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

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.

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.

Expression Analysis of Hairpin RNA Carrying Sugarcane mosaic virus (SCMV) Derived Sequences and Transgenic Resistance Development in a Model Rice Plant

Developing transgenic resistance in monocotyledonous crops against pathogens remains a challenging area of research. Sugarcane mosaic virus (SCMV) is a serious pathogen of many monocotyledonous crops including sugarcane. The objective of present study was to analyze transgenic expression of hairpin RNA (hpRNA), targeting simultaneously CP (Coat Protein) and Hc-Pro (helper component-proteinase) genes of SCMV, in a model rice plant. Conserved nucleotide sequences, exclusive for DAG (Aspartic acid-Alanine-Glycine) and KITC (Lycine-Isoleucine-Threonine-Cysteine) motifs, derived from SCMV CP and Hc-Pro genes, respectively, were fused together and assembled into the hpRNA cassette under maize ubiquitin promoter to form Ubi-hpCP:Hc-Pro construct. The same CP:Hc-Pro sequence was fused with the β-glucuronidase gene (GUS) at the 3' end under CaMV 35S promoter to develop 35S-GUS:CP:Hc-Pro served as a target reporter gene construct. When delivered into rice callus tissues by particle bombardment, the Ubi-hpCP:Hc-Pro construct induced strong silencing of 35S-GUS:CP:Hc-Pro. Transgenic rice plants, containing Ubi-hpCP:Hc-Pro construct, expressed high level of 21-24 nt small interfering RNAs, which induced specific suppression against GUS:CP:Hc-Pro delivered by particle bombardment and conferred strong resistance to mechanically inoculated SCMV. It is concluded that fusion hpRNA approach is an affordable method for developing resistance against SCMV in model rice plant and it could confer SCMV resistance when transformed into sugarcane.

Citrus psorosis virus coat protein-derived hairpin construct confers stable transgenic resistance in citrus against psorosis A and B syndromes

Citrus psorosis virus (CPsV) is the causal agent of psorosis, a serious and widespread citrus disease. Two syndromes of psorosis, PsA and PsB, have been described. PsB is the most aggressive and rampant form. Previously, we obtained Pineapple sweet orange plants transformed with a hairpin construct derived from the CPsV coat protein gene (ihpCP). Some of these plants were resistant to CPsV 90-1-1, a PsA isolate homologous to the transgene. In this study, we found that expression of the ihpCP transgene and siRNA production in lines ihpCP-10 and -15 were stable with time and propagation. In particular, line ihpCP-15 has been resistant for more than 2 years, even after re-inoculation. The ihpCP plants were also resistant against a heterologous CPsV isolate that causes severe PsB syndrome. Line ihpCP-15 manifested complete resistance while line ihpCP-10 was tolerant to the virus, although with variable behaviour, showing delay and attenuation in PsB symptoms. These lines are promising for a biotech product aimed at eradicating psorosis.

Mechanisms, applications, and perspectives of antiviral RNA silencing in plants

Viral diseases of plants cause important economic losses due to reduction in crop quality and quantity to the point of threatening food security in some countries. Given the reduced availability of natural sources, genetic resistance to viruses has been successfully engineered for some plant-virus combinations. A sound understanding of the basic mechanisms governing plant-virus interactions, including antiviral RNA silencing, is the foundation to design better management strategies and biotechnological approaches to engineer and implement antiviral resistance in plants. In this review, we present current molecular models to explain antiviral RNA silencing and its application in basic plant research, biotechnology and genetic engineering.

Fighting Sharka in Peach: Current Limitations and Future Perspectives

Sharka, caused by Plum Pox Virus (PPV), is by far the most important infectious disease of peach [P. persica (L.) Batsch] and other Prunus species. The progressive spread of the virus in many important growing areas throughout Europe poses serious issues to the economic sustainability of stone fruit crops, peach in particular. The adoption of internationally agreed-upon rules for diagnostic tests, strain-specific monitoring schemes and spatial-temporal modeling of virus spread, are all essential for a more effective sharka containment. The EU regulations on nursery activity should be modified based on the zone delimitation of PPV presence, limiting open-field production of propagation materials only to virus-free areas. Increasing the efficiency of preventive measures should be augmented by the short-term development of resistant cultivars. Putative sources of resistance/tolerance have been recently identified in peach germplasm, although the majority of novel resistant sources to PPV-M have been found in almond. However, the complexity of introgression from related-species imposes the search for alternative strategies. The use of genetic engineering, particularly RNA interference (RNAi)-based approaches, appears as one of the most promising perspectives to introduce a durable resistance to PPV in peach germplasm, notwithstanding the well-known difficulties of in vitro plant regeneration in this species. In this regard, rootstock transformation to induce RNAi-mediated systemic resistance would avoid the transformation of numerous commercial cultivars, and may alleviate consumer resistance to the use of GM 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.

Biotechnological strategies and tools for Plum pox virus resistance: trans-, intra-, cis-genesis, and beyond

Plum pox virus (PPV) is the etiological agent of sharka, the most devastating and economically important viral disease affecting Prunus species. It is widespread in most stone fruits producing countries even though eradication and quarantine programs are in place. The development of resistant cultivars and rootstocks remains the most ecologically and economically suitable approach to achieve long-term control of sharka disease. However, the few PPV resistance genetic resources found in Prunus germplasm along with some intrinsic biological features of stone fruit trees pose limits for efficient and fast breeding programs. This review focuses on an array of biotechnological strategies and tools, which have been used, or may be exploited to confer PPV resistance. A considerable number of scientific studies clearly indicate that robust and predictable resistance can be achieved by transforming plant species with constructs encoding intron-spliced hairpin RNAs homologous to conserved regions of the PPV genome. In addition, we discuss how recent advances in our understanding of PPV biology can be profitably exploited to develop viral interference strategies. In particular, genetic manipulation of host genes by which PPV accomplishes its infection cycle already permits the creation of intragenic resistant plants. Finally, we review the emerging genome editing technologies based on ZFN, TALEN and CRISPR/Cas9 engineered nucleases and how the knockout of host susceptibility genes will open up next generation of PPV resistant plants.

Control of pome and stone fruit virus diseases

Many different systemic pathogens, including viruses, affect pome and stone fruits causing diseases with adverse effects in orchards worldwide. The significance of diseases caused by these pathogens on tree health and fruit shape and quality has resulted in the imposition of control measures both nationally and internationally. Control measures depend on the identification of diseases and their etiological agents. Diagnosis is the most important aspect of controlling fruit plant viruses. Early detection of viruses in fruit trees or in the propagative material is a prerequisite for their control and to guarantee a sustainable agriculture. Many quarantine programs are in place to reduce spread of viruses among countries during international exchange of germplasm. All these phytosanitary measures are overseen by governments based on agreements produced by international organizations. Also certification schemes applied to fruit trees allow the production of planting material of known variety and plant health status for local growers by controlling the propagation of pathogen-tested mother plants. They ensure to obtain propagative material not only free of "quarantine" organisms under the national legislation but also of important "nonquarantine" pathogens. The control of insect vectors plays an important role in the systemic diseases management, but it must be used together with other control measures as eradication of infected plants and use of certified propagation material. Apart from the control of the virus vector and the use of virus-free material, the development of virus-resistant cultivars appears to be the most effective approach to achieve control of plant viruses, especially for perennial crops that are more exposed to infection during their long life span. The use of resistant or tolerant cultivars and/or rootstocks could be potentially the most important aspect of virus disease management, especially in areas in which virus infections are endemic. The conventional breeding for virus-tolerant or resistant fruit tree cultivars using available germplasm is a long-term strategy, and the development and production of these cultivars may take decades, if successful. Genetic engineering allows the introduction of specific DNA sequences offering the opportunity to obtain existing fruit tree cultivars improved for the desired resistance trait. Unfortunately, genetic transformation of pome and stone fruits is still limited to few commercial genotypes. Research carried out and the new emerging biotechnological approaches to obtain fruit tree plants resistant or tolerant to viruses are discussed.

Robust RNA silencing-mediated resistance to Plum pox virus under variable abiotic and biotic conditions

Some abiotic and biotic conditions are known to have a negative impact on post-transcriptional gene silencing (PTGS), thus representing a potential concern for the production of stable engineered virus resistance traits. However, depending on the strategy followed to achieve PTGS of the transgene, different responses to external conditions can be expected. In the present study, we utilized the Nicotiana benthamiana–Plum pox virus (PPV) pathosystem to evaluate in detail the stability of intron-hairpin(ihp)-mediated virus resistance under conditions known to adversely affect PTGS. The ihp plants grown at low or high temperatures were fully resistant to multiple PPV challenges, different PPV inoculum concentrations and even to a PPV isolate differing from the ihp construct by more than 28% at the nucleotide level. In addition, infections of ihp plants with viruses belonging to Cucumovirus, Potyvirus or Tombusvirus, all known to affect PTGS at different steps, were not able to defeat PPV resistance. Low temperatures did not affect the accumulation of transgenic small interfering RNAs (siRNAs), whereas a clear increase in the amount of siRNAs was observed during infections sustained by Cucumber mosaic virus and Potato virus Y. Our results show that the above stress factors do not represent an important concern for the production,through ihp-PTGS technology, of transgenic plants having robust virus resistance traits.

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.

Exploiting the combination of natural and genetically engineered resistance to cassava mosaic and cassava brown streak viruses impacting cassava production in Africa

Cassava brown streak disease (CBSD) and cassava mosaic disease (CMD) are currently two major viral diseases that severely reduce cassava production in large areas of Sub-Saharan Africa. Natural resistance has so far only been reported for CMD in cassava. CBSD is caused by two virus species, Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV). A sequence of the CBSV coat protein (CP) highly conserved between the two virus species was used to demonstrate that a CBSV-CP hairpin construct sufficed to generate immunity against both viral species in the cassava model cultivar (cv. 60444). Most of the transgenic lines showed high levels of resistance under increasing viral loads using a stringent top-grafting method of inoculation. No viral replication was observed in the resistant transgenic lines and they remained free of typical CBSD root symptoms 7 month post-infection. To generate transgenic cassava lines combining resistance to both CBSD and CMD the hairpin construct was transferred to a CMD-resistant farmer-preferred Nigerian landrace TME 7 (Oko-Iyawo). An adapted protocol allowed the efficient Agrobacterium-based transformation of TME 7 and the regeneration of transgenic lines with high levels of CBSV-CP hairpin-derived small RNAs. All transgenic TME 7 lines were immune to both CBSV and UCBSV infections. Further evaluation of the transgenic TME 7 lines revealed that CBSD resistance was maintained when plants were co-inoculated with East African cassava mosaic virus (EACMV), a geminivirus causing CMD. The innovative combination of natural and engineered virus resistance in farmer-preferred landraces will be particularly important to reducing the increasing impact of cassava viral diseases in Africa.

Clarifying omics concepts, challenges, and opportunities for Prunus breeding in the postgenomic era

The recent sequencing of the complete genome of the peach, together with the availability of new high-throughput genome, transcriptome, proteome, and metabolome analysis technologies, offers new possibilities for Prunus breeders in what has been described as the postgenomic era. In this context, new biological challenges and opportunities for the application of these technologies in the development of efficient marker-assisted selection strategies in Prunus breeding include genome resequencing using DNA-Seq, the study of RNA regulation at transcriptional and posttranscriptional levels using tilling microarray and RNA-Seq, protein and metabolite identification and annotation, and standardization of phenotype evaluation. Additional biological opportunities include the high level of synteny among Prunus genomes. Finally, the existence of biases presents another important biological challenge in attaining knowledge from these new high-throughput omics disciplines. On the other hand, from the philosophical point of view, we are facing a revolution in the use of new high-throughput analysis techniques that may mean a scientific paradigm shift in Prunus genetics and genomics theories. The evaluation of scientific progress is another important question in this postgenomic context. Finally, the incommensurability of omics theories in the new high-throughput analysis context presents an additional philosophical challenge.

A rep-based hairpin inhibits replication of diverse maize streak virus isolates in a transient assay

Maize streak disease, caused by the A strain of the African endemic geminivirus, maize streak mastrevirus (MSV-A), threatens the food security and livelihoods of subsistence farmers throughout sub-Saharan Africa. Using a well-established transient expression assay, this study investigated the potential of a spliceable-intron hairpin RNA (hpRNA) approach to interfere with MSV replication. Two strategies were explored: (i) an inverted repeat of a 662 bp region of the MSV replication-associated protein gene (rep), which is essential for virus replication and is therefore a good target for post-transcriptional gene silencing; and (ii) an inverted repeat of the viral long intergenic region (LIR), considered for its potential to trigger transcriptional silencing of the viral promoter region. After co-bombardment of cultured maize cells with each construct and an infectious partial dimer of the cognate virus genome (MSV-Kom), followed by viral replicative-form-specific PCR, it was clear that, whilst the hairpin rep construct (pHPrepΔI(662)) completely inhibited MSV replication, the LIR hairpin construct was ineffective in this regard. In addition, pHPrepΔI(662) inhibited or reduced replication of six MSV-A genotypes representing the entire breadth of known MSV-A diversity. Further investigation by real-time PCR revealed that the pHPrepΔI(662) inverted repeat was 22-fold more effective at reducing virus replication than a construct containing the sense copy, whilst the antisense copy had no effect on replication when compared with the wild type. This is the first indication that an hpRNA strategy targeting MSV rep has the potential to protect transgenic maize against diverse MSV-A genotypes found throughout sub-Saharan Africa.