P3N-PIPO but not P3 is the avirulence determinant in melon carrying the Wmr resistance against watermelon mosaic virus, although they contain a common genetic determinant.

Viruses employ a series of diverse translational strategies to expand their coding capacity, which produces viral proteins with common domains and entangles virus-host interactions. P3N-PIPO, which is a transcriptional slippage product from the P3 cistron, is a potyviral protein dedicated to intercellular movement. Here, we show that P3N-PIPO from watermelon mosaic virus (WMV) triggers cell death when transiently expressed in Cucumis melo accession PI 414723 carrying the Wmr resistance gene. Surprisingly, expression of the P3N domain, shared by both P3N-PIPO and P3, can alone induce cell death, whereas expression of P3 fails to activate cell death in PI 414723. Confocal microscopy analysis revealed that P3N-PIPO targets plasmodesmata (PD) and P3N associates with PD, while P3 localizes in endoplasmic reticulum in melon cells. We also found that mutations in residues L35, L38, P41, and I43 of the P3N domain individually disrupt the cell death induced by P3N-PIPO, but do not affect the PD localization of P3N-PIPO. Furthermore, WMV mutants with L35A or I43A can systemically infect PI 414723 plants. These key residues guide us to discover some WMV isolates potentially breaking the Wmr resistance. Through searching the NCBI database, we discovered some WMV isolates with variations in these key sites, and one naturally occurring I43V variation enables WMV to systemically infect PI 414723 plants. Taken together, these results demonstrate that P3N-PIPO, but not P3, is the avirulence determinant recognized by Wmr, although the shared N terminal P3N domain can alone trigger cell death.IMPORTANCEThis work reveals a novel viral avirulence (Avr) gene recognized by a resistance (R) gene. This novel viral Avr gene is special because it is a transcriptional slippage product from another virus gene, which means that their encoding proteins share the common N-terminal domain but have distinct C-terminal domains. Amazingly, we found that it is the common N-terminal domain that determines the Avr-R recognition, but only one of the viral proteins can be recognized by the R protein to induce cell death. Next, we found that these two viral proteins target different subcellular compartments. In addition, we discovered some virus isolates with variations in the common N-terminal domain and one naturally occurring variation that enables the virus to overcome the resistance. These results show how viral proteins with common domains interact with a host resistance protein and provide new evidence for the arms race between plants and viruses.

First Report of Enterobacter mori Causing Bacterial Wilt on Tomato in China.

Tomato (Solanum lycopersicum L.) is one of the most important vegetable crops in China. In October 2023, a new bacterial disease was discovered on tomato plants in a 0.3-acre farm's greenhouse (35.514806N, 118.996106E) in Longshan Town, Shandong Province, China. Over 50% of the tomato plants showed symptoms of stem rot, leaf wilt, or plant death. Three diseased tomato plants were collected for pathogen isolation and purification. Two leaf samples, each about 1 cm2, were cut from the junction area of healthy and diseased parts and disinfected with 75% alcohol for 60 s, followed by 0.1% HgCl2 for 90 s, and then washed three times with sterilized H2O. The samples were subsequently ground with 1.0 mL sterilized H2O. The ground samples were diluted to 10-4, 10-5, and 10-6 and then were plated on a potato dextrose agar (PDA) plate, respectively. White mucous bacterial colonies appeared at 28℃ for 24~48 h, no fungal colony was observed. Six bacterial colonies were randomly selected for gram staining and found to be gram-negative. To further determine their species classification, fragments of the 16SrDNA, hsp60, gyrB, and rpoB genes were separately amplified using previously reported PCR conditions and with primer pairs, including 27F/1492R (Wu et al., 2023), HSP60-F /HSP60-R (Gül et al., 2023), gyrB UP-1 / gyrB UP-2r (Yamamoto et al., 1995) and rpoB CM81-F / rpoB CM32b-R (Brady et al., 2008). Sequence analysis showed that the obtained sequences of the 16SrDNA, hsp60, gyrB, and rpoB genes among these six colonies were identical and 100%, 100%, 99.31%, and 99.36% similar to those of Enterobacter mori accessions OP601841 (with a coverage of 100%), MT199160 (83%), OP676246 (100%), and MN594495 (100%), at nucleotide level, respectively. Sequences of the above four genes of 23LSFQ were submitted to GenBank under the accession numbers PP461247, PP474090, PP136037, and PP136038, respectively. We selected one of these six colonies, 23LSFQ, for further analysis. The phylogenetic tree based on the concatenated sequences of the above four genes using the maximum likelihood method with MEGAX software showed that 23LSFQ is grouped with E. mori LMG25706 (NCBI: txid980518). To determine the pathogenicity of 23LSFQ , we sprayed 23LSFQ (OD600=0.8) onto five 30-day-old healthy plants of the tomato cultivars Alisa Craig, Jinpeng NO.1, and Chaobei, respectively. These seedlings were incubated in a chamber at 28°C with a 16 h light/ 8h dark photoperiod and 60% relative humidity. The leaves of the inoculated plants became curled and wilted at two days post inoculation (dpi) and appeared necrotic at 10 dpi. The symptoms were similar to those observed in field-infected tomato plants. No symptoms were observed on the plants inoculated with water. We further sequenced the re-isolated bacteria from the symptomatic inoculated seedlings. Results showed that they belong to E. mori. The experiment was repeated three times. E. mori has been found to cause diseases on peaches (Ahmad et al., 2021), watermelons (Wu et al., 2023), Canna indica, (Zhang et al., 2023), and strawberries (Ji et al., 2023). E. cloacae has been found to cause diseases on tomatoes in Heilongjiang province (Jin et al., 2023). This is the first report of E. mori causing leaf yellowing and wilting on tomatoes in China. These results are significant for the safe production and disease control of greenhouse tomatoes.

Development of an attenuated potato virus Y mutant carrying multiple mutations in helper-component protease for cross-protection.

Tobacco (Nicotiana tabacum) is one of the major cash crops in China. Potato virus Y (PVY), a representative member of the genus Potyvirus, greatly reduces the quality and yield of tobacco leaves by inducing veinal necrosis. Mild strain-mediated cross-protection is an attractive method of controlling diseases caused by PVY. Currently, there is a lack of effective and stable attenuated PVY mutants. Potyviral helper component-protease (HC-Pro) is a likely target for the development of mild strains. Our previous studies showed that the residues lysine at positions 124 and 182 (K124 and K182) in HC-Pro were involved in PVY virulence, and the conserved KITC motif in HC-Pro was involved in aphid transmission. In this study, to improve the stability of PVY mild strains, K at position 50 (K50) in KITC motif, K124, and K182 were separately substituted with glutamic acid (E), leucine (L), and arginine (R), resulting in a triple-mutant PVY-HCELR. The mutant PVY-HCELR had attenuated virulence and did not induce leaf veinal necrosis symptoms in tobacco plants and could not be transmitted by Myzus persicae. Furthermore, PVY-HCELR mutant was genetically stable after six serial passages, and only caused mild mosaic symptoms in tobacco plants even at 90 days post inoculation. The tobacco plants cross-protected by PVY-HCELR mutant showed high resistance to the wild-type PVY. This study showed that PVY-HCELR mutant was a promising mild mutant for cross-protection to control PVY.

A natural substitution of a conserved amino acid in eIF4E confers resistance against multiple potyviruses.

Eukaryotic translation initiation factor 4E (eIF4E), which plays a pivotal role in initiating translation in eukaryotic organisms, is often hijacked by the viral genome-linked protein to facilitate the infection of potyviruses. In this study, we found that the naturally occurring amino acid substitution D71G in eIF4E is widely present in potyvirus-resistant watermelon accessions and disrupts the interaction between watermelon eIF4E and viral genome-linked protein of papaya ringspot virus-watermelon strain, zucchini yellow mosaic virus or watermelon mosaic virus. Multiple sequence alignment and protein modelling showed that the amino acid residue D71 located in the cap-binding pocket of eIF4E is strictly conserved in many plant species. The mutation D71G in watermelon eIF4E conferred resistance against papaya ringspot virus-watermelon strain and zucchini yellow mosaic virus, and the equivalent mutation D55G in tobacco eIF4E conferred resistance to potato virus Y. Therefore, our finding provides a potential precise target for breeding plants resistant to multiple potyviruses.

Potato virus Y viral protein 6K1 inhibits the interaction between defense proteins during virus infection.

14-3-3 proteins play vital roles in plant defense against various pathogen invasions. To date, how 14-3-3 affects virus infections in plants remains largely unclear. In this study, we found that Nicotiana benthamiana 14-3-3h interacts with TRANSLATIONALLY CONTROLLED TUMOR PROTEIN (TCTP), a susceptibility factor of potato virus Y (PVY). Silencing of Nb14-3-3h facilitates PVY accumulation, whereas overexpression of Nb14-3-3h inhibits PVY replication. The antiviral activities of 3 Nb14-3-3h dimerization defective mutants are significantly decreased, indicating that dimerization of Nb14-3-3h is indispensable for restricting PVY infection. Our results also showed that the mutant Nb14-3-3hE16A, which is capable of dimerizing but not interacting with NbTCTP, has reduced anti-PVY activity; the mutant NbTCTPI65A, which is unable to interact with Nb14-3-3h, facilitates PVY replication compared with the wild-type NbTCTP, indicating that dimeric Nb14-3-3h restricts PVY infection by interacting with NbTCTP and preventing its proviral function. As a counter-defense, PVY 6K1 interferes with the interaction between Nb14-3-3h and NbTCTP by competitively binding to Nb14-3-3h and rescues NbTCTP to promote PVY infection. Our results provide insights into the arms race between plants and potyviruses.

Corrigendum: Development and Evaluation of Stable Sugarcane Mosaic Virus Mild Mutants for Cross-Protection Against Infection by Severe Strain.

[This corrects the article DOI: 10.3389/fpls.2021.788963.].

A maize triacylglycerol lipase inhibits sugarcane mosaic virus infection.

Triacylglycerol lipase (TGL) plays critical roles in providing energy for seed germination and plant development. However, the role of TGL in regulating plant virus infection is largely unknown. In this study, we adopted affinity purification coupled with mass spectrometry and identified that a maize (Zea mays) pathogenesis-related lipase protein Z. mays TGL (ZmTGL) interacted with helper component-proteinase (HC-Pro) of sugarcane mosaic virus (SCMV). Yeast two-hybrid, luciferase complementation imaging, and bimolecular fluorescence complementation assays confirmed that ZmTGL directly interacted with SCMV HC-Pro in vitro and in vivo. The 101-460 residues of SCMV HC-Pro were important for its interaction with ZmTGL. ZmTGL and SCMV HC-Pro co-localized at the mitochondria. Silencing of ZmTGL facilitated SCMV infection, and over-expression of ZmTGL reduced the RNA silencing suppression activity, most likely through reducing HC-Pro accumulation. Our results provided evidence that the lipase hydrolase activity of ZmTGL was associated with reducing HC-Pro accumulation, activation of salicylic acid (SA)-mediated defense response, and inhibition of SCMV infection. We show that ZmTGL inhibits SCMV infection by reducing HC-Pro accumulation and activating the SA pathway.

A Predicted Stem Loop in Coat Protein-Coding Sequence of Tobacco Vein Banding Mosaic Virus Is Required for Efficient Replication.

Potyviral coat protein (CP) is involved in the replication and movement of potyviruses. However, little information is available on the roles of CP-coding sequence in potyviral infection. Here, we introduced synonymous substitutions to the codon C574G575C576 coding conserved residue arginine at position 192 (R192) of tobacco vein banding mosaic virus (TVBMV) CP. Substitution of the codon C574G575C576 to A574G575A576 or A574G575G576, but not C574G575A576, C574G575T576, or C574G575G576, reduced the replication, cell-to-cell movement, and accumulation of TVBMV in Nicotiana benthamiana plants, suggesting that C574 was critical for replication of TVBMV. Nucleotides 531 to 576 of the TVBMV CP-coding sequence were predicted to form a stem-loop structure, in which four consecutive C-G base pairs (C576-G531, C532-G575, C574-G533, and C534-G573) were located at the stem. Synonymous substitutions of R178-codon C532G533C534 to A532G533A534 and A532G533G534, but not C532G533A534, C532G533T534, or C532G533G534, reduced the replication levels, cell-to-cell, and systemic movement of TVBMV, suggesting that C532 was critical for TVBMV replication. Synonymous substitutions disrupting base pairs C576-G531 and C534-G573 did not affect viral accumulation. After three serial-passage inoculations, the accumulation of spontaneous mutant viruses was restored, and codons A532G533A534, A532G533G534, A574G575A576, or A574G575G576 of mutants were each separately changed to C532G533A534, C532G533G534, C574G575A576, or C574G575G576. Synonymous mutation of R178 and R192 also reduced viral accumulation in N. tabacum plants. Therefore, we concluded that the two consecutive C532-G575 and C574-G533 base pairs played critical roles in TVBMV replication via maintaining the stability of the stem-loop structures formed by nucleotides 531 to 576 of the CP-coding sequence.

The chloroplast ribosomal protein large subunit 1 interacts with viral polymerase and promotes virus infection.

Chloroplasts play an indispensable role in the arms race between plant viruses and hosts. Chloroplast proteins are often recruited by plant viruses to support viral replication and movement. However, the mechanism by which chloroplast proteins regulate potyvirus infection remains largely unknown. In this study, we observed that Nicotiana benthamiana ribosomal protein large subunit 1 (NbRPL1), a chloroplast ribosomal protein, localized to the chloroplasts via its N-terminal 61 amino acids (transit peptide), and interacted with tobacco vein banding mosaic virus (TVBMV) nuclear inclusion protein b (NIb), an RNA-dependent RNA polymerase. Upon TVBMV infection, NbRPL1 was recruited into the 6K2-induced viral replication complexes in chloroplasts. Silencing of NbRPL1 expression reduced TVBMV replication. NbRPL1 competed with NbBeclin1 to bind NIb, and reduced the NbBeclin1-mediated degradation of NIb. Therefore, our results suggest that NbRPL1 interacts with NIb in the chloroplasts, reduces NbBeclin1-mediated NIb degradation, and enhances TVBMV infection.

Identification of genetic determinants of tomato brown rugose fruit virus that enable infection of plants harbouring the Tm-22 resistance gene.

Tomato cultivars containing the Tm-22 resistance gene have been widely known to resist tobacco mosaic virus (TMV) and tomato mosaic virus. Tomato brown rugose fruit virus (ToBRFV), a new emerging tobamovirus, can infect tomato plants carrying the Tm-22 gene. However, the virulence determinant of ToBRFV that overcomes the resistance conferred by the Tm-22 gene remains unclear. In this study, we substituted the movement protein (MP) encoding sequences between ToBRFV and TMV infectious clones and conducted infectivity assays. The results showed that MP was the virulence determinant for ToBRFV to infect Tm-22 transgenic Nicotiana benthamiana plants and Tm-22 -carrying tomato plants. A TMV MP chimera with amino acid residues 60-186 of ToBRFV MP failed to induce hypersensitive cell death in the leaves of Tm-22 transgenic N. benthamiana plants. Chimeric TMV containing residues 60-186 of ToBRFV MP could, but chimeric ToBRFV containing 61-187 residues of TMV MP failed to infect Tm-22 transgenic N. benthamiana plants, indicating that 60-186 residues of MP were important for ToBRFV to overcome Tm-22 gene-mediated resistance. Further analysis showed that six amino acid residues, H67 , N125 , K129 , A134 , I147 , and I168 of ToBRFV MP, were critical in overcoming Tm-22 -mediated resistance in transgenic N. benthamiana plants and tomato plants. These results increase our understanding of the mechanism by which ToBRFV overcomes Tm-22 -mediated resistance.

DNA Hypermethylation Induced by L-Methionine Leads to Oligodendroglial and Myelin Deficits and Schizophrenia-Like Behaviors in Adolescent Mice.

Increasing evidence has demonstrated that in addition to dysfunction of neuronal circuitry, oligodendroglial dysfunction and/or disruption of white matter integrity are found in the brains of patients with schizophrenia. DNA methylation, a well-established risk factor for schizophrenia, has been demonstrated to cause neuronal dysfunction; however, whether dysregulation of DNA methylation contributes to oligodendroglial/myelin deficits in the pathogenesis of schizophrenia remains unclear. In the present study, by using L-methionine-treated mice, we confirmed that mice with DNA hypermethylation exhibited an anxious phenotype, impaired sociability, and sensorimotor gating deficits. Notably, DNA hypermethylation in oligodendroglial cells led to dysregulation of multiple oligodendroglia-specific transcription factors, which indicated disruption of the transcriptional architecture. Furthermore, DNA hypermethylation caused a reduction of oligodendroglial lineage cells and myelin integrity in the frontal white matter of mice. Taken together, these results indicate that DNA hypermethylation leads to oligodendroglial and/or myelin deficits, which may, at least in part, contribute to schizophrenia-like behaviors in mice. This study provides new insights into the possibility that precise modulation of DNA methylation status in oligodendroglia could be beneficial for the white matter pathology in schizophrenia.

A tobacco ringspot virus-based vector system for gene and microRNA function studies in cucurbits.

Cucurbits are economically important crops worldwide. The genomic data of many cucurbits are now available. However, functional analyses of cucurbit genes and noncoding RNAs have been impeded because genetic transformation is difficult for many cucurbitaceous plants. Here, we developed a set of tobacco ringspot virus (TRSV)-based vectors for gene and microRNA (miRNA) function studies in cucurbits. A TRSV-based expression vector could simultaneously express GREEN FLUORESCENT PROTEIN (GFP) and heterologous viral suppressors of RNA silencing in TRSV-infected plants, while a TRSV-based gene silencing vector could knock down endogenous genes exemplified by PHYTOENE DESATURASE (PDS) in Cucumis melo, Citrullus lanatus, Cucumis sativus, and Nicotiana benthamiana plants. We also developed a TRSV-based miRNA silencing vector to dissect the functions of endogenous miRNAs. Four representative miRNAs, namely, miR159, miR166, miR172, and miR319, from different cucurbits were inserted into the TRSV vector using a short tandem target mimic strategy and induced characteristic phenotypes in TRSV-miRNA-infected plants. This TRSV-based vector system will facilitate functional genomic studies in cucurbits.

Residues R192 and K225 in RNA-Binding Pocket of Tobacco Vein Banding Mosaic Virus CP Control Virus Cell-to-Cell Movement and Replication.

Potyviruses move to neighboring cells in the form of virus particles or a coat protein (CP)-containing ribonucleoprotein complex. However, the precise roles of RNA-binding residues in potyviral CP in viral cell-to-cell movement remain to be elucidated. In this study, we predicted the three-dimensional model of tobacco vein banding mosaic virus (TVBMV)-encoded CP and found nine residues presumably located in the CP RNA-binding pocket. Substitutions of the two basic residues at positions 192 and 225 (R192 and K225) with either alanine, cysteine, or glutamic acid abolished TVBMV cell-to-cell and systemic movement in Nicotiana benthamiana plants. These substitutions also reduced the replication of the mutant viruses. Results from the electrophoretic mobility shift assay showed that the RNA-binding activity of mutant CPs derived from R192 or K225 substitutions was significantly lower than that of wild-type CP. Analysis of purified virus particles showed that mutant viruses with R192 or K225 substitutions formed RNA-free virus-like particles. Mutations of R192 and K225 did not change the CP plasmodesmata localization. The wild-type TVBMV CP could rescue the deficient cell-to-cell movement of mutant viruses. Moreover, deletion of any of the other seven residues also abolished TVBMV cell-to-cell movement and reduced the CP RNA-binding activity. The corresponding nine residues in watermelon mosaic virus CP were also found to play essential roles in virus cell-to-cell movement. In conclusion, residues R192 and K225 in the CP RNA-binding pocket are critical for viral RNA binding and affect both virus replication and cell-to-cell movement.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Development and Evaluation of Stable Sugarcane Mosaic Virus Mild Mutants for Cross-Protection Against Infection by Severe Strain.

Sugarcane mosaic virus (SCMV; genus Potyvirus) induces maize dwarf mosaic disease that has caused serious yield losses of maize in China. Cross-protection is one of the efficient strategies to fight against severe virus strains. Although many mild strains have been identified, the spontaneous mutation is one of the challenging problems affecting their application in cross-protection. In this study, we found that the substitution of cysteine (C) at positions 57 or 60 in the zinc finger-like motif of HC-Pro with alanine (A; C57A or C60A) significantly reduced its RNA silencing suppression activity and SCMV virulence. To reduce the risk of mild strains mutating to virulent ones by reverse or complementary mutations, we obtained attenuated SCMV mutants with double-mutations in the zinc finger-like and FRNK motifs of HC-Pro and evaluated their potential application in cross-protection. The results showed that the maize plants infected with FKNK/C60A double-mutant showed symptomless until 95 days post-inoculation and FKNK/C60A cross-protected plants displayed high resistance to severe SCMV strain. This study provides theoretical and material bases for the control of SCMV through cross-protection.

The conserved aromatic residue W122 is a determinant of potyviral coat protein stability, replication, and cell-to-cell movement in plants.

Coat proteins (CPs) play critical roles in potyvirus cell-to-cell movement. However, the underlying mechanism controlling them remains unclear. Here, we show that substitutions of alanine, glutamic acid, or lysine for the conserved residue tryptophan at position 122 (W122 ) in tobacco vein banding mosaic virus (TVBMV) CP abolished virus cell-to-cell movement in Nicotiana benthamiana plants. In agroinfiltrated N. benthamiana leaf patches, both the CP and RNA accumulation levels of three W122 mutant viruses were significantly reduced compared with those of wild-type TVBMV, and CP accumulated to a low level similar to that of a replication-deficient mutant. The results of polyprotein transient expression experiments indicated that CP instability was responsible for the significantly low CP accumulation levels of the three W122 mutant viruses. The substitution of W122 did not affect CP plasmodesmata localization or virus particle formation; however, the substitution significantly reduced the number of virus particles. The wild-type TVBMV CP could complement the reduced replication and abolished cell-to-cell movement of the mutant viruses. When the codon for W122 was mutated to that for a different aromatic residue, phenylalanine or tyrosine, the resultant mutant viruses moved systemically and accumulated up to 80% of the wild-type TVBMV level. Similar results were obtained for the corresponding amino acids of W122 in the watermelon mosaic virus and potato virus Y CPs. Therefore, we conclude that the aromatic ring in W122 in the core domain of the potyviral CP is critical for cell-to-cell movement through the effects on CP stability and viral replication.

Effectiveness of Kinesio taping on peripheral facial paralysis: A protocol for systematic review and meta-analysis.

BACKGROUND: Peripheral facial paralysis is a rapid unilateral facial paralysis or paralysis of unknown etiology. Nearly 30% of patients leave sequela that have a negative impact on the patient's quality of life, both physically and psychologically. As its safety, convenience and effectiveness, Kinesio taping has been gradually used in the rehabilitation of peripheral facial paralysis. However, whether Kinesio taping is effective for peripheral facial paralysis is still unknown. The purpose of this systematic review (SR) and meta-analysis will summarize the current evidence of Kinesio taping used as an intervention for peripheral facial paralysis. METHODS AND ANALYSIS: We will search the following electronic databases for randomized controlled trials (RCTs) and controlled clinical trials (CCTs) to evaluate the effectiveness of Kinesio taping in treating peripheral facial paralysis: China National Knowledge Infrastructure (CNKI), Wanfang Date, SinoMed, Technology Periodical Database (VIP), PubMed, Embase, Web of Science, and The Cochrane Library. Each database will be searched from inception to April 2020. Studies that present clear descriptions of Kinesio taping in treating peripheral facial paralysis administration are published in peer-reviewed journals in any languages and are published in full will be taken into consideration. The entire process will include study selection, data extraction, risk of bias assessment and meta-analyses. Assessment of risk of bias and data synthesis will be conducted using Review Manager 5.3 software. RESULTS: The current evidence on the Kinesio taping for managing peripheral facial paralysis will be illustrated using subjective reports and objective measures of performance. The primary outcome is the effective rate. Secondary outcomes include House-Brackmann scale, Portmann score, facial nerve conduction velocity, Facial Disability Index, Facial Disability Index include Facial Function score and social Function score. CONCLUSION: This protocol will present evidence on the efficacy of Kinesio taping in relieving peripheral facial paralysis. ETHICS AND DISSEMINATION: Since all the data used in this SR and meta-analysis have been published, ethical approval is not required for this review. The results of this SR will be published in a peer-reviewed journal or presented at conferences. INPLASY ID:: (INPLASY2020100008).

A Spontaneous Complementary Mutation Restores the RNA Silencing Suppression Activity of HC-Pro and the Virulence of Sugarcane Mosaic Virus.

Cross-protection is a promising measure to control plant viral diseases. Reverse genetics had been recently adopted to generate attenuated mutants that have potential in cross-protection. But studies on the variability of the progeny viruses of the attenuated mutants are scarce. Sugarcane mosaic virus (SCMV; genus Potyvirus, family Potyviridae) is the prevalent virus inducing maize dwarf mosaic disease in China. Here, we showed that the substitution of arginine with isoleucine in the FRNK motif at position 184 of helper component-proteinase (HC-Pro) abolished its RNA silencing suppression (RSS) activity, drastically reduced the virulence and accumulation level of SCMV, and impaired the synergism between SCMV and maize chlorotic mottle virus. The attenuated mutant could protect maize plants from a severe infection of SCMV. However, a spontaneous mutation of glycine at position 440 to arginine in HC-Pro rescued the virulence and synergism with maize chlorotic mottle virus of SCMV and the RSS activity of HC-Pro. Similar results were obtained with tobacco vein banding mosaic virus and watermelon mosaic virus. These results provide novel evidence for the complementary mutation of potyviruses in maintaining the HC-Pro RSS activity and potyviral virulence and remind us of evaluating the potential risk of attenuated mutants thoroughly before applying for the control of plant viral diseases via cross-protection.