N-terminal of Papaya ringspot virus type-W (PRSV-W) helper component proteinase (HC-Pro) is essential for PRSV systemic infection in zucchini

Mutagenesis of the melon Prv gene by CRISPR/Cas9 breaks papaya ringspot virus resistance and generates an autoimmune allele with constitutive defense responses

The majority of plant disease resistance (R) genes encode nucleotide binding-leucine-rich repeat (NLR) proteins. In melon, two closely linked NLR genes, Fom-1 and Prv, were mapped and identified as candidate genes that control resistance to Fusarium oxysporum f.sp. melonis races 0 and 2, and to papaya ringspot virus (PRSV), respectively. In this study, we validated the function of Prv and showed that it is essential for providing resistance against PRSV infection. We generated CRISPR/Cas9 [clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated protein 9] mutants using Agrobacterium-mediated transformation of a PRSV-resistant melon genotype, and the T1 progeny proved susceptible to PRSV, showing strong disease symptoms and viral spread upon infection. Three alleles having 144, 154, and ~3 kb deletions, respectively, were obtained, all of which caused loss of resistance. Interestingly, one of the Prv mutant alleles, prvΔ154, encoding a truncated product, caused an extreme dwarf phenotype, accompanied by leaf lesions, high salicylic acid levels, and defense gene expression. The autoimmune phenotype observed at 25 °C proved to be temperature dependent, being suppressed at 32 °C. This is a first report on the successful application of CRISPR/Cas9 to confirm R gene function in melon. Such validation opens up new opportunities for molecular breeding of disease resistance in this important vegetable crop.

Modification of the N-terminal FWKG-αH1 element of potyviral HC-Pro affects its multiple functions and generates effective attenuated mutants for cross-protection

Control of plant viruses by cross-protection is limited by the availability of effective protective strains. Incorporation of an NIa-protease processing site in the extreme N-terminal region of the helper component protease (HC-Pro) of turnip mosaic virus (TuMV) resulted in a mutant virus TuHN^D I that induced highly attenuated symptoms. Recombination analysis verified that two variations, F7I mutation and amino acid 7-upstream-deletion, in HC-Pro co-determined TuHN^D I attenuation. TuHN^D I provided complete protection to Nicotiana benthamiana and Brassica campestris subsp. chinensis plants against infection by the severe parental strain. Aphid transmission tests revealed that TuHN^D I was not aphid-transmissible. An RNA silencing suppression (RSS) assay by agroinfiltration suggested the RSS-defective nature of the mutant HC-Pro. In the context (amino acids 3-17) encompassing the two variations of HC-Pro, we uncovered an FWKG-α-helix 1 (αH1) element that influenced the functions of aphid transmission and RSS, whose motifs were located far downstream. We further demonstrated that HC-Pro F7 was a critical residue on αH1 for HC-Pro functions and that reinstating αH1 in the RSS-defective HC-Pro of TuHN^D I restored the protein's RSS function. Yeast two-hybrid and bimolecular fluorescence complementation assays indicated the FWKG-αH1 element as an integral part of the HC-Pro self-interaction domain. The possibility of regulation of the mechanistically independent functions of RSS and aphid transmission by the FWKG-αH1 element is discussed. Extension of TuMV HC-Pro FWKG-αH1 variations to another potyvirus, zucchini yellow mosaic virus, also generated nonaphid-transmissible cross-protective mutant viruses. Hence, the modification of the FWKG-αH1 element can generate effective attenuated viruses for the control of potyviruses by cross-protection.

Cucurbitaceae genome evolution, gene function and molecular breeding

The Cucurbitaceae is one of the most genetically diverse plant families in the world. Many of them are important vegetables or medicinal plants and are widely distributed worldwide. The rapid development of sequencing technologies and bioinformatic algorithms has enabled the generation of genome sequences of numerous important Cucurbitaceae species. This has greatly facilitated research on gene identification, genome evolution, genetic variation and molecular breeding of cucurbit crops. So far, genome sequences of 18 different cucurbit species belonging to tribes Benincaseae, Cucurbiteae, Sicyoeae, Momordiceae and Siraitieae have been deciphered. This review summarizes the genome sequence information, evolutionary relationship, and functional genes associated with important agronomic traits (e.g., fruit quality). The progress of molecular breeding in cucurbit crops and prospects for future applications of Cucurbitaceae genome information are also discussed.

Molecular and biological characterization of Potato virus Y detected in zucchini in China

Potato virus Y (PVY) is an important viral pathogen of the plant family Solanaceae. However, it has not been reported that PVY infects zucchini (Cucurbita pepo L). Zucchini plants showing symptoms of leaf mosaic and fruit ringspot were collected from Inner Mongolia, China. Using RT-PCR, we amplified the PVY-specific bands in zucchini (PVY-Zu isolate). Further, we found that the viral pathogen could infect Nicotiana benthamiana via mechanical inoculation with the extract of the zucchini leaves. Filamentous virions were observed in infected N. benthamiana plants under a transmission electron microscope. Additionally, the complete genomic fragment of the PVY-Zu isolate was obtained via RT-PCR and RACE analysis, containing 9662 nucleotides (nt) excluding the 3'-terminal poly (A) tail. The 5' and 3' untranslated regions (UTRs) were 150 nt and 329 nt, respectively, while the putative polyprotein contained 3061 amino acids. Comparison of the PVY-Zu isolate with 19 other PVY isolates revealed that they shared nucleotide sequence identities of ~ 83.1-99.8% at the complete genomic level. Based on the complete genome sequences, phylogenetic analysis showed that PVY-Zu clustered with PVY strain N605 (X97895-PVY), a Swiss necrotic isolate of PVY. Compared with the complete genome sequence of X97895-PVY, the PVY-Zu isolate had a deletion of a 39-nt fragment in its 5' UTR and 22 mutated nucleotides without changes in the amino acids. Moreover, although the similarity in the identities of the PVY-Zu isolate and PVY N isolate N-Jg (AY166867) was about 96.2%, the former had a deletion of a 38-nt fragment in its 5' UTR and 334 mutations among the viral genome, which resulted in 61 amino acid changes. However, we could not detect any recombination throughout the PVY-Zu genome. To our knowledge, this is the first study reporting that PVY could infect zucchini plants, thus broadening the host diversity of the viral pathogen.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13337-020-00647-2.

Application of Reverse Genetics in Functional Genomics of Potyvirus

Numerous potyvirus studies, including virus biology, transmission, viral protein function, as well as virus–host interaction, have greatly benefited from the utilization of reverse genetic techniques. Reverse genetics of RNA viruses refers to the manipulation of viral genomes, transfection of the modified cDNAs into cells, and the production of live infectious progenies, either wild-type or mutated. Reverse genetic technology provides an opportunity of developing potyviruses into vectors for improving agronomic traits in plants, as a reporter system for tracking virus infection in hosts or a production system for target proteins. Therefore, this review provides an overview on the breakthroughs achieved in potyvirus research through the implementation of reverse genetic systems.

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.

The HCPro from the Potyviridae family: an enviable multitasking Helper Component that every virus would like to have

RNA viruses have very compact genomes and so provide a unique opportunity to study how evolution works to optimize the use of very limited genomic information. A widespread viral strategy to solve this issue concerning the coding space relies on the expression of proteins with multiple functions. Members of the family Potyviridae, the most abundant group of RNA viruses in plants, offer several attractive examples of viral factors which play roles in diverse infection-related pathways. The Helper Component Proteinase (HCPro) is an essential and well-characterized multitasking protein for which at least three independent functions have been described: (i) viral plant-to-plant transmission; (ii) polyprotein maturation; and (iii) RNA silencing suppression. Moreover, multitudes of host factors have been found to interact with HCPro. Intriguingly, most of these partners have not been ascribed to any of the HCPro roles during the infectious cycle, supporting the idea that this protein might play even more roles than those already established. In this comprehensive review, we attempt to summarize our current knowledge about HCPro and its already attributed and putative novel roles, and to discuss the similarities and differences regarding this factor in members of this important viral family.

Helper component-proteinase enhances the activity of 1-deoxy-D-xylulose-5-phosphate synthase and promotes the biosynthesis of plastidic isoprenoids in Potato virus Y-infected tobacco

Virus-infected plants show strong morphological and physiological alterations. Many physiological processes in chloroplast are affected, including the plastidic isoprenoid biosynthetic pathway [the 2C-methyl-D-erythritol-4-phosphate (MEP) pathway]; indeed, isoprenoid contents have been demonstrated to be altered in virus-infected plants. In this study, we found that the levels of photosynthetic pigments and abscisic acid (ABA) were altered in Potato virus Y (PVY)-infected tobacco. Using yeast two-hybrid assays, we demonstrated an interaction between virus protein PVY helper component-proteinase (HC-Pro) and tobacco chloroplast protein 1-deoxy-D-xylulose-5-phosphate synthase (NtDXS). This interaction was confirmed using bimolecular fluorescence complementation (BiFC) assays and pull-down assays. The Transket_pyr domain (residues 394-561) of NtDXS was required for interaction with HC-Pro, while the N-terminal region of HC-Pro (residues 1-97) was necessary for interaction with NtDXS. Using in vitro enzyme activity assays, PVY HC-Pro was found to promote the synthase activity of NtDXS. We observed increases in photosynthetic pigment contents and ABA levels in transgenic plants with HC-Pro accumulating in the chloroplasts. During virus infection, the enhancement of plastidic isoprenoid biosynthesis was attributed to the enhancement of DXS activity by HC-Pro. Our study reveals a new role of HC-Pro in the host plant metabolic system and will contribute to the study of host-virus relationships.

The ER quality control and ER associated degradation machineries are vital for viral pathogenesis

The endoplasmic reticulum (ER) is central to protein production and membrane lipid synthesis. The unfolded protein response (UPR) supports cellular metabolism by ensuring protein quality control in the ER. Most positive strand RNA viruses cause extensive remodeling of membranes and require active membrane synthesis to promote infection. How viruses interact with the cellular machinery controlling membrane metabolism is largely unknown. Furthermore, there is mounting data pointing to the importance of the UPR and ER associated degradation (ERAD) machineries in viral pathogenesis in eukaryotes emerging topic. For many viruses, the UPR is an early event that is essential for persistent infection and benefits virus replication. In addition, many viruses are reported to commandeer ER resident chaperones to contribute to virus replication and intercellular movement. In particular, calreticulin, the ubiquitin machinery, and the 26S proteasome are most commonly identified components of the UPR and ERAD machinery that also regulate virus infection. In addition, researchers have noted a link between UPR and autophagy. It is well accepted that positive strand RNA viruses use autophagic membranes as scaffolds to support replication and assembly. However this topic has yet to be explored using plant viruses. The goal of research on this topic is to uncover how viruses interact with this ER-related machinery and to use this information for designing novel strategies to boost immune responses to virus infection.

The ER quality control and ER associated degradation machineries are vital for viral pathogenesis

The endoplasmic reticulum (ER) is central to protein production and membrane lipid synthesis. The unfolded protein response (UPR) supports cellular metabolism by ensuring protein quality control in the ER. Most positive strand RNA viruses cause extensive remodeling of membranes and require active membrane synthesis to promote infection. How viruses interact with the cellular machinery controlling membrane metabolism is largely unknown. Furthermore, there is mounting data pointing to the importance of the UPR and ER associated degradation (ERAD) machineries in viral pathogenesis in eukaryotes emerging topic. For many viruses, the UPR is an early event that is essential for persistent infection and benefits virus replication. In addition, many viruses are reported to commandeer ER resident chaperones to contribute to virus replication and intercellular movement. In particular, calreticulin, the ubiquitin machinery, and the 26S proteasome are most commonly identified components of the UPR and ERAD machinery that also regulate virus infection. In addition, researchers have noted a link between UPR and autophagy. It is well accepted that positive strand RNA viruses use autophagic membranes as scaffolds to support replication and assembly. However this topic has yet to be explored using plant viruses. The goal of research on this topic is to uncover how viruses interact with this ER-related machinery and to use this information for designing novel strategies to boost immune responses to virus infection.

Viral and cellular factors involved in Phloem transport of plant viruses

Phloem transport of plant viruses is an essential step in the setting-up of a complete infection of a host plant. After an initial replication step in the first cells, viruses spread from cell-to-cell through mesophyll cells, until they reach the vasculature where they rapidly move to distant sites in order to establish the infection of the whole plant. This last step is referred to as systemic transport, or long-distance movement, and involves virus crossings through several cellular barriers: bundle sheath, vascular parenchyma, and companion cells for virus loading into sieve elements (SE). Viruses are then passively transported within the source-to-sink flow of photoassimilates and are unloaded from SE into sink tissues. However, the molecular mechanisms governing virus long-distance movement are far from being understood. While most viruses seem to move systemically as virus particles, some viruses are transported in SE as viral ribonucleoprotein complexes (RNP). The nature of the cellular and viral factors constituting these RNPs is still poorly known. The topic of this review will mainly focus on the host and viral factors that facilitate or restrict virus long-distance movement.

Mapping the self-interacting domains of TuMV HC-Pro and the subcellular localization of the protein

The helper component-proteinase (HC-Pro) of potyviruses is a multifunctional protein involved in aphid transmission, polyprotein processing, cell-to-cell and long-distance movement, genome amplification and symptom expression. The HC-Pros of several potyviruses interact with themselves but the key domains responsible for self-interaction are apparently not conserved. In our experiments, yeast two-hybrid assays and bimolecular fluorescence complementation showed that Turnip mosaic virus (TuMV) HC-Pro interacted with itself in yeast cells, plant cells and insect cells. It was also shown that the central and C-terminal regions of the HC-Pro participated in these self-interactions. Fluorescence microscopy showed that TuMV HC-Pro was present in the cytoplasm and formed aggregates along the ER.

Genomics of helper component proteinase reveals effective strategy for papaya ringspot virus resistance

Papaya ringspot virus (PRSV) causes severe economic losses in both cucurbits and papaya throughout the tropics and subtropics. Development of PRSV-resistant transgenic plants faces a major hurdle in achieving resistance against geographically distinct isolates. One of the major reasons of failing to achieve the broad-spectrum PRSV resistance is the involvement of silencing suppressor proteins of viral origin. Here, based on sequence profile of silencing suppressor protein, HcPro, we show that PRSV-HcPro, acts as a suppressor of RNA silencing through micro RNA binding in a dose- dependent manner. In planta expression of PRSV-HcPro affects developmental biology of plants, suggesting the interference of suppressor protein in micro RNA-directed regulatory pathways of plants. Besides facilitating the establishment of PRSV, it showed strong positive synergism with other heterologous viruses as well. This study provides a strategy to develop effective and stable PRSV-resistant transgenic plants.