Selective modification of rice (Oryza sativa) gene expression by rice stripe virus infection

Transcriptome analysis of genes involved in the pathogenesis mechanism of potato virus Y in potato cultivar YouJin

Introduction

Potatoes (Solanum tuberosum L.) can be infected by various viruses, but out of all of viruses, the potato virus Y (PVY) is the most detrimental. Research shows that the potato cultivar YouJin is especially vulnerable to PVY and displays severe symptoms, including leaf vein chlorosis, curled leaf margins, large necrotic spots on the leaf blades, and the growth of small new leaves.

Methods

PVY infection in potato cultivar YouJin was confirmed through symptom observation, RT-PCR, and Western blot analysis. Transcriptome sequencing was used to analyze the genes associated with PVY pathogenesis in this cultivar.

Result

Transcriptome analysis of differential genes was conducted in this study to examine the pathogenesis of PVY on YouJin. The results showed that 1,949 genes were differentially regulated, including 853 upregulated genes and 1,096 downregulated genes. The Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis indicated that carbohydrate synthesis and metabolism pathways were suppressed, and electron transferase and hydrolase activities were reduced. Moreover, there were increased expression levels of protein kinase genes. By focusing on plant-pathogen interaction pathways, six core genes all upregulating the WARK family of transcription factors were obtained. Additionally, a constructed PPI network revealed the identification of key modular differential genes, such as downregulated photosynthesis-related protein genes and upregulated AP2/ERF-ERF transcription factors. Functional network enrichment analysis revealed that PVY infection limited RNA metabolism, glutathionylation, and peroxiredoxin activity while triggering the expression of associated defense genes in YouJin. After analyzing the above, 26 DEGs were screened and 12 DEGs were confirmed via RT-qPCR.

Conclusion

These results establish a hypothetical framework for clarifying the pathogenesis of PVY in the YouJin variety of potatoes, which will help design the disease resistance of YouJin.

Ammonium protects rice against rice stripe virus by activating HDA703/OsBZR1-mediated BR signaling

Ammonium (NH₄⁺) is a major inorganic nitrogen source for plants and also as a signal regulates plant growth and defense. Brassinosteroids (BRs) are a class of steroid hormones that control plant developmental and physiological processes through its signaling pathway. Rice is a kind of NH₄⁺-preferring plant which responds to virus infection involving in the regulation of BR biosynthesis and signaling. However, the BR-mediated regulatory mechanisms in rice-virus interactions are not fully understood. In addition, it remains unknown whether there is a direct link between NH₄⁺ and BRs in regulating rice response to virus. HDA703, a histone deacetylase and OsBZR1, a transcription factor, are two positive regulator of BR signaling and interact with each other. In this study, we show that rice plants grown with NH₄⁺ as the sole N source have enhanced resistance to rice stripe virus (RSV), one of the most devastating viruses of rice, than those grown with NO₃^- as the sole N source. We also show that in contrast to NO₃^-, NH₄⁺ does not affect BR biosynthesis but promotes BR signaling by upregulating the expression of HDA703 and promoting the accumulation of OsBZR1 in rice shoots. We further show that BR biosynthesis and signaling is required for rice defense against RSV and BR-mediated resistance to RSV attributes to activating HDA703/OsBZR1 module, then decreasing the expression of Ghd7, a direct target of HDA703/OsBZR1. Consistently, increase of the expression of HDA703 or decrease of the expression of Ghd7 enhances rice resistance to RSV. Together, our study reveals that activation of HDA703/OsBZR1-Ghd7 signaling cascade is an undescribed mechanism conferring BR-mediated RSV resistance and NH₄⁺ protects rice against RSV by activating HDA703/OsBZR1-Ghd7-mediated BR signaling in rice.

Transcriptional and post-transcriptional regulation of RNAi-related gene expression during plant-virus interactions

As sessile organisms, plants encounter diverse invasions from pathogens including viruses. To survive and thrive, plants have evolved multilayered defense mechanisms to combat virus infection. RNAi, also known as RNA silencing, is an across-kingdom innate immunity and gene regulatory machinery. Molecular framework and crucial roles of RNAi in antiviral defense have been well-characterized. However, it is largely unknown that how RNAi is transcriptionally regulated to initiate, maintain and enhance cellular silencing under normal or stress conditions. Recently, insights into the transcriptional and post-transcriptional regulation of RNAi-related genes in different physiological processes have been emerging. In this review, we integrate these new findings to provide updated views on how plants modulate RNAi machinery at the (post-) transcriptional level to respond to virus infection.

Rice stripe virus: Exploring Molecular Weapons in the Arsenal of a Negative-Sense RNA Virus

Rice stripe disease caused by Rice stripe virus (RSV) is one of the most devastating plant viruses of rice and causes enormous losses in production. RSV is transmitted from plant to plant by the small brown planthopper (Laodelphax striatellus) in a circulative-propagative manner. The recent reemergence of this pathogen in East Asia since 2000 has made RSV one of the most studied plant viruses over the past two decades. Extensive studies of RSV have resulted in substantial advances regarding fundamental aspects of the virus infection. Here, we compile and analyze recent information on RSV with a special emphasis on the strategies that RSV has adopted to establish infections. These advances include RSV replication and movement in host plants and the small brown planthopper vector, innate immunity defenses against RSV infection, epidemiology, and recent advances in the management of rice stripe disease. Understanding these issues will facilitate the design of novel antiviral therapies for management and contribute to a more detailed understanding of negative-sense virus-host interactions at the molecular level.

The titers of rice tungro bacilliform virus dictate the expression levels of genes related to cell wall dynamics in rice plants affected by tungro disease

Rice tungro disease (RTD) is a devastating disease of rice caused by combined infection with rice tungro bacilliform virus (RTBV) and rice tungro spherical virus (RTSV), with one of the main symptoms being stunting. To dissect the molecular events responsible for RTD-induced stunting, the expression patterns of 23 cell-wall-related genes were examined in different rice lines with the same titers of RTSV but different titers of RTBV and in lines where only RTBV was present. Genes encoding cellulose synthases, expansins, glycosyl hydrolases, exostosins, and xyloglucan galactosyl transferase showed downregulation, whereas those encoding defensin or defensin-like proteins showed upregulation with increasing titers of RTBV. RTSV titers did not affect the expression levels of these genes. A similar relationship was seen for the reduction in the cellulose and pectin content and the accumulation of lignin. In silico analysis of promoters of the genes indicated a possible link to transcription factors reported earlier to respond to viral titers in rice. These results suggest a common network in which the genes related to the cell wall components are affected during infection with diverse viruses in rice.

Virus and Viroid-Derived Small RNAs as Modulators of Host Gene Expression: Molecular Insights Into Pathogenesis

Virus-derived siRNAs (vsiRNAs) generated by the host RNA silencing mechanism are effectors of plant’s defense response and act by targeting the viral RNA and DNA in post-transcriptional gene silencing (PTGS) and transcriptional gene silencing (TGS) pathways, respectively. Contrarily, viral suppressors of RNA silencing (VSRs) compromise the host RNA silencing pathways and also cause disease-associated symptoms. In this backdrop, reports describing the modulation of plant gene(s) expression by vsiRNAs via sequence complementarity between viral small RNAs (sRNAs) and host mRNAs have emerged. In some cases, silencing of host mRNAs by vsiRNAs has been implicated to cause characteristic symptoms of the viral diseases. Similarly, viroid infection results in generation of sRNAs, originating from viroid genomic RNAs, that potentially target host mRNAs causing typical disease-associated symptoms. Pathogen-derived sRNAs have been demonstrated to have the propensity to target wide range of genes including host defense-related genes, genes involved in flowering and reproductive pathways. Recent evidence indicates that vsiRNAs inhibit host RNA silencing to promote viral infection by acting as decoy sRNAs. Nevertheless, it remains unclear if the silencing of host transcripts by viral genome-derived sRNAs are inadvertent effects due to fortuitous pairing between vsiRNA and host mRNA or the result of genuine counter-defense strategy employed by viruses to enhance its survival inside the plant cell. In this review, we analyze the instances of such cross reaction between pathogen-derived vsiRNAs and host mRNAs and discuss the molecular insights regarding the process of pathogenesis.

Barley yellow dwarf virus-GAV-derived vsiRNAs are involved in the production of wheat leaf yellowing symptoms by targeting chlorophyll synthase

BACKGROUND

Wheat yellow dwarf virus disease is infected by barley yellow dwarf virus (BYDV), which causes leaf yellowing and dwarfing symptoms in wheat, thereby posing a serious threat to China's food production. The infection of plant viruses can produce large numbers of vsiRNAs, which can target host transcripts and cause symptom development. However, few studies have been conducted to explore the role played by vsiRNAs in the interaction between BYDV-GAV and host wheat plants.

METHODS

In this study, small RNA sequencing was conducted to profile vsiRNAs in BYDV-GAV-infected wheat plants. The putative targets of vsiRNAs were predicted by the bioinformatics software psRNATarget. RT-qPCR and VIGS were employed to identify the function of selected target transcripts. To confirm the interaction between vsiRNA and the target, 5' RACE was performed to analyze the specific cleavage sites.

RESULTS

From the sequencing data, we obtained a total of 11,384 detected vsiRNAs. The length distribution of these vsiRNAs was mostly 21 and 22 nt, and an A/U bias was observed at the 5' terminus. We also observed that the production region of vsiRNAs had no strand polarity. The vsiRNAs were predicted to target 23,719 wheat transcripts. GO and KEGG enrichment analysis demonstrated that these targets were mostly involved in cell components, catalytic activity and plant-pathogen interactions. The results of RT-qPCR analysis showed that most chloroplast-related genes were downregulated in BYDV-GAV-infected wheat plants. Silencing of a chlorophyll synthase gene caused leaf yellowing that was similar to the symptoms exhibited by BYDV-GAV-inoculated wheat plants. A vsiRNA from an overlapping region of BYDV-GAV MP and CP was observed to target chlorophyll synthase for gene silencing. Next, 5' RACE validated that vsiRNA8856 could cleave the chlorophyll synthase transcript in a sequence-specific manner.

CONCLUSIONS

This report is the first to demonstrate that BYDV-GAV-derived vsiRNAs can target wheat transcripts for symptom development, and the results of this study help to elucidate the molecular mechanisms underlying leaf yellowing after viral infection.

Transcriptome Analysis Shows Activation of Stress and Defense Responses by Silencing of Chlorophyll Biosynthetic Enzyme CHLI in Transgenic Tobacco

In the present study, we have shown the transcriptional changes in a chlorosis model transgenic tobacco plant, i-amiCHLI, in which an artificial micro RNA is expressed in a chemically inducible manner to silence the expression of CHLI genes encoding a subunit of a chlorophyll biosynthetic enzyme. Comparison to the inducer-treated and untreated control non-transformants and untreated i-amiCHLI revealed that 3568 and 3582 genes were up- and down-regulated, respectively, in the inducer-treated i-amiCHLI plants. Gene Ontology enrichment analysis of these differentially expressed genes indicated the upregulation of the genes related to innate immune responses, and cell death pathways, and the downregulation of genes for photosynthesis, plastid organization, and primary and secondary metabolic pathways in the inducer-treated i-amiCHLI plants. The cell death in the chlorotic tissues with a preceding H2O2 production was observed in the inducer-treated i-amiCHLI plants, confirming the activation of the immune response. The involvement of activated innate immune response in the chlorosis development was supported by the comparative expression analysis between the two transgenic chlorosis model systems, i-amiCHLI and i-hpHSP90C, in which nuclear genes encoding different chloroplast proteins were similarly silenced.

Impaired Expression of Chloroplast HSP90C Chaperone Activates Plant Defense Responses with a Possible Link to a Disease-Symptom-Like Phenotype

RNA-seq analysis of a transgenic tobacco plant, i-hpHSP90C, in which chloroplast HSP90C genes can be silenced in an artificially inducible manner resulting in the development of chlorosis, revealed the up- and downregulation of 2746 and 3490 genes, respectively. Gene ontology analysis of these differentially expressed genes indicated the upregulation of ROS-responsive genes; the activation of the innate immunity and cell death pathways; and the downregulation of genes involved in photosynthesis, plastid organization, and cell cycle. Cell death was confirmed by trypan blue staining and electrolyte leakage assay, and the H2O2 production was confirmed by diaminobenzidine staining. The results collectively suggest that the reduced levels of HSP90C chaperone lead the plant to develop chlorosis primarily through the global downregulation of chloroplast- and photosynthesis-related genes and additionally through the light-dependent production of ROS, followed by the activation of immune responses, including cell death.

Dynamic Transcriptional Profiles of Arabidopsis thaliana Infected by Tomato spotted wilt virus

Tomato spotted wilt virus (TSWV) is a negative-stranded RNA virus that infects hundreds of plant species, causing great economic loss. Infected Arabidopsis thaliana plants develop symptoms including chlorosis and wilt, which can lead to cell death. From 9 to 15 days after TSWV infection, symptoms progress through a three-stage process of appearance, severity, and death. In this study, deep sequencing technology was first used to explore gene expression in response to TSWV infection in model plant A. thaliana at different symptom development stages. We found that plant immune defense and protein degradation are induced by TSWV infection and that both inductions became stronger over time. The photosynthesis pathway was attenuated with TSWV infection. Cell wall metabolism had a large extent of downregulation while some genes were upregulated. These results illustrate the dynamic nature of TSWV infection in A. thaliana at the whole-transcriptome level. The link between biological processes and subpathway metabolism was further analyzed. Our study provides new insight into host regulatory networks and dynamic processes in response to TSWV infection.

Stvb-i, a Rice Gene Conferring Durable Resistance to Rice stripe virus, Protects Plant Growth From Heat Stress

Disease resistance is affected by temperature. A rice gene, Stvb-i, is known to have conferred sustained resistance to Rice stripe virus (RSV) despite global warming. Stvb-i protects plants from growth stunting caused by RSV. The underlying resistance mechanism is unclear. Here, Stvb-i showed stable RSV resistance for 20 years in laboratory experiments. This gene encodes a protein distinct from well-studied plant disease-resistance proteins. It has a domain homologous to the histidine kinase/heat-shock protein 90-like ATPase superfamily. Rice has three paralogous genes including Stvb-i. The genes are expressed mainly in meristematic tissues. In the initial period after viral inoculation, RSV multiplication enhanced Stvb-i, whereas Stvb-i suppressed RSV multiplication. Stvb-i silencing inhibited plant growth regardless of viral infection, and silencing of the other paralogous gene that located closely to Stvb-i caused morphological abnormalities. The results suggested that the Stvb-i and its paralogs are related to plant development; especially, Stvb-i supports meristem growth, resulting in plant growth stabilizing. Growth stunting in the Stvb-i-silenced plants was more severe under repetitive heat stress, suggesting that Stvb-i contributed to the attenuation of heat damage in plant development. The symptoms of RSV infection (chlorosis, wilting, stunting, fewer tillers, and defective panicles) were similar to those of heat damage, suggesting that RSV multiplication induces heat-like stress in meristematic cells. Our findings suggest that the mechanism of meristem growth protection conferred by Stvb-i allows plants to withstand both heat stress and RSV multiplication. The suppression of RSV multiplication by the Stvb-i function in meristems results in durable resistance.

Symptoms and yield loss caused by rice stripe mosaic virus

Background

Rice stripe mosaic virus (RSMV) is a tentative new Cytorhabdovirus species in family Rhabdoviridae transmitted by the leafhopper Recilia dorsalis. Although the virus was first detected in southern China in 2015, few studies have investigated rice symptoms and yield losses caused by RSMV infection.

Methods

In this study, we observed and systematically compared symptoms of three virally infected, representative varieties of indica, hybrid and japonica rice and determined the yield parameters of the artificially inoculated plants.

Results

The three RSMV-infected cultivated rice varieties exhibited slight dwarfing, striped mosaicism, stiff, crinkled or even twisted leaves, an increased number of tillers, delayed heading, cluster-shaped shortening of panicles and mostly unfilled grains. Slight differences in symptom occurrence time were observed under different environmental conditions. For example, mosaic symptoms appeared earlier and crinkling symptoms appeared later, with both symptoms later receding in some infected plants. Yield losses due to RSMV also differed among varieties. The most serious yield reduction was experienced by indica rice (cv. Meixiangzhan), followed by hybrid indica rice (cv. Wuyou 1179) and then japonica (cv. Nipponbare). Single panicle weight, seed setting rate and 1000-kernel weight were reduced in the three infected varieties compared with healthy plants—by 85.42, 94.85 and 31.56% in Meixiangzhan; 52.43, 53.06 and 25.65% in Wuyou 1179 and 25.53, 49.32 and 23.86% in Nipponbare, respectively.

Conclusions

Our findings contribute basic data for field investigations, formulation of prevention and control strategies and further study of the pathogenesis of RSMV.

Analysis of Symptom Development in Relation to Quantity of Rice stripe virus in Rice (Oryza sativa) to Simplify Evaluation of Resistance

Rice stripe virus (RSV) is one of the most devastating pathogens of rice (Oryza sativa) in rice-growing regions of East Asia. We analyzed the increase in RSV accumulation in infected rice plants over time and evaluated the association between disease severity and RSV accumulation with the aim of establishing an experimental system for accurate and efficient evaluation of RSV resistance in rice. As an index of RSV accumulation in plants, relative concentration of RNA corresponding to the coat protein gene region was measured using reverse-transcription quantitative polymerase chain reaction. Actin and elongation factor 1a were used as the host reference genes. RSV concentrations tended to increase with time from 7 to 28 days after inoculation, and a strong positive correlation was observed between the log RSV concentrations in the midsections of the uppermost leaves and in the stems at the first leaf sheath position. We analyzed RSV concentrations at these two locations 21 days after inoculation with RSV and assessed severity of disease symptoms based on a commonly used scale (Washio's six-grade scale) rated as A (most severe), B, Bt, C, Cr, or D (mild symptoms). RSV concentrations at both locations were high in plants graded A, B, or Bt, with no significant difference in concentration of RSV among the three grades, but concentrations were significantly higher in the three grades compared with that in the plants in grade D. RSV concentrations were highly variable among plants in grades C and Cr. On the basis of these data, we propose a new formula to estimate the range of disease severities with greater ease and practical value. The values calculated by the new formula corresponded well to those based on Washio's six-grade scale.

Revealing shared differential co-expression profiles in rice infected by virus from reoviridae and sequiviridae group

Differential co-expression is a cutting-edge approach to analyze gene expression data and identify both shared and divergent expression patterns. The availability of high-throughput gene expression datasets and efficient computational approaches have unfolded the opportunity to a systems level understanding of functional genomics of different stresses with respect to plants. We performed the meta-analysis of the available microarray data for reoviridae and sequiviridae infection in rice with the aim to identify the shared gene co-expression profile. The microarray data were downloaded from ArrayExpress and analyzed through a modified Weighted Gene Co-expression Network Analysis (WGCNA) protocol. WGCNA clustered the genes based on the expression intensities across the samples followed by identification of modules, eigengenes, principal components, topology overlap, module membership and module preservation. The module preservation analysis identified 4 modules; salmon (638 genes), midnightblue (584 genes), lightcyan (686 genes) and red (562 genes), which are highly preserved in both the cases. The networks in case of reoviridae infection showed neatly packed clusters whereas, in sequiviridae, the clusters were loosely connected which is due to the differences in the correlation values. We also identified 83 common transcription factors targeting the hub genes from all the identified modules. This study provides a coherent view of the comparative aspect of the expression of common genes involved in different virus infections which may aid in the identification of novel targets and development of new intervention strategy against the virus.

Jasmonate-Induced Defense Mechanisms in the Belowground Antagonistic Interaction Between Pythium arrhenomanes and Meloidogyne graminicola in Rice

Next to their essential roles in plant growth and development, phytohormones play a central role in plant immunity against pathogens. In this study we studied the previously reported antagonism between the plant-pathogenic oomycete Pythium arrhenomanes and the root-knot nematode Meloidogyne graminicola, two root pathogens that co-occur in aerobic rice fields. In this manuscript, we investigated if the antagonism is related to imbalances in plant hormone levels, which could be involved in activation of plant defense. Hormone measurements and gene expression analyses showed that the jasmonate (JA) pathway is induced early upon P. arrhenomanes infection. Exogenous application of methyl-jasmonate (MeJA) on the plant confirmed that JA is needed for basal defense against both P. arrhenomanes and M. graminicola in rice. Whereas M. graminicola suppresses root JA levels to increase host susceptibility, Pythium inoculation boosts JA in a manner that prohibits JA repression by the nematode in double-inoculated plants. Exogenous MeJA supply phenocopied the defense-inducing capacity of Pythium against the root-knot nematode, whereas the antagonism was weakened in JA-insensitive mutants. Transcriptome analysis confirmed upregulation of JA biosynthesis and signaling genes upon P. arrhenomanes infection, and additionally revealed induction of genes involved in biosynthesis of diterpenoid phytoalexins, consistent with strong activation of the gene encoding the JA-inducible transcriptional regulator DITERPENOID PHYTOALEXIN FACTOR. Altogether, the here-reported data indicate an important role for JA-induced defense mechanisms in this antagonistic interaction. Next to that, our results provide evidence for induced expression of genes encoding ERF83, and related PR proteins, as well as auxin depletion in P. arrhenomanes infected rice roots, which potentially further contribute to the reduced nematode susceptibility seen in double-infected plants.

Combined small RNA and gene expression analysis revealed roles of miRNAs in maize response to rice black-streaked dwarf virus infection

Maize rough dwarf disease, caused by rice black-streaked dwarf virus (RBSDV), is a devastating disease in maize (Zea mays L.). MicroRNAs (miRNAs) are known to play critical roles in regulation of plant growth, development, and adaptation to abiotic and biotic stresses. To elucidate the roles of miRNAs in the regulation of maize in response to RBSDV, we employed high-throughput sequencing technology to analyze the miRNAome and transcriptome following RBSDV infection. A total of 76 known miRNAs, 226 potential novel miRNAs and 351 target genes were identified. Our dataset showed that the expression patterns of 81 miRNAs changed dramatically in response to RBSDV infection. Transcriptome analysis showed that 453 genes were differentially expressed after RBSDV infection. GO, COG and KEGG analysis results demonstrated that genes involved with photosynthesis and metabolism were significantly enriched. In addition, twelve miRNA-mRNA interaction pairs were identified, and six of them were likely to play significant roles in maize response to RBSDV. This study provided valuable information for understanding the molecular mechanism of maize disease resistance, and could be useful in method development to protect maize against RBSDV.

Changes in maize transcriptome in response to maize Iranian mosaic virus infection

Background

Maize Iranian mosaic virus (MIMV, genus Nucleorhabdovirus, family Rhabdoviridae) causes an economically important disease in maize and other gramineous crops in Iran. MIMV negative-sense RNA genome sequence of 12,426 nucleotides has recently been completed. Maize Genetics and Genomics database shows that 39,498 coding genes and 4,976 non-coding genes of maize have been determined, but still some transcripts could not be annotated. The molecular host cell responses of maize to MIMV infection including differential gene expression have so far not been elucidated.

Methodology/Principal findings

Complementary DNA libraries were prepared from total RNA of MIMV-infected and mock-inoculated maize leaves and sequenced using Illumina HiSeq 2500. Cleaned raw transcript reads from MIMV-infected maize were mapped to reads from uninfected maize and to a maize reference genome. Differentially expressed transcripts were characterized by gene ontology and biochemical pathway analyses. Transcriptome data for selected genes were validated by real-time quantitative PCR.

Conclusion/Significance

Approximately 42 million clean reads for each treatment were obtained. In MIMV-infected maize compared to uninfected plants, 1689 transcripts were up-regulated and 213 transcripts were down-regulated. In response to MIMV infection, several pathways were activated in maize including immune receptor signaling, metabolic pathways, RNA silencing, hormone-mediated pathways, protein degradation, protein kinase and ATP binding activity, and fatty acid metabolism. Also, several transcripts including those encoding hydrophobic protein RCI2B, adenosylmethionine decarboxylase NAC transcription factor and nucleic acid binding, leucine-rich repeat, heat shock protein, 26S proteasome, oxidoreductases and endonuclease activity protein were up-regulated. These data will contribute to the identification of genes and pathways involved in plant-virus interactions that may serve as future targets for improved disease control.

Rice Stripe Virus Interferes with S-acylation of Remorin and Induces Its Autophagic Degradation to Facilitate Virus Infection

Remorins are plant-specific membrane-associated proteins and were proposed to play crucial roles in plant-pathogen interactions. However, little is known about how pathogens counter remorin-mediated host responses. In this study, by quantitative whole-proteome analysis we found that the remorin protein (NbREM1) is downregulated early in Rice stripe virus (RSV) infection. We further discovered that the turnover of NbREM1 is regulated by S-acylation modification and its degradation is mediated mainly through the autophagy pathway. Interestingly, RSV can interfere with the S-acylation of NbREM1, which is required to negatively regulate RSV infection by restricting virus cell-to-cell trafficking. The disruption of NbREM1 S-acylation affects its targeting to the plasma membrane microdomain, and the resulting accumulation of non-targeted NbREM1 is subjected to autophagic degradation, causing downregulation of NbREM1. Moreover, we found that RSV-encoded movement protein, NSvc4, alone can interfere with NbREM1 S-acylation through binding with the C-terminal domain of NbREM1 the S-acylation of OsREM1.4, the homologous remorin of NbREM1, and thus remorin-mediated defense against RSV in rice, the original host of RSV, indicating that downregulation of the remorin protein level by interfering with its S-acylation is a common strategy adopted by RSV to overcome remorin-mediated inhibition of virus movement.

Target of rapamycin signaling orchestrates growth-defense trade-offs in plants

Plant defense to microbial pathogens is often accompanied by significant growth inhibition. How plants merge immune system function with normal growth and development is still poorly understood. Here, we investigated the role of target of rapamycin (TOR), an evolutionary conserved serine/threonine kinase, in the plant defense response. We used rice as a model system and applied a combination of chemical, genetic, genomic and cell-based analyses. We demonstrate that ectopic expression of TOR and Raptor (regulatory-associated protein of mTOR), a protein previously demonstrated to interact with TOR in Arabidopsis, positively regulates growth and development in rice. Transcriptome analysis of rice cells treated with the TOR-specific inhibitor rapamycin revealed that TOR not only dictates transcriptional reprogramming of extensive gene sets involved in central and secondary metabolism, cell cycle and transcription, but also suppresses many defense-related genes. TOR overexpression lines displayed increased susceptibility to both bacterial and fungal pathogens, whereas plants with reduced TOR signaling displayed enhanced resistance. Finally, we found that TOR antagonizes the action of the classic defense hormones salicylic acid and jasmonic acid. Together, these results indicate that TOR acts as a molecular switch for the activation of cell proliferation and plant growth at the expense of cellular immunity.

Overexpression of OsCIPK30 Enhances Plant Tolerance to Rice stripe virus

Rice stripe virus (RSV) causes a severe disease in Oryza sativa (rice) in many Eastern Asian countries. The NS3 protein of RSV is a viral suppressor of RNA silencing, but plant host factors interacting with NS3 have not been reported yet. Here, we present evidence that expression of RSV NS3 in Arabidopsis thaliana causes developmental abnormalities. Through yeast two-hybrid screening and a luciferase complementation imaging assay, we demonstrate that RSV NS3 interacted with OsCIPK30, a CBL (calcineurin B-like proteins)-interaction protein kinase protein. Furthermore, OsCIPK30 was overexpressed to investigate the function of OsCIPK30 in rice. Our investigation showed that overexpression of OsCIPK30 in rice could delay the RSV symptoms and show milder RSV symptoms. In addition, the expression of pathogenesis-related genes was increased in OsCIPK30 transgenic rice. These results suggest that overexpression of OsCIPK30 positively regulates pathogenesis-related genes to enhance the tolerance to RSV in rice. Our findings provide new insight into the molecular mechanism underlying resistance to RSV disease.

NbEXPA1, an α-expansin, is plasmodesmata-specific and a novel host factor for potyviral infection

Plasmodesmata (PD), unique to the plant kingdom, are structurally complex microchannels that cross the cell wall to establish symplastic communication between neighbouring cells. Viral intercellular movement occurs through PD. To better understand the involvement of PD in viral infection, we conducted a quantitative proteomic study on the PD-enriched fraction from Nicotiana benthamiana leaves in response to infection by Turnip mosaic virus (TuMV). We report the identification of a total of 1070 PD protein candidates, of which 100 (≥2-fold increase) and 48 (≥2-fold reduction) are significantly differentially accumulated in the PD-enriched fraction, when compared with protein levels in the corresponding healthy control. Among the differentially accumulated PD protein candidates, we show that an α-expansin designated NbEXPA1, a cell wall loosening protein, is PD-specific. TuMV infection downregulates NbEXPA1 mRNA expression and protein accumulation. We further demonstrate that NbEXPA1 is recruited to the viral replication complex via the interaction with NIb, the only RNA-dependent RNA polymerase of TuMV. Silencing of NbEXPA1 inhibits plant growth and TuMV infection, whereas overexpression of NbEXPA1 promotes viral replication and intercellular movement. These data suggest that NbEXPA1 is a host factor for potyviral infection. This study not only generates a PD-proteome dataset that is useful in future studies to expound PD biology and PD-mediated virus-host interactions but also characterizes NbEXPA1 as the first PD-specific cell wall loosening protein and its essential role in potyviral infection.

Transcriptomic changes in Nicotiana benthamiana plants inoculated with the wild-type or an attenuated mutant of Tobacco vein banding mosaic virus

Tobacco vein banding mosaic virus (TVBMV) is a potyvirus which mainly infects solanaceous crops. The helper component proteinase (HCpro) of a potyvirus is an RNA silencing suppressor protein and determines the severity of disease symptoms caused by different potyviruses, including TVBMV. It has been shown that substitution mutations introduced into the HCpro open reading frame (ORF) in a TVBMV infectious clone result in changes of Asp189 to Lys or Ile250 -Gln251 to Asp-Glu (Asp, aspartic acid; Gln, glutamine; Glu, glutamic acid; Ile, isoleucine). These amino acid changes eliminate the RNA silencing suppression activity of the mutant HCpro (HCm) and attenuate the disease symptoms caused by the mutant TVBMV (T-HCm) in Nicotiana benthamiana plants. Here, we used RNA-sequencing technology to compare gene expression in plants inoculated with the wild-type TVBMV (T-WT) with that in plants inoculated with T-HCm at 1, 2 and 10 days post-agroinfiltration (dpai). At 1 and 2 dpai, N. benthamiana genes related to the translation machinery were up-regulated, whereas genes related to lipid biosynthesis and metabolism or to responses to extracellular/external stimuli were down-regulated in leaves inoculated with T-WT or T-HCm. At 10 dpai, T-WT infection repressed photosynthesis-related genes. T-WT and T-HCm infections differentially perturbed the genes involved in the RNA silencing pathway. The salicylic acid and ethylene signalling pathways were induced, but the jasmonic acid signalling pathway was repressed after T-WT infection. Infections of T-WT and T-HCm also differentially regulated the genes involved in auxin signalling transduction, which is known to associate with the stunting phenotypes caused by TVBMV. These results illustrate the dynamic nature of TVBMV infection in N. benthamiana at the transcriptomic level.

Stunted Growth Caused by Blast Disease in Rice Seedlings Is Associated with Changes in Phytohormone Signaling Pathways

In response to pathogen attack, plants prioritize defense reactions generally at the expense of plant growth. In this work, we report that changes in phytohormone signaling pathways are associated with the stunted plant growth caused by blast disease in rice seedlings. Infection of rice seedlings with blast fungus Magnaporthe oryzae (race 007.0) at the four-leaf stage (three true leaves) resulted in considerable inhibition of the growth of the upper uninfected distal leaves; the length of leaf blade and leaf sheath of the sixth and seventh leaf was reduced by 27 and 82%, and 88 and 72%, respectively, compared to that in the uninoculated plant control. Interestingly, cutting off the blast-infected fourth leaf blade within 2 days post inoculation (dpi) significantly rescued the inhibition of leaf growth, implying that an inhibitory substance(s) and/or signal was generated in the blast-infected leaves (fourth leaf) and transmitted to the upper distal leaves (sixth and seventh) during the 2-dpi period that induced growth inhibition. Expression analysis of marker genes for phytohormone pathways revealed acute activation of the jasmonate (JA) and abscisic acid (ABA) signaling pathways, and repression of auxin, gibberellic acid (GA) and salicylic acid (SA) signaling pathways, in the sixth leaf. The genes related to cell wall expansion were also significantly downregulated. In the blast-infected fourth leaf, JA pathway was activated within 2 dpi, followed by activation of ABA pathway 3 dpi. Further, leaf inhibition caused by blast infection was partially rescued in the rice mutant line coleoptile photomorphogenesis 2 (cpm2), which is defective in the gene encoding allene oxide cyclase (OsAOC). These results indicate that the JA signaling pathway is at least partly involved in the growth inhibition processes. Collectively, our data suggest that, upon pathogen attack, rice seedlings prioritize defense reactions against the infecting pathogen by temporarily ceasing plant growth through the systemic control of phytohormone pathways.

Inducible transgenic tobacco system to study the mechanisms underlying chlorosis mediated by the silencing of chloroplast heat shock protein 90

Chlorosis is one of the most common symptoms of plant diseases, including those caused by viruses and viroids. Recently, a study has shown that Peach latent mosaic viroid (PLMVd) exploits host RNA silencing machinery to modulate the virus disease symptoms through the silencing of chloroplast-targeted heat shock protein 90 (Hsp90C). To understand the molecular mechanisms of chlorosis in this viroid disease, we established an experimental system suitable for studying the mechanism underlying the chlorosis induced by the RNA silencing of Hsp90C in transgenic tobacco. Hairpin RNA of the Hsp90C-specific region was expressed under the control of a dexamethasone-inducible promoter, resulted in the silencing of Hsp90C gene in 2 days and the chlorosis along with growth suppression phenotypes. Time course study suggests that a sign of chlorosis can be monitored as early as 2 days, suggesting that this experimental model is suitable for studying the molecular events taken place before and after the onset of chlorosis. During the early phase of chlorosis development, the chloroplast- and photosynthesis-related genes were downregulated. It should be noted that some pathogenesis related genes were upregulated during the early phase of chlorosis in spite of the absence of any pathogen-derived molecules in this system.

Transcriptome analysis of Cucumis sativus infected by Cucurbit chlorotic yellows virus

BACKGROUND

Cucurbit chlorotic yellows virus (CCYV) is a recently reported bipartite crinivirus that causes chlorotic leaf spots and yellowing symptoms on the leaves of cucurbit plants. The virus-host interaction of CCYV remains to be elucidated, and the influence of criniviruses on the host gene transcriptome requires analysis.

METHODS

We used transcriptome sequencing to analyse the differentially expressed genes (DEGs) caused by CCYV infection.

RESULTS

CCYV infection resulted in 865 DEGs. The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis identified 67 pathways, and the three major enrichment pathways (according to the P-values) were photosynthesis-antenna proteins (KO00196), phenylalanine metabolism (KO00360a), and phenylpropanoid biosynthesis (KO00940). Of the 13 DEGs identified in phenylalanine metabolism, 11 genes encode disease resistance-related phenylalanine ammonia-lyase (PAL) genes. Using quantitative real-time PCR, we validated the differential expression of 12 genes.

CONCLUSIONS

Our study based on the CCYV-cucumber interaction provides comprehensive transcriptomic information, and will improve our understanding of host-crinivirus interactions.

RNA-seq-based digital gene expression analysis reveals modification of host defense responses by rice stripe virus during disease symptom development in Arabidopsis

Background

Virus infection induces and suppresses host gene expression on a global level. Rice stripe virus (RSV) is the type species of the genus Tenuivirus and infects rice and Arabidopsis plants. Microarray-based and next generation sequencing-based transcriptomic approaches have been used to study rice-RSV interactions. However, our knowledge of the response of Arabidopsis plants to RSV infection is limited, and it requires further investigation to determine the similarities (or differences) in virus-host interactions between monocot and dicot hosts infected with RSV.

Methods

We characterized transcriptome changes in Arabidopsis thaliana infected with rice stripe virus (RSV) with RNA-seq based digital gene expression (DGE) analysis. The transcriptomes of RSV-infected samples were compared to those of mock-treated samples at 14 and 21 days post-infection (dpi) during different stages of symptom development.

Results

We identified 624 differentially expressed genes (DEGs) in Arabidopsis influenced by RSV at 14 dpi and 21 dpi, among which at 14 dpi, 255 transcripts were induced, and 38 were repressed; at 21 dpi, 146 were induced, and 237 were repressed. Functional annotation indicated that these DEGs were related to multiple biological functions, including defense response, secondary metabolism, protein amino acid phosphorylation and response to abiotic stress.

Conclusions

Importantly, the transcription of genes related to host defense systems was activated by RSV infection at an early stage of symptom development (14 dpi), whereas over the infection period (21 dpi), the host defense response systems were suppressed. A total of 52 genes were continuously differentially expressed between the two time points, indicating that the majority of DEGs were transient and unique to a particular time point during symptom development. The DEGs, particularly the defense response genes, identified in this study are candidates suitable for further functional analysis during the RSV-Arabidopsis interaction.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-016-0663-7) contains supplementary material, which is available to authorized users.

Chloroplast in Plant-Virus Interaction

In plants, the chloroplast is the organelle that conducts photosynthesis. It has been known that chloroplast is involved in virus infection of plants for approximate 70 years. Recently, the subject of chloroplast-virus interplay is getting more and more attention. In this article we discuss the different aspects of chloroplast-virus interaction into three sections: the effect of virus infection on the structure and function of chloroplast, the role of chloroplast in virus infection cycle, and the function of chloroplast in host defense against viruses. In particular, we focus on the characterization of chloroplast protein-viral protein interactions that underlie the interplay between chloroplast and virus. It can be summarized that chloroplast is a common target of plant viruses for viral pathogenesis or propagation; and conversely, chloroplast and its components also can play active roles in plant defense against viruses. Chloroplast photosynthesis-related genes/proteins (CPRGs/CPRPs) are suggested to play a central role during the complex chloroplast-virus interaction.

Suppression of Jasmonic Acid-Mediated Defense by Viral-Inducible MicroRNA319 Facilitates Virus Infection in Rice

MicroRNAs (miRNAs) are pivotal modulators of plant development and host-virus interactions. However, the roles and action modes of specific miRNAs involved in viral infection and host susceptibility remain largely unclear. In this study, we show that Rice ragged stunt virus (RRSV) infection caused increased accumulation of miR319 but decreased expression of miR319-regulated TCP (TEOSINTE BRANCHED/CYCLOIDEA/PCF) genes, especially TCP21, in rice plants. Transgenic rice plants overexpressing miR319 or downregulating TCP21 exhibited disease-like phenotypes and showed significantly higher susceptibility to RRSV in comparison with the wild-type plants. In contrast, only mild disease symptoms were observed in RRSV-infected lines overexpressing TCP21 and especially in the transgenic plants overexpressing miR319-resistant TCP21. Both RRSV infection and overexpression of miR319 caused the decreased endogenous jasmonic acid (JA) levels along with downregulated expression of JA biosynthesis and signaling-related genes in rice. However, treatment of rice plants with methyl jasmonate alleviated disease symptoms caused by RRSV and reduced virus accumulation. Taken together, our results suggest that the induction of miR319 by RRSV infection in rice suppresses JA-mediated defense to facilitate virus infection and symptom development.

Dual transcriptome analysis reveals insights into the response to Rice black-streaked dwarf virus in maize

Maize rough dwarf disease (MRDD) is a viral infection that results in heavy yield losses in maize worldwide, particularly in the summer maize-growing regions of China. MRDD is caused by the Rice black-streaked dwarf virus (RBSDV). In the present study, analyses of microRNAs (miRNAs), the degradome, and transcriptome sequences were used to elucidate the RBSDV-responsive pathway(s) in maize. Genomic analysis indicated that the expression of three non-conserved and 28 conserved miRNAs, representing 17 known miRNA families and 14 novel miRNAs, were significantly altered in response to RBSDV when maize was inoculated at the V3 (third leaf) stage. A total of 99 target transcripts from 48 genes of 10 known miRNAs were found to be responsive to RBSDV infection. The annotations of these target genes include a SQUAMOSA promoter binding (SPB) protein, a P450 reductase, an oxidoreductase, and a ubiquitin-related gene, among others. Characterization of the entire transcriptome suggested that a total of 28 and 1085 differentially expressed genes (DEGs) were detected at 1.5 and 3.0 d, respectively, after artificial inoculation with RBSDV. The expression patterns of cell wall- and chloroplast-related genes, and disease resistance- and stress-related genes changed significantly in response to RBSDV infection. The negatively regulated genes GRMZM2G069316 and GRMZM2G031169, which are the target genes for miR169i-p5 and miR8155, were identified as a nucleolin and a NAD(P)-binding Rossmann-fold superfamily protein in maize, respectively. The gene ontology term GO:0003824, including GRMZM2G031169 and other 51 DEGs, was designated as responsive to RBSDV.

Overexpression of BSR1 confers broad-spectrum resistance against two bacterial diseases and two major fungal diseases in rice

Broad-spectrum disease resistance against two or more types of pathogen species is desirable for crop improvement. In rice, Xanthomonas oryzae pv. oryzae (Xoo), the causal bacteria of rice leaf blight, and Magnaporthe oryzae, the fungal pathogen causing rice blast, are two of the most devastating pathogens. We identified the rice BROAD-SPECTRUM RESISTANCE 1 (BSR1) gene for a BIK1-like receptor-like cytoplasmic kinase using the FOX hunting system, and demonstrated that BSR1-overexpressing (OX) rice showed strong resistance to the bacterial pathogen, Xoo and the fungal pathogen, M. oryzae. Here, we report that BSR1-OX rice showed extended resistance against two other different races of Xoo, and to at least one other race of M. oryzae. In addition, the rice showed resistance to another bacterial species, Burkholderia glumae, which causes bacterial seedling rot and bacterial grain rot, and to Cochliobolus miyabeanus, another fungal species causing brown spot. Furthermore, BSR1-OX rice showed slight resistance to rice stripe disease, a major viral disease caused by rice stripe virus. Thus, we demonstrated that BSR1-OX rice shows remarkable broad-spectrum resistance to at least two major bacterial species and two major fungal species, and slight resistance to one viral pathogen.

Identification and regulation of host genes related to Rice stripe virus symptom production

Viral infections cause plant chlorosis, stunting, necrosis or other symptoms. The down-regulation of chloroplast-related genes (ChRGs) is assumed to be responsible for chlorosis. We identified the differentially expressed genes (DEGs) in Rice stripe virus (RSV)-infected Nicotiana benthamiana, and examined the contribution of 75 down-regulated DEGs to RSV symptoms by silencing them one by one using Tobacco rattle virus (TRV)-induced gene silencing. Silencing of 11 of the 75 down-regulated DEGs caused plant chlorosis, and nine of the 11 were ChRGs. Silencing of a down-regulated DEG encoding the eukaryotic translation initiation factor 4A (eIF4A) caused leaf-twisting and stunting that were visible on RSV-infected N. benthamiana. A region of RSV RNA4 was complementary to part of eIF4A mRNA and virus-derived small interfering (vsiRNAs) from that region were present in infected N. benthamiana. When expressed as artificial microRNAs, those vsiRNAs could target NbeIF4A mRNA for regulation. We provide experimental evidence supporting the association of ChRGs with chlorosis and show that eIF4A is involved in RSV symptom development. This is also the first report demonstrating that siRNA derived directly from a plant virus can target a host gene for regulation.

Integrative Analysis of the microRNAome and Transcriptome Illuminates the Response of Susceptible Rice Plants to Rice Stripe Virus

Rice stripe virus (RSV) is one of the most serious rice viruses in East Asia. To investigate how rice responds to RSV infection, we integrated miRNA expression with parallel mRNA transcription profiling by deep sequencing. A total of 570 miRNAs were identified of which 69 miRNAs (56 up-regulated and 13 down-regulated) were significantly modified by RSV infection. Digital gene expression (DGE) analysis showed that 1274 mRNAs (431 up-regulated and 843 down-regulated genes) were differentially expressed as a result of RSV infection. The differential expression of selected miRNAs and mRNAs was confirmed by qRT-PCR. Gene ontology (GO) and pathway enrichment analysis showed that a complex set of miRNA and mRNA networks were selectively regulated by RSV infection. In particular, 63 differentially expressed miRNAs were found to be significantly and negatively correlated with 160 target mRNAs. Interestingly, 22 up-regulated miRNAs were negatively correlated with 24 down-regulated mRNAs encoding disease resistance-related proteins, indicating that the host defense responses were selectively suppressed by RSV infection. The suppression of both osa-miR1423-5p- and osa-miR1870-5p-mediated resistance pathways was further confirmed by qRT-PCR. Chloroplast functions were also targeted by RSV, especially the zeaxanthin cycle, which would affect the stability of thylakoid membranes and the biosynthesis of ABA. All these modifications may contribute to viral symptom development and provide new insights into the pathogenicity mechanisms of RSV.

Chloroplast in Plant-Virus Interaction

In plants, the chloroplast is the organelle that conducts photosynthesis. It has been known that chloroplast is involved in virus infection of plants for approximate 70 years. Recently, the subject of chloroplast-virus interplay is getting more and more attention. In this article we discuss the different aspects of chloroplast-virus interaction into three sections: the effect of virus infection on the structure and function of chloroplast, the role of chloroplast in virus infection cycle, and the function of chloroplast in host defense against viruses. In particular, we focus on the characterization of chloroplast protein-viral protein interactions that underlie the interplay between chloroplast and virus. It can be summarized that chloroplast is a common target of plant viruses for viral pathogenesis or propagation; and conversely, chloroplast and its components also can play active roles in plant defense against viruses. Chloroplast photosynthesis-related genes/proteins (CPRGs/CPRPs) are suggested to play a central role during the complex chloroplast-virus interaction.

Assessment of reference gene stability in Rice stripe virus and Rice black streaked dwarf virus infection rice by quantitative Real-time PCR

BACKGROUND

Stably expressed reference gene(s) normalization is important for the understanding of gene expression patterns by quantitative Real-time PCR (RT-qPCR), particularly for Rice stripe virus (RSV) and Rice black streaked dwarf virus (RBSDV) that caused seriously damage on rice plants in China and Southeast Asia.

METHODS

The expression of fourteen common used reference genes of Oryza sativa L. were evaluated by RT-qPCR in RSV and RBSDV infected rice plants. Suitable normalization reference gene(s) were identified by geNorm and NormFinder algorithms.

RESULTS

UBQ 10 + GAPDH and UBC + Actin1 were identified as suitable reference genes for RT-qPCR normalization under RSV and RBSDV infection, respectively. When using multiple reference genes, the expression patterns of OsPRIb and OsWRKY, two virus resistance genes, were approximately similar with that reported previously. Comparatively, by using single reference gene (TIP41-Like), a weaker inducible response was observed.

CONCLUSIONS

We proposed that the combination of two reference genes could obtain more accurate and reliable normalization of RT-qPCR results in RSV- and RBSDV-infected plants. This work therefore sheds light on establishing a standardized RT-qPCR procedure in RSV- and RBSDV-infected rice plants, and might serve as an important point for discovering complex regulatory networks and identifying genes relevant to biological processes or implicated in virus.

Primary metabolism plays a central role in moulding silicon-inducible brown spot resistance in rice

Over recent decades, a multitude of studies have shown the ability of silicon (Si) to protect various plants against a range of microbial pathogens exhibiting different lifestyles and infection strategies. Despite this relative wealth of knowledge, an understanding of the action mechanism of Si is still in its infancy, which hinders its widespread application for agricultural purposes. In an attempt to further elucidate the molecular underpinnings of Si-induced disease resistance, we studied the transcriptome of control and Si-treated rice plants infected with the necrotrophic brown spot fungus Cochliobolus miyabeanus. Analysis of brown spot-infected control plants suggested that C. miyabeanus represses plant photosynthetic processes and nitrate reduction in order to trigger premature senescence and cause disease. In Si-treated plants, however, these pathogen-induced metabolic alterations are strongly impaired, suggesting that Si alleviates stress imposed by the pathogen. Interestingly, Si also significantly increased photorespiration rates in brown spot-infected plants. Although photorespiration is often considered as a wasteful process, recent studies have indicated that this metabolic bypass also enhances resistance during abiotic stress and pathogen attack by protecting the plant's photosynthetic machinery. In view of these findings, our results favour a scenario in which Si enhances brown spot resistance by counteracting C. miyabeanus-induced senescence and cell death via increased photorespiration. Moreover, our results shed light onto the mechanistic basis of Si-induced disease control and support the view that, in addition to activating plant immune responses, Si can also reduce disease severity by interfering with pathogen virulence strategies.

Interaction of HSP20 with a viral RdRp changes its sub-cellular localization and distribution pattern in plants

Small heat shock proteins (sHSPs) perform a fundamental role in protecting cells against a wide array of stresses but their biological function during viral infection remains unknown. Rice stripe virus (RSV) causes a severe disease of rice in Eastern Asia. OsHSP20 and its homologue (NbHSP20) were used as baits in yeast two-hybrid (YTH) assays to screen an RSV cDNA library and were found to interact with the viral RNA-dependent RNA polymerase (RdRp) of RSV. Interactions were confirmed by pull-down and BiFC assays. Further analysis showed that the N-terminus (residues 1-296) of the RdRp was crucial for the interaction between the HSP20s and viral RdRp and responsible for the alteration of the sub-cellular localization and distribution pattern of HSP20s in protoplasts of rice and epidermal cells of Nicotiana benthamiana. This is the first report that a plant virus or a viral protein alters the expression pattern or sub-cellular distribution of sHSPs.

Time-Course RNA-Seq Analysis Reveals Transcriptional Changes in Rice Plants Triggered by Rice stripe virus Infection

Rice stripe virus (RSV) has become a major pathogen of rice. To determine how the rice transcriptome is modified in response to RSV infection, we used RNA-Seq to perform a genome-wide gene expression analysis of a susceptible rice cultivar. The transcriptomes of RSV-infected samples were compared to those of mock-treated samples at 3, 7, and 15 days post-infection (dpi). From 8 to 11% of the genes were differentially expressed (>2-fold difference in expression) in RSV-infected vs. noninfected rice. Among them, 532 genes were differentially expressed at all three time points. Surprisingly, 37.6% of the 532 genes are related to transposons. Gene ontology enrichment analysis revealed that many chloroplast genes were down-regulated in infected plants at 3 and 15 dpi. Expression of genes associated with cell differentiation and flowering was significantly down-regulated in infected plants at 15 dpi. In contrast, most of the up-regulated genes in infected plants concern the cell wall, plasma membrane, and vacuole and are known to function in various metabolic pathways and stress responses. In addition, transcripts of diverse transcription factors gradually accumulated in infected plants with increasing infection time. We also confirmed that the expression of gene subsets (including NBS-LRR domain-containing genes, receptor-like kinase genes, and genes involving RNA silencing) was changed by RSV infection. Taken together, we demonstrated that down-regulation of genes related to photosynthesis and flowering was strongly associated with disease symptoms caused by RSV and that up-regulation of genes involved in metabolic pathways, stress responses, and transcription was related to host defense mechanisms.

High-Throughput Sequencing Identifies Novel and Conserved Cucumber (Cucumis sativus L.) microRNAs in Response to Cucumber Green Mottle Mosaic Virus Infection

Seedlings of Cucumis sativus L. (cv. 'Zhongnong 16') were artificially inoculated with Cucumber green mottle mosaic virus (CGMMV) at the three-true-leaf stage. Leaf and flower samples were collected at different time points post-inoculation (10, 30 and 50 d), and processed by high throughput sequencing analysis to identify candidate miRNA sequences. Bioinformatic analysis using screening criteria, and secondary structure prediction, indicated that 8 novel and 23 known miRNAs (including 15 miRNAs described for the first time in vivo) were produced by cucumber plants in response to CGMMV infection. Moreover, gene expression profiles (p-value <0.01) validated the expression of 3 of the novel miRNAs and 3 of the putative candidate miRNAs and identified a further 82 conserved miRNAs in CGMMV-infected cucumbers. Gene ontology (GO) analysis revealed that the predicted target genes of these 88 miRNAs, which were screened using the psRNATarget and miRanda algorithms, were involved in three functional categories: 2265 in molecular function, 1362 as cellular components and 276 in biological process. The subsequent Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis revealed that the predicted target genes were frequently involved in metabolic processes (166 pathways) and genetic information processes (40 pathways) and to a lesser degree the biosynthesis of secondary metabolites (12 pathways). These results could provide useful clues to help elucidate host-pathogen interactions in CGMMV and cucumber, as well as for the screening of resistance genes.

Comparative analysis of antiviral responses in Brachypodium distachyon and Setaria viridis reveals conserved and unique outcomes among C3 and C4 plant defenses

Viral diseases cause significant losses in global agricultural production, yet little is known about grass antiviral defense mechanisms. We previously reported on host immune responses triggered by Panicum mosaic virus (PMV) and its satellite virus (SPMV) in the model C3 grass Brachypodium distachyon. To aid comparative analyses of C3 and C4 grass antiviral defenses, here, we establish B. distachyon and Setaria viridis (a C4 grass) as compatible hosts for seven grass-infecting viruses, including PMV and SPMV, Brome mosaic virus, Barley stripe mosaic virus, Maize mild mottle virus, Sorghum yellow banding virus, Wheat streak mosaic virus (WSMV), and Foxtail mosaic virus (FoMV). Etiological and molecular characterization of the fourteen grass-virus pathosystems showed evidence for conserved crosstalk among salicylic acid (SA), jasmonic acid, and ethylene pathways in B. distachyon and S. viridis. Strikingly, expression of PHYTOALEXIN DEFICIENT4, an upstream modulator of SA signaling, was consistently suppressed during most virus infections in B. distachyon and S. viridis. Hierarchical clustering analyses further identified unique antiviral responses triggered by two morphologically similar viruses, FoMV and WSMV, and uncovered other host-dependent effects. Together, the results of this study establish B. distachyon and S. viridis as models for the analysis of plant-virus interactions and provide the first framework for conserved and unique features of C3 and C4 grass antiviral defenses.

Transcriptome profiling confirmed correlations between symptoms and transcriptional changes in RDV infected rice and revealed nucleolus as a possible target of RDV manipulation

BACKGROUND

Rice dwarf virus (RDV) is the causal agent of rice dwarf disease, which limits rice production in many areas of south East Asia. Transcriptional changes of rice in response to RDV infection have been characterized by Shimizu et al. and Satoh et al.. Both studies found induction of defense related genes and correlations between transcriptional changes and symptom development in RDV-infected rice. However, the same rice cultivar, namely Nipponbare belonging to the Japonic subspecies of rice was used in both studies.

METHODS

Gene expression changes of the indica subspecies of rice, namely Oryza sativa L. ssp. indica cv Yixiang2292 that show moderate resistance to RDV, in response to RDV infection were characterized using an Affymetrix Rice Genome Array. Differentially expressed genes (DEGs) were classified according to their Gene Ontology (GO) annotation. The effects of transient expression of Pns11 in Nicotiana benthaminana on the expression of nucleolar genes were studied using real-time PCR (RT-PCR).

RESULTS

856 genes involved in defense or other physiological processes were identified to be DEGs, most of which showed up-regulation. Ribosome- and nucleolus related genes were significantly enriched in the DEGs. Representative genes related to nucleolar function exhibited altered expression in N. benthaminana plants transiently expressing Pns11 of RDV.

CONCLUSIONS

Induction of defense related genes is common for rice infected with RDV. There is a co-relation between symptom severity and transcriptional alteration in RDV infected rice. Besides ribosome, RDV may also target nucleolus to manipulate the translation machinery of rice. Given the tight links between nucleolus and ribosome, it is intriguing to speculate that RDV may enhance expression of ribosomal genes by targeting nucleolus through Pns11.

Infection of apple by apple stem grooving virus leads to extensive alterations in gene expression patterns but no disease symptoms

To understand the molecular basis of viral diseases, transcriptome profiling has been widely used to correlate host gene expression change patterns with disease symptoms during viral infection in many plant hosts. We used infection of apple by Apple stem grooving virus (ASGV), which produces no disease symptoms, to assess the significance of host gene expression changes in disease development. We specifically asked the question of whether such asymptomatic infection is attributed to limited changes in host gene expression. Using RNA-seq, we identified a total of 184 up-regulated and 136 down-regulated genes in apple shoot cultures permanently infected by ASGV in comparison with virus-free shoot cultures. As in most plant hosts showing disease symptoms during viral infection, these differentially expressed genes encode known or putative proteins involved in cell cycle, cell wall biogenesis, response to biotic and abiotic stress, development and fruit ripening, phytohormone function, metabolism, signal transduction, transcription regulation, translation, transport, and photosynthesis. Thus, global host gene expression changes do not necessarily lead to virus disease symptoms. Our data suggest that the general approaches to correlate host gene expression changes under viral infection conditions to specific disease symptom, based on the interpretation of transcription profiling data and altered individual gene functions, may have limitations depending on particular experimental systems.

Quantitative transcriptional changes associated with chlorosis severity in mosaic leaves of tobacco plants infected with Cucumber mosaic virus

Cucumber mosaic virus (CMV) causes mosaic disease in inoculated tobacco plants. Coat protein (CP) is one of the major virulence determinants of CMV, and an amino acid substitution at residue 129 in CP alters the severity of chlorosis, such as pale green chlorosis and white chlorosis, in symptomatic tissues of mosaic leaves of infected tobacco. In this study, we compared the transcriptomes of chlorotic tissues infected with the wild-type pepo strain of CMV and two strains carrying CP mutants with diverse chlorosis severity. Differential gene expression analysis showed that CMV inoculation appeared to have similar effects on the transcriptional expression profiles of the symptomatic chlorotic tissues, and only the magnitude of expression differed among the different CMVs. Gene ontology analysis with biological process and cellular component terms revealed that many nuclear genes related to abiotic stress responses, including responses to cadmium, heat, cold and salt, were up-regulated, whereas chloroplast- and photosynthesis-related genes (CPRGs) were down-regulated, in the chlorotic tissues. Interestingly, the level of CPRG down-regulation was correlated with the severity of chlorosis. These results indicate that CP mutation governs the repression level and mRNA accumulation of CPRGs, which are closely associated with the induction of chlorosis.

Suppression of cell wall-related genes associated with stunting of Oryza glaberrima infected with Rice tungro spherical virus

Rice tungro disease is a complex disease caused by the interaction between Rice tungro bacilliform virus and Rice tungro spherical virus (RTSV). RTSV alone does not cause recognizable symptoms in most Asian rice (Oryza sativa) plants, whereas some African rice (O. glaberrima) plants were found to become stunted by RTSV. Stunting of rice plants by virus infections usually accompanies the suppression of various cell wall-related genes. The expression of cell wall-related genes was examined in O. glaberrima and O. sativa infected with RTSV to see the relationship between the severity of stunting and the suppression of cell wall-related genes by RTSV. The heights of four accessions of O. glaberrima were found to decline by 14-34% at 28 days post-inoculation (dpi) with RTSV, whereas the height reduction of O. sativa plants by RTSV was not significant. RTSV accumulated more in O. glaberrima plants than in O. sativa plants, but the level of RTSV accumulation was not correlated with the degree of height reduction among the four accessions of O. glaberrima. Examination for expression of genes for cellulose synthase A5 (CESA5) and A6 (CESA6), cellulose synthase-like A9 (CSLA9) and C7, and α-expansin 1 (expansin 1) and 15 precursors in O. glaberrima and O. sativa plants between 7 and 28 dpi with RTSV showed that the genes such as those for CESA5, CESA6, CSLA9, and expansin 1were more significantly suppressed in stunted plants of O. glaberrima at 14 dpi with RTSV than in O. sativa, suggesting that stunting of O. glaberrima might be associated with these cell wall-related genes suppressed by RTSV. Examination for expression of these genes in O. sativa plants infected with other rice viruses in previous studies indicated that the suppression of the expansin 1 gene is likely to be a signature response commonly associated with virus-induced stunting of Oryza species. These results suggest that stunting of O. glaberrima by RTSV infection might be associated with the suppression of these cell wall-related genes at the early stage of infection with RTSV.

Deep sequencing-based transcriptome profiling reveals comprehensive insights into the responses of Nicotiana benthamiana to beet necrotic yellow vein virus infections containing or lacking RNA4

BACKGROUND

Beet necrotic yellow vein virus (BNYVV), encodes either four or five plus-sense single stranded RNAs and is the causal agent of sugar beet rhizomania disease, which is widely distributed in most regions of the world. BNYVV can also infect Nicotiana benthamiana systemically, and causes severe curling and stunting symptoms in the presence of RNA4 or mild symptoms in the absence of RNA4.

RESULTS

Confocal laser scanning microscopy (CLSM) analyses showed that the RNA4-encoded p31 protein fused to the red fluorescent protein (RFP) accumulated mainly in the nuclei of N. benthamiana epidermal cells. This suggested that severe RNA4-induced symptoms might result from p31-dependent modifications of the transcriptome. Therefore, we used next-generation sequencing technologies to analyze the transcriptome profile of N. benthamiana in response to infection with different isolates of BNYVV. Comparisons of the transcriptomes of mock, BN3 (RNAs 1+2+3), and BN34 (RNAs 1+2+3+4) infected plants identified 3,016 differentially expressed transcripts, which provided a list of candidate genes that potentially are elicited in response to virus infection. Our data indicate that modifications in the expression of genes involved in RNA silencing, ubiquitin-proteasome pathway, cellulose synthesis, and metabolism of the plant hormone gibberellin may contribute to the severe symptoms induced by RNA4 from BNYVV.

CONCLUSIONS

These results expand our understanding of the genetic architecture of N. benthamiana as well as provide valuable clues to identify genes potentially involved in resistance to BNYVV infection. Our global survey of gene expression changes in infected plants reveals new insights into the complicated molecular mechanisms underlying symptom development, and aids research into new strategies to protect crops against viruses.

Gene response in rice plants treated with continuous fog influenced by pH, was similar to that treated with biotic stress

BACKGROUND

Throughout Asia, including Japan, rice plants are cultivated in a wide range of areas from lowlands to highlands and are frequently exposed to fog, including acid fog. Some physiological studies have shown that acid fog can be a stress factor for plants. We analyzed the gene expression profiles of rice plants treated with artificially prepared simulated acid fog (SiAF) or simulated neutral fog (SiNF) for 1 or 7 days.

RESULTS

Microarray analysis results suggested that both the SiAF and the SiNF treatments induced the expression of genes involved in the defense and stress responses in rice plants. Induction of such genes was detected in plants treated with SiAF for 1 day, and the number of induced genes increased in plants treated with SiAF for 7 days. The genes for defense and stress responses were also induced by SiNF for 7 days, although they were not induced by SiNF for 1 day. The gene expression profiles of the SiAF-treated and the SiNF-treated plants were compared to those of plants treated with other stress factors. The comparison revealed that both SiAF and SiNF treatments have similar effects to biotic stresses and ozone stress. The genes encoding NADPH oxidase and germin, which function in apoplasts, were also induced by SiAF, SiNF and biotic stresses.

CONCLUSIONS

These findings suggest that both the SiAF and the SiNF treatments may result in oxidative stress through the apoplastic production of reactive oxygen species.

Comparative transcriptome analysis of two rice varieties in response to rice stripe virus and small brown planthoppers during early interaction

Rice stripe, a virus disease, transmitted by a small brown planthopper (SBPH), has greatly reduced production of japonica rice in East Asia, especially in China. Although we have made great progress in mapping resistance genes, little is known about the mechanism of resistance.

By de novo transcriptome assembling, we gained sufficient transcript data to analyze changes in gene expression of early interaction in response to SBPH and RSV infection in rice. Respectively 648 and 937 DEGs were detected from the disease-resistant (Liaonong 979) and the susceptible (Fengjin) varieties, most of which were up-regulated. We found 37 genes related to insect resistance, which mainly included genes for jasmonate-induced protein, TIFY protein, lipoxygenase, as well as trypsin inhibitor genes and transcription factor genes. In the interaction process between RSV and rice, 87 genes were thought to be related to RSV resistance; these primarily included 12 peroxidase biosynthesis genes, 12 LRR receptor-like protein kinase genes, 6 genes coding pathogenesis-related proteins, 4 glycine-rich cell wall structural protein genes, 2 xyloglucan hydrolase genes and a cellulose synthase. The results indicate that the rice-pathogen interaction happened both in disease-resistant and susceptible varieties, and some genes related to JA biosynthesis played key roles in the interaction between SBPHs and rice. When rice was infected by RSV a hypersensitive reaction (HR) in the disease-resistant variety was suppressed, which resulted from an increase in peroxidase expression and down-regulation of LRR receptor-like protein kinase and pathogenesis-related proteins, while, the changes of peroxidase biosynthesis, glycine-rich cell wall structural protein, cellulose synthase and xyloglucan endotransglucosylase/hydrolase could lead to the strengthening of physical barriers of rice, which may be an important resistance mechanism to RSV in rice.

Relationship between gene responses and symptoms induced by Rice grassy stunt virus

Rice grassy stunt virus (RGSV) is a serious threat to rice production in Southeast Asia. RGSV is a member of the genus Tenuivirus, and it induces leaf yellowing, stunting, and excess tillering on rice plants. Here we examined gene responses of rice to RGSV infection to gain insight into the gene responses which might be associated with the disease symptoms. The results indicated that (1) many genes related to cell wall synthesis and chlorophyll synthesis were predominantly suppressed by RGSV infection; (2) RGSV infection induced genes associated with tillering process; (3) RGSV activated genes involved in inactivation of gibberellic acid and indole-3-acetic acid; and (4) the genes for strigolactone signaling were suppressed by RGSV. These results suggest that these gene responses to RGSV infection account for the excess tillering specific to RGSV infection as well as other symptoms by RGSV, such as stunting and leaf chlorosis.

Identification of host proteins modulated by the virulence factor AC2 of Tomato chlorotic mottle virus in Nicotiana benthamiana

Tomato, one of the most important crops cultivated worldwide, has been severely affected by begomoviruses such as the Tomato chlorotic mottle virus (ToCMoV). Virulence factor AC2 is considered crucial for a successful virus-plant interaction and is known to act as a transcriptional activator and in some begomoviruses to function as an RNA silencing suppressor factor. However, the exact functions of the AC2 protein of the begomovirus ToCMoV are not yet established. The aim of the present study was to identify differentially expressed proteins of the model plant Nicotiana benthamiana in response to the expression of the AC2 gene, isolated from ToCMoV. N. benthamiana plants were inoculated with Agrobacterium tumefaciens containing the viral vector Potato virus X (PVX) and with the PVX-AC2 construction. 2DE was performed and proteins were identified by MS. The results showed that the expression of ToCMoV AC2 alters the levels of several host proteins, which are important for normal plant development, causing an imbalance in cellular homeostasis. This study highlights the effect of AC2 in the modulation of plant defense processes by increasing the expression of several oxidative stress-related and pathogenesis-related proteins, as well as its role in modulating the proteome of the photosynthesis and energy production systems.

Discovering host genes involved in the infection by the Tomato Yellow Leaf Curl Virus complex and in the establishment of resistance to the virus using Tobacco Rattle Virus-based post transcriptional gene silencing

The development of high-throughput technologies allows for evaluating gene expression at the whole-genome level. Together with proteomic and metabolomic studies, these analyses have resulted in the identification of plant genes whose function or expression is altered as a consequence of pathogen attacks. Members of the Tomato yellow leaf curl virus (TYLCV) complex are among the most important pathogens impairing production of agricultural crops worldwide. To understand how these geminiviruses subjugate plant defenses, and to devise counter-measures, it is essential to identify the host genes affected by infection and to determine their role in susceptible and resistant plants. We have used a reverse genetics approach based on Tobacco rattle virus-induced gene silencing (TRV-VIGS) to uncover genes involved in viral infection of susceptible plants, and to identify genes underlying virus resistance. To identify host genes with a role in geminivirus infection, we have engineered a Nicotiana benthamiana line, coined 2IRGFP, which over-expresses GFP upon virus infection. With this system, we have achieved an accurate description of the dynamics of virus replication in space and time. Upon silencing selected N. benthamiana genes previously shown to be related to host response to geminivirus infection, we have identified eighteen genes involved in a wide array of cellular processes. Plant genes involved in geminivirus resistance were studied by comparing two tomato lines: one resistant (R), the other susceptible (S) to the virus. Sixty-nine genes preferentially expressed in R tomatoes were identified by screening cDNA libraries from infected and uninfected R and S genotypes. Out of the 25 genes studied so far, the silencing of five led to the total collapse of resistance, suggesting their involvement in the resistance gene network. This review of our results indicates that TRV-VIGS is an exquisite reverse genetics tool that may provide new insights into the molecular mechanisms underlying plant infection and resistance to infection by begomoviruses.