Histone deacetylase OsHDA706 orchestrates rice broad-spectrum antiviral immunity and is impeded by a viral effector.

Lysine acetylation is a dynamic post-translational modification of proteins. Extensive studies have revealed that the acetylation modulated by histone acetyltransferases and histone deacetylases (HDACs) plays a crucial role in regulating protein function. However, there has been limited focus on how HDACs regulate jasmonic acid (JA) biosynthesis in plants. Here, we uncover that the protein stability of OsLOX14, a critical enzyme involved in JA biosynthesis, is regulated by a histone deacetylase, OsHDA706, and is hindered by a viral protein. Our results show that OsHDA706 deacetylates OsLOX14 and enhances the stability of OsLOX14, leading to JA accumulation and an improved broad-spectrum rice antiviral defense. Furthermore, we found that the viral protein P2, encoded by the destructive rice stripe virus, disrupts the association of OsHDA706-OsLOX14, promoting viral infection. Overall, our findings reveal how HDAC manipulates the interplay of deacetylation and protein stability of a JA biosynthetic enzyme to enhance plant antiviral responses.

Two different viral proteins suppress NUCLEAR FACTOR-YC-mediated antiviral immunity during infection in rice.

Plant viruses have multiple strategies to counter and evade the host's antiviral immune response. However, limited research has been conducted on the antiviral defense mechanisms commonly targeted by distinct types of plant viruses. In this study, we discovered that NUCLEAR FACTOR-YC (NF-YC) and NUCLEAR FACTOR-YA (NF-YA), 2 essential components of the NF-Y complex, were commonly targeted by viral proteins encoded by 2 different rice (Oryza sativa L.) viruses, rice stripe virus (RSV, Tenuivirus) and southern rice black streaked dwarf virus (SRBSDV, Fijivirus). In vitro and in vivo experiments showed that OsNF-YCs associate with OsNF-YAs and inhibit their transcriptional activation activity, resulting in the suppression of OsNF-YA-mediated plant susceptibility to rice viruses. Different viral proteins RSV P2 and SRBSDV SP8 directly disrupted the association of OsNF-YCs with OsNF-YAs, thereby suppressing the antiviral defense mediated by OsNF-YCs. These findings suggest an approach for conferring broad-spectrum disease resistance in rice and reveal a common mechanism employed by viral proteins to evade the host's antiviral defense by hindering the antiviral capabilities of OsNF-YCs.

Alternative splicing impacts the rice stripe virus response transcriptome.

Alternative splicing (AS) is an important form of post transcriptional modification present in both animals and plants. However, little information was obtained about AS events in response to plant virus infection. In this study, we conducted a genome-wide transcriptome analysis on AS change in rice infected by a devastating virus, Rice stripe virus (RSV). KEGG analysis was performed on the differentially expressed (DE) genes and differentially alternative spliced (DAS) genes. The results showed that DE genes were significantly enriched in the pathway of interaction with plant pathogens. The DAS genes were mainly enriched in basal metabolism and RNA splicing pathways. The heat map clustering showed that DEGs clusters were mainly enriched in regulation of transcription and defense response while differential transcript usage (DTU) clusters were strongly enriched in mRNA splicing and calcium binding. Overall, our results provide a fundamental basis for gene-wide AS changes in rice after RSV infection.

Different viral effectors suppress hormone-mediated antiviral immunity of rice coordinated by OsNPR1.

Salicylic acid (SA) and jasmonic acid (JA) are plant hormones that typically act antagonistically in dicotyledonous plants and SA and JA signaling is often manipulated by pathogens. However, in monocotyledonous plants, the detailed SA-JA interplay in response to pathogen invasion remains elusive. Here, we show that different types of viral pathogen can disrupt synergistic antiviral immunity mediated by SA and JA via OsNPR1 in the monocot rice. The P2 protein of rice stripe virus, a negative-stranded RNA virus in the genus Tenuivirus, promotes OsNPR1 degradation by enhancing the association of OsNPR1 and OsCUL3a. OsNPR1 activates JA signaling by disrupting the OsJAZ-OsMYC complex and boosting the transcriptional activation activity of OsMYC2 to cooperatively modulate rice antiviral immunity. Unrelated viral proteins from different rice viruses also interfere with the OsNPR1-mediated SA-JA interplay to facilitate viral pathogenicity, suggesting that this may be a more general strategy in monocot plants. Overall, our findings highlight that distinct viral proteins convergently obstruct JA-SA crosstalk to facilitate viral infection in monocot rice.

A secreted salivary effector from Riptortus pedestris impairs soybean defense through modulating phytohormone signaling pathways.

Riptortus pedestris (Fabricius), one of the major piercing-sucking insects in soybeans, causes delayed plant senescence and abnormal pods, known as staygreen syndrome. Recent research has shown that direct feeding of this insect is the major cause of soybean staygreen syndrome. However, it remains unclear whether R. pedestris salivary proteins play vital roles in insect infestation. Here, we found that 4 secretory salivary proteins can induce cell death in Nicotiana benthamiana by transient heterologous expression. The cell death induced by Rp2155 relies on the nucleotide-binding leucine-rich repeat helper, HSP90. Tissue-specificity assays indicated that Rp2155 is specifically expressed in the salivary gland of R. pedestris and is significantly induced during insect feeding. The expression of salicylic acid (SA)-, jasmonic acid (JA)-related genes was increased in soybean when fed by Rp2155-silenced R. pedestris. More importantly, soybean staygreen symptoms caused by R. pedestris were significantly alleviated when Rp2155 was silenced. Together, these results suggest that the salivary effector Rp2155 is involved in promoting insect infestation by suppressing the JA and SA pathways, and it can be considered as a potential RNA interference target for insect control.

A Rice Receptor-like Protein Negatively Regulates Rice Resistance to Southern Rice Black-Streaked Dwarf Virus Infection.

Plants rely on various receptor-like proteins and receptor-like kinases to recognize and defend against invading pathogens. However, research on the role of receptor-like proteins in plant antiviral defense, particularly in rice-virus interactions, is limited. In this study, we identified a receptor-like gene, OsBAP1, which was significantly induced upon infection with southern rice black-streaked dwarf virus (SRBSDV) infection. A viral inoculation assay showed that the OsBAP1 knockout mutant exhibited enhanced resistance to SRBSDV infection, indicating that OsBAP1 plays a negatively regulated role in rice resistance to viral infection. Transcriptome analysis revealed that the genes involved in plant-pathogen interactions, plant hormone signal transduction, oxidation-reduction reactions, and protein phosphorylation pathways were significantly enriched in OsBAP1 mutant plants (osbap1-cas). Quantitative real-time PCR (RT-qPCR) analysis further demonstrated that some defense-related genes were significantly induced during SRBSDV infection in osbap1-cas mutants. Our findings provide new insights into the role of receptor-like proteins in plant immune signaling pathways, and demonstrate that OsBAP1 negatively regulates rice resistance to SRBSDV infection.

A salivary secretory protein from Riptortus pedestris facilitates pest infestation and soybean staygreen syndrome.

The bean bug (Riptortus pedestris), one of the most important pests of soybean, causes staygreen syndrome, delaying plant maturation and affecting pod development, resulting in severe crop yield loss. However, little is known about the underlying mechanism of this pest. In this study, we found that a salivary secretory protein, Rp614, induced cell death in nonhost Nicotiana benthamiana leaves. NbSGT1 and NbNDR1 are involved in Rp614-induced cell death. Tissue specificity analysis showed that Rp614 is mainly present in salivary glands and is highly induced during pest feeding. RNA interference experiments showed that staygreen syndrome caused by R. pedestris was significantly attenuated when Rp614 was silenced. Together, our results indicate that Rp614 plays an essential role in R. pedestris infestation and provide a promising RNA interference target for pest control.

Transcriptome analysis of auxin transcription factor OsARF17-mediated rice stripe mosaic virus response in rice.

Introduction: Plant auxin response factors (ARFs) play an irreplaceable role in regulating the expression of auxin response genes. Our previous studies have indicated that auxin response factor OsARF17 plays a crucial role in plant defense against diverse rice viruses. Methods: Utilizing a comparative transcriptome analysis of Rice stripe mosaic virus (RSMV)-inoculated OsARF17 mutant rice plants, to further elucidate the molecular mechanism of OsARF17 in antiviral defense pathway. Results: KEGG enrichment analyses showed that the down-regulated differentially expressed genes (DEGs) belonged to plant-pathogen interaction and plant hormone signal transduction pathways were markedly enriched in OsARF17 mutants under RSMV inoculation. Furthermore, Gene ontology (GO) analyses revealed that these genes were enriched in a variety of hormone biosynthetic process, including jasmonic acid (JA), auxin, and abscisic acid (ABA). RT-qPCR assays showed that the induction of plant defense-related genes, such as WRKY transcription factors, OsAHT2 and OsDR8, and JA-related genes, were significantly suppressed in OsARF17 mutants in response to RSMV. Discussion: Our study reveals that OsARF17-mediated antiviral immunity may be achieved through affecting the interaction between different phytohormones and regulating defense gene expression in rice. This study provides new insights into the molecular mechanisms of auxin signaling in the rice-virus interaction.

Transcriptional profiling reveals a critical role of GmFT2a in soybean staygreen syndrome caused by the pest Riptortus pedestris.

Soybean staygreen syndrome, characterized by delayed leaf and stem senescence, abnormal pods, and aborted seeds, has recently become a serious and prominent problem in soybean production. Although the pest Riptortus pedestris has received increasing attention as the possible cause of staygreen syndrome, the mechanism remains unknown. Here, we clarify that direct feeding by R. pedestris, not transmission of a pathogen by this pest, is the primary cause of typical soybean staygreen syndrome and that critical feeding damage occurs at the early pod stage. Transcriptome profiling of soybean indicated that many signal transduction pathways, including photoperiod, hormone, defense response, and photosynthesis, respond to R. pedestris infestation. Importantly, we discovered that members of the FLOWERING LOCUS T (FT) gene family were suppressed by R. pedestris infestation, and overexpression of floral inducer GmFT2a attenuates staygreen symptoms by mediating soybean defense response and photosynthesis. Together, our findings systematically illustrate the association between pest infestation and soybean staygreen syndrome and provide the basis for establishing a targeted soybean pest prevention and control system.

Independently evolved viral effectors convergently suppress DELLA protein SLR1-mediated broad-spectrum antiviral immunity in rice.

Plant viruses adopt diverse virulence strategies to inhibit host antiviral defense. However, general antiviral defense directly targeted by different types of plant viruses have rarely been studied. Here, we show that the single rice DELLA protein, SLENDER RICE 1 (SLR1), a master negative regulator in Gibberellin (GA) signaling pathway, is targeted by several different viral effectors for facilitating viral infection. Viral proteins encoded by different types of rice viruses all directly trigger the rapid degradation of SLR1 by promoting association with the GA receptor OsGID1. SLR1-mediated broad-spectrum resistance was subverted by these independently evolved viral proteins, which all interrupted the functional crosstalk between SLR1 and jasmonic acid (JA) signaling. This decline of JA antiviral further created the advantage of viral infection. Our study reveals a common viral counter-defense strategy in which different types of viruses convergently target SLR1-mediated broad-spectrum resistance to benefit viral infection in the monocotyledonous crop rice.

Insights Into the Effect of Rice Stripe Virus P2 on Rice Defense by Comparative Proteomic Analysis.

Rice stripe virus (RSV) has a serious effect on rice production. Our previous research had shown that RSV P2 plays important roles in RSV infection, so in order to further understand the effect of P2 on rice, we used Tandem Mass Tag (TMT) quantitative proteomics experimental system to analyze the changes of protein in transgenic rice expressing P2 for the first time. The results of proteomics showed that a total of 4,767 proteins were identified, including 198 up-regulated proteins and 120 down-regulated proteins. Functional classification results showed that differentially expressed proteins (DEPs) were mainly localized in chloroplasts and mainly involved in the metabolic pathways. Functional enrichment results showed that DEPs are mainly involved in RNA processing and splicing. We also verified the expression of several DEPs at the mRNA level and the interaction of a transcription factor (B7EPB8) with RSV P2. This research is the first time to use proteomics technology to explore the mechanism of RSV infection in rice with the RSV P2 as breakthrough point. Our findings provide valuable information for the study of RSV P2 and RSV infection mechanism.

The C-Terminal Transmembrane Domain of Cowpea Mild Mottle Virus TGBp2 Is Critical for Plasmodesmata Localization and for Its Interaction With TGBp1 and TGBp3.

The movement of some plant RNA viruses is mediated by triple gene block (TGB) proteins, which cooperate to transfer the viral genome from cell to cell through plasmodesmata. Here, we investigated the function of the TGB proteins of cowpea mild mottle virus (CPMMV; genus Carlavirus, family Betaflexiviridae), which causes severe damage to soybean production. Subcellular localization experiments demonstrated that TGBp1 and TGBp3 were localized to the endoplasmic reticulum (ER), plasmodesmata (PD) and nucleus in Nicotiana benthamiana leaves. TGBp2 was unusually localized to PD. In protein interaction assays TGBp2 significantly enhanced the interaction between TGBp3 and TGBp1. Interaction assays using deletion mutants showed that the C-terminal transmembrane (TM) domain of TGBp2 is critical for its localization to PD and for its interaction with TGBp1 and TGBp3.

NF-YA transcription factors suppress jasmonic acid-mediated antiviral defense and facilitate viral infection in rice.

NF-Y transcription factors are known to play many diverse roles in the development and physiological responses of plants but little is known about their role in plant defense. Here, we demonstrate the negative roles of rice NF-YA family genes in antiviral defense against two different plant viruses, Rice stripe virus (RSV, Tenuivirus) and Southern rice black-streaked dwarf virus (SRBSDV, Fijivirus). RSV and SRBSDV both induced the expression of OsNF-YA family genes. Overexpression of OsNF-YAs enhanced rice susceptibility to virus infection, while OsNF-YAs RNAi mutants were more resistant. Transcriptome sequencing showed that the expression of jasmonic acid (JA)-related genes was significantly decreased in plants overexpressing OsNF-YA when they were infected by viruses. qRT-PCR and JA sensitivity assays confirmed that OsNF-YAs play negative roles in regulating the JA pathway. Further experiments showed that OsNF-YAs physically interact with JA signaling transcription factors OsMYC2/3 and interfere with JA signaling by dissociating the OsMYC2/3-OsMED25 complex, which inhibits the transcriptional activation activity of OsMYC2/3. Together, our results reveal that OsNF-YAs broadly inhibit plant antiviral defense by repressing JA signaling pathways, and provide new insight into how OsNF-YAs are directly associated with the JA pathway.

Genome-Wide Identification and Gene Expression Analysis of the OTU DUB Family in Oryza sativa.

Ovarian tumor domain (OTU)-containing deubiquitinating enzymes (DUBs) are an essential DUB to maintain protein stability in plants and play important roles in plant growth development and stress response. However, there is little genome-wide identification and analysis of the OTU gene family in rice. In this study, we identified 20 genes of the OTU family in rice genome, which were classified into four groups based on the phylogenetic analysis. Their gene structures, conserved motifs and domains, chromosomal distribution, and cis elements in promoters were further studied. In addition, OTU gene expression patterns in response to plant hormone treatments, including SA, MeJA, NAA, BL, and ABA, were investigated by RT-qPCR analysis. The results showed that the expression profile of OsOTU genes exhibited plant hormone-specific expression. Expression levels of most of the rice OTU genes were significantly changed in response to rice stripe virus (RSV), rice black-streaked dwarf virus (RBSDV), Southern rice black-streaked dwarf virus (SRBSDV), and Rice stripe mosaic virus (RSMV). These results suggest that the rice OTU genes are involved in diverse hormone signaling pathways and in varied responses to virus infection, providing new insights for further functional study of OsOTU genes.

A rice LRR receptor-like protein associates with its adaptor kinase OsSOBIR1 to mediate plant immunity against viral infection.

Plants sense pathogen attacks using a variety of receptors at the cell surface. The LRR receptor-like proteins (RLP) and receptor-like kinases (RLK) are widely reported to participate in plant defence against bacterial and fungal pathogen invasion. However, the role of RLP and RLK in plant antiviral defence has rarely been reported. We employed a high-throughput-sequencing approach, transgenic rice plants and viral inoculation assays to investigate the role of OsRLP1 and OsSOBIR1 proteins in rice immunity against virus infection. The transcript of a rice LRR receptor-like protein, OsRLP1, was markedly up-regulated following infection by RBSDV, a devastating pathogen of rice and maize. Viral inoculation on various OsRLP1 mutants demonstrated that OsRLP1 modulates rice resistance against RBSDV infection. It was also shown that OsRLP1 is involved in the RBSDV-induced defence response by positively regulating the activation of MAPKs and PTI-related gene expression. OsRLP1 interacted with a receptor-like kinase OsSOBIR1, which was shown to regulate the PTI response and rice antiviral defence. Our results offer a novel insight into how a virus-induced receptor-like protein and its adaptor kinase activate the PTI response and antiviral defence in rice.

Identification of a New Genetic Clade of Cowpea Mild Mottle Virus and Characterization of Its Interaction With Soybean Mosaic Virus in Co-infected Soybean.

Cowpea mild mottle virus (CPMMV; genus Carlavirus) can be a destructive pathogen of soybean but there is little information about its distribution on soybean in China. Here, we collected soybean plants with virus-like symptoms from 11 fields widely scattered within China, and used high-throughput sequencing to determine their virome. Most samples (8/11) were co-infected by the well-studied potyvirus soybean mosaic virus (SMV) and CPMMV, and the remaining three samples were singly infected with CPMMV. The near-complete genome sequences of the 11 CPMMV isolates were determined and phylogenetic analysis showed that they constituted a new genetic clade. One recombination event was detected among the CPMMV sequences, and the isolate CPMMV_JL_CC was identified as recombinant. In mechanical inoculation assays, co-infection by CPMMV and SMV resulted in an enhancement of disease symptoms, but decreased the expression level of the genomic RNAs and CP of CPMMV, without significantly affecting SMV accumulation. The interaction between these viruses needs further investigation.

Genome-Wide Analysis of the RAV Transcription Factor Genes in Rice Reveals Their Response Patterns to Hormones and Virus Infection.

The RAV family is part of the B3 superfamily and is one of the most abundant transcription factor families in plants. Members have highly conserved B3 or AP2 DNA binding domains. Although the RAV family genes of several species have been systematically identified from genome-wide studies, there has been no comprehensive study to identify rice RAV family genes. Here, we identified 15 genes of the RAV family in the rice genome and analyzed their phylogenetic relationships, gene structure, conserved domains, and chromosomal distribution. Based on domain similarity and phylogenetic topology, rice RAV transcription factors were phylogenetically clustered into four groups. qRT-PCR analyses showed that expression of these RAV genes was significantly up-regulated or down-regulated by plant hormone treatments, including BL, NAA, IAA, MeJA, and SA. Most of the rice RAV genes were dramatically down-regulated in response to rice stripe virus (RSV) and mostly up-regulated in response to Southern rice black-streaked dwarf virus (SRBSDV). These results suggest that the rice RAV genes are involved in diverse signaling pathways and in varied responses to virus infection.

A class of independently evolved transcriptional repressors in plant RNA viruses facilitates viral infection and vector feeding.

Plant viruses employ diverse virulence strategies to achieve successful infection, but there are few known general strategies of viral pathogenicity and transmission used by widely different plant viruses. Here, we report a class of independently evolved virulence factors in different plant RNA viruses which possess active transcriptional repressor activity. Rice viruses in the genera Fijivirus, Tenuivirus, and Cytorhabdovirus all have transcriptional repressors that interact in plants with the key components of jasmonic acid (JA) signaling, namely mediator subunit OsMED25, OsJAZ proteins, and OsMYC transcription factors. These transcriptional repressors can directly disassociate the OsMED25-OsMYC complex, inhibit the transcriptional activation of OsMYC, and then combine with OsJAZ proteins to cooperatively attenuate the JA pathway in a way that benefits viral infection. At the same time, these transcriptional repressors efficiently enhanced feeding by the virus insect vectors by repressing JA signaling. Our findings reveal a common strategy in unrelated plant viruses in which viral transcriptional repressors hijack and repress the JA pathway in favor of both viral pathogenicity and vector transmission.

Occurrence of Soybean Yellow Common Mosaic Virus in Soybean in China Showing Yellow Common Mosaic Disease.

Soybean yellow common mosaic virus (SYCMV), a positive sense ssRNA virus classified in the genus Sobemovirus, was first reported and characterized in Korea (Nam et al., 2012). Currently, its only known host is soybean (Nam et al., 2012) on which it causes bright yellow mosaic and crinkling of the leaves (Lim et al., 2016). During a field survey in July 2019, bright yellow mosaic and mild crinkling symptoms were observed on soybean leaves (cv. Zhonghuang 13) in the Hubei province of China. To identify the possible pathogen(s) associated to the disease symptoms, leaves from five symptomatic plants were collected, pooled and total RNA was extracted using TRIzol® Reagent (Invitrogen, CA, USA). 10 µg of the total RNA was purified via magnetic beads (Thermo Fischer Scientific, USA) and a TruSeq RNA Sample Prep Kit (Illumina, San Diego, CA, USA) was then used to construct an RNA sequencing library. Transcriptome sequencing was performed on an Illumina HiSeq 4000 (LC Sciences, USA). The average insert size for the paired-end library was 300 ± 50 bp. After quality control, a total of 47.5 million clean reads were obtained and assembled using the Trinity software (version 2.8.5). The assembled contigs were searched against NCBI virus RefSeqs (ftp://ftp.ncbi.nlm.nih.gov/refseq/release/viral) by the BLASTx algorithm with a cutoff E value of ≤10-5. 12 contigs sized from 3,421 to 4,093 bp were found to share a sequence identity of 77.5%-94.1% with SYCMV isolates from Japan (LC332541) and South Korea (JF495127.1). No other virus matches were identified. The largest contig (4,093 bp, MT816507) covers 99% of the expected complete genome of SYCMV (4,121 bp, KX096577). To verify the accuracy of the sequence assembled, RT-PCR-Sanger sequencing was performed on a single field plant sample using primers designed for SYCMV (Forward, 5'-GAACAAAGAGTCTGGATCTT-3'; Reverse, 5'-TCCTTCCAAAACCTCGCGGG-3'). The sequence of the amplicon (3854 bp, MT997092) exhibited an identity of 99.9% to the HTS-derived SYCMV contig sequence. Phylogenetic analysis of the amplicon sequence revealed that the SYCMV isolate from China formed a distinct branch in the tree (Fig. S1). Sap from symptomatic field plants was used to mechanically inoculate two soybean cultivars (Jiunong 9 and Kefeng 1, 10 plants per cultivar), and leaves inoculated with phosphate buffer saline (PBS, 0.01 M, pH 7.5) served as a control (3 plants per cultivar). All but the control plants developed systemic bright yellow mosaic symptoms 10 days after inoculation (Fig. S2A). The infection of the soybean plants with SYCMV was confirmed by RT-PCR with the newly designed primers for SYCMV (Forward, 5'- CCTACAGGCATTGGTTTCGT-3'; Reverse, 5'-CGTGAGGTTCTTGCTTCACA-3', anticipated amplicon size: 2,210 bp) (Fig. S2B) and by amplicon sequencing (100% sequence identity with MT9979092). In addition, the infection was further confirmed by immuno-blotting using an antibody against SYCMV coat protein (synthesized by GenScript, USA) (Fig. S2C). Together, the results demonstrate that SYCMV is the causal agent of the bright yellow mosaic symptoms in soybean observed in the field. To the best of our knowledge, this is the first report of SYCMV on soybean in China. These findings shall not only alert local growers to a potential new threat to soybean production in their region, but also provide new insights on the transmission, epidemiology and pathological properties of SYCMV in China.

Rice stripe virus coat protein induces the accumulation of jasmonic acid, activating plant defence against the virus while also attracting its vector to feed.

The jasmonic acid (JA) pathway plays crucial roles in plant defence against pathogens and herbivores. Rice stripe virus (RSV) is the type member of the genus Tenuivirus. It is transmitted by the small brown planthopper (SBPH) and causes damaging epidemics in East Asia. The role(s) that JA may play in the tripartite interaction against RSV, its host, and vector are poorly understood. Here, we found that the JA pathway was induced by RSV infection and played a defence role against RSV. The coat protein (CP) was the major viral component responsible for inducing the JA pathway. Methyl jasmonate treatment attracted SBPHs to feed on rice plants while a JA-deficient mutant was less attractive than wild-type rice. SBPHs showed an obvious preference for feeding on transgenic rice lines expressing RSV CP. Our results demonstrate that CP is an inducer of the JA pathway that activates plant defence against RSV while also attracting SBPHs to feed and benefitting viral transmission. This is the first report of the function of JA in the tripartite interaction between RSV, its host, and its vector.