OUP accepted manuscript

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.

Identification of Barley yellow mosaic virus Isolates Breaking rym3 Resistance in Japan

In early spring 2018, significant mosaic disease symptoms were observed for the first time on barley leaves (Hordeum vulgare L., cv. New Sachiho Golden) in Takanezawa, Tochigi Prefecture, Japan. This cultivar carries the resistance gene rym3 (rym; resistance to yellow mosaic). Through RNA-seq analysis, Barley yellow mosaic virus (BaYMV-Takanezawa) was identified in the roots of all five plants (T01-T05) in the field. Phylogenetic analysis of RNA1, encompassing known BaYMV pathotypes I through V, revealed that it shares the same origin as isolate pathotype IV (BaYMV-Ohtawara pathotype). However, RNA2 analysis of isolates revealed the simultaneous presence of two distinct BaYMV isolates, BaYMV-Takanezawa-T01 (DRR552862, closely related to pathotype IV) and BaYMV-Takanezawa-T02 (DRR552863, closely related to pathotype III). The amino acid sequences of the BaYMV-Takanezawa isolates displayed variations, particularly in the VPg and N-terminal region of CP, containing mutations not found in other domains of the virus genome. Changes in the CI (RNA1 amino acid residue 459) and CP (RNA1 amino acid residue 2138) proteins correlated with pathogenicity. These findings underscore the importance of monitoring and understanding the genetic diversity of BaYMV for effective disease management strategies in crop breeding.

Identification of yam mosaic virus as the main cause of yam mosaic diseases in Ethiopia

Yam (Dioscorea spp.) is a staple food crop with cultural, nutritional and economic significance for millions of small-scale farmers in sub-Saharan Africa. While various virus-like symptoms such as mosaic and chlorosis are frequently observed in yam fields in Ethiopia, little information is available on the prevalence, distribution, and molecular characteristics of viruses causing these symptoms. The aim of this study was to investigate the incidence and distribution of yam viruses and determine the primary cause of yam mosaic diseases (YMD) in Ethiopia. Both symptomatic (n = 280) and asymptomatic (n = 110) yam leaf samples were collected and tested for potyviruses using ACP-ELISA. In addition, the symptomatic leaf samples were screened for yam mosaic virus (YMV), yam mild mosaic virus (YMMV), and cucumber mosaic virus (CMV) by DAS-ELISA. Subsequently, total RNA was extracted from 130 leaf samples comprising 94 symptomatic and 36 asymptomatic samples representing the different study areas. The representative RT-PCR amplicons (n = 6) were Sanger sequenced. The ACP-ELISA and DAS-ELISA results showed 9.2%, and 12.9% YMV infection, respectively, while the RT-PCR analysis showed 28.5% YMV positivity rate. Both CMV and YMMV were not detected in any of the samples tested. Thus, YMV is confirmed as the primary cause of YMD in Ethiopia. YMV isolates from Ethiopia shared 92-93% nucleotide identity among themselves and 85-99% with other YMV isolates from the GenBank. Phylogenetic analysis revealed that YMV isolates from Ethiopia, South America, and west-central Africa have the most recent common ancestor, while isolates from China and Japan are clustered as sister groups. This study enhances our understanding of YMV's genetic diversity and provides valuable information regarding the first report of YMV in Ethiopia.

Rubisco small subunit (RbCS) is co-opted by potyvirids as the scaffold protein in assembling a complex for viral intercellular movement

Plant viruses must move through plasmodesmata (PD) to complete their life cycles. For viruses in the Potyviridae family (potyvirids), three viral factors (P3N-PIPO, CI, and CP) and few host proteins are known to participate in this event. Nevertheless, not all the proteins engaging in the cell-to-cell movement of potyvirids have been discovered. Here, we found that HCPro2 encoded by areca palm necrotic ring spot virus (ANRSV) assists viral intercellular movement, which could be functionally complemented by its counterpart HCPro from a potyvirus. Affinity purification and mass spectrometry identified several viral factors (including CI and CP) and host proteins that are physically associated with HCPro2. We demonstrated that HCPro2 interacts with both CI and CP in planta in forming PD-localized complexes during viral infection. Further, we screened HCPro2-associating host proteins, and identified a common host protein in Nicotiana benthamiana-Rubisco small subunit (NbRbCS) that mediates the interactions of HCPro2 with CI or CP, and CI with CP. Knockdown of NbRbCS impairs these interactions, and significantly attenuates the intercellular and systemic movement of ANRSV and three other potyvirids (turnip mosaic virus, pepper veinal mottle virus, and telosma mosaic virus). This study indicates that a nucleus-encoded chloroplast-targeted protein is hijacked by potyvirids as the scaffold protein to assemble a complex to facilitate viral movement across cells.

Viral communities in millipede guts: Insights into the diversity and potential role in modulating the microbiome

Millipedes are important detritivores harbouring a diverse microbiome. Previous research focused on bacterial and archaeal diversity, while the virome remained neglected. We elucidated the DNA and RNA viral diversity in the hindguts of two model millipede species with distinct microbiomes: the tropical Epibolus pulchripes (methanogenic, dominated by Bacillota) and the temperate Glomeris connexa (non-methanogenic, dominated by Pseudomonadota). Based on metagenomic and metatranscriptomic assembled viral genomes, the viral communities differed markedly and preferentially infected the most abundant prokaryotic taxa. The majority of DNA viruses were Caudoviricetes (dsDNA), Cirlivirales (ssDNA) and Microviridae (ssDNA), while RNA viruses consisted of Leviviricetes (ssRNA), Potyviridae (ssRNA) and Eukaryotic viruses. A high abundance of subtypes I-C, I-B and II-C CRISPR-Cas systems was found, primarily from Pseudomonadota, Bacteroidota and Bacillota. In addition, auxiliary metabolic genes that modulate chitin degradation, vitamins and amino acid biosynthesis and sulphur metabolism were also detected. Lastly, we found low virus-to-microbe-ratios and a prevalence of lysogenic viruses, supporting a Piggyback-the-Winner dynamic in both hosts.

Mixed infection of ITPase-encoding potyvirid and secovirid in Mercurialis perennis: evidences for a convergent euphorbia-specific viral counterstrike

BACKGROUND

In cellular organisms, inosine triphosphate pyrophosphatases (ITPases) prevent the incorporation of mutagenic deaminated purines into nucleic acids. These enzymes have also been detected in the genomes of several plant RNA viruses infecting two euphorbia species. In particular, two ipomoviruses produce replicase-associated ITPases to cope with high concentration of non-canonical nucleotides found in cassava tissues.

METHOD

Using high-throughput RNA sequencing on the wild euphorbia species Mercurialis perennis, two new members of the families Potyviridae and Secoviridae were identified. Both viruses encode for a putative ITPase, and were found in mixed infection with a new partitivirid. Following biological and genomic characterization of these viruses, the origin and function of the phytoviral ITPases were investigated.

RESULTS

While the potyvirid was shown to be pathogenic, the secovirid and partitivirid could not be transmitted. The secovirid was found belonging to a proposed new Comovirinae genus tentatively named "Mercomovirus", which also accommodates other viruses identified through transcriptome mining, and for which an asymptomatic pollen-associated lifestyle is suspected. Homology and phylogenetic analyses inferred that the ITPases encoded by the potyvirid and secovirid were likely acquired through independent horizontal gene transfer events, forming lineages distinct from the enzymes found in cassava ipomoviruses. Possible origins from cellular organisms are discussed for these proteins. In parallel, the endogenous ITPase of M. perennis was predicted to encode for a C-terminal nuclear localization signal, which appears to be conserved among the ITPases of euphorbias but absent in other plant families. This subcellular localization is in line with the idea that nucleic acids remain protected in the nucleus, while deaminated nucleotides accumulate in the cytoplasm where they act as antiviral molecules.

CONCLUSION

Three new RNA viruses infecting M. perennis are described, two of which encoding for ITPases. These enzymes have distinct origins, and are likely required by viruses to circumvent high level of cytoplasmic non-canonical nucleotides. This putative plant defense mechanism has emerged early in the evolution of euphorbias, and seems to specifically target certain groups of RNA viruses infecting perennial hosts.

The nanovirus U2 protein suppresses RNA silencing via three conserved cysteine residues

Nanoviruses have multipartite, circular, single-stranded DNA genomes and cause huge production losses in legumes and other crops. No viral suppressor of RNA silencing (VSR) has yet been reported from a member of the genus Nanovirus. Here, we demonstrate that the nanovirus U2 protein is a VSR. The U2 protein of milk vetch dwarf virus (MDV) suppressed the silencing of the green fluorescent protein (GFP) gene induced by single-stranded and double-stranded RNA, and the systemic spread of the GFP silencing signal. An electrophoretic mobility shift assay showed that the U2 protein was able to bind double-stranded 21-nucleotide small interfering RNA (siRNA). The cysteine residues at positions 43, 79 and 82 in the MDV U2 protein are critical to its nuclear localization, self-interaction and siRNA-binding ability, and were essential for its VSR activity. In addition, expression of the U2 protein via a potato virus X vector induced more severe necrosis symptoms in Nicotiana benthamiana leaves. The U2 proteins of other nanoviruses also acted as VSRs, and the three conserved cysteine residues were indispensable for their VSR activity.

Methyltransferase-like (METTL) homologues participate in Nicotiana benthamiana antiviral responses

Methyltransferase (MTase) enzymes catalyze the addition of a methyl group to a variety of biological substrates. MTase-like (METTL) proteins are Class I MTases whose enzymatic activities contribute to the epigenetic and epitranscriptomic regulation of multiple cellular processes. N⁶-adenosine methylation (m⁶A) is a common chemical modification of eukaryotic and viral RNA whose abundance is jointly regulated by MTases and METTLs, demethylases, and m⁶A binding proteins. m⁶A affects various cellular processes including RNA degradation, post-transcriptional processing, and antiviral immunity. Here, we used Nicotiana benthamiana and plum pox virus (PPV), an RNA virus of the Potyviridae family, to investigated the roles of MTases in plant-virus interaction. RNA sequencing analysis identified MTase transcripts that are differentially expressed during PPV infection; among these, accumulation of a METTL gene was significantly downregulated. Two N. benthamiana METTL transcripts (NbMETTL1 and NbMETTL2) were cloned and further characterized. Sequence and structural analyses of the two encoded proteins identified a conserved S-adenosyl methionine (SAM) binding domain, showing they are SAM-dependent MTases phylogenetically related to human METTL16 and Arabidopsis thaliana FIONA1. Overexpression of NbMETTL1 and NbMETTL2 caused a decrease of PPV accumulation. In sum, our results indicate that METTL homologues participate in plant antiviral responses.

Phylogeny and Genetic Divergence among Sorghum Mosaic Virus Isolates Infecting Sugarcane

Sorghum mosaic virus (SrMV, the genus Potyvirus of the family Potyviridae) is a causal agent of common mosaic in sugarcane and poses a threat to the global sugar industry. In this study, a total of 901 sugarcane leaf samples with mosaic symptom were collected from eight provinces in China and were detected via RT-PCR using a primer pair specific to the SrMV coat protein (CP). These leaf samples included 839 samples from modern cultivars (Saccharum spp. hybrids) and 62 samples from chewing cane (S. officinarum). Among these, 632 out of 901 (70.1%) samples were tested positive for SrMV. The incidences of SrMV infection were 72.3% and 40.3% in modern cultivars and chewing cane, respectively. Phylogenetic analysis showed that all tested SrMV isolates were clustered into three clades consisting of six phylogenetic groups based on 306 CP sequences (this study = 265 and GenBank database = 41). A total of 10 SrMV isolates from South America (the United States and Argentina) along with 106 isolates from China were clustered in group D, while the remaining 190 SrMV isolates from Asia (China and Vietnam) were dispersed in five groups. The SrMV isolates in group F were limited to Yunnan province in China, and those in group A were spread over eight provinces. A significant genetic heterogeneity was elucidated in the nucleotide sequence identities of all SrMV CPs, ranging from 69.0% to 100%. A potential recombination event was postulated among SrMV isolates based on CP sequences. All tested SrMV CPs underwent dominant negative selection. Geographical isolation (South America vs. Asia) and host types (modern cultivars vs. chewing cane) are important factors promoting the genetic differentiation of SrMV populations. Overall, this study contributes to the global understanding of the genetic evolution of SrMV and provides a valuable resource for the epidemiology and management of the mosaic in sugarcane.

Quantitative Analysis of RNA Polymerase Slippages for Production of P3N-PIPO Trans-frame Fusion Proteins in Potyvirids

Potyvirids, members of the family Potyviridae, produce the P3N-PIPO protein, which is crucial for the cell-to-cell transport of viral genomic RNAs. The production of P3N-PIPO requires an adenine (A) insertion caused by RNA polymerase slippage at a conserved GAAAAAA (GA6) sequence preceding the PIPO open reading frame. Presently, the slippage rate of RNA polymerase has been estimated in only a few potyvirids, ranging from 0.8 to 2.1%. In this study, we analyzed publicly available plant RNA-seq data and identified 19 genome contigs from 13 distinct potyvirids. We further investigated the RNA polymerase slippage rates at the GA6 motif. Our analysis revealed that the frequency of the A insertion variant ranges from 0.53 to 4.07% in 11 potyviruses (genus Potyvirus). For the two macluraviruses (genus Macluravirus), the frequency of the A insertion variant was found to be 0.72% and 10.96% respectively. Notably, the estimated RNA polymerase slippage rates for 12 out of the 13 investigated potyvirids were reported for the first time in this study. Our findings underscore the value of plant RNA-seq data for quantitative analysis of potyvirid genome variants, specifically at the GA6 slippage site, and contribute to a more comprehensive understanding of the RNA polymerase slippage phenomenon in potyvirids.

The Single Distinct Leader Protease Encoded by Alpinia oxyphylla Mosaic Virus (Genus Macluravirus) Suppresses RNA Silencing Through Interfering with Double-Stranded RNA Synthesis

The genomic 5'-terminal regions of viruses in the family Potyviridae (potyvirids) encode two types of leader proteases: serine-protease (P1) and cysteine-protease (HCPro), which differ greatly in the arrangement and sequence composition among inter-genus viruses. Most potyvirids have the same tandemly arranged P1 and HCPro, whereas viruses in the genus Macluravirus encode a single distinct leader protease, a truncated version of HCPro with yet-unknown functions. We investigated the RNA silencing suppression (RSS) activity and its underpinning mechanism of the distinct HCPro from alpinia oxyphylla mosaic macluravirus (aHCPro). Sequence analysis revealed that macluraviral HCPros have obvious truncations in the N-terminal and middle regions when aligned to their counterparts in potyviruses (well-characterized viral suppressors of RNA silencing). Nearly all defined elements essential for the RSS activity of potyviral counterparts are not distinguished in macluraviral HCPros. Here, we demonstrated that aHCPro exhibits a similar anti-silencing activity with the potyviral counterpart. However, aHCPro fails to block both the local and systemic spreading of RNA silencing. In line, aHCPro interferes with the dsRNA synthesis, an upstream step in the RNA silencing pathway. Affinity-purification and NanoLC-MS/MS analysis revealed that aHCPro has no association with core components or their potential interactors involving in dsRNA synthesis from the protein layer. Instead, the ectopic expression of aHCPro significantly reduces the transcript abundance of RDR2, RDR6, SGS3, and SDE5. This study represents the first report on the anti-silencing function of Macluravirus-encoded HCPro and the underlying molecular mechanism.

CryoEM and stability analysis of virus-like particles of potyvirus and ipomovirus infecting a common host

Sweet potato feathery mottle virus (SPFMV) and Sweet potato mild mottle virus (SPMMV) are members of the genera Potyvirus and Ipomovirus, family Potyviridae, sharing Ipomoea batatas as common host, but transmitted, respectively, by aphids and whiteflies. Virions of family members consist of flexuous rods with multiple copies of a single coat protein (CP) surrounding the RNA genome. Here we report the generation of virus-like particles (VLPs) by transient expression of the CPs of SPFMV and SPMMV in the presence of a replicating RNA in Nicotiana benthamiana. Analysis of the purified VLPs by cryo-electron microscopy, gave structures with resolutions of 2.6 and 3.0 Å, respectively, showing a similar left-handed helical arrangement of 8.8 CP subunits per turn with the C-terminus at the inner surface and a binding pocket for the encapsidated ssRNA. Despite their similar architecture, thermal stability studies reveal that SPMMV VLPs are more stable than those of SPFMV.

The Mechanism of Resistance of EUROPEAN Plum to Plum pox virus Mediated by Hypersensitive Response Is Linked to VIRAL NIa and Its Protease Activity

Plum pox virus (PPV) infects Prunus trees across the globe, causing the serious Sharka disease. Breeding programs in the past 20 years have been successful, generating plum varieties hypersensitive to PPV that show resistance in the field. Recently, a single tree displaying typical PPV symptoms was detected in an orchard of resistant plums. The tree was eradicated, and infected material was propagated under controlled conditions to study the new PPV isolate. Performing overlapping PCR analysis, the viral sequence was reconstructed, cloned and tested for infectivity in different 'Jojo'-based resistant plums. The results confirmed that the isolate, named PPV-D 'Herrenberg' (PPVD-H), was able to infect all these varieties. Analyses of chimeras between PPVD-H and a PPV-D standard isolate (PPVD) revealed that the NIa region of PPD-H, carrying three amino acid changes, was enough to break the resistance of these plums. Experiments with single and double mutants showed that all changes were essential to preserve the escaping phenotype. Additionally, one of the changes at the VPg-NIapro junction suggested the involvement of controlled endopeptidase cleavage in the viral response. Transient expression experiments in Nicotiana benthamiana confirmed that NIa cleavage in PPVD-H was reduced, compared to PPVD, linking the observed behavior to an NIa cleavage modulation.

A Zinc Finger Motif in the P1 N Terminus, Highly Conserved in a Subset of Potyviruses, Is Associated with the Host Range and Fitness of Telosma Mosaic Virus

P1 is the first protein translated from the genomes of most viruses in the family Potyviridae, and it contains a C-terminal serine-protease domain that cis-cleaves the junction between P1 and HCPro in most cases. Intriguingly, P1 is the most divergent among all mature viral factors, and its roles during viral infection are still far from understood. In this study, we found that telosma mosaic virus (TelMV, genus Potyvirus) in passion fruit, unlike TelMV isolates present in other hosts, has two stretches at the P1 N terminus, named N1 and N2, with N1 harboring a Zn finger motif. Further analysis revealed that at least 14 different potyviruses, mostly belonging to the bean common mosaic virus subgroup, encode a domain equivalent to N1. Using the newly developed TelMV infectious cDNA clones from passion fruit, we demonstrated that N1, but not N2, is crucial for viral infection in both Nicotiana benthamiana and passion fruit. The regulatory effects of N1 domain on P1 cis cleavage, as well as the accumulation and RNA silencing suppression (RSS) activity of its cognate HCPro, were comprehensively investigated. We found that N1 deletion decreases HCPro abundance at the posttranslational level, likely by impairing P1 cis cleavage, thus reducing HCPro-mediated RSS activity. Remarkably, disruption of the Zn finger motif in N1 did not impair P1 cis cleavage and HCPro accumulation but severely debilitated TelMV fitness. Therefore, our results suggest that the Zn finger motif in P1s plays a critical role in viral infection that is independent of P1 protease activity and self-release, as well as HCPro accumulation and silencing suppression. IMPORTANCE Viruses belonging to the family Potyviridae represent the largest group of plant-infecting RNA viruses, including a variety of agriculturally and economically important viral pathogens. Like all picorna-like viruses, potyvirids employ polyprotein processing as the gene expression strategy. P1, the first protein translated from most potyvirid genomes, is the most variable viral factor and has attracted great scientific interest. Here, we defined a Zn finger motif-encompassing domain (N1) at the N terminus of P1 among diverse potyviruses phylogenetically related to bean common mosaic virus. Using TelMV as a model virus, we demonstrated that the N1 domain is key for viral infection, as it is involved both in regulating the abundance of its cognate HCPro and in an as-yet-undefined key function unrelated to protease processing and RNA silencing suppression. These results advance our knowledge of the hypervariable potyvirid P1s and highlight the importance for infection of a previously unstudied Zn finger domain at the P1 N terminus.

Development of tobacco rattle virus-based platform for dual heterologous gene expression and CRISPR/Cas reagent delivery

A large number of viral delivery systems have been developed for characterizing functional genes and producing heterologous recombinant proteins in plants, and but most of them are unable to co-express two fusion-free foreign proteins in the whole plant for extended periods of time. In this study, we modified tobacco rattle virus (TRV) as a TRVe dual delivery vector, using the strategy of gene substitution. The reconstructed TRVe had the capability to simultaneously produce two fusion-free foreign proteins at the whole level of Nicotiana benthamiana, and maintained the genetic stability for the insert of double foreign genes. Moreover, TRVe allowed systemic expression of two foreign proteins with the total lengths up to ∼900 aa residues. In addition, Cas12a protein and crRNA were delivered by the TRVe expression system for site-directed editing of genomic DNA in N. benthamiana 16c line constitutively expressing green fluorescent protein (GFP). Taker together, the TRV-based delivery system will be a simple and powerful means to rapidly co-express two non-fused foreign proteins at the whole level and facilitate functional genomics studies in plants.

AlkB RNA demethylase homologues and N6 -methyladenosine are involved in Potyvirus infection

Proteins of the alkylation B (AlkB) superfamily show RNA demethylase activity removing methyl adducts from N6 -methyladenosine (m6 A). m6 A is a reversible epigenetic mark of RNA that regulates human virus replication but has unclear roles in plant virus infection. We focused on Potyvirus-the largest genus of plant RNA viruses-and report here the identification of AlkB domains within P1 of endive necrotic mosaic virus (ENMV) and an additional virus of a putative novel species within Potyvirus. We show that Nicotiana benthamiana m6 A levels are reduced by infection of plum pox virus (PPV) and potato virus Y (PVY). The two potyviruses lack AlkB and the results suggest a general involvement of RNA methylation in potyvirus infection and evolution. Methylated RNA immunoprecipitation sequencing of virus-infected samples showed that m6 A peaks are enriched in plant transcript 3' untranslated regions and in discrete internal and 3' terminal regions of PPV and PVY genomes. Down-regulation of N. benthamiana AlkB homologues of the plant-specific ALKBH9 clade caused a significant decrease in PPV and PVY accumulation. In summary, our study provides evolutionary and experimental evidence that supports the m6 A implication and the proviral roles of AlkB homologues in Potyvirus infection.

AtGSTU19 and AtGSTU24 as Moderators of the Response of Arabidopsis thaliana to Turnip mosaic virus

Plants produce glutathione as a response to the intercellular redox state. Glutathione actively participates in the reactive oxygen species (ROS)-dependent signaling pathway, especially under biotic stress conditions. Most of the glutathione S-transferases (GSTs) are induced in cells during the defense response of plants not only through highly specific glutathione-binding abilities but also by participating in the signaling function. The tau class of GSTs has been reported to be induced as a response under stress conditions. Although several studies have focused on the role of the tau class of GSTs in plant–pathogen interactions, knowledge about their contribution to the response to virus inoculation is still inadequate. Therefore, in this study, the response of Atgstu19 and Atgstu24 knockout mutants to mechanical inoculation of Turnip mosaic virus (TuMV) was examined. The systemic infection of TuMV was more dynamically promoted in Atgstu19 mutants than in wild-type (Col-0) plants, suggesting the role of GSTU19 in TuMV resistance. However, Atgstu24 mutants displayed virus limitation and downregulation of the relative expression of TuMV capsid protein, accompanied rarely by TuMV particles only in vacuoles, and ultrastructural analyses of inoculated leaves revealed the lack of virus cytoplasmic inclusions. These findings indicated that Atgstu24 mutants displayed a resistance-like reaction to TuMV, suggesting that GSTU24 may suppress the plant resistance. In addition, these findings confirmed that GSTU1 and GSTU24 are induced and contribute to the susceptible reaction to TuMV in the Atgstu19–TuMV interaction. However, the upregulation of GSTU19 and GSTU13 highly correlated with virus limitation in the resistance-like reaction in the Atgstu24–TuMV interaction. Furthermore, the highly dynamic upregulation of GST and glutathione reductase (GR) activities resulted in significant induction (between 1 and 14 days post inoculation [dpi]) of the total glutathione pool (GSH + GSSG) in response to TuMV, which was accompanied by the distribution of active glutathione in plant cells. On the contrary, in Atgstu19, which is susceptible to TuMV interaction, upregulation of GST and GR activity only up to 7 dpi symptom development was reported, which resulted in the induction of the total glutathione pool between 1 and 3 dpi. These observations indicated that GSTU19 and GSTU24 are important factors in modulating the response to TuMV in Arabidopsis thaliana. Moreover, it was clear that glutathione is an important component of the regulatory network in resistance and susceptible response of A. thaliana to TuMV. These results help achieve a better understanding of the mechanisms regulating the Arabidopsis–TuMV pathosystem.

Assembly of plant virus agroinfectious clones using biological material or DNA synthesis

Infectious clone technology is universally applied for biological characterization and engineering of viruses. This protocol describes procedures that implement synthetic biology advances for streamlined assembly of virus infectious clones. Here, I detail homology-based cloning using biological material, as well as SynViP assembly using type IIS restriction enzymes and chemically synthesized DNA fragments. The assembled virus clones are based on compact T-DNA binary vectors of the pLX series and are delivered to host plants by Agrobacterium-mediated inoculation. For complete details on the use and execution of this protocol, please refer to Pasin et al. (2017, 2018) and Pasin (2021).

Simultaneously induced mutations in eIF4E genes by CRISPR/Cas9 enhance PVY resistance in tobacco

Tobacco is an important commercial crop and a rich source of alkaloids for pharmaceutical and agricultural applications. However, its yield can be reduced by up to 70% due to virus infections, especially by a potyvirus Potato virus Y (PVY). The replication of PVY relies on host factors, and eukaryotic translation initiation factor 4Es (eIF4Es) have already been identified as recessive resistance genes against potyviruses in many plant species. To investigate the molecular basis of PVY resistance in the widely cultivated allotetraploid tobacco variety K326, we developed a dual guide RNA CRISPR/Cas9 system for combinatorial gene editing of two clades, eIF4E1 (eIF4E1-S and eIF4E1-T) and eIF4E2 (eIF4E2-S and eIF4E2-T) in the eIF4E gene family comprising six members in tobacco. We screened for CRISPR/Cas9-induced mutations by heteroduplex analysis and Sanger sequencing, and monitored PVY^O accumulation in virus challenged regenerated plants by DAS-ELISA both in T0 and T1 generations. We found that all T0 lines carrying targeted mutations in the eIF4E1-S gene displayed enhanced resistance to PVY^O confirming previous reports. More importantly, our combinatorial approach revealed that eIF4E1-S is necessary but not sufficient for complete PVY resistance. Only the quadruple mutants harboring loss-of-function mutations in eIF4E1-S, eIF4E1-T, eIF4E2-S and eIF4E2-T showed heritable high-level resistance to PVY^O in tobacco. Our work highlights the importance of understanding host factor redundancy in virus replication and provides a roadmap to generate virus resistance by combinatorial CRISPR/Cas9-mediated editing in non-model crop plants with complex genomes.

Complete Genomic RNA Sequence of Tuberose Mild Mosaic Virus and Tuberose Mild Mottle Virus Acquired by High-Throughput Sequencing

Tuberose (Polianthes tuberosa) is an ornamental flowering crop of the Amaryllidaceae family. Tuberose mild mosaic virus (TuMMV) and tuberose mild mottle virus (TuMMoV), members of the genus Potyvirus, are ubiquitously distributed in most tuberose growing countries worldwide with low biological incidence. Here, we report the first coding-complete genomic RNA of TuMMV and TuMMoV obtained through high-throughput sequencing (HTS) and further, the presence of both the viruses were confirmed using virus-specific primers in RT-PCR assays. Excluding the poly (A) tail, the coding-complete genomic RNA of TuMMV and TuMMoV was 9485 and 9462 nucleotides (nts) in length, respectively, and contained a single large open reading frame (ORF). Polyprotein encoded by both the viral genomes contained nine putative cleavage sites. BLASTn analysis of TuMMV and TuMMoV genomes showed 72.40–76.80% and 67.95–77% nucleotide sequence similarities, respectively, with the existing potyviral sequences. Phylogenetic analysis based on genome sequences showed that TuMMV and TuMMoV clustered in a distinct clade to other potyviruses. Further studies are required to understand the mechanism of symptom development, distribution, genetic variability, and their possible threat to tuberose production in India.