The melon Fom-1-Prv resistance gene pair: Correlated spatial expression and interaction with a viral protein

The head-to-head oriented pair of melon resistance genes, Fom-1 and Prv, control resistance to Fusarium oxysporum races 0 and 2 and papaya ringspot virus (PRSV), respectively. They encode, via several RNA splice variants, TIR-NBS-LRR proteins, and Prv has a C-terminal extra domain with a second NBS homologous sequence. In other systems, paired R-proteins were shown to operate by "labor division," with one protein having an extra integrated domain that directly binds the pathogen's Avr factor, and the second protein executing the defense response. We report that the expression of the two genes in two pairs of near-isogenic lines was higher in the resistant isoline and inducible by F. oxysporum race 2 but not by PRSV. The intergenic DNA region separating the coding sequences of the two genes acted as a bi-directional promoter and drove GUS expression in transgenic melon roots and transgenic tobacco plants. Expression of both genes was strong in melon root tips, around the root vascular cylinder, and the phloem and xylem parenchyma of tobacco stems and petioles. The pattern of GUS expression suggests coordinated expression of the two genes. In agreement with the above model, Prv's extra domain was shown to interact with the cylindrical inclusion protein of PRSV both in yeast cells and in planta.

Knockout of elF4E using CRISPR/Cas9 for large-scale production of resistant cucumber cultivar against WMV, ZYMV, and PRSV

CRISPR/Cas9 is one of the most robust technologies for plant breeding enabling precise and efficient modifications in a genome. This technology is being used for the manipulation of target genes in a host to develop resistance against the plant pathogens. Cucumis sativus elF4E is one of the target genes playing a key role in viral infection during interaction with potyvirus viral proteins genome linked (VPg). Nevertheless, the allelic and positional effect of elF4E mutations in C. sativus is to be clarified in elF4E-VPg interaction. In addition, there are entanglements in the massive production of pathogen-resistant cultivars suitable for commercial production using CRISPR/Cas9 technology. Therefore, we targeted different positions of the elF4E in G27 and G247 inbred lines, using specific gRNA1 and gRNA2 for the first and third exons, respectively, and 1,221 transgene-free plants were selected in segregated T1 generation, where 192 G27 and 79 G247 plants had the least mutation at Cas9 cleavage site of gRNA1 or gRNA2. Crossing was performed to see allelic effects of elfF4E mutations in F1 populations, which were homozygous and heterozygous single (elF4E_1^DEL or elF4E_3^DEL) and double (elF4E_1-3^DEL) mutants. Disease symptoms of watermelon mosaic virus (WMV), papaya ringspot virus (PRSV), and zucchini yellow mosaic virus (ZYMV) were evaluated in both non-edited and edited F1 plants, and we did not observe any symptom in homozygous elF4E_1-3^DEL and elF4E_1^DEL mutants. However, homozygous elF4E_3^DEL was positive in reverse transcription polymerase chain reaction (RT-PCR), even if there were no significant symptoms on the inoculated leaves. ELISA and qRT-PCR indicated lower viral accumulation in homozygous elF4E_3^DEL than heterozygous and non-edited plants. Regeneration and transformation protocols were also optimized comprehensively for both the genotypes. The average number of shoots/100 explants was determined for both G27 and G247 as 13.6 and 18.0, respectively. We could not detect any distinguishing difference between the non-edited and edited F1 plants for yield and morphology. Our results demonstrate an effective route for mass production of viral resistant cultivars of cucumber to WMV, ZYMV, and PRSV. In this way, the pathogen-resistant cultivars could be generated to reduce the losses caused by these pathogens in cucumber production.

Interactive Effect of Biological Agents Chitosan, Lentinan and Ningnanmycin on Papaya Ringspot Virus Resistance in Papaya (Carica papaya L.)

The papaya industry is mainly impacted by viral diseases, especially papaya ringspot disease (PRSD) caused by papaya ringspot virus (PRSV). So far, research on the interaction between Chitosan, Lentinan and Ningnanmycin on PRSD has not been reported. This research studied the controlled and interactive effect of three biological agents, namely, Chitosan (C), Lentinan (L) and Ningnanmycin (N), on PRSV in papaya, individually and collectively. The changes in disease index, controlled effect, Peroxidase (POD), Polyphenol oxidase (PPO), Superoxide dismutase (SOD), growth and development of plants were observed at the seedling stage, in pots, and at the fruiting stage, in the field. The appearance and nutrient contents of fruits were measured during the fruit stage. The disease index of PRSV, at seedling and fruiting stages, was significantly lower for chitosan, lentinan and ningnanmycin and their interactive effect, compared to a control check treatment. The activity of the defense enzymes could be improved by the three kinds of biological agents and their interactive effect, especially lentinan and ningnanmycin. The chlorophyll content, plant height, stem diameter and fruit quality rose significantly under chitosan, lentinan and ningnanmycin treatments. The interaction of LN could inhibit PRSV disease at the seedling and fruiting stages of papaya, and promote the growth of plants and the quality of fruit at the fruit stage. Hence, this study provides the theoretical foundation for the biological control of papaya ringspot disease.

Differential Accumulation of Innate- and Adaptive-Immune-Response-Derived Transcripts during Antagonism between Papaya Ringspot Virus and Papaya Mosaic Virus

Papaya ringspot virus (PRSV), a common potyvirus infecting papaya plants worldwide, can lead to either antagonism or synergism in mixed infections with Papaya mosaic virus (PapMV), a potexvirus. These two unrelated viruses produce antagonism or synergism depending on their order of infection in the plant. When PRSV is inoculated first or at the same time as PapMV, the viral interaction is synergistic. However, an antagonistic response is observed when PapMV is inoculated before PRSV. In the antagonistic condition, PRSV is deterred from the plant and its drastic effects are overcome. Here, we examine differences in gene expression by high-throughput RNA sequencing, focused on immune system pathways. We present the transcriptomic expression of single and mixed inoculations of PRSV and PapMV leading to synergism and antagonism. Upregulation of dominant and hormone-mediated resistance transcripts suggests that the innate immune system participates in synergism. In antagonism, in addition to innate immunity, upregulation of RNA interference-mediated resistance transcripts suggests that adaptive immunity is involved.

DsRNA-mediated protection against two isolates of Papaya ringspot virus through topical application of dsRNA in papaya

Papaya ringspot virus (PRSV) infections in papaya result in heavy yield losses, severely affecting the papaya industry worldwide, and hence warranting for effective control measures. In the past, transgenic papaya cultivars were developed that overexpressed parts of the PRSV genome and exhibited high levels of virus resistance. In the present study, a non-transgenic approach was employed, in which in vitro produced dsRNA molecules derived from a PRSV isolate from South India (PRSV-Tirupati) was tested for dsRNA-mediated protection against two isolates of PRSV through topical application of the dsRNA on papaya. The results showed that the dsRNA molecules from both the coat protein (CP) and helper component-proteinase (HC-Pro) genes of the PRSV-Tirupati isolate conferred 100 % resistance against PRSV-Tirupati infection. Further, the same dsRNA molecules were highly effective against the PRSV-Delhi isolate on the papaya cv. Pusa Nanha, conferring a resistance of 94 % and 81 %, respectively. Systemic papaya leaves of the dsRNA-treated plants were virus-free at 14 days post-inoculation, confirming the robustness of this non-transgenic virus control strategy. In contrast, the control TMV dsRNA did not protect against the PRSV infection. This study on the topical application of dsRNA opened up a new avenue for the control of papaya ringspot disease worldwide.

Cross protection against the watermelon strain of Papaya ringspot virus through modification of viral RNA silencing suppressor

Papaya ringspot virus watermelon strain (PRSV-W) causes huge economic losses to cucurbits production. Here, we constructed an infectious clone of PRSV-W, pCamPRSV-W, which can induce similar symptoms and accumulate to same levels as wild type virus in plants of Cucurbita pepo, Cucumis melo, Citrullus lanatus and Cucumis sativus. The green fluorescence protein gene gfp was cloned into pCamPRSV-W to produce pCamPRSV-W-GFP, which produced strong green fluorescence in systemic leaves of inoculated Cucurbita pepo, Cucumis melo, Citrullus lanatus and Cucumis sativus plants, indicating that pCamPRSV-W can be used to express foreign genes. Ten mutants of PRSV-W, obtained by site-directed mutagenesis in the RNA silencing suppressor helper-component proteinase encoding region, produced dramatically attenuated symptoms in plants of Cucumis melo. The Cucumis melo plants pre-infected with mutants K125D and G317 K showed effective protection against the challenge inoculation of wild type PRSV-W. The attenuated mutants generated in this study will be helpful for the eco-friendly control of PRSV-W.

Generation of stable infectious clones of plant viruses by using Rhizobium radiobacter for both cloning and inoculation

A novel Rhizobium radiobacter (synonym Agrobacterium tumefaciens)-mediated approach was developed to generate stable infectious clones of plant viruses. This method uses R. radiobacter for both cloning and inoculation of infectious clones, bypassing the requirement of cloning in E. coli to avoid the instability. Only three steps are included in this method: (i) construct viral genome-encoding plasmids in vitro by one-step Gibson assembly; (ii) transform the assembled DNA products into R. radiobacter; (iii) inoculate plants with the R. radiobacter clones containing the viral genome. Stable infectious clones were obtained from two potyviruses papaya ringspot virus (PRSV) and papaya leaf distortion mosaic virus (PLDMV) using this method, whereas attempts utilizing "classical" E. coli cloning system failed repeatedly. This method is simple and efficient, and is promising for a wide application in generation of infectious clones of plant virus, especially for those which are instable in E. coli.

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A novel approach was developed to generate infectious clones of plant viruses.

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It uses R. radiobacter for both cloning and inoculation of infectious clones.

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It bypasses the requirement of cloning in E. coli to avoid the instability.

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Stable infectious clones of PRSV and PLDMV were obtained using this method.

A case study to determine the geographical origin of unknown GM papaya in routine food sample analysis, followed by identification of papaya events 16-0-1 and 18-2-4

During routine monitoring for GMOs in food in the Netherlands, papaya-containing food supplements were found positive for the genetically modified (GM) elements P-35S and T-nos. The goal of this study was to identify the unknown and EU unauthorised GM papaya event(s). A screening strategy was applied using additional GM screening elements including a newly developed PRSV coat protein PCR. The detected PRSV coat protein PCR product was sequenced and the nucleotide sequence showed identity to PRSV YK strains indigenous to China and Taiwan. The GM events 16-0-1 and 18-2-4 could be identified by amplifying and sequencing events-specific sequences. Further analyses showed that both papaya event 16-0-1 and event 18-2-4 were transformed with the same construct. For use in routine analysis, derived TaqMan qPCR methods for events 16-0-1 and 18-2-4 were developed. Event 16-0-1 was detected in all samples tested whereas event 18-2-4 was detected in one sample. This study presents a strategy for combining information from different sources (literature, patent databases) and novel sequence data to identify unknown GM papaya events.

Antagonism or synergism between papaya ringspot virus and papaya mosaic virus in Carica papaya is determined by their order of infection

Antagonism between unrelated plant viruses has not been thoroughly described. Our studies show that two unrelated viruses, papaya ringspot virus (PRSV) and papaya mosaic virus (PapMV) produce different symptomatic outcomes during mixed infection depending on the inoculation order. Synergism occurs in plants infected first with PRSV or in plants infected simultaneously with PRSV and PapMV, and antagonism occurs in plants infected first with PapMV and later inoculated with PRSV. During antagonism, elevated pathogenesis-related (PR-1) gene expression and increased reactive oxygen species production indicated the establishment of a host defense resulting in the reduction in PRSV titers. Polyribosomal fractioning showed that PRSV affects translation of cellular eEF1α, PR-1, β-tubulin, and PapMV RNAs in planta, suggesting that its infection could be related to an imbalance in the translation machinery. Our data suggest that primary PapMV infection activates a defense response against PRSV and establishes a protective relationship with the papaya host.

Repeated Dose 90-Day Feeding Study of Whole Fruits of Genetically Modified Papaya Resistant to Papaya Ringspot Virus in Rats

Genetically modified (GM) papaya plants resistant to infection by Papaya ringspot virus (PRSV) have been successfully generated by cloning the coat protein (CP) gene of PRSV to increase fruit production. In this study, the GM papaya line 823-2210 was used to conduct a 90-day feeding toxicity study and compared to its parent plant of non-GM papaya, Tainung-2 (TN-2) based on the experimental guidance reported by the European Food Safety Authority.1 Ten male and 10 female Sprague-Dawley albino rats were gavaged at low (1 g/kg bw) and high (2 g/kg bw) doses of non-GM and GM lyophilized papaya fruits for 90 days. Hematology, coagulation, biochemistry, urinalysis, and pathology were examined in all animals. Although some differences were found in feed consumption, hematology, and serum chemistry examinations between non-GM and GM papaya, the results were within historical control values and not considered biologically significant in rats. In addition, there were no treatment-related gross or microscopic lesions in male or female rats attributable to the non-GM or GM papaya fruit. This 90-day feeding study of GM papaya fruit did not reveal adverse effects in rats and indicates that GM papaya fruits may be substantially equivalent to their non-GM parent plants.