OsRFPH2-10, a ring-H2 finger E3 ubiquitin ligase, is involved in rice antiviral defense in the early stages of rice dwarf virus infection

A distinct protein posttranslational modifications-linked OsATL32-OsPPKL2-OsGSK2 loop modulates rice immunity against blast disease

Protein posttranslational modifications play crucial roles in plant immunity through modulating a complicated signaling network mediated by different hormones. We previously demonstrated that OsATL32, an ATL-type E3 ligase, negatively contributes to rice immunity against Magnaporthe oryzae. Here, we show that OsATL32 forms a loop with OsPPKL2 and OsGSK2 through distinct protein posttranslational modifications to modulate rice immunity. OsATL32 ubiquitinates OsPPKL2, a protein phosphatase with Kelch-like repeat domains that exerts positive roles in regulating rice immunity against M. oryzae and chitin-triggered immune responses, for degradation. The glycogen synthase kinase 2 (OsGSK2), which acts as a negative regulator of rice immunity against M. oryzae and chitin-triggered immune responses, phosphorylates OsATL32 to elevate its protein stability and E3 ligase activity on OsPPKL2. Moreover, OsPPKL2 directly dephosphorylates OsGSK2, affecting its kinase activity on substrates including OsATL32 for phosphorylation. Like OsGSK2 as a BR signaling repressor, OsATL32 negatively regulates BR signaling; conversely, OsPPKL2 plays a positive role in BR signaling. These findings provide a molecular mechanism in which OsATL32 serves as a node connecting BR signaling and immunity by associating with OsPPKL2 and OsGSK2, assembling into a distinct protein posttranslational modifications-linked loop that functions in rice BR signaling and immunity.

OsATL32 ubiquitinates the reactive oxygen species-producing OsRac5-OsRbohB module to suppress rice immunity

Ubiquitination-mediated protein degradation is integral to plant immunity, with E3 ubiquitin ligases acting as key factors in this process. Here, we report the functions of OsATL32, a plasma membrane-localized Arabidopsis Tóxicos En Levadura (ATL)-type E3 ubiquitin ligase, in rice (Oryza sativa) immunity and its associated regulatory network. We found that the expression of OsATL32 is downregulated in both compatible and incompatible interactions between rice and the rice blast fungus Magnaporthe oryzae. The OsATL32 protein level declines in response to infection by a compatible M. oryzae strain or to chitin treatment. OsATL32 negatively regulates rice resistance to blast and bacterial leaf blight diseases, as well as chitin-triggered immunity. Biochemical and genetic studies revealed that OsATL32 suppresses pathogen-induced reactive oxygen species (ROS) accumulation by mediating ubiquitination and degradation of the ROS-producing OsRac5-OsRbohB module, which enhances rice immunity against M. oryzae. The protein phosphatase PHOSPHATASE AND TENSIN HOMOLOG enhances rice blast resistance by dephosphorylating OsATL32 and promoting its degradation, preventing its negative effect on rice immunity. This study provides insights into the molecular mechanism by which the E3 ligase OsATL32 targets a ROS-producing module to undermine rice immunity.

Rice stripe mosaic virus hijacks rice heading-related gene to promote the overwintering of its insect vector

Rice stripe mosaic virus (RSMV) is an emerging pathogen which significantly reduces rice yields in the southern region of China. It is transmitted by the leafhopper Recilia dorsalis, which overwinters in rice fields. Our field investigations revealed that RSMV infection causes delayed rice heading, resulting in a large number of green diseased plants remaining in winter rice fields. This creates a favorable environment for leafhoppers and viruses to overwinter, potentially contributing to the rapid spread and epidemic of the disease. Next, we explored the mechanism by which RSMV manipulates the developmental processes of the rice plant. A rice heading-related E3 ubiquitin ligase, Heading date Associated Factor 1 (HAF1), was found to be hijacked by the RSMV-encoded P6. The impairment of HAF1 function affects the ubiquitination and degradation of downstream proteins, HEADING DATE 1 and EARLY FLOWERING3, leading to a delay in rice heading. Our results provide new insights into the development regulation-based molecular interactions between virus and plant, and highlights the importance of understanding virus-vector-plant tripartite interactions for effective disease management strategies.

Plant virology in the 21st century in China: Recent advances and future directions

Plant viruses are a group of intracellular pathogens that persistently threaten global food security. Significant advances in plant virology have been achieved by Chinese scientists over the last 20 years, including basic research and technologies for preventing and controlling plant viral diseases. Here, we review these milestones and advances, including the identification of new crop-infecting viruses, dissection of pathogenic mechanisms of multiple viruses, examination of multilayered interactions among viruses, their host plants, and virus-transmitting arthropod vectors, and in-depth interrogation of plant-encoded resistance and susceptibility determinants. Notably, various plant virus-based vectors have also been successfully developed for gene function studies and target gene expression in plants. We also recommend future plant virology studies in China.

N-terminal acetylation of the βC1 protein encoded by the betasatellite of tomato yellow leaf curl China virus is critical for its viral pathogenicity

N-terminal acetylation (N-acetylation) is one of the most common protein modifications and plays crucial roles in viability and stress responses in animals and plants. However, very little is known about N-acetylation of viral proteins. Here, we identified the Thr residue at position 2 (Thr-2) in the βC1 protein encoded by the betasatellite of tomato yellow leaf curl China virus (TYLCCNB-βC1) as a novel N-acetylation site. Furthermore, the effects of TYLCCNB-βC1 N-acetylation on its function as a pathogenicity factor were determined via N-acetylation mutants in Nicotiana benthamiana plants. We found that N-acetylation of TYLCCNB-βC1 is critical for its self-interaction in the nucleus and viral pathogenesis, and that removal of N-acetylation of TYLCCNB-βC1 attenuated tomato yellow leaf curl China virus-induced symptoms and led to accelerated degradation of TYLCCNB-βC1 through the ubiquitin-proteasome system. Our data reveal a protective effect of N-acetylation of TYLCCNB-βC1 on its pathogenesis and demonstrate an antagonistic crosstalk between N-acetylation and ubiquitination in this geminiviral protein.

The Great Game between Plants and Viruses: A Focus on Protein Homeostasis

Plant viruses are tiny pathogenic obligate parasites that cause significant damage to global crop production. They exploit and manipulate the cellular components of host plants to ensure their own survival. In response, plants activate multiple defense signaling pathways, such as gene silencing and plant hormone signaling, to hinder virus propagation. Growing evidence suggests that the regulation of protein homeostasis plays a vital role in the ongoing battle between plants and viruses. The ubiquitin-proteasome-degradation system (UPS) and autophagy, as two major protein-degradation pathways, are widely utilized by plants and viruses in their arms race. One the one hand, these pathways act as essential components of plant's antiviral defense system by facilitating the degradation of viral proteins; on the other hand, viruses exploit the UPS and autophagy to create a favorable intracellular environment for viral infection. This review aims to provide a comprehensive summary of the events involved in protein homeostasis regulation during viral infection in plants. Gaining knowledge in this area will enhance our understanding of the complex interplay between plants and viruses.

The RING-Finger Protein NbRFP1 Contributes to Regulating the Host Hypersensitive Response Induced by Oat Dwarf Virus RepA

Our previous study identified that the RepA protein encoded by the oat dwarf virus (ODV) was responsible for inducing a strong hypersensitive response (HR) during the virus infection in non-host tobacco plants. However, little was known about the molecular mechanism of the RepA-elicited HR. Here, a RING-finger protein, which is described as NbRFP1 and is mainly located in the cytoplasm and nucleus in Nicotiana benthamiana cells, was confirmed to interact with RepA. In addition, the accumulation level of NbRFP1 in N. benthamiana leaves was enhanced by either ODV infection or by only RepA expression. The knockdown of NbRFP1 by a TRV-mediated virus-induced gene silencing markedly delayed the ODV or RepA-elicited HR. By contrast, the overexpression of NbRFP1 in N. benthamiana conferred enhanced resistance to ODV infection and promoted RepA-induced HR. Further mutation analysis showed that a RING-finger domain located in NbRFP1 plays important roles in modulating RepA-induced HR, as well as in mediating the interaction between NbRFP1 and RepA.

The NAC transcription factor ONAC083 negatively regulates rice immunity against Magnaporthe oryzae by directly activating transcription of the RING-H2 gene OsRFPH2-6

NAC transcription factors (TFs) play critical roles in plant immunity by modulating the expression of downstream genes via binding to specific cis-elements in promoters. Here, we report the function and regulatory network of a pathogen- and defense phytohormone-inducible NAC TF gene, ONAC083, in rice (Oryza sativa) immunity. ONAC083 localizes to the nucleus and exhibits transcriptional activation activity that depends on its C-terminal region. Knockout of ONAC083 enhances rice immunity against Magnaporthe oryzae, strengthening pathogen-induced defense responses, and boosting chitin-induced pattern-triggered immunity (PTI), whereas ONAC083 overexpression has opposite effects. We identified ONAC083-binding sites in the promoters of 82 genes, and showed that ONAC083 specifically binds to a conserved element with the core sequence ACGCAA. ONAC083 activated the transcription of the genes OsRFPH2-6, OsTrx1, and OsPUP4 by directly binding to the ACGCAA element. OsRFPH2-6, encoding a RING-H2 protein with an N-terminal transmembrane region and a C-terminal typical RING domain, negatively regulated rice immunity against M. oryzae and chitin-triggered PTI. These data demonstrate that ONAC083 negatively contributes to rice immunity against M. oryzae by directly activating the transcription of OsRFPH2-6 through the ACGCAA element in its promoter. Overall, our study provides new insight into the molecular regulatory network of NAC TFs in rice immunity.

Genome-Wide Association Study (GWAS) and genome prediction of seedling salt tolerance in bread wheat (Triticum aestivum L.)

BACKGROUND

Salinity tolerance in wheat is imperative for improving crop genetic capacity in response to the expanding phenomenon of soil salinization. However, little is known about the genetic foundation underlying salinity tolerance at the seedling growth stage of wheat. Herein, a GWAS analysis was carried out by the random-SNP-effect mixed linear model (mrMLM) multi-locus model to uncover candidate genes responsible for salt tolerance at the seedling stage in 298 Iranian bread wheat accessions, including 208 landraces and 90 cultivars.

RESULTS

A total of 29 functional marker-trait associations (MTAs) were detected under salinity, 100 mM NaCl (sodium chloride). Of these, seven single nucleotide polymorphisms (SNPs) including rs54146, rs257, rs37983, rs18682, rs55629, rs15183, and rs63185 with R² ≥ 10% were found to be linked with relative water content, root fresh weight, root dry weight, root volume, shoot high, proline, and shoot potassium (K⁺), respectively. Further, a total of 27 candidate genes were functionally annotated to be involved in response to the saline environment. Most of these genes have key roles in photosynthesis, response to abscisic acid, cell redox homeostasis, sucrose and carbohydrate metabolism, ubiquitination, transmembrane transport, chromatin silencing, and some genes harbored unknown functions that all together may respond to salinity as a complex network. For genomic prediction (GP), the genomic best linear unbiased prediction (GBLUP) model reflected genetic effects better than both bayesian ridge regression (BRR) and ridge regression-best linear unbiased prediction (RRBLUP), suggesting GBLUP as a favorable tool for wheat genomic selection.

CONCLUSION

The SNPs and candidate genes identified in the current work can be used potentially for developing salt-tolerant varieties at the seedling growth stage by marker-assisted selection.

An ORFeome of rice E3 ubiquitin ligases for global analysis of the ubiquitination interactome

BACKGROUND

Ubiquitination is essential for many cellular processes in eukaryotes, including 26S proteasome-dependent protein degradation, cell cycle progression, transcriptional regulation, and signal transduction. Although numerous ubiquitinated proteins have been empirically identified, their cognate ubiquitin E3 ligases remain largely unknown.

RESULTS

Here, we generate a complete ubiquitin E3 ligase-encoding open reading frames (UbE3-ORFeome) library containing 98.94% of the 1515 E3 ligase genes in the rice (Oryza sativa L.) genome. In the test screens with four known ubiquitinated proteins, we identify both known and new E3s. The interaction and degradation between several E3s and their substrates are confirmed in vitro and in vivo. In addition, we identify the F-box E3 ligase OsFBK16 as a hub-interacting protein of the phenylalanine ammonia lyase family OsPAL1-OsPAL7. We demonstrate that OsFBK16 promotes the degradation of OsPAL1, OsPAL5, and OsPAL6. Remarkably, we find that overexpression of OsPAL1 or OsPAL6 as well as loss-of-function of OsFBK16 in rice displayed enhanced blast resistance, indicating that OsFBK16 degrades OsPALs to negatively regulate rice immunity.

CONCLUSIONS

The rice UbE3-ORFeome is the first complete E3 ligase library in plants and represents a powerful proteomic resource for rapid identification of the cognate E3 ligases of ubiquitinated proteins and establishment of functional E3-substrate interactome in plants.

Root Transcriptional and Metabolic Dynamics Induced by the Plant Growth Promoting Rhizobacterium (PGPR) Bacillus subtilis Mbi600 on Cucumber Plants

Bacillus subtilis MBI600 is a commercialized plant growth-promoting bacterial species used as a biocontrol agent in many crops, controlling various plant pathogens via direct or indirect mechanisms. In the present study, a detailed transcriptomic analysis of cucumber roots upon response to the Bs MBI600 strain is provided. Differentially expressed genes (DEGs) analysis showed altered gene expression in more than 1000 genes at 24 and 48 h post-application of Bs MBI600. Bs MBI600 induces genes involved in ISR and SAR signaling. In addition, genes involved in phytohormone production and nutrient availability showed an upregulation pattern, justifying the plant growth promotion. Biocontrol ability of Bs MBI600 seems also to be related to the activation of defense-related genes, such as peroxidase, endo-1,3(4)-beta-glucanase, PR-4, and thaumatin-like. Moreover, KEGG enriched results showed that differentially expressed genes were classified into biocontrol-related pathways. To further investigate the plant's response to the presence of PGPR, a profile of polar metabolites of cucumber treated with Bs MBI600 was performed and compared to that of untreated plants. The results of the current study gave insights into the mechanisms deployed by this biocontrol agent to promote plant resistance, helping to understand the molecular interactions in this system.

SAMDC3 enhances resistance to Barley stripe mosaic virus by promoting the ubiquitination and proteasomal degradation of viral γb protein

Posttranslational modifications (PTMs) play important roles in virus-host interplay. We previously demonstrated that Barley stripe mosaic virus (BSMV) γb protein is phosphorylated by different host kinases to support or impede viral infection. However, whether and how other types of PTMs participate in BSMV infection remains to be explored. Here, we report that S-adenosylmethionine decarboxylase 3 (SAMDC3) from Nicotiana benthamiana or wheat (Triticum aestivum) interacts with γb. BSMV infection induced SAMDC3 expression. Overexpression of SAMDC3 led to the destabilization of γb and reduction in viral infectivity, whereas knocking out NbSAMDC3 increased susceptibility to BSMV. NbSAMDC3 positively regulated the 26S proteasome-mediated degradation of γb via its PEST domain. Further mechanistic studies revealed that γb can be ubiquitinated in planta and that NbSAMDC3 promotes the proteasomal degradation of γb by increasing γb ubiquitination. We also found evidence that ubiquitination occurs at nonlysine residues (Ser-133 and Cys-144) within γb. Together, our results provide a function for SAMDC3 in defence against BSMV infection through targeting of γb abundance, which contributes to our understanding of how a plant host deploys the ubiquitin-proteasome system to mount defences against viral infections.

Manipulation of auxin signalling by plant viruses

Compatible plant-virus interactions result in dramatic changes of the plant transcriptome and morphogenesis, and are often associated with rapid alterations in plant hormone homeostasis and signalling. Auxin controls many aspects of plant organogenesis, development, and growth; therefore, plants can rapidly perceive and respond to changes in the cellular auxin levels. Auxin signalling is a tightly controlled process and, hence, is highly vulnerable to changes in the mRNA and protein levels of its components. There are several core nuclear components of auxin signalling. In the nucleus, the interaction of auxin response factors (ARFs) and auxin/indole acetic acid (Aux/IAA) proteins is essential for the control of auxin-regulated pathways. Aux/IAA proteins are negative regulators, whereas ARFs are positive regulators of the auxin response. The interplay between both is essential for the transcriptional regulation of auxin-responsive genes, which primarily regulate developmental processes but also modulate the plant immune system. Recent studies suggest that plant viruses belonging to different families have developed various strategies to disrupt auxin signalling, namely by (a) changing the subcellular localization of Aux/IAAs, (b) preventing degradation of Aux/IAAs by stabilization, or (c) inhibiting the transcriptional activity of ARFs. These interactions perturb auxin signalling and experimental evidence from various studies highlights their importance for virus replication, systemic movement, interaction with vectors for efficient transmission, and symptom development. In this microreview, we summarize and discuss the current knowledge on the interaction of plant viruses with auxin signalling components of their hosts.

Evolutionary and Characteristic Analysis of RING-DUF1117 E3 Ubiquitin Ligase Genes in Gossypium Discerning the Role of GhRDUF4D in Verticillium dahliae Resistance

Verticillium wilt, primarily induced by the soil-borne fungus Verticillium dahliae, is a serious threat to cotton fiber production. There are a large number of really interesting new gene (RING) domain-containing E3 ubiquitin ligases in Arabidopsis, of which three (At2g39720 (AtRHC2A), At3g46620 (AtRDUF1), and At5g59550 (AtRDUF2)) have a domain of unknown function (DUF) 1117 domain in their C-terminal regions. This study aimed to detect and characterize the RDUF members in cotton, to gain an insight into their roles in cotton’s adaptation to environmental stressors. In this study, a total of 6, 7, 14, and 14 RDUF (RING-DUF1117) genes were detected in Gossypium arboretum, G. raimondii, G. hirsutum, and G. barbadense, respectively. These RDUF genes were classified into three groups. The genes in each group were highly conserved based on gene structure and domain analysis. Gene duplication analysis revealed that segmental duplication occurred during cotton evolution. Expression analysis revealed that the GhRDUF genes were widely expressed during cotton growth and under abiotic stresses. Many cis-elements related to hormone response and environment stressors were identified in GhRDUF promoters. The predicted target miRNAs and transcription factors implied that GhRDUFs might be regulated by gra-miR482c, as well as by transcription factors, including MYB, C2H2, and Dof. The GhRDUF genes responded to cold, drought, and salt stress and were sensitive to jasmonic acid, salicylic acid, and ethylene signals. Meanwhile, GhRDUF4D expression levels were enhanced after V. dahliae infection. Subsequently, GhRDUF4D was verified by overexpression in Arabidopsis and virus-induced gene silencing treatment in upland cotton. We observed that V. dahliae resistance was significantly enhanced in transgenic Arabidopsis, and weakened in GhRDUF4D silenced plants. This study conducted a comprehensive analysis of the RDUF genes in Gossypium, hereby providing basic information for further functional studies.

Auxin response factors (ARFs) differentially regulate rice antiviral immune response against rice dwarf virus

There are 25 auxin response factors (ARFs) in the rice genome, which play critical roles in regulating myriad aspects of plant development, but their role (s) in host antiviral immune defense and the underneath mechanism remain largely unknown. By using the rice-rice dwarf virus (RDV) model system, here we report that auxin signaling enhances rice defense against RDV infection. In turn, RDV infection triggers increased auxin biosynthesis and accumulation in rice, and that treatment with exogenous auxin reduces OsIAA10 protein level, thereby unleashing a group of OsIAA10-interacting OsARFs to mediate downstream antiviral responses. Strikingly, our genetic data showed that loss-of-function mutants of osarf12 or osarf16 exhibit reduced resistance whereas osarf11 mutants display enhanced resistance to RDV. In turn, OsARF12 activates the down-stream OsWRKY13 expression through direct binding to its promoter, loss-of-function mutants of oswrky13 exhibit reduced resistance. These results demonstrated that OsARF 11, 12 and 16 differentially regulate rice antiviral defense. Together with our previous discovery that the viral P2 protein stabilizes OsIAA10 protein via thwarting its interaction with OsTIR1 to enhance viral infection and pathogenesis, our results reveal a novel auxin-IAA10-ARFs-mediated signaling mechanism employed by rice and RDV for defense and counter defense responses.

The phytohormone auxin is often critical for plant growth and orchestrates many developmental processes. Here we find that rice accumulates more auxin upon RDV infection and treatment with exogenous auxin enhances rice tolerance to RDV infection. Auxin treatment reduces the protein level of OsIAA10, thus releasing a group of OsIAA10-interacting OsARFs to mediate downstream antiviral responses. Among the 25 ARFs in the rice genome, their functions on regulation of rice antiviral defense are diversified. Our findings elucidate a novel auxin-OsIAA10-ARFs-mediated signaling mechanism employed by rice and RDV for defense and counter defense responses. These findings significantly deepen our understanding of virus-host interactions and provide novel targets for molecular breeding (or engineering) rice cultivars resistant to RDV.

Molecular insight into cotton leaf curl geminivirus disease resistance in cultivated cotton (Gossypium hirsutum)

Cultivated cotton (Gossypium hirsutum) is the most important fibre crop in the world. Cotton leaf curl disease (CLCuD) is the major limiting factor and a threat to textile industry in India and Pakistan. All the local cotton cultivars exhibit moderate to no resistance against CLCuD. In this study, we evaluated an exotic cotton accession Mac7 as a resistance source to CLCuD by challenging it with viruliferous whiteflies and performing qPCR to evaluate the presence/absence and relative titre of CLCuD-associated geminiviruses/betasatellites. The results indicated that replication of pathogenicity determinant betasatellite is significantly attenuated in Mac7 and probably responsible for resistance phenotype. Afterwards, to decipher the genetic basis of CLCuD resistance in Mac7, we performed RNA sequencing on CLCuD-infested Mac7 and validated RNA-Seq data with qPCR on 24 independent genes. We performed co-expression network and pathway analysis for regulation of geminivirus/betasatellite-interacting genes. We identified nine novel modules with 52 hubs of highly connected genes in network topology within the co-expression network. Analysis of these hubs indicated the differential regulation of auxin stimulus and cellular localization pathways in response to CLCuD. We also analysed the differential regulation of geminivirus/betasatellite-interacting genes in Mac7. We further performed the functional validation of selected candidate genes via virus-induced gene silencing (VIGS). Finally, we evaluated the genomic context of resistance responsive genes and found that these genes are not specific to A or D sub-genomes of G. hirsutum. These results have important implications in understanding CLCuD resistance mechanism and developing a durable resistance in cultivated cotton.

Comparative Transcriptome Analysis in Eggplant Reveals Selection Trends during Eggplant Domestication

Eggplant (Solanum melongena L.) is an economically and nutritionally important fruit crop of the Solanaceae family, which was domesticated in India and southern China. However, the genome regions subjected to selective sweeps in eggplant remain unknown. In the present study, we performed comparative transcriptome analysis of cultivated and wild eggplant species with emphasis on the selection pattern during domestication. In total, 44,073 (S. sisymbriifolium) to 58,677 (S. melongena cultivar S58) unigenes were generated for the six eggplant accessions with total lengths of 36.6-46 Mb. The orthologous genes were assessed using the ratio of nonsynonymous (K a) to synonymous (K s) nucleotide substitutions to characterize selective patterns during eggplant domestication. We identified 19 genes under positive selection across the phylogeny that were classified into four groups. The gene (OG12205) under positive selection was possibly associated with fruit-related traits in eggplant, which may have resulted from human manipulation. Eight positive selected genes were potentially involved in stress tolerance or disease resistance, suggesting that environmental changes and biotic stresses were important selective pressures in eggplant domestication. Taken together, our results shed light on the effects of artificial and natural selection on the transcriptomes of eggplant and its wild relatives. Identification of the selected genes will facilitate the understanding of genetic architecture of domesticated-related traits and provide resources for resistant breeding in eggplant.

The Xanthomonas effector XopK harbours E3 ubiquitin-ligase activity that is required for virulence

Xanthomonas oryzae pv. oryzae is the causative agent of rice bacterial leaf blight. While the type III secretion system of X. oryzae pv. oryzae is essential for virulence, the biochemical activities and virulence mechanisms of non-transcription activator-like (non-TAL) effectors delivered by this system are largely unknown. Here, by screening for non-TAL effectors that contribute to X. oryzae pv. oryzae virulence, we revealed that Xanthomonas outer protein K (XopK) inhibits pathogen-associated molecular pattern-triggered immunity upstream of mitogen-activated protein kinase cascades. Specifically, XopK interacted with and directly ubiquitinated rice somatic embryogenic receptor kinase 2 (OsSERK2), resulting in its degradation. Accordingly, mutation of a putative ubiquitin-conjugation enzyme (E2) binding site abolished XopK-induced degradation of OsSERK2 and compromised XopK-dependent virulence. As crucial immune regulators associated with a multitude of immune receptors, SERKs have been shown to be perturbed by Pseudomonas effectors via different mechanisms. Our study revealed a distinct perturbation mechanism of SERK activity via ubiquitination achieved by Xanthomonas non-TAL effector.

A viral protein promotes host SAMS1 activity and ethylene production for the benefit of virus infection

Ethylene plays critical roles in plant development and biotic stress response, but the mechanism of ethylene in host antiviral response remains unclear. Here, we report that Rice dwarf virus (RDV) triggers ethylene production by stimulating the activity of S-adenosyl-L-methionine synthetase (SAMS), a key component of the ethylene synthesis pathway, resulting in elevated susceptibility to RDV. RDV-encoded Pns11 protein specifically interacted with OsSAMS1 to enhance its enzymatic activity, leading to higher ethylene levels in both RDV-infected and Pns11-overexpressing rice. Consistent with a counter-defense role for ethylene, Pns11-overexpressing rice, as well as those overexpressing OsSAMS1, were substantially more susceptible to RDV infection, and a similar effect was observed in rice plants treated with an ethylene precursor. Conversely, OsSAMS1-knockout mutants, as well as an osein2 mutant defective in ethylene signaling, resisted RDV infection more robustly. Our findings uncover a novel mechanism which RDV manipulates ethylene biosynthesis in the host plants to achieve efficient infection.

Rice provides food for billions of people all over the world, but diseases caused by plant-infecting viruses cause serious risks to the production of rice. As a result, there is an urgent demand for developing new and impactful ways to help defend rice plants from harmful viruses. Toward this goal, it will be important to better understand how viruses actually cause diseases in plants.

Plants make chemicals known as hormones to control their own development, and hormone production is often disturbed when viruses infect rice plants. Many viruses cause infected plants to make more of a gaseous hormone called ethylene, which benefits the viruses. Yet, it is still not known how virus infection induces the production of more ethylene.

Zhao, Hong et al. have exposed rice plants to infection with a virus called rice dwarf virus. Infected plants made more ethylene than normal, which did indeed help the virus to infect. Further experiments then showed that an enzyme that makes one of the building blocks needed to produce ethylene became more active after infection with this virus. Next, Zhao, Hong et al. engineered rice plants to make more or less of this building block – which is known as S-adenosyl-L-methionine or SAM for short. Plants with too much SAM were less able to defend themselves against the virus, while plants that lacked SAM were better able to fight off viral infection.

Zhao, Hong et al. suggest that engineering rice plants to make less of the SAM-producing enzyme could make them more resistant to viruses. Further work will also be needed to find out why ethylene benefits viral infection, and to confirm whether ethylene also performs similar roles when rice is infected with other viruses.

ROS accumulation and antiviral defence control by microRNA528 in rice

MicroRNAs (miRNAs) are key regulators of plant-pathogen interactions. Modulating miRNA function has emerged as a new strategy to produce virus resistance traits^1-5. However, the miRNAs involved in antiviral defence and the underlying mechanisms remain largely elusive. We previously demonstrated that sequestration by Argonaute (AGO) proteins plays an important role in regulating miRNA function in antiviral defence pathways⁶. Here we reveal that cleavage-defective AGO18 complexes sequester microRNA528 (miR528) upon viral infection. We show that miR528 negatively regulates viral resistance in rice by cleaving L-ascorbate oxidase (AO) messenger RNA, thereby reducing AO-mediated accumulation of reactive oxygen species. Upon viral infection, miR528 becomes preferentially associated with AGO18, leading to elevated AO activity, higher basal reactive oxygen species accumulation and enhanced antiviral defence. Our findings reveal a mechanism in which antiviral defence is boosted through suppression of an miRNA that negatively regulates viral resistance. This mechanism could be manipulated to engineer virus-resistant crop plants.

Rice Dwarf Virus P2 Protein Hijacks Auxin Signaling by Directly Targeting the Rice OsIAA10 Protein, Enhancing Viral Infection and Disease Development

The phytohormone auxin plays critical roles in regulating myriads of plant growth and developmental processes. Microbe infection can disturb auxin signaling resulting in defects in these processes, but the underlying mechanisms are poorly understood. Auxin signaling begins with perception of auxin by a transient co-receptor complex consisting of an F-box transport inhibitor response 1/auxin signaling F-box (TIR1/AFB) protein and an auxin/indole-3-acetic acid (Aux/IAA) protein. Auxin binding to the co-receptor triggers ubiquitination and 26S proteasome degradation of the Aux/IAA proteins, leading to subsequent events, including expression of auxin-responsive genes. Here we report that Rice dwarf virus (RDV), a devastating pathogen of rice, causes disease symptoms including dwarfing, increased tiller number and short crown roots in infected rice as a result of reduced sensitivity to auxin signaling. The RDV capsid protein P2 binds OsIAA10, blocking the interaction between OsIAA10 and OsTIR1 and inhibiting 26S proteasome-mediated OsIAA10 degradation. Transgenic rice plants overexpressing wild-type or a dominant-negative (degradation-resistant) mutant of OsIAA10 phenocopy RDV symptoms are more susceptible to RDV infection; however, knockdown of OsIAA10 enhances the resistance of rice to RDV infection. Our findings reveal a previously unknown mechanism of viral protein reprogramming of a key step in auxin signaling initiation that enhances viral infection and pathogenesis.

Auxin regulates plant growth and development through auxin signaling, which begins with the interaction of an F-box transport inhibitor response 1/auxin signaling F-box (TIR1/AFB) protein and an auxin/indole-3-acetic acid (Aux/IAA) protein co-receptor. Auxin binding to the co-receptor complex triggers ubiquitination and 26S proteasome degradation of Aux/IAA proteins, leading to a downstream signaling cascade that induces the expression of auxin-responsive genes. Auxin signaling is manipulated by plant pathogens to maximize their own multiplication, but the underlying mechanisms are poorly understood. Here we report that the P2 capsid protein encoded by Rice dwarf virus (RDV) sabotages auxin signaling by interacting with the rice Aux/IAA protein, OsIAA10, thereby shielding it from degradation and causing infected plants to display typical RDV symptoms including dwarfism, excessive tillering and stunted crown roots. Importantly, these symptoms are phenocopied by transgenic rice plants overexpressing OsIAA10 or its degradation-resistant mutant. Conversely, down-regulating OsIAA10 expression in rice led to milder RDV infection. Together these findings reveal a novel mechanism by which RDV reprograms auxin signaling, leading to enhanced viral infection.

Tobacco RING E3 Ligase NtRFP1 Mediates Ubiquitination and Proteasomal Degradation of a Geminivirus-Encoded βC1

The βC1 protein encoded by the Tomato yellow leaf curl China virus-associated betasatellite functions as a pathogenicity determinant. To better understand the molecular basis whereby βC1 functions in pathogenicity, a yeast two-hybrid screen of a tobacco cDNA library was carried out using βC1 as the bait. The screen revealed that βC1 interacts with a tobacco RING-finger protein designated NtRFP1, which was further confirmed by the bimolecular fluorescence complementation and co-immunoprecipitation assays in Nicotiana benthamiana cells. Expression of NtRFP1 was induced by βC1, and in vitro ubiquitination assays showed that NtRFP1 is a functional E3 ubiquitin ligase that mediates βC1 ubiquitination. In addition, βC1 was shown to be ubiquitinated in vivo and degraded by the plant 26S proteasome. After viral infection, plants overexpressing NtRFP1 developed attenuated symptoms, whereas plants with silenced expression of NtRFP1 showed severe symptoms. Other lines of evidence showed that NtRFP1 attenuates βC1-induced symptoms through promoting its degradation by the 26S proteasome. Taken together, our results suggest that tobacco RING E3 ligase NtRFP1 attenuates disease symptoms by interacting with βC1 to mediate its ubiquitination and degradation via the ubiquitin/26S proteasome system.

NtRING1, putative RING-finger E3 ligase protein, is a positive regulator of the early stages of elicitin-induced HR in tobacco

KEY MESSAGE

NtRING1 is a RING-finger protein with a putative E3 ligase activity. NtRING1 regulates HR establishment against different pathogens. Loss-/gain-of-function of NtRING1 altered early stages of HR phenotype establishment. Plant defence responses against pathogens often involve the restriction of pathogens by inducing a hypersensitive response (HR). cDNA clones DD11-39, DD38-11 and DD34-26 were previously obtained from a differential screen aimed at characterising tobacco genes with an elicitin-induced HR-specific pattern of expression. Our precedent observations suggested that DD11-39, DD38-11 and DD34-26 might play roles in the HR establishment. Only for DD11-39 a full-length cDNA sequence was obtained and the corresponding protein encoded for a type-HC RING-finger/putative E3 ligase protein which we termed NtRING1. The expression of NtRING1 was upregulated upon HR induction by elicitin, Ralstonia solanacearum, or tobacco mosaic virus (TMV) in tobacco. Silencing of NtRING1 remarkably delayed the establishment of elicitin-induced HR in tobacco as well as the expression of different early induction genes in tissues undergoing HR. Accordingly, transient overexpression of NtRING1 accelerated the HR launching upon elicitin treatment. Taking together, our data suggests that NtRING1 plays a functional role in the early establishment of HR.

Simultaneous determination of entecavir and lamivudine in rat plasma by UPLC-MS/MS and its application to a pharmacokinetic study

The study's aim is to develop and validate a rapid, selective and sensitive ultra-performance liquid chromatography-tandem mass spectrometry with multiple reaction monitoring (MRM) mode method for the simultaneous determination of entecavir and lamivudine in rat plasma.

An ATL78-Like RING-H2 Finger Protein Confers Abiotic Stress Tolerance through Interacting with RAV2 and CSN5B in Tomato

RING finger proteins play an important role in plant adaptation to abiotic stresses. In the present study, a wild tomato (Solanum habrochaites) cold-induced RING-H2 finger gene, ShATL78L, was isolated, which has been identified as an abiotic stress responsive gene in tomato. The results showed that ShATL78L was constitutively expressed in various tissues such as root, leaf, petiole, stem, flower, and fruit. Cold stress up-regulated ShATL78L in the cold-tolerant S. habrochaites compared to the susceptible cultivated tomato (S. lycopersicum). Furthermore, ShATL78L expression was also regulated under different stresses such as drought, salt, heat, wound, osmotic stress, and exogenous hormones. Functional characterization showed that cultivated tomato overexpressing ShATL78L had improved tolerance to cold, drought and oxidative stresses compared to the wild-type and the knockdown lines. To understand the underlying molecular mechanism of ShATL78L regulating abiotic stress responses, we performed yeast one-hybrid and two-hybrid assays and found that RAV2 could bind to the promoter of ShATL78L and activates/alters its transcription, and CSN5B could interact with ShATL78L to regulate abiotic stress responses. Taken together, these results show that ShATL78L plays an important role in regulating plant adaptation to abiotic stresses through bound by RAV2 and interacting with CSN5B. Highlight: RAV2 binds to the promoter of ShATL78L to activates/alters its transcription to adapt the environmental conditions; furthermore, ShATL78L interacts with CSN5B to regulate the stress tolerance.

OsRDR6 plays role in host defense against double-stranded RNA virus, Rice Dwarf Phytoreovirus

RNAi is a major antiviral defense response in plant and animal model systems. RNA-dependent RNA polymerase 6 (RDR6) is an essential component of RNAi, which plays an important role in the resistance against viruses in the model plants. We found previously that rice RDR6 (OsRDR6) functioned in the defense against Rice stripe virus (RSV), and Rice Dwarf Phytoreovirus (RDV) infection resulted in down-regulation of expression of RDR6. Here we report our new findings on the function of OsRDR6 against RDV. Our result showed that down-regulation of OsRDR6 through the antisense (OsRDR6AS) strategy increased rice susceptibility to RDV infection while over-expression of OsRDR6 had no effect on RDV infection. The accumulation of RDV vsiRNAs was reduced in the OsRDR6AS plants. In the OsRDR6 over-expressed plants, the levels of OsRDR6 RNA transcript and protein were much higher than that in the control plants. Interestingly, the accumulation level of OsRDR6 protein became undetectable after RDV infection. This finding indicated that the translation and/or stability of OsRDR6 protein were negatively impacted upon RDV infection. This new finding provides a new light on the function of RDR6 in plant defense response and the cross-talking between factors encoded by host plant and double-stranded RNA viruses.