected portion of the C5 plant and in C6 PPV susceptible plants only the approximately 21-22 nt siRNAs was detected. Cytosine-methylation was confirmed in C5 plants both uninfected and showing PPV symptoms. The 25-26 nt siRNA normally present in C5 was absent in PPV-infected C5 tissues confirming the critical role of this siRNA in the resistance of clone C5 to PPV infection. We also show that this PPV infection was limited and transient. It was only detected in one plant at one of four post-dormancy sampling dates and did not appear to affect the overall PPV resistance of the C5 clone.

Assessment of the diversity and dynamics of Plum pox virus and aphid populations in transgenic European plums under Mediterranean conditions

The molecular variability of Plum pox virus (PPV) populations was compared in transgenic European plums (Prunus domestica L.) carrying the coat protein (CP) gene of PPV and non-transgenic plums in an experimental orchard in Valencia, Spain. A major objective of this study was to detect recombination between PPV CP transgene transcripts and infecting PPV RNA. Additionally, we assessed the number and species of PPV aphid vectors that visited transgenic and non-transgenic plum trees. Test trees consisted of five different P. domestica transgenic lines, i.e. the PPV-resistant C5 'HoneySweet' line and the PPV-susceptible C4, C6, PT6 and PT23 lines, and non-transgenic P. domestica and P. salicina Lind trees. No significant difference in the genetic diversity of PPV populations infecting transgenic and conventional plums was detected, in particular no recombinant between transgene transcripts and incoming viral RNA was found at detectable levels. Also, no significant difference was detected in aphid populations, including viruliferous individuals, that visited transgenic and conventional plums. Our data indicate that PPV-CP transgenic European plums exposed to natural PPV infection over an 8 year period caused limited, if any, risk beyond the cultivation of conventional plums under Mediterranean conditions in terms of the emergence of recombinant PPV and diversity of PPV and aphid populations.

Field Trials of Plum Clones Transformed with the Plum pox virus Coat Protein (PPV-CP) Gene

Transgenic clones C2, C3, C4, C5, C6, and PT-6, of plum (Prunus domestica L.) transformed with the coat protein (CP) gene of Plum pox virus (PPV), PT-23 transformed with marker genes only, and nontransgenic B70146 were evaluated for sharka resistance under high infection pressure in field trials in Poland and Spain. These sites differed in climatic conditions and virus isolates. Transgenic clone C5 showed high resistance to PPV at both sites. None of the C5 trees became naturally infected by aphids during seven (Spain) or eight (Poland) years of the test, although up to 100% of other plum trees (transgenic clones and nontransgenic control plants) grown in the same conditions showed disease symptoms and tested positively for PPV. Although highly resistant, C5 trees could be infected artificially by chip budding or via susceptible rootstock. Infected C5 trees showed only a few mild symptoms on single, isolated shoots, even up to 8 years post inoculation. These results clearly indicate the long-term nature and high level of resistance to PPV obtained through genetically engineered resistance.

Post-transcriptional gene silencing of the p23 silencing suppressor of Citrus tristeza virus confers resistance to the virus in transgenic Mexican lime

Previously, we have shown that most Mexican limes (Citrus aurantifolia (Christ.) Swing.) expressing the p23 gene of Citrus tristeza virus (CTV) exhibit aberrations resembling viral leaf symptoms. Here we report that five independent transgenic lines having normal phenotype displayed characteristics typical of post-transcriptional gene silencing (PTGS): multiple copies of the transgene, low levels of the corresponding mRNA, methylation of the silenced transgene, and accumulation of p23-specific small interfering RNAs (siRNAs). When graft- or aphid-inoculated with CTV, some propagations of these silenced lines were immune: they neither expressed symptoms nor accumulated virions and viral RNA as estimated by DAS-ELISA and Northern blot hybridization, respectively. Other propagations were moderately resistant because they became infected later and showed attenuated symptoms compared to controls. The susceptible propagations, in addition to symptom expression and elevated virus titer, accumulated p23-specific siRNAs at levels significantly higher than immune or non-inoculated propagations, and showed transgene demethylation. This variable response among clonal transformants indicates that factors other than the genetic background of the transgenic plants play a key role in PTGS-mediated resistance.

Accumulation of the long class of siRNA is associated with resistance to Plum pox virus in a transgenic woody perennial plum tree

We investigated the hallmarks of posttranscription gene silencing (PTGS) in mature plants, embryos, and seedlings of the transgenic plum trees (Prunus sp.) that are resistant to Plum pox virus (PPV). We previously demonstrated that the transgene insert and resistance to PPV were mutually inherited in progeny of line C5. We show here that C5 constitutively produces a short (22 nt) and a long (25 to 26 nt) species of short interfering (si)RNA from embryo to mature plant in the absence of PPV inoculation. Unlike siRNA, methylation and transcription of the PPV-coat protein transgene were 're-set' following seed germination. Uninoculated transgenic susceptible clones did not display DNA methylation, nor did they produce detectable levels of siRNA. Upon infection, susceptible clones, transgenic or untransformed, did produce siRNA but only the short 22-nt species. These findings show that plum trees respond to virus infection by initiating PTGS-like mechanisms that involve the production of siRNA. We further suggest that high-level virus resistance in transgenic Prunus species requires the production of the long-size class of siRNA. The research adds new insights into PTGS silencing in woody perennial plant species.