Differential roles of Hsp70 and Hsp90 in the assembly of the replicase complex of a positive-strand RNA plant virus

Exploration of the truncated cytosolic Hsp70 in plants - unveiling the diverse T1 lineage and the conserved T2 lineage

The 70-kDa heat shock proteins (Hsp70s) are chaperone proteins involved in protein folding processes. Truncated Hsp70 (Hsp70T) refers to the variant lacking a conserved C-terminal motif, which is crucial for co-chaperone interactions or protein retention. Despite their significance, the characteristics of Hsp70Ts in plants remain largely unexplored. In this study, we performed a comprehensive genome-wide analysis of 192 sequenced plant and green algae genomes to investigate the distribution and features of Hsp70Ts. Our findings unveil the widespread occurrence of Hsp70Ts across all four Hsp70 forms, including cytosolic, endoplasmic reticulum, mitochondrial, and chloroplast Hsp70s, with cytosolic Hsp70T being the most prevalent and abundant subtype. Cytosolic Hsp70T is characterized by two distinct lineages, referred to as T1 and T2. Among the investigated plant and green algae species, T1 genes were identified in approximately 60% of cases, showcasing a variable gene count ranging from one to several dozens. In contrast, T2 genes were prevalent across the majority of plant genomes, usually occurring in fewer than five gene copies per species. Sequence analysis highlights that the putative T1 proteins exhibit higher similarity to full-length cytosolic Hsp70s in comparison to T2 proteins. Intriguingly, the T2 lineage demonstrates a higher level of conservation within their protein sequences, whereas the T1 lineage presents a diverse range in the C-terminal and SBDα region, leading to categorization into four distinct subtypes. Furthermore, we have observed that T1-rich species characterized by the possession of 15 or more T1 genes exhibit an expansion of T1 genes into tandem gene clusters. The T1 gene clusters identified within the Laurales order display synteny with clusters found in a species of the Chloranthales order and another species within basal angiosperms, suggesting a conserved evolutionary relationship of T1 gene clusters among these plants. Additionally, T2 genes demonstrate distinct expression patterns in seeds and under heat stress, implying their potential roles in seed development and stress response.

Mode of Action of Heat Shock Protein (HSP) Inhibitors against Viruses through Host HSP and Virus Interactions

Misfolded proteins after stress-induced denaturation can regain their functions through correct re-folding with the aid of molecular chaperones. As a molecular chaperone, heat shock proteins (HSPs) can help client proteins fold correctly. During viral infection, HSPs are involved with replication, movement, assembly, disassembly, subcellular localization, and transport of the virus via the formation of macromolecular protein complexes, such as the viral replicase complex. Recent studies have indicated that HSP inhibitors can inhibit viral replication by interfering with the interaction of the virus with the HSP. In this review, we describe the function and classification of HSPs, the transcriptional mechanism of HSPs promoted by heat shock factors (HSFs), discuss the interaction between HSPs and viruses, and the mode of action of HSP inhibitors at two aspects of inhibiting the expression of HSPs and targeting the HSPs, and elaborate their potential use as antiviral agents.

Identification and Functional Analyses of Host Proteins Interacting with the P3a Protein of Brassica Yellows Virus

Viruses are obligate parasites that only undergo genomic replication in their host organisms. ORF3a, a newly identified non-AUG-initiated ORF encoded by members of the genus Polerovirus, is required for long-distance movement in plants. However, its interactions with host proteins still remain unclear. Here, we used Brassica yellows virus (BrYV)-P3a as bait to screen a plant split-ubiquitin-based membrane yeast two-hybrid (MYTH) cDNA library to explain the functional role of P3a in viral infections. In total, 138 genes with annotations were obtained. Bioinformatics analyses revealed that the genes from carbon fixation in photosynthetic, photosynthesis pathways, and MAPK signaling were affected. Furthermore, Arabidopsis thaliana purine permease 14 (AtPUP14), glucosinolate transporter 1 (AtGTR1), and nitrate transporter 1.7 (AtNRT1.7) were verified to interact with P3a in vivo. P3a and these three interacting proteins mainly co-localized in the cytoplasm. Expression levels of AtPUP14, AtGTR1, and AtNRT1.7 were significantly reduced in response to BrYV during the late stages of viral infection. In addition, we characterized the roles of AtPUP14, AtGTR1, and AtNRT1.7 in BrYV infection in A. thaliana using T-DNA insertion mutants, and the pup14, gtr1, and nrt1.7 mutants influenced BrYV infection to different degrees.

HSP90s are required for hypocotyl elongation during skotomorphogenesis and thermomorphogenesis via the COP1-ELF3-PIF4 pathway in Arabidopsis

Light and temperature are two key environmental signals that share several molecular components that, in turn, regulate plant growth. Darkness and high ambient temperatures promote skoto- and thermomorphogenesis, including stem elongation. Heat shock proteins 90 (HSP90s) facilitate the adaptation of organisms to various adverse environmental stimuli. Here, we showed that HSP90s are required for hypocotyl elongation during both skoto- and thermomorphogenesis. When HSP90s activities are impaired by the knockdown of HSP90s expression or the application of HSP90 inhibitors, the expression levels and protein abundance of PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) markedly decreased. EARLY FLOWERING 3 (ELF3) deficiency was resistant to the inhibition of HSP90s activities. Furthermore, HSP90s interacted with and destabilized ELF3. In the CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) mutant, the changes in endogenous PIF4 and ELF3 protein levels caused by the inhibition of HSP90s activities were abolished. HSP90s enhanced the interaction between COP1 and ELF3, reduced ELF3 functional effects on PIF4 and modulated hypocotyl elongation during skoto- and thermomorphogenesis. Our results indicated that HSP90s participate in light and temperature signalling via the COP1-ELF3-PIF4 module to regulate hypocotyl growth in changing environments.

Discovery of a Closterovirus Infecting Jujube Plants Grown at Aksu Area in Xinjiang of China

Chinese jujube (Ziziphus jujuba Mill.) is a widely grown fruit crop at Aksu in Xinjiang Uygur Autonomous Region of China. Viral disease-like symptoms are common on jujube plants. Here, for the first time, we report a virus tentatively named persimmon ampelovirus jujube isolate (PAmpV-Ju) infecting jujube plants. The virus was identified using high-throughput sequencing from a jujube plant (ID: AKS15) and molecularly related to viruses in the family Closteroviridae. The genomic sequences of two PAmpV-Ju variants named AKS15-20 and AKS15-17 were determined by RT-PCR amplifications. The genome structure of PAmpV-Ju was identical to that of a recently reported persimmon ampelovirus (PAmpV) and consisted of seven open reading frames. The genomes of AKS15-20 and AKS15-17 shared 83.7% nt identity with each other, and the highest nt sequence identity of 79% with two variants of PAmpV. The incidence of PAmpV-Ju on Aksu jujube plants was evaluated by RT-PCR assays. The phylogenetic analysis of amplified partial sequences coding for polymerase, HSP70h, and CP revealed two phylogenetic clades represented by AKS15-20 and AKS15-17. Our study provides important evidence for understanding viruses infecting jujube plants and establishing efficient measures to prevent virus spread.

Regulation of heat shock proteins 70 and their role in plant immunity

Heat shock proteins 70 (HSP70s) are steadily gaining more attention in the field of plant biotic interactions. Though their regulation and activity in plants are much less well characterized than are those of their counterparts in mammals, accumulating evidence indicates that the role of HSP70-mediated defense mechanisms in plant cells is indispensable. In this review, we summarize current knowledge of HSP70 post-translational control in plants. We comment on the phytohormonal regulation of HSP70 expression and protein abundance, and identify a prominent role for cytokinin in HSP70 control. We outline HSP70s' subcellular localizations, chaperone activity, and chaperone-mediated protein degradation. We focus on the role of HSP70s in plant pathogen-associated molecular pattern-triggered immunity and effector-triggered immunity, and discuss the contribution of different HSP70 subfamilies to plant defense against pathogens.

Serratia marcescens-S3 inhibits Potato virus Y by activating ubiquitination of molecular chaperone proteins NbHsc70-2 in Nicotiana benthamiana

The potato virus Y (PVY) is a plant virus that causes massive crop losses globally, especially in Solanaceae crops. A strain of the plant growth-promoting rhizobacterium (PGPR), Serratia marcescens-S3 was found to inhibit PVY replication in Nicotiana benthamiana. However, there have been no in-depth studies demonstrating the underlying mechanism. In the current study, we found that ubiquitination of NbHsc70-2 is an important way for Serratia marcescens-S3 to trigger induced systemic resistance (ISR). After the treatment with S. marcescens-S3, the protein level of NbHsc70-2 reduced significantly. Inhibiting of ubiquitination increased the accumulation of NbHsc70-2 in plants and reduced S. marcescens-S3-mediated resistance to PVY. Furthermore, transgenic engineered Nicotiana benthamiana NbHsc70-2KO and NbHsc70-2USM were constructed using CRISPR-Cas9-mediated NbHsc70-2 knock-out and ubiquitination respectively. S. marcescens-S3 significantly reduced the inhibition of NbHsc70-2 protein accumulation in NbHsc70-2KO and NbHsc70-2USM . The virulence of PVY was stronger in NbHsc70-2USM than the wild-type plants. These results showed that S. marcescens-S3 increases the ubiquitination of NbHsc70-2 to inhibit the recruitment of molecular chaperone NbHsc70-2 to reduce its replication and infection of PVY.

Global Screening and Functional Identification of Major HSPs Involved in PVY Infection in Potato

HSP40 (also known as DnaJ), HSP70, and HSP90 are major heat shock protein (HSP) families that play critical roles in plant growth and development and stress adaption. Recently, several members of the three HSP families were reported to be widely involved in the plant host-virus interactions. However, their global expression profiles and core members recruited by viruses are largely unknown. In this study, a total of 89 StDnaJs were identified from a genome-wide survey, and their classification, phylogenetic relationships, chromosomal locations, and gene duplication events were further analyzed. Together with 20 StHSP70s and 7 StHSP90s previously identified in the potato genome, the global expression patterns of the members in 3 HSP families were investigated in 2 potato cultivars during Potato virus Y (PVY) infection using RNA-seq data. Of them, 16 genes (including 8 StDnaJs, 6 StHSP70s, and 2 StHSP90s) were significantly up- or downregulated. Further analysis using qRT-PCR demonstrated that 7 of the 16 genes (StDnaJ06, StDnaJ17, StDnaJ21, StDnaJ63, StHSP70-6, StHSP70-19, and StHSP90.5) were remarkably upregulated in the potato cultivar 'Eshu 3' after PVY infection, implying their potential roles in the potato-PVY compatible interaction. Subsequent virus-induced gene silencing (VIGS) assays showed that silencing of the homologous genes of StDnaJ17, StDnaJ21, StHSP70-6, and StHSP90.5 in Nicotiana. benthamiana plants dramatically reduced the accumulation of PVY, which indicated the four genes may function as susceptibility factors in PVY infection. This study provides candidate genes for exploring the mechanism of potato-PVY compatible interaction and benefits breeding work aiming to produce new cultivars with the ability to grow healthily under PVY infection.

Transcriptome and Small RNA Profiling of Potato Virus Y Infected Potato Cultivars, Including Systemically Infected Russet Burbank

Potatoes are the world’s most produced non-grain crops and an important food source for billions of people. Potatoes are susceptible to numerous pathogens that reduce yield, including Potato virus Y (PVY). Genetic resistance to PVY is a sustainable way to limit yield and quality losses due to PVY infection. Potato cultivars vary in their susceptibility to PVY and include susceptible varieties such as Russet Burbank, and resistant varieties such as Payette Russet. Although the loci and genes associated with PVY-resistance have been identified, the genes and mechanisms involved in limiting PVY during the development of systemic infections have yet to be fully elucidated. To increase our understanding of PVY infection, potato antiviral responses, and resistance, we utilized RNA sequencing to characterize the transcriptomes of two potato cultivars. Since transcriptional responses associated with the extreme resistance response occur soon after PVY contact, we analyzed the transcriptome and small RNA profile of both the PVY-resistant Payette Russet cultivar and PVY-susceptible Russet Burbank cultivar 24 h post-inoculation. While hundreds of genes, including terpene synthase and protein kinase encoding genes, exhibited increased expression, the majority, including numerous genes involved in plant pathogen interactions, were downregulated. To gain greater understanding of the transcriptional changes that occur during the development of systemic PVY-infection, we analyzed Russet Burbank leaf samples one week and four weeks post-inoculation and identified similarities and differences, including higher expression of genes involved in chloroplast function, photosynthesis, and secondary metabolite production, and lower expression of defense response genes at those time points. Small RNA sequencing identified different populations of 21- and 24-nucleotide RNAs and revealed that the miRNA profiles in PVY-infected Russet Burbank plants were similar to those observed in other PVY-tolerant cultivars and that during systemic infection ~32% of the NLR-type disease resistance genes were targeted by 21-nt small RNAs. Analysis of alternative splicing in PVY-infected potato plants identified splice variants of several hundred genes, including isoforms that were more dominant in PVY-infected plants. The description of the PVYN-Wi-associated transcriptome and small RNA profiles of two potato cultivars described herein adds to the body of knowledge regarding differential outcomes of infection for specific PVY strain and host cultivar pairs, which will help further understanding of the mechanisms governing genetic resistance and/or virus-limiting responses in potato plants.

Heat shock cognate protein 70 is required for rice stripe tenuivirus accumulation and transmission in small brown planthopper

Rice stripe tenuivirus (RSV) is mainly transmitted by the insect vector small brown planthopper (SBPH, Laodelphax striatellus) in a persistent-propagative manner. Virus transmission is dependent on the interplay between viral proteins and vector factors. Pc2, a nonstructural protein of RSV, plays an important role in virus transmission. However, the vector proteins that interact with Pc2 are unknown. In this study, we identified three SBPH proteins that interact with the N-terminal 381 amino acids of Pc2 (Pc2N) by using a yeast two-hybrid system (Y2H). The interaction of Pc2N with heat shock protein cognate 70 (HSC70) was studied further. HSC70 was verified to interact with RSV Pc2N by biomolecular fluorescence complementation and co-immunoprecipitation assays. HSC70 colocalized with RSV Pc2N in both Sf9 cells and the hemocytes of SBPHs. Inhibition of HSC70 expression via RNA interference reduced virus levels in hemolymph and salivary glands of SBPHs and resulted in decreased virus transmission efficiency. These data provide evidence that a vector protein, HSC70, is employed by RSV to facilitate virus accumulation in the hemolymph and thereby promote virus transmission. These findings are important for a better understanding of the interactions between plant viruses and insect vectors.

Effects of the noncoding subgenomic RNA of red clover necrotic mosaic virus in virus infection

In recent years, a new class of viral noncoding subgenomic RNA (ncsgRNA) has been identified. This RNA is generated as a stable degradation product via an exoribonuclease-resistant RNA (xrRNA) structure, which blocks the progression of 5′→3′ exoribonuclease on viral RNAs in infected cells. Here, we assess the effects of the ncsgRNA of red clover necrotic mosaic virus (RCNMV), called SR1f, in infected plants. We demonstrate the following: (i) the absence of SR1f reduces symptoms and decreases viral RNA accumulation in Nicotiana benthamiana and Arabidopsis thaliana plants; (ii) SR1f has an essential function other than suppression of RNA silencing; and (iii) the cytoplasmic exoribonuclease involved in mRNA turnover, XRN4, is not required for SR1f production or virus infection. A comparative transcriptomic analysis in N. benthamiana infected with wild-type RCNMV or an SR1f-deficient mutant RCNMV revealed that wild-type RCNMV infection, which produces SR1f and much higher levels of virus, has a greater and more significant impact on cellular gene expression than the SR1f-deficient mutant. Upregulated pathways include plant hormone signaling, plant-pathogen interaction, MAPK signaling, and several metabolic pathways, while photosynthesis-related genes were downregulated. We compare this to host genes known to participate in infection by other tombusvirids. Viral reads revealed a 10- to 100-fold ratio of positive to negative strand, and the abundance of reads of both strands mapping to the 3′ region of RCNMV RNA1 support the premature transcription termination mechanism of synthesis for the coding sgRNA. These results provide a framework for future studies of the interactions and functions of noncoding RNAs of plant viruses.

IMPORTANCE Knowledge of how RNA viruses manipulate host and viral gene expression is crucial to our understanding of infection and disease. Unlike viral protein-host interactions, little is known about the control of gene expression by viral RNA. Here, we begin to address this question by investigating the noncoding subgenomic RNA (ncsgRNA) of red clover necrotic mosaic virus (RCNMV), called SR1f. Similar exoribonuclease-resistant RNAs of flaviviruses are well studied, but the roles of plant viral ncsgRNAs, and how they arise, are poorly understood. Surprisingly, we find the likely exonuclease candidate, XRN4, is not required to generate SR1f, and we assess the effects of SR1f on virus accumulation and symptom development. Finally, we compare the effects of infection by wild-type RCNMV versus an SR1f-deficient mutant on host gene expression in Nicotiana benthamiana, which reveals that ncsgRNAs such as SR1f are key players in virus-host interactions to facilitate productive infection.

RNA-Protein Interaction Analysis of SARS-CoV-2 5' and 3' Untranslated Regions Reveals a Role of Lysosome-Associated Membrane Protein-2a during Viral Infection

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a positive-strand RNA virus. The viral genome is capped at the 5′ end, followed by an untranslated region (UTR). There is a poly(A) tail at the 3′ end, preceded by a UTR. The self-interaction between the RNA regulatory elements present within the 5′ and 3′ UTRs and their interaction with host/virus-encoded proteins mediate the function of the 5′ and 3′ UTRs. Using an RNA-protein interaction detection (RaPID) assay coupled to liquid chromatography with tandem mass spectrometry, we identified host interaction partners of SARS-CoV-2 5′ and 3′ UTRs and generated an RNA-protein interaction network. By combining these data with the previously known protein-protein interaction data proposed to be involved in virus replication, we generated the RNA-protein-protein interaction (RPPI) network, likely to be essential for controlling SARS-CoV-2 replication. Notably, bioinformatics analysis of the RPPI network revealed the enrichment of factors involved in translation initiation and RNA metabolism. Lysosome-associated membrane protein-2a (Lamp2a), the receptor for chaperone-mediated autophagy, is one of the host proteins that interact with the 5′ UTR. Further studies showed that the Lamp2 level is upregulated in SARS-CoV-2-infected cells and that the absence of the Lamp2a isoform enhanced the viral RNA level whereas its overexpression significantly reduced the viral RNA level. Lamp2a and viral RNA colocalize in the infected cells, and there is an increased autophagic flux in infected cells, although there is no change in the formation of autophagolysosomes. In summary, our study provides a useful resource of SARS-CoV-2 5′ and 3′ UTR binding proteins and reveals the role of Lamp2a protein during SARS-CoV-2 infection.

IMPORTANCE Replication of a positive-strand RNA virus involves an RNA-protein complex consisting of viral genomic RNA, host RNA(s), virus-encoded proteins, and host proteins. Dissecting out individual components of the replication complex will help decode the mechanism of viral replication. 5′ and 3′ UTRs in positive-strand RNA viruses play essential regulatory roles in virus replication. Here, we identified the host proteins that associate with the UTRs of SARS-CoV-2, combined those data with the previously known protein-protein interaction data (expected to be involved in virus replication), and generated the RNA-protein-protein interaction (RPPI) network. Analysis of the RPPI network revealed the enrichment of factors involved in translation initiation and RNA metabolism, which are important for virus replication. Analysis of one of the interaction partners of the 5′-UTR (Lamp2a) demonstrated its role in reducing the viral RNA level in SARS-CoV-2-infected cells. Collectively, our study provides a resource of SARS-CoV-2 UTR-binding proteins and identifies an important role for host Lamp2a protein during viral infection.

Effects of heat treatment on metabolism of tobacco plants infected with Potato virus Y

Many factors affect successful virus propagation and plant defence responses. Heat shock protein (Hsp) expression after heat shock plays an ambiguous role in viral infection. On the one hand, Hsp70 participates in plant defence response; on the other hand, Hsp70 could interact with viral proteins and facilitate virus propagation. Here, we studied metabolic adaptations of Nicotiana tabacum L. subjected to heat shock (42 °C, 2 h) before or after inoculating the plants with Potato virus Y (potyvirus). RT-qPCR and ELISA were used for potyvirus quantification. Hsp70 and Hsp90 isoforms were analysed by Western blotting. Salicylic, quinic and chlorogenic acid content was determined by LC-MS. The activity of Hatch-Slack enzymes (as markers of potyviral infection in tobacco) and glycosidases was assayed. Application of heat shock before or after inoculation showed accelerated potyviral propagation in comparison with only inoculated plants. Plants exposed to heat shock and concurrently inoculated showed higher potyviral content, higher amount of Hsp70, together with late decline of quinic acid content and low chlorogenic acid content. Spread of potyviral infection correlated with enhanced salicylic acid content and activities of enzymes of the Hatch-Slack cycle, α- and β-galactosidase, α-mannosidase, α-glucosidase and β-N-acetylhexosaminidase. Heat shock proteins accelerate potyviral propagation. The lower weight cytosolic and mitochondrial Hsp70 (~50-75 kDa) persist throughout the viral infection. Also, the plant defense response results in increase of salicylic and chlorogenic acids but decrease of quinic acid content.

Changes in Subcellular Localization of Host Proteins Induced by Plant Viruses

Viruses are dependent on host factors at all parts of the infection cycle, such as translation, genome replication, encapsidation, and cell-to-cell and systemic movement. RNA viruses replicate their genome in compartments associated with the endoplasmic reticulum, chloroplasts, and mitochondria or peroxisome membranes. In contrast, DNA viruses replicate in the nucleus. Viral infection causes changes in plant gene expression and in the subcellular localization of some host proteins. These changes may support or inhibit virus accumulation and spread. Here, we review host proteins that change their subcellular localization in the presence of a plant virus. The most frequent change is the movement of host cytoplasmic proteins into the sites of virus replication through interactions with viral proteins, and the protein contributes to essential viral processes. In contrast, only a small number of studies document changes in the subcellular localization of proteins with antiviral activity. Understanding the changes in the subcellular localization of host proteins during plant virus infection provides novel insights into the mechanisms of plant–virus interactions and may help the identification of targets for designing genetic resistance to plant viruses.

Comparative RNA-Seq analysis unfolds a complex regulatory network imparting yellow mosaic disease resistance in mungbean [Vigna radiata (L.) R. Wilczek]

Yellow Mosaic Disease (YMD) in mungbean [Vigna radiata (L.) R. Wilczek] is one of the most damaging diseases in Asia. In the northern part of India, the YMD is caused by Mungbean Yellow Mosaic India Virus (MYMIV), while in southern India this is caused by Mungbean Yellow Mosaic Virus (MYMV). The molecular mechanism of YMD resistance in mungbean remains largely unknown. In this study, RNA-seq analysis was conducted between a resistant (PMR-1) and a susceptible (Pusa Vishal) mungbean genotype under infected and control conditions to understand the regulatory network operating between mungbean-YMV. Overall, 76.8 million raw reads could be generated in different treatment combinations, while mapping rate per library to the reference genome varied from 86.78% to 93.35%. The resistance to MYMIV showed a very complicated gene network, which begins with the production of general PAMPs (pathogen-associated molecular patterns), then activation of various signaling cascades like kinases, jasmonic acid (JA) and brassinosteroid (BR), and finally the expression of specific genes (like PR-proteins, virus resistance and R-gene proteins) leading to resistance response. The function of WRKY, NAC and MYB transcription factors in imparting the resistance against MYMIV could be established. The string analysis also revealed the role of proteins involved in kinase, viral movement and phytoene synthase activity in imparting YMD resistance. A set of novel stress-related EST-SSRs are also identified from the RNA-Seq data which may be used to find the linked genes/QTLs with the YMD resistance. Also, 11 defence-related transcripts could be validated through quantitative real-time PCR analysis. The identified gene networks have led to an insight about the defence mechanism operating against MYMIV infection in mungbean which will be of immense use to manage the YMD resistance in mungbean.

iTRAQ-based protein analysis provides insight into heterologous superinfection exclusion with TMV-43A against CMV in tobacco (Nicotiana benthamiana) plants

Heterologous superinfection exclusion (HSE) is a phenomenon of an initial virus infection which prevents reinfection by a distantly related or unrelated challenger virus strain in the same host. Here, we demonstrate that a mild strain mutant of Tobacco mosaic virus (TMV-43A) can protect Nicotiana benthamiana plants against infection by a challenger Cucumber mosaic virus (CMV)-Fny strain. The isobaric tags for relative and absolute quantification (iTRAQ) technique was used to investigate proteome of N. benthamiana plant during HSE. Our results indicated that in superinfected plants, the PSI and PSII proteins in the photosynthetic pathway increased in abundance, providing sufficient energy to plants for survival. The fatty acid synthesis-related proteins acetyl-CoA carboxylase 1-like and fatty acid synthase were decreased in abundance, affecting the formation of virus replication complex, which in turn reduced CMV replication and lessen hijacking of basic building blocks of RNA transcription and protein synthesis required for normal host functions. This is the first analyses of host proteins that are correlated to HSE between two unrelated plant viruses TMV-43A and CMV in N. benthamiana plants. BIOLOGICAL SIGNIFICANCE: CMV is one of the most studied host-virus interaction models in plants. It infects both monocot and dicot crop plants, causing significant economic losses. Superinfection exclusion (also known as cross protection) is one of the methods to combat virus infection. However, there is lack of proteome information of heterologous superinfection exclusion between two taxonomically unrelated plant viruses (such as between CMV and TMV). An iTRAQ-based quantitative approach was used to study proteomics of superinfection, where TMV-43A acts as a protector of N. benthamiana plants against its challenger CMV. Results showed that TMV-43A protects host plants and prevents plant death from CMV infection. This study provided insights into host responses involving multiple host pathways: photosynthesis, plant defence, carbon metabolism, translation and protein processing, fatty acid metabolism and amino acid biosynthesis. The findings provide a reference database for other viruses and increase our knowledge in host proteins that are correlated to superinfection.

Correlation Analysis of Expression Profile and Quantitative iTRAQ-LC-MS/MS Proteomics Reveals Resistance Mechanism Against TuMV in Chinese Cabbage (Brassica rapa ssp. pekinensis)

The arms race between plants and viruses never ceases. Chinese cabbage, an important type of Brassica vegetable crop, is vulnerable to plant virus infection, especially to Turnip mosaic virus (TuMV). To better examine the molecular mechanisms behind the virus infection, we conducted the correlation analysis of RNA-Seq and quantitative iTRAQ-LC-MS/MS in TuMV-infected and in healthy Chinese cabbage leaves. There were 757 differentially expressed genes and 75 differentially expressed proteins that were screened in Chinese cabbage plants infected with TuMV. These genes were enriched in many pathways, and among them, the plant hormone signal transduction, plant-pathogen interaction, and protein processing in the endoplasmic reticulum pathways were suggested to be closely related pathways. The correlation analysis between RNA-Seq and quantitative iTRAQ-LC-MS/MS was then further explored. Finally, we obtained a preliminary network of several candidate genes associated with TuMV infection, and we found that they mainly belonged to calcium signaling pathways, heat shock proteins, WRKY transcription factors, and non-specific lipid transfer proteins. These results may lead to a better understanding of antiviral mechanisms and of disease-resistant breeding.

Novel combined biological antiviral agents Cytosinpeptidemycin and Chitosan oligosaccharide induced host resistance and changed movement protein subcellular localization of tobacco mosaic virus

Plant viral diseases cause severe economic losses in agricultural production. Development of microorganism-derived antiviral agents provides an alternative strategy to efficiently control plant viral diseases. In this study, the antiviral effect and mechanism of a combined biological agent Cytosinpeptidemycin and Chitosan oligosaccharide (CytPM-COS) were investigated. CytPM-COS effectively inhibited tobacco mosaic virus (TMV) in Nicotiana glutinosa, suppressed viral RNA and CP accumulation in BY-2 protoplast and affected the subcellular localization as well as punctate formation of TMV MP in N. benthamiana leaves. In addition, CytPM-COS triggered reactive oxygen species (ROS) production and induced up-regulation of various defense responsive genes including PR-1, PR-5, FLS2, Hsp70. Our results indicated that CytPM-COS can potentially act as a pesticide for integrated control of plant viruses in the future.

H2O2Accumulation, Host Cell Death and Differential Levels of Proteins Related to Photosynthesis, Redox Homeostasis, and Required for Viral Replication Explain the Resistance of EMS-mutagenized Cowpea to Cowpea Severe Mosaic Virus

Infection with Cowpea severe mosaic virus (CPSMV) represents one of the main limitations for cowpea (Vigna unguiculata L. Walp.) productivity due to the severity of the disease symptoms, frequency of incidence, and difficulties in dissemination control. This study aimed to identify the proteins and metabolic pathways associated with the susceptibility and resistance of cowpea plants to CPSMV. Therefore, we treated the seeds of a naturally susceptible cowpea genotype (CE-31) with the mutagenic agent ethyl methane sulfonate (EMS) and compared the secondary leaf proteomic profile of the mutagenized resistant plants inoculated with CPSMV (MCPI plant group) to those of the naturally susceptible cowpea genotype CE-31 inoculated (CPI) and noninoculated (CPU) with CPSMV. MCPI responded to CPSMV by accumulating proteins involved in the oxidative burst, increasing H₂O₂ generation, promoting leaf cell death (LCD), increasing the synthesis of defense proteins, and decreasing host factors important for the establishment of CPSMV infection. In contrast, CPI accumulated several host factors that favor CPSMV infection and did not accumulate H₂O₂ or present LCD, which allowed CPSMV replication and systemic dissemination. Based on these results, we propose that the differential abundance of defense proteins and proteins involved in the oxidative burst, LCD, and the decrease in cowpea protein factors required for CPSMV replication are associated with the resistance trait acquired by the MCPI plant group.

Hsp70 plays a role in programmed cell death during the remodelling of leaves of the lace plant (Aponogeton madagascariensis)

Lace plant leaves utilize programmed cell death (PCD) to form perforations during development. The role of heat shock proteins (Hsps) in PCD during lace plant leaf development is currently unknown. Hsp70 amounts were measured throughout lace plant leaf development, and the results indicate that it is highest before and during PCD. Increased Hsp70 amounts correlate with raised anthocyanin content and caspase-like protease (CLP) activity. To investigate the effects of Hsp70 on leaf development, whole plants were treated with either of the known regulators of PCD [reactive oxygen species (ROS) or antioxidants] or an Hsp70 inhibitor, chlorophenylethynylsulfonamide (PES-Cl). ROS treatment significantly increased Hsp70 2-fold and CLP activity in early developing leaves, but no change in anthocyanin and the number of perforations formed was observed. Antioxidant treatment significantly decreased Hsp70, anthocyanin, and CLP activity in early leaves, resulting in the fewest perforations. PES-Cl (25 μM) treatment significantly increased Hsp70 4-fold in early leaves, while anthocyanin, superoxide, and CLP activity significantly declined, leading to fewer perforations. Results show that significantly increased (4-fold) or decreased Hsp70 amounts lead to lower anthocyanin and CLP activity, inhibiting PCD induction. Our data support the hypothesis that Hsp70 plays a role in regulating PCD at a threshold in lace plant leaf development. Hsp70 affects anthocyanin content and caspase-like protease activity, and helps regulate PCD during the remodelling of leaves of lace plant, Aponogeton madagascariensis.

The RNA-Dependent RNA Polymerase NIb of Potyviruses Plays Multifunctional, Contrasting Roles during Viral Infection

Potyviruses represent the largest group of known plant RNA viruses and include many agriculturally important viruses, such as Plum pox virus, Soybean mosaic virus, Turnip mosaic virus, and Potato virus Y. Potyviruses adopt polyprotein processing as their genome expression strategy. Among the 11 known viral proteins, the nuclear inclusion protein b (NIb) is the RNA-dependent RNA polymerase responsible for viral genome replication. Beyond its principal role as an RNA replicase, NIb has been shown to play key roles in diverse virus–host interactions. NIb recruits several host proteins into the viral replication complexes (VRCs), which are essential for the formation of functional VRCs for virus multiplication, and interacts with the sumoylation pathway proteins to suppress NPR1-mediated immunity response. On the other hand, NIb serves as a target of selective autophagy as well as an elicitor of effector-triggered immunity, resulting in attenuated virus infection. These contrasting roles of NIb provide an excellent example of the complex co-evolutionary arms race between plant hosts and potyviruses. This review highlights the current knowledge about the multifunctional roles of NIb in potyvirus infection, and discusses future research directions.

Heat Shock Proteins: Dynamic Biomolecules to Counter Plant Biotic and Abiotic Stresses

Due to the present scenario of climate change, plants have to evolve strategies to survive and perform under a plethora of biotic and abiotic stresses, which restrict plant productivity. Maintenance of plant protein functional conformation and preventing non-native proteins from aggregation, which leads to metabolic disruption, are of prime importance. Plant heat shock proteins (HSPs), as chaperones, play a pivotal role in conferring biotic and abiotic stress tolerance. Moreover, HSP also enhances membrane stability and detoxifies the reactive oxygen species (ROS) by positively regulating the antioxidant enzymes system. Additionally, it uses ROS as a signal to molecules to induce HSP production. HSP also enhances plant immunity by the accumulation and stability of pathogenesis-related (PR) proteins under various biotic stresses. Thus, to unravel the entire plant defense system, the role of HSPs are discussed with a special focus on plant response to biotic and abiotic stresses, which will be helpful in the development of stress tolerance in plant crops.

Expression and Localization Patterns of a Small Heat Shock Protein that Interacts with the Helicase Domain of Cucumber Green Mottle Mosaic Virus

Cucumber green mottle mosaic virus (CGMMV), a member of the genus Tobamovirus (family Virgaviridae), is an economically important virus that has detrimental effects on cucurbit crops worldwide. Understanding the interaction between host factors and CGMMV viral proteins will facilitate the design of new strategies for disease control. In this study, a yeast two-hybrid assay revealed that the CGMMV helicase (HEL) domain interacts with a Citrullus lanatus small heat shock protein (sHSP), and we verified this observation by performing in vitro GST pull-down and in vivo coimmunoprecipitation assays. Measurement of the levels of accumulated sHSP transcript revealed that sHSP is upregulated on initial CGMMV infection in both Nicotiana benthamiana and C. lanatus plants, although not in the systemically infected leaves. We also found that the subcellular localization of the sHSP was altered after CGMMV infection. To further validate the role of sHSP in CGMMV infection, we produced and assayed N. benthamiana transgenic plants with up- and down-regulated sHSP expression. Overexpression of sHSP inhibited viral RNA accumulation and retarded disease development, whereas sHSP silencing had no marked effect on CGMMV infection. Therefore, we postulate that the identified sHSP may be one of the factors modulating host defense mechanisms in response to CGMMV infection and that the HEL domain interaction may inhibit this sHSP function to promote viral infection.

In planta proximity-dependent biotin identification (BioID) identifies a TMV replication co-chaperone NbSGT1 in the vicinity of 126 kDa replicase

Tobacco mosaic virus (TMV) is a positive, single-stranded RNA virus. It encodes two replicases (126 kDa and 183 kDa), a movement protein and a coat protein. These proteins interact with host proteins for successful infection. Some host proteins such as eEF1α, Tm-1, TOM1, 14-3-3 proteins directly interact with Tobamovirus replication proteins. There are host proteins in the virus replication complex which do not interact with viral replicases directly, such as pyruvate kinase and glyceraldehyde-3-phosphate dehydrogenase. We have used Proximity-dependent biotin identification (BioID) technique to screen for transient or weak protein interactions of host proteins and viral replicase in vivo. We transiently expressed BirA* tagged TMV 126 kDa replicase in TMV infected Nicotiana benthamiana plants. Among 18 host proteins, we identified NbSGT1 as a potential target for further characterization. Silencing of NbSGT1 in N. benthamiana plants increased its susceptibility to TMV infection, and overexpression of NbSGT1 increased resistance to TMV infection. There were weak interactions between NbSGT1 and TMV replicases but no interaction between them was found in Y2H assay. It suggests that the interaction might be transient or indirect. Therefore, the BioID technique is a valuable method to identify weak or transient/indirect interaction(s) between pathogen proteins and host proteins in plants. BIOLOGICAL SIGNIFICANCE: TMV is a well characterized positive-strand RNA virus model for study of virus-plant host interactions. It infects >350 plant species and is one of the significant pathogens of crop loss globally. Many host proteins are involved in TMV replication complex formation. To date there are few techniques available for identifying interacting host proteins to viral proteins. There is limited knowledge on transient or non-interacting host proteins during virus infection/replication. In this study, we used agroinfiltration-mediated in planta BioID technique to identify transiently or non-interacting host proteins to viral proteins in TMV-infected N. benthamiana plants. This technique allowed us to identify potential candidate proteins in the vicinity of TMV 126 kDa replicase. We have selected NbSGT1 and its overexpression suppresses TMV replication and increase plant resistance. NbSGT1 is believed to interact transiently or indirectly with TMV replicases in the presence of Hsp90/Hsp70. BioID is a novel and powerful technique to identify transiently or indirectly interacting proteins in the study of pathogen-host interactions.

Key checkpoints in the movement of plant viruses through the host

Plant viruses cannot exploit any of the membrane fusion-based routes of entry described for animal viruses. In addition, one of the distinctive structures of plant cells, the cell wall, acts as the first barrier against the invasion of pathogens. To overcome the rigidity of the cell wall, plant viruses normally take advantage of the way of life of different biological vectors. Alternatively, the physical damage caused by environmental stresses can facilitate virus entry. Once inside the cell and taking advantage of the characteristic symplastic continuity of plant cells, viruses need to remodel and/or modify the restricted pore size of the plasmodesmata (channels that connect plant cells). In a successful interaction for the virus, it can reach the vascular tissue to systematically invade the plant. The connections between the different cell types in this path are not designed to allow the passage of molecules with the complexity of viruses. During this process, viruses face different cell barriers that must be overcome to reach the distal parts of the plant. In this review, we highlight the current knowledge about how plant RNA viruses enter plant cells, move between them to reach vascular cells and overcome the different physical and cellular barriers that the phloem imposes. Finally, we update the current research on cellular organelles as key regulator checkpoints in the long-distance movement of plant viruses.

Comparative proteomics of Tobacco mosaic virus-infected Nicotiana tabacum plants identified major host proteins involved in photosystems and plant defence

Tobacco mosaic virus (TMV) is a positive single-stranded RNA virus. Its 5' end ORF codes for the replicase proteins, namely 126 kDa and 183 kDa, respectively. These proteins interact with many host proteins to form a virus replication complex (VRC). This study aims to dissect the proteome profile of TMV-infected Nicotiana tabacum in host cellular and molecular pathways. We used the isobaric tags for relative and absolute quantification (iTRAQ) technique to analyse the differential global proteomic profile of TMV infected and mock infected plants. Out of 1897 total proteins, we identified 407 differentially abundant proteins and grouped them into three functional categories, namely metabolism, cellular processes and signalling processing. Our results showed that photosynthesis, carbon metabolism, plant defence, protein synthesis, and protein processing in the endoplasmic reticulum were significantly altered. Carbon metabolism and photosynthesis were present in very low abundance, whereas accumulation of reactive oxygen species and misfolded proteins lead to the accumulation of thioredoxin H-type 1. In conclusion, we identified several key host proteins that are involved in TMV infection/replication in N. tabacum plants. SIGNIFICANCE OF THE STUDY: TMV is one of the most widely studied plant virus. It is used as a tool to study host-virus interaction. There are several host proteins reported that facilitate VRC formation and replication of TMV. However, there is limited knowledge in the expression regulation of these host proteins upon TMV infection. This study is the first report that investigates the response of host protein expression involved in TMV infection through a quantitative proteomics technique iTRAQ, combined with LC-MS/MS analysis. We used TMV-infected Nicotiana tabacum plants to investigate the effects of TMV infection on host proteins. Our results revealed differential abundance of proteins involving various pathways in protein translation, protein processing, photosynthesis and plant defence. There was a high abundance of thioredoxin H-type 1, a protein that counters oxidative stress and accelerated regulation of fatty acid synthesis to provide additional lipid molecules for VRC formation. There was a significant reduction in abundance of psaA and psbB proteins in the photosynthetic pathways. Our results identified key candidate host proteins involved in TMV-infected N. tabacum for functional studies in future.

Manipulating Cellular Factors to Combat Viruses: A Case Study From the Plant Eukaryotic Translation Initiation Factors eIF4

Genes conferring resistance to plant viruses fall in two categories; the dominant genes that mostly code for proteins with a nucleotide binding site and leucine rich repeats (NBS-LRR), and that directly or indirectly, recognize viral avirulence factors (Avr), and the recessive genes. The latter provide a so-called recessive resistance. They represent roughly half of the known resistance genes and are alleles of genes that play an important role in the virus life cycle. Conversely, all cellular genes critical for the viral infection virtually represent recessive resistance genes. Based on the well-documented case of recessive resistance mediated by eukaryotic translation initiation factors of the 4E/4G family, this review is intended to summarize the possible approaches to control viruses via their host interactors. Classically, resistant crops have been developed through introgression of natural variants of the susceptibility factor from compatible relatives or by random mutagenesis and screening. Transgenic methods have also been applied to engineer improved crops by overexpressing the translation factor either in its natural form or after directed mutagenesis. More recently, innovative approaches like silencing or genome editing have proven their great potential in model and crop plants. The advantages and limits of these different strategies are discussed. This example illustrates the need to identify and characterize more host factors involved in virus multiplication and to assess their application potential in the control of viral diseases.

Hijacking a host scaffold protein, RACK1, for replication of a plant RNA virus

Receptor for activated C kinase 1 (RACK1) is strictly conserved across eukaryotes and acts as a versatile scaffold protein involved in various signaling pathways. Plant RACK1 is known to exert important functions in innate immunity against fungal and bacterial pathogens. However, the role of the RACK1 in plant-virus interactions remains unknown. Here, we addressed the role of RACK1 of Nicotiana benthamiana during infection by red clover necrotic mosaic virus (RCNMV), a plant positive-stranded RNA virus. NbRACK1 was shown to be recruited by the p27 viral replication protein into endoplasmic reticulum-derived aggregated structures (possible replication sites). Downregulation of NbRACK1 by virus-induced gene silencing inhibited viral cap-independent translation and p27-mediated reactive oxygen species (ROS) accumulation, which are prerequisite for RCNMV replication. We also found that NbRACK1 interacted with a host calcium-dependent protein kinase (NbCDPKiso2) that activated a ROS-generating enzyme. Interestingly, NbRACK1 was required for the interaction of p27 with NbCDPKiso2, suggesting that NbRACK1 acts as a bridge between the p27 viral replication protein and NbCDPKiso2. Collectively, our findings provide an example of a viral strategy in which a host multifaceted scaffold protein RACK1 is highjacked for promoting viral protein-triggered ROS production necessary for robust viral replication.

Killing two birds with one stone: How do Plant Viruses Break Down Plant Defenses and Manipulate Cellular Processes to Replicate Themselves?

As simple organisms with a parasite nature, viruses have become masters in manipulating and subvert cellular components, including host proteins and organelles, to improve viral replication. Therefore, the understanding of viral strategies to manipulate cell function disrupting plant defenses and enhancing viral infection cycles is fundamental to the production of virus-resistant plant lines. After invading susceptible plants, viruses create conditions that favor local and systemic infections by suppressing multiple layers of innate host defenses while use cellular machinery to own benefit. Viral interference in interlinked essential cellular functions results in phenotypic changes and disease symptoms, which debilitates plants favoring infection establishment. Herein in this review, the novelty it will be the discussion about the strategies used by (+) single strand RNA viruses to affect cellular processes and components to improve viral replication, in parallel to overcome plant defenses, favoring disease establishment by applying in one action using the same viral protein to coordinate viral replication and breaking down plant defense. This focus on plant-virus interaction was never done before, and this knowledge has the potential to help in the development of new strategies to produce resistant plants.

A resistant cowpea (Vigna unguiculata [L.] Walp.) genotype became susceptible to cowpea severe mosaic virus (CPSMV) after exposure to salt stress

In nature, plants are simultaneously challenged by biotic and abiotic stresses. However, little is known about the effects of these combined stresses for most crops. This work aimed to evaluate the responsed of the virus-resistant cowpea genotype BRS-Marataoã to the exposure of salt stress combined with CPSMV infection. Cowpea plants were exposed to 200 mM NaCl either simultaneously (SV plant group) or 24 h prior to the CPSMV infection [S(24 h)V plant group]. Physiological, biochemical, and proteomic analyses at 2 and 6 days post salt stress (DPS) revealed that cowpea significantly reprogrammed its cellular metabolism. Indeed, plant size, photosynthetic parameters (net photosynthesis, transpiration rate, stomatal conductance, and internal CO₂ partial pressure) and chlorophyll and carotenoid contents were reduced in S(24 h)V compared to SV. Moreover, accumulation of viral particles at 6 DPS in S(24 h)V was observed indicating that the salt stress imposed prior to virus infection favors viral particle proliferation. Proteomic analysis showed differential contents of 403 and 330 proteins at 2 DPS and 6 DPS, respectively, out of 733 differentially abundant proteins between the two plant groups. The altered leaf proteins are involved in energy and metabolism, photosynthesis, stress response, and oxidative burst. BIOLOGICAL SIGNIFICANCE: This is an original study in which a virus-resistant cowpea genotype (BRS-Marataoã) was (i) exposed simultaneously to 200 mM NaCl and inoculation with CPSMV (SV plant group) or (ii) exposed to 200 mM NaCl stress 24 h prior to inoculation with CPSMV [S(24 h)V plant group]. The purpose was to shed light on how this CPSMV resistant cowpea responded to the combined stresses. Numerous key proteins and associated pathways were altered in the cowpea plants challenged with both stresses, but unexpectedly, the salt stress imposed 24 h prior to CPSMV inoculation allowed viral proliferation, turning the cowpea genotype from resistant to susceptible.

Molecular Co-Chaperone SGT1 Is Critical for Cell-to-Cell Movement and Systemic Infection of Tomato Spotted Wild Virus in Nicotiana benthamiana

Tospovirus is a tripartite negative stranded RNA virus and is considered as one of the most devastating plant viruses. Successful virus infection in plant requires many host factors. To date, very few host factors have been identified as important in Tospovirus infection in plants. We reported earlier that NSm protein encoded by Tomato spotted wilt virus (TSWV), a type species of the genus Orthotospovirus, plays critical roles in viral cell-to-cell and long-distance movement. In this study, we determined that molecular co-chaperone NbSGT1 interacted with TSWV NSm in Nicotianabenthamiana. TSWV infection significantly upregulated the expression of NbSGT1 gene and transient overexpression of NbSGT1 in N.benthamiana leaves accelerated TSWV infection. In contrast, silencing the NbSGT1 gene expression using a virus-induced gene silencing (VIGS) approach strongly inhibited TSWV NSm cell-to-cell movement, as well as TSWV local and systemic infection in N.benthamiana plants. Furthermore, NbSGT1 was found to regulate the infection of both American and Euro/Asia type tospoviruses in N.benthamiana plant. Collectively, our findings presented in this paper and the results published previously indicated that molecular co-chaperone NbSGT1 plays important roles in modulating both positive stranded and tripartite negative stranded RNA virus infection in plants.

Susceptibility Genes to Plant Viruses

Plant viruses use cellular factors and resources to replicate and move. Plants respond to viral infection by several mechanisms, including innate immunity, autophagy, and gene silencing, that viruses must evade or suppress. Thus, the establishment of infection is genetically determined by the availability of host factors necessary for virus replication and movement and by the balance between plant defense and viral suppression of defense responses. Host factors may have antiviral or proviral activities. Proviral factors condition susceptibility to viruses by participating in processes essential to the virus. Here, we review current advances in the identification and characterization of host factors that condition susceptibility to plant viruses. Host factors with proviral activity have been identified for all parts of the virus infection cycle: viral RNA translation, viral replication complex formation, accumulation or activity of virus replication proteins, virus movement, and virion assembly. These factors could be targets of gene editing to engineer resistance to plant viruses.

Hsc70-2 is required for Beet black scorch virus infection through interaction with replication and capsid proteins

Dissecting the complex molecular interplay between the host plant and invading virus improves our understanding of the mechanisms underlying viral pathogenesis. In this study, immunoprecipitation together with the mass spectrometry analysis revealed that the heat shock protein 70 (Hsp70) family homolog, Hsc70-2, was co-purified with beet black scorch virus (BBSV) replication protein p23 and coat protein (CP), respectively. Further experiments demonstrated that Hsc70-2 interacts directly with both p23 and CP, whereas there is no interaction between p23 and CP. Hsc70-2 expression is induced slightly during BBSV infection of Nicotiana benthamiana, and overexpression of Hsc70-2 promotes BBSV accumulation, while knockdown of Hsc70-2 in N. benthamiana leads to drastic reduction of BBSV accumulation. Infection experiments revealed that CP negatively regulates BBSV replication, which can be mitigated by overexpression of Hsc70-2. Further experiments indicate that CP impairs the interaction between Hsc70-2 and p23 in a dose-dependent manner. Altogether, we provide evidence that besides specific functions of Hsp70 family proteins in certain aspects of viral infection, they can serve as a mediator for the orchestration of virus infection by interacting with different viral components. Our results provide new insight into the role of Hsp70 family proteins in virus infection.

The Stoichiometric Interaction of the Hsp90-Sgt1-Rar1 Complex by CD and SRCD Spectroscopy

While the molecular details by which Hsp90 interacts with Sgt1 and Rar1 were previously described the exact stoichiometric complex that is formed remains elusive. Several possibilities remain that include two asymmetric complexes, Sgt1₂-Hsp90₂-Rar1₂ (two molecules of Sgt1 and Rar1 and one Hsp90 dimer) or Sgt1₂-Hsp90₂-Rar1₁ (with a single Rar1 molecule) and an asymmetric complex (Sgt1₁-Hsp90₂-Rar1₁). The Hsp90-mediated activation of NLR receptors (Nucleotide-binding domain and Leucine-rich Repeat) in the innate immunity of both plants and animals is dependent on the co-chaperone Sgt1 and in plants on Rar1, a cysteine- and histidine-rich domain (CHORD)-containing protein. The exact stoichiometry of such a complex may have a direct impact on NLR protein oligomerization and thus ultimately on the mechanism by which NLRs are activated. CD spectroscopy was successfully used to determine the stoichiometry of a ternary protein complex among Hsp90, Sgt1, and Rar1 in the presence of excess ADP. The results indicated that a symmetric Sgt1₂-Hsp90₂-Rar1₁ complex was formed that could allow two NLR molecules to simultaneously bind. The stoichiometry of this complex has implications on, and might promote, the dimerization of NLR proteins following their activation.

Dual function of a cis-acting RNA element that acts as a replication enhancer and a translation repressor in a plant positive-stranded RNA virus

The genome of red clover necrotic mosaic virus is divided into two positive-stranded RNA molecules of RNA1 and RNA2, which have no 5' cap structure and no 3' poly(A) tail. Previously, we showed that any mutations in the cis-acting RNA replication elements of RNA2 abolished its cap-independent translational activity, suggesting a strong link between RNA replication and translation. Here, we investigated the functions of the 5' untranslated region (UTR) of RNA2 and revealed that the basal stem-structure (5'BS) predicted in the 5' UTR is essential for robust RNA replication. Interestingly, RNA2 mutants with substitution or deletion in the right side of the 5'BS showed strong translational activity, despite their impaired replication competency. Furthermore, nucleotide sequences other than the 5'BS of the 5' UTR were essential to facilitate the replication-associated translation. Overall, these cis-acting RNA elements seem to coordinately regulate the balance between RNA replication and replication-associated translation.

Chloroplast Hsp70 Isoform Is Required for Age-Dependent Tissue Preference of Bamboo mosaic virus in Mature Nicotiana benthamiana Leaves

Plant viruses may exhibit age-dependent tissue preference in their hosts but the underlying mechanisms are not well understood. In this study, we provide several lines of evidence to reveal the determining role of a protein of the Nicotiana benthamiana chloroplast Hsp70 (NbcpHsp70) family, NbcpHsp70-2, involved in the preference of Bamboo mosaic virus (BaMV) to infect older tissues. NbcpHsp70 family proteins were identified in complexes pulled down with BaMV replicase as the bait. Among the isoforms of NbcpHsp70, only the specific silencing of NbcpHsp70-2 resulted in the significant decrease of BaMV RNA in N. benthamiana protopalsts, indicating that NbcpHsp70-2 is involved in the efficient replication of BaMV RNA. We further identified the age-dependent import regulation signal contained in the transit peptide of NbcpHsp70-2. Deletion, overexpression, and substitution experiments revealed that the signal in the transit peptide of NbcpHsp70-2 is crucial for both the import of NbcpHsp70-2 into older chloroplasts and the preference of BaMV for infecting older leaves of N. benthamiana. Together, these data demonstrated that BaMV may exploit a cellular age-dependent transportation mechanism to target a suitable environment for viral replication.

Roles of superoxide anion and hydrogen peroxide during replication of two unrelated plant RNA viruses in Nicotiana benthamiana

Reactive oxygen species (ROS), including superoxide anion (O2-), hydrogen peroxide (H2O2), and hydroxyl radical, act as signaling molecules to transduce biotic and abiotic stimuli into stress adaptations in plants. A respiratory burst oxidase homolog B of Nicotiana benthamiana (NbRBOHB) is responsible for O2- production to inhibit pathogen infection during plant innate immunity. RBOH-derived O2- can be immediately converted into H2O2 by the action of superoxide dismutase. Interestingly, we recently showed that red clover necrotic mosaic virus (RCNMV), a plant positive-strand RNA [(+)RNA] virus, hijacks the host's ROS-generating machinery during infection. An RCNMV replication protein associates with NbRBOHB and triggers intracellular ROS bursts. These bursts are required for robust viral RNA replication. However, what types of ROS are required for viral replication is currently unknown. Here, we found that RCNMV replication was sensitive to an O2- scavenger but insensitive to an H2O2 scavenger. Interestingly, replication of another plant (+)RNA virus, brome mosaic virus, was sensitive to both types of scavengers. These results indicate a virus-specific pattern requirement of O2- and H2O2 for (+)RNA virus replication and suggest a conserved nature of the roles of ROS in (+)RNA virus replication.

The Involvement of Heat Shock Proteins in the Establishment of Tomato Yellow Leaf Curl Virus Infection

Tomato yellow leaf curl virus (TYLCV), a begomovirus, induces protein aggregation in infected tomatoes and in its whitefly vector Bemisia tabaci. The interactions between TYLCV and HSP70 and HSP90 in plants and vectors are necessity for virus infection to proceed. In infected host cells, HSP70 and HSP90 are redistributed from a soluble to an aggregated state. These aggregates contain, together with viral DNA/proteins and virions, HSPs and components of the protein quality control system such as ubiquitin, 26S proteasome subunits, and the autophagy protein ATG8. TYLCV CP can form complexes with HSPs in tomato and whitefly. Nonetheless, HSP70 and HSP90 play different roles in the viral cell cycle in the plant host. In the infected host cell, HSP70, but not HSP90, participates in the translocation of CP from the cytoplasm into the nucleus. Viral amounts decrease when HSP70 is inhibited, but increase when HSP90 is downregulated. In the whitefly vector, HSP70 impairs the circulative transmission of TYLCV; its inhibition increases transmission. Hence, the efficiency of virus acquisition by whiteflies depends on the functionality of both plant chaperones and their cross-talk with other protein mechanisms controlling virus-induced aggregation.

An Atypical Thioredoxin Imparts Early Resistance to Sugarcane Mosaic Virus in Maize

Sugarcane mosaic virus (SCMV) causes substantial losses of grain yield and forage biomass in susceptible maize worldwide. A major quantitative trait locus, Scmv1, has been identified to impart strong resistance to SCMV at the early infection stage. Here, we demonstrate that ZmTrxh, encoding an atypical h-type thioredoxin, is the causal gene at Scmv1, and that its transcript abundance correlated strongly with maize resistance to SCMV. ZmTrxh alleles, whether they are resistant or susceptible, share the identical coding/proximal promoter regions, but vary in the upstream regulatory regions. ZmTrxh lacks two canonical cysteines in the thioredoxin active-site motif and exists uniquely in the maize genome. Because of this, ZmTrxh is unable to reduce disulfide bridges but possesses a strong molecular chaperone-like activity. ZmTrxh is dispersed in maize cytoplasm to suppress SCMV viral RNA accumulation. Moreover, ZmTrxh-mediated maize resistance to SCMV showed no obvious correlation with the salicylic acid- and jasmonic acid-related defense signaling pathways. Taken together, our results indicate that ZmTrxh exhibits a distinct defense profile in maize resistance to SCMV, differing from previously characterized dominant or recessive potyvirus resistance genes.

Harnessing host ROS-generating machinery for the robust genome replication of a plant RNA virus

As sessile organisms, plants have to accommodate to rapid changes in their surrounding environment. Reactive oxygen species (ROS) act as signaling molecules to transduce biotic and abiotic stimuli into plant stress adaptations. It is established that a respiratory burst oxidase homolog B of Nicotiana benthamiana (NbRBOHB) produces ROS in response to microbe-associated molecular patterns to inhibit pathogen infection. Plant viruses are also known as causative agents of ROS induction in infected plants; however, the function of ROS in plant-virus interactions remains obscure. Here, we show that the replication of red clover necrotic mosaic virus (RCNMV), a plant positive-strand RNA [(+)RNA] virus, requires NbRBOHB-mediated ROS production. The RCNMV replication protein p27 plays a pivotal role in this process, redirecting the subcellular localization of NbRBOHB and a subgroup II calcium-dependent protein kinase of N. benthamiana (NbCDPKiso2) from the plasma membrane to the p27-containing intracellular aggregate structures. p27 also induces an intracellular ROS burst in an RBOH-dependent manner. NbCDPKiso2 was shown to be an activator of the p27-triggered ROS accumulations and to be required for RCNMV replication. Importantly, this RBOH-derived ROS is essential for robust viral RNA replication. The need for RBOH-derived ROS was demonstrated for the replication of another (+)RNA virus, brome mosaic virus, suggesting that this characteristic is true for plant (+)RNA viruses. Collectively, our findings revealed a hitherto unknown viral strategy whereby the host ROS-generating machinery is diverted for robust viral RNA replication.

Recessive Resistance to Plant Viruses: Potential Resistance Genes Beyond Translation Initiation Factors

The ability of plant viruses to propagate their genomes in host cells depends on many host factors. In the absence of an agrochemical that specifically targets plant viral infection cycles, one of the most effective methods for controlling viral diseases in plants is taking advantage of the host plant’s resistance machinery. Recessive resistance is conferred by a recessive gene mutation that encodes a host factor critical for viral infection. It is a branch of the resistance machinery and, as an inherited characteristic, is very durable. Moreover, recessive resistance may be acquired by a deficiency in a negative regulator of plant defense responses, possibly due to the autoactivation of defense signaling. Eukaryotic translation initiation factor (eIF) 4E and eIF4G and their isoforms are the most widely exploited recessive resistance genes in several crop species, and they are effective against a subset of viral species. However, the establishment of efficient, recessive resistance-type antiviral control strategies against a wider range of plant viral diseases requires genetic resources other than eIF4Es. In this review, we focus on recent advances related to antiviral recessive resistance genes evaluated in model plants and several crop species. We also address the roles of next-generation sequencing and genome editing technologies in improving plant genetic resources for recessive resistance-based antiviral breeding in various crop species.

EXA1, a GYF domain protein, is responsible for loss-of-susceptibility to plantago asiatica mosaic virus in Arabidopsis thaliana

One of the plant host resistance machineries to viruses is attributed to recessive alleles of genes encoding critical host factors for virus infection. This type of resistance, also referred to as recessive resistance, is useful for revealing plant-virus interactions and for breeding antivirus resistance in crop plants. Therefore, it is important to identify a novel host factor responsible for robust recessive resistance to plant viruses. Here, we identified a mutant from an ethylmethane sulfonate (EMS)-mutagenized Arabidopsis population which confers resistance to plantago asiatica mosaic virus (PlAMV, genus Potexvirus). Based on map-based cloning and single nucleotide polymorphism analysis, we identified a premature termination codon in a functionally unknown gene containing a GYF domain, which binds to proline-rich sequences in eukaryotes. Complementation analyses and robust resistance to PlAMV in a T-DNA mutant demonstrated that this gene, named Essential for poteXvirus Accumulation 1 (EXA1), is indispensable for PlAMV infection. EXA1 contains a GYF domain and a conserved motif for interaction with eukaryotic translation initiation factor 4E (eIF4E), and is highly conserved among monocot and dicot species. Analysis using qRT-PCR and immunoblotting revealed that EXA1 was expressed in all tissues, and was not transcriptionally responsive to PlAMV infection in Arabidopsis plants. Moreover, accumulation of PlAMV and a PlAMV-derived replicon was drastically diminished in the initially infected cells by the EXA1 deficiency. Accumulation of two other potexviruses also decreased in exa1-1 mutant plants. Our results provided a functional annotation to GYF domain-containing proteins by revealing the function of the highly conserved EXA1 gene in plant-virus interactions.

Tombusvirus-Host Interactions: Co-Opted Evolutionarily Conserved Host Factors Take Center Court

Plant positive-strand (+)RNA viruses are intracellular infectious agents that reorganize subcellular membranes and rewire the cellular metabolism of host cells to achieve viral replication in elaborate replication compartments. This review describes the viral replication process based on tombusviruses, highlighting common strategies with other plant and animal viruses. Overall, the works on Tomato bushy stunt virus (TBSV) have revealed intriguing and complex functions of co-opted cellular translation factors, heat shock proteins, DEAD-box helicases, lipid transfer proteins, and membrane-deforming proteins in virus replication. The emerging picture is that many of the co-opted host factors are from highly expressed and conserved protein families. By hijacking host proteins, phospholipids, sterols, and the actin network, TBSV exerts supremacy over the host cell to support viral replication in large replication compartments. Altogether, these advances in our understanding of tombusvirus-host interactions are broadly applicable to many other viruses, which also usurp conserved host factors for various viral processes.

Replication of Tobamovirus RNA

Tobacco mosaic virus and other tobamoviruses have served as models for studying the mechanisms of viral RNA replication. In tobamoviruses, genomic RNA replication occurs via several steps: (a) synthesis of viral replication proteins by translation of the genomic RNA; (b) translation-coupled binding of the replication proteins to a 5'-terminal region of the genomic RNA; (c) recruitment of the genomic RNA by replication proteins onto membranes and formation of a complex with host proteins TOM1 and ARL8; (d) synthesis of complementary (negative-strand) RNA in the complex; and (e) synthesis of progeny genomic RNA. This article reviews current knowledge on tobamovirus RNA replication, particularly regarding how the genomic RNA is specifically selected as a replication template and how the replication proteins are activated. We also focus on the roles of the replication proteins in evading or suppressing host defense systems.

Activation of senescence-associated Dark-inducible (DIN) genes during infection contributes to enhanced susceptibility to plant viruses

Virus infections in plants cause changes in host gene expression that are common to other environmental stresses. In this work, we found extensive overlap in the transcriptional responses between Arabidopsis thaliana plants infected with Tobacco rattle virus (TRV) and plants undergoing senescence. This is exemplified by the up-regulation during infection of several senescence-associated Dark-inducible (DIN) genes, including AtDIN1 (Senescence 1, SEN1), AtDIN6 (Asparagine synthetase 1, AtASN1) and AtDIN11. DIN1, DIN6 and DIN11 homologues were also activated in Nicotiana benthamiana in response to TRV and Potato virus X (PVX) infection. Reduced TRV levels in RNA interference (RNAi) lines targeting AtDIN11 indicate that DIN11 is an important modulator of susceptibility to TRV in Arabidopsis. Furthermore, low accumulation of TRV in Arabidopsis protoplasts from RNAi lines suggests that AtDIN11 supports virus multiplication in this species. The effect of DIN6 on virus accumulation was negligible in Arabidopsis, perhaps as a result of gene or functional redundancy. However, TRV-induced silencing of NbASN, the DIN6 homologue in N. benthamiana, compromises TRV and PVX accumulation in systemically infected leaves. Interestingly, NbASN inactivation correlates with the appearance of morphological defects in infected leaves. We found that DIN6 and DIN11 regulate virus multiplication in a step prior to the activation of plant defence responses. We hypothesize on the possible roles of DIN6 and DIN11 during virus infection.

Cucumber Necrosis Virus Recruits Cellular Heat Shock Protein 70 Homologs at Several Stages of Infection

RNA viruses often depend on host factors for multiplication inside cells due to the constraints of their small genome size and limited coding capacity. One such factor that has been exploited by several plant and animal viruses is heat shock protein 70 (HSP70) family homologs which have been shown to play roles for different viruses in viral RNA replication, viral assembly, disassembly, and cell-to-cell movement. Using next generation sequence analysis, we reveal that several isoforms of Hsp70 and Hsc70 transcripts are induced to very high levels during cucumber necrosis virus (CNV) infection of Nicotiana benthamiana and that HSP70 proteins are also induced by at least 10-fold. We show that HSP70 family protein homologs are co-opted by CNV at several stages of infection. We have found that overexpression of Hsp70 or Hsc70 leads to enhanced CNV genomic RNA, coat protein (CP), and virion accumulation, whereas downregulation leads to a corresponding decrease. Hsc70-2 was found to increase solubility of CNV CP in vitro and to increase accumulation of CNV CP independently of viral RNA replication during coagroinfiltration in N. benthamiana. In addition, virus particle assembly into virus-like particles in CP agroinfiltrated plants was increased in the presence of Hsc70-2. HSP70 was found to increase the targeting of CNV CP to chloroplasts during infection, reinforcing the role of HSP70 in chloroplast targeting of host proteins. Hence, our findings have led to the discovery of a highly induced host factor that has been co-opted to play multiple roles during several stages of the CNV infection cycle.

IMPORTANCE

Because of the small size of its RNA genome, CNV is dependent on interaction with host cellular components to successfully complete its multiplication cycle. We have found that CNV induces HSP70 family homologs to a high level during infection, possibly as a result of the host response to the high levels of CNV proteins that accumulate during infection. Moreover, we have found that CNV co-opts HSP70 family homologs to facilitate several aspects of the infection process such as viral RNA, coat protein and virus accumulation. Chloroplast targeting of the CNV CP is also facilitated, which may aid in CNV suppression of host defense responses. Several viruses have been shown to induce HSP70 during infection and others to utilize HSP70 for specific aspects of infection such as replication, assembly, and disassembly. We speculate that HSP70 may play multiple roles in the infection processes of many viruses.