The role of hydrogen peroxide and nitric oxide in the induction of plant-encoded RNA-dependent RNA polymerase 1 in the basal defense against Tobacco mosaic virus

Phytohormones and emerging plant growth regulators in tailoring plant immunity against viral infections

Viral infections are major contributors to crop yield loss and represent a significant threat to sustainable agriculture. Plants respond to virus attacks by activating sophisticated signalling cascades that initiate multiple defence mechanisms. Notably, several phytohormones, including salicylic acid (SA), jasmonic acid (JA), abscisic acid (ABA), and ethylene (ET), are known to shape these defence responses. In recent years, various plant growth regulators (PGRs) such as melatonin, carrageenans, sulfated fucan oligosaccharides, nitric oxide (NO), brassinosteroids (BRs), and hydrogen sulfide (H2S) have also emerged as crucial regulators of plant defence responses against virus infections. Emerging evidence indicates that these PGRs coordinate with phytohormones to activate various defence strategies, including (1) stomatal closure to limit pathogen entry, (2) callose deposition to block plasmodesmata and restrict viral spread within host tissues, (3) attenuation of viral replication, and (4) activation of RNA interference (RNAi), a crucial antiviral defence response. However, the interactions and crosstalk between PGRs and phytohormones remain largely underexplored, thereby limiting our ability to develop innovative strategies for managing viral diseases. This review discusses the diverse functions and crosstalk among various phytohormones and PGRs in orchestrating the plant defence mechanisms, highlighting their impact on viral replication, movement, and intercellular transport.

Defense signaling pathways in resistance to plant viruses: Crosstalk and finger pointing

Resistance to infection by plant viruses involves proteins encoded by plant resistance (R) genes, viz., nucleotide-binding leucine-rich repeats (NLRs), immune receptors. These sensor NLRs are activated either directly or indirectly by viral protein effectors, in effector-triggered immunity, leading to induction of defense signaling pathways, resulting in the synthesis of numerous downstream plant effector molecules that inhibit different stages of the infection cycle, as well as the induction of cell death responses mediated by helper NLRs. Early events in this process involve recognition of the activation of the R gene response by various chaperones and the transport of these complexes to the sites of subsequent events. These events include activation of several kinase cascade pathways, and the syntheses of two master transcriptional regulators, EDS1 and NPR1, as well as the phytohormones salicylic acid, jasmonic acid, and ethylene. The phytohormones, which transit from a primed, resting states to active states, regulate the remainder of the defense signaling pathways, both directly and by crosstalk with each other. This regulation results in the turnover of various suppressors of downstream events and the synthesis of various transcription factors that cooperate and/or compete to induce or suppress transcription of either other regulatory proteins, or plant effector molecules. This network of interactions results in the production of defense effectors acting alone or together with cell death in the infected region, with or without the further activation of non-specific, long-distance resistance. Here, we review the current state of knowledge regarding these processes and the components of the local responses, their interactions, regulation, and crosstalk.

Exploration of Piezo Channels in Bread Wheat (Triticum aestivum L.)

Piezo channels belong to an important class of cell membrane-bound, Ca2+-permeable, mechanosensitive channels consisting of a pore and multiple transmembrane helices. In plants, the functional aspects of Piezo channels have been less studied than other mechanosensitive channels. However, a few studies that have been carried out indicate the involvement of Piezo channels in stress response and developmental processes. In our analysis, we identified a total of three Piezo genes in the Triticum aestivum genome. The phylogenetic analysis revealed the monocot and dicot-specific clustering of Piezo proteins. The gene and protein structure analysis indicated their conserved architecture. The promoter region of each of the three Piezo genes contained light-, growth-and development-, hormone-, and stress-responsive cis-regulatory elements. Moreover, the differential expression of Piezo genes in tissue developmental stages and under abiotic and biotic stress conditions indicated their probable role in plant growth and development and various stresses. The quantitative real-time polymerase chain reaction (qRT-PCR) analysis suggested that TaPiezo1-D might be involved in Ca2+ homeostasis. In addition, protein–protein interaction indicated their precise role in glucose, hormone and stress responses. The miRNA interaction analysis further suggested their participation in signaling cascades and biological processes. The present study will extend our understanding about Piezo channels in Ca2+ mediated signaling in plants under various stresses and provide a path for the functional validation of TaPiezo genes in future research.

Genome-Wide Identification and Characterization of Argonaute, Dicer-like and RNA-Dependent RNA Polymerase Gene Families and Their Expression Analyses in Fragaria spp

In the growth and development of plants, some non-coding small RNAs (sRNAs) not only mediate RNA interference at the post-transcriptional level, but also play an important regulatory role in chromatin modification at the transcriptional level. In these processes, the protein factors Argonaute (AGO), Dicer-like (DCL), and RNA-dependent RNA polymerase (RDR) play very important roles in the synthesis of sRNAs respectively. Though they have been identified in many plants, the information about these gene families in strawberry was poorly understood. In this study, using a genome-wide analysis and a phylogenetic approach, 13 AGO, six DCL, and nine RDR genes were identified in diploid strawberry Fragaria vesca. We also identified 33 AGO, 18 DCL, and 28 RDR genes in octoploid strawberry Fragaria × ananassa, studied the expression patterns of these genes in various tissues and developmental stages of strawberry, and researched the response of these genes to some hormones, finding that almost all genes respond to the five hormone stresses. This study is the first report of a genome-wide analysis of AGO, DCL, and RDR gene families in Fragaria spp., in which we provide basic genomic information and expression patterns for these genes. Additionally, this study provides a basis for further research on the functions of these genes and some evidence for the evolution between diploid and octoploid strawberries.

The Modulatory Role of sti-1 in Methylmercury-Induced Toxicity in Caenorhabditis elegans

Human exposure to the neurotoxin methylmercury (MeHg) poses a significant health risk to the development of the nervous system. The mechanisms of MeHg-induced neurotoxicity are associated with the disruption of cellular homeostasis, and include oxidative stress, loss of calcium homeostasis, and impaired protein quality control. The stress inducible protein 1 (STI-1) is involved in the regulation of protein quality control by acting as a protein cochaperone to maintain optimal protein unfolding and refolding. Here, we utilized the Caenorhabditis elegans (C. elegans) model of MeHg toxicity to characterize the role of the sti-1 gene in MeHg-induced toxicity. We showed that lifespan and developmental milestone timings were significantly altered in sti-1 knockout (KO) animals with MeHg exposure. However, knocking down sti-1 by RNAi did not result in an analogous effect for lifespan, but did still sensitize to delays in developmental milestone progression by acute MeHg, suggesting that insufficiency of sti-1 does not recapitulate all phenotypes of the null mutation. Furthermore, inhibition of ATP levels by MeHg exposure was modulated by sti-1. Considering that the skn-1/gst-4 pathway is highly involved in metal's toxicity, such pathway was also explored in our model. We showed that sti-1 mutant worms exhibited impaired capacity to upregulate the antioxidant genes skn-1/gst-4, highlighting a central role of sti-1 in modulating antioxidant response. Lastly, we showed that loss-of-function mutation in the rrf-3 gene, which encodes a putative RNA-directed RNA polymerase, has significant effect in altering MeHg-induced toxicity by potentiating the animal's detoxification system. Altogether, our novel data show an indispensable role of protein quality control in the defense against MeHg toxicity.

RNA-dependent RNA polymerase 1 delays the accumulation of viroids in infected plants

RNA-dependent RNA polymerase 1 (RDR1) is essential for plant antiviral defence, but its role in plant defence against viroid infection remains unknown. The present study aimed to identify the function and mechanism of RDR1 in plant resistance to viroid infection. Overexpression of Nicotiana tabacum RDR1 (NtRDR1) delayed the accumulation of potato spindle tuber viroid (PSTVd) genomic RNA and PSTVd-derived small RNA (sRNA) in Nicotiana benthamiana plants at the early invasion stage, but not in the late stage of infection. Conversely, virus-induced gene silencing of tomato RDR1 (SlRDR1a) increased the susceptibility to PSTVd infection (increased viroid accumulation). Salicylic acid (SA) pretreatment induced SlRDR1a expression and enhanced the defence against PSTVd infection in tomato plants. Our study demonstrated that RDR1 is involved in SA-mediated defence and restricts the early systemic invasion by PSTVd in plants. The decreased PSTVd accumulation in N. benthamiana was not caused by efficient accumulation of PSTVd sRNAs. These results deepen our understanding of the mechanism of RDR1 in plant defence responses to viroid attack.

Impact of Wheat Streak Mosaic Virus on Peroxisome Proliferation, Redox Reactions, and Resistance Responses in Wheat

Although peroxisomes play an essential role in viral pathogenesis, and viruses are known to change peroxisome morphology, the role of genotype in the peroxisomal response to viruses remains poorly understood. Here, we analyzed the impact of wheat streak mosaic virus (WSMV) on the peroxisome proliferation in the context of pathogen response, redox homeostasis, and yield in two wheat cultivars, Patras and Pamir, in the field trials. We observed greater virus content and yield losses in Pamir than in Patras. Leaf chlorophyll and protein content measured at the beginning of flowering were also more sensitive to WSMV infection in Pamir. Patras responded to the WSMV infection by transcriptional up-regulation of the peroxisome fission genes PEROXIN 11C (PEX11C), DYNAMIN RELATED PROTEIN 5B (DRP5B), and FISSION1A (FIS1A), greater peroxisome abundance, and activation of pathogenesis-related proteins chitinase, and β-1,3-glucanase. Oppositely, in Pamir, WMSV infection suppressed transcription of peroxisome biogenesis genes and activity of chitinase and β-1,3-glucanase, and did not affect peroxisome abundance. Activity of ROS scavenging enzymes was higher in Patras than in Pamir. Thus, the impact of WMSV on peroxisome proliferation is genotype-specific and peroxisome abundance can be used as a proxy for the magnitude of plant immune response.

A novel glycoprotein from Streptomyces sp. triggers early responses of plant defense

GP-1, a novel glycoprotein from Streptomyces sp. ZX01 has a plant immunity-inducing effect. GP-1-treated plants exhibited enhanced systemic resistance with a significant reduction in TMV lesions on tobacco leaves, but its antiviral mechanism remains unclear. In this study, early plant defense-related responses, such as Ca2+ influx, callose apposition, oxidative burst, hypersensitive response, programmed cell death, increase in nitric oxide (NO), and stomatal closure, were investigated under GP-1 treatment, and the mechanism of how GP-1 induces viral resistance in Nicotiana benthamiana was studied. Results showed that GP-1 induced [Ca2+]cyt rapidly in tobacco leaves and suspended cells, followed by reactive oxygen species (ROS) and NO elevation. Transcriptome analysis showed significant differences in expression levels between the GP-1-treated N. benthamiana and the control and showed significantly upregulated and enriched pathways including defense and immune reaction. Similar to typical pathogen-associated molecular patterns, GP-1 induced callose deposition and stomatal closure to form defense barriers against pathogen invasion. The expression of defense-related genes further confirmed the above conclusions. By analyzing transcriptome in N. benthamiana and the contents of salicylic acid (SA) and jasmonic acid (JA), GP-1 enhanced viral resistance of tobacco by improving the SA and JA contents, strengthening plant secondary metabolites activities, promoting systemic accumulation of pathogenesis-related proteins in TMV- inoculated tobacco there by producing antiviral activity.

Epigallocatechin-3-gallate enhances tomato resistance to tobacco mosaic virus by modulating RBOH1-dependent H2O2 signaling

Tobacco mosaic virus (TMV) is one of the most damaging plant viruses from an economic and research point of view. Epigallocatechin-3-Gallate (EGCG), a flavonoid type secondary metabolite can selectively improve plant defense against pathogens; however, the effect of EGCG on plant defense against TMV and the underlying mechanism(s) remain elusive. In this study, exogenous EGCG application increased plant resistance to TMV as revealed by significantly decreased transcript levels of TMV-coat protein (CP) in tomato leaves. A time-course of H₂O₂ concentrations in tomato leaves showed that TMV inoculation rapidly increased the H₂O₂ accumulation, reaching its peak at 3 days post-inoculation (dpi) which remained the highest until 6 dpi. However, the combined treatment of EGCG and TMV remarkably decreased the concentrations of H₂O₂ at 3 and 6 dpi. Meanwhile, the transcript levels of RESPIRATORY BURST OXIDASE HOMOLOG 1 (SlRBOH1) were significantly increased by either EGCG or TMV inoculation, but the EGCG treatment along with TMV caused a further upregulation in the SlRBOH1 transcripts compared with that in only TMV-inoculated plants. Chemical scavenging of H₂O₂ or silencing SlRBOH1 both compromised the EGCG-induced enhanced resistance to TMV. Furthermore, EGCG-induced elevation in the activity of antioxidant enzymes was abolished by SlRBOH1 silencing, suggesting that EGCG enhanced defense against TMV by increasing the antioxidant enzyme activity via RBOH1-dependent H₂O₂ signaling. Taken together, our results suggest that EGCG functioned to maintain a delicate balance between ROS signaling and ROS scavenging via RBOH1, which enhanced tomato resistance to TMV.

Signaling pathway played by salicylic acid, gentisic acid, nitric oxide, polyamines and non-enzymatic antioxidants in compatible and incompatible Solanum-tomato mottle mosaic virus interactions

Plants are exposed to a vast array of pathogens. The interaction between them may be classified in compatible and incompatible. Polyamines (PAs) are involved in defense responses, as well as salicylic acid (SA), gentisic acid (GA) and nitric oxide (NO), which can increase the content of reactive oxygen species (ROS), creating a harsh environment to the pathogen. ROS can also damage the host cell and they can be controlled by ascorbate and glutathione. Among phytopathogens, one of the major threats to tomato crops is tomato mottle mosaic virus (ToMMV). Resistance against this virus probably involves the Tm-2² gene. This work aimed to analyze signaling and antioxidant molecules in the defense response against ToMMV in Solanum pimpinellifolium and in S. lycopersicum 'VFNT'. In S. pimpinellifolium plants inoculated with ToMMV, an increase in NO, SA, GA, ascorbate and oxidized glutathione and a decrease in the content of PAs were observed. Characteristic symptoms of diseased plants and high absorbance values in PTA-ELISA indicated a compatible interaction. In VFNT-inoculated plants, less significant differences were noticed. Symptoms and viral concentration were not detected, indicating an incompatible interaction, possibly associated with the effector-triggered immunity (ETI) response.

Molecular Regulation of Host Defense Responses Mediated by Biological Anti-TMV Agent Ningnanmycin

Ningnanmycin (NNM) belongs to microbial pesticides that display comprehensive antiviral activity against plant viruses. NNM treatment has been shown to efficiently delay or suppress the disease symptoms caused by tobacco mosaic virus (TMV) infection in local-inoculated or systemic-uninoculated tobacco leaves, respectively. However, the underlying molecular mechanism of NNM-mediated antiviral activity remains to be further elucidated. In this study, 414 differentially expressed genes (DEGs), including 383 which were up-regulated and 31 down-regulated, caused by NNM treatment in TMV-infected BY-2 protoplasts, were discovered by RNA-seq. In addition, KEGG analysis indicated significant enrichment of DEGs in the plant-pathogen interaction and MAPK signaling pathway. The up-regulated expression of crucial DEGs, including defense-responsive genes, such as the receptor-like kinase FLS2, RLK1, and the mitogen-activated protein kinase kinase kinase MAPKKK, calcium signaling genes, such as the calcium-binding protein CML19, as well as phytohormone responsive genes, such as the WRKY transcription factors WRKY40 and WRKY70, were confirmed by RT-qPCR. These findings provided valuable insights into the antiviral mechanisms of NNM, which indicated that the agent induces tobacco systemic resistance against TMV via activating multiple plant defense signaling pathways.

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

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

Plant defense signals: Players and pawns in plant-virus-vector interactions

Plant viruses face an array of host defenses. Well-studied responses that protect against viruses include effector-triggered immunity, induced resistance (such as systemic acquired resistance mediated by salicylic acid), and RNA silencing. Recent work shows that viruses are also affected by non-host resistance mechanisms; previously thought to affect only bacteria, oomycetes and fungi. However, an enduring puzzle is how viruses are inhibited by several inducible host resistance mechanisms. Many viruses have been shown to encode factors that inhibit antiviral silencing. A number of these, including the cucumoviral 2b protein, the poytviral P1/HC-Pro and, respectively, geminivirus or satellite DNA-encoded proteins such as the C2 or βC1, also inhibit defensive signaling mediated by salicylic acid and jasmonic acid. This helps to explain how viruses can, in some cases, overcome host resistance. Additionally, interference with defensive signaling provides a means for viruses to manipulate plant-insect interactions. This is important because insects, particularly aphids and whiteflies, transmit many viruses. Indeed, there is now substantial evidence that viruses can enhance their own transmission through their effects on hosts. Even more surprisingly, it appears that viruses may be able to manipulate plant interactions with beneficial insects by, for example, 'paying back' their hosts by attracting pollinators.

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

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

Silencing the tobacco gene for RNA-dependent RNA polymerase 1 and infection by potato virus Y cause remodeling of cellular organelles

RNA-dependent RNA polymerase 1 (RDR1) has been shown to be involved in DNA methylation, RNA silencing and regulating expression of other genes. RDR1 gene expression is stimulated by infection with potato virus Y° (PVY). Transgenic Nicotiana tabacum plants silenced for RDR1 gene expression showed morphological changes in mesophyll cells, associated with remodeling of the nuclei, chloroplasts and mitochondria. RDR1 silencing led to decreased nuclear size, increased heterochromatin content and aggregation, decreased numbers of chloroplasts, plus changes in shape, internal structures and integrity of chloroplasts and mitochondria. RDR1-silenced transgenic plants showed increased PVY accumulation and ultrastructural remodeling was intensified in both chloroplasts and mitochondria of PVY-infected, RDR1-silenced plants. By contrast, heterochromatin condensation was reduced by PVY infection, and in non-transgenic plants the nuclei were translucent and lacked morphology after PVY infection. Thus, RDR1 regulates gene expression leading to remodeling of chromosomes, and PVY infection counteracts these effects on chromosomal remodeling.

Nitric oxide associated protein 1 is associated with chloroplast perturbation and disease symptoms in Nicotiana benthamiana infected with South African cassava mosaic virus

Nitric oxide associated 1 (NOA1) in plants is a cyclic GTPase involved in protein translation in the chloroplast and has been indirectly linked to nitric oxide (NO) accumulation and response to biotic stress. The association between NOA1 and NO accumulation in Arabidopsis noa1 mutants has been linked to the inability of noa1 mutants to accumulate carbon reserves such as fumarate, leading to chloroplast dysfunction and a pale green leaf phenotype. To understand the role played by NOA1 in response to South African cassava mosaic virus infection in Nicotiana benthamiana, the expression of NbNOA1 and the accumulation of NO in leaf samples was compared between south african cassava mosaic (SACMV)-infected and mock-infected plants at 14 and 28 dpi. Real-time qPCR was used to measure SACMV viral load which increased significantly by 20% from 14 to 28 dpi as chlorosis and symptom severity progressed. At 14 and 28 dpi, NbNOA1 expression was significantly lower than mock inoculated plants (2-fold lower at 14 dpi, p-value=0.01 and 5-fold lower at 28, p-value=0.00). At 14 dpi, NO accumulation remained unchanged in infected leaf tissue compared to mock inoculated, while at 28 dpi, NO accumulation was 40% lower (p-value=0.01). At 28 dpi, the decrease in NbNOA1 expression and NO accumulation was accompanied by chloroplast dysfunction, evident from the significant reduction in chlorophylls a and b and carotenoids in SACMV-infected leaves. Furthermore, the expression of chloroplast translation factors (chloroplast RNA binding, chloroplast elongation factor G, translation elongation factor Tu, translation initiation factor 3-2, plastid-specific ribosomal protein 6 and plastid ribosome recycling factor) were found to be repressed in infected N. benthamiana. GC-MS analysis showed a decrease in fumarate and an increase in glucose in SACMV-infected N. benthamiana in comparison to mock samples suggesting a decrease in carbon stores. Collectively, these results provide evidence that in response to SACMV infection, a decrease in photopigments and carbon stores, accompanied by an increase in glucose and decrease in fumarate, leads to a decline in NbNOA1expression and NO levels. This is manifested by suppressed translation factors and disruption of chloroplast function, thereby contributing to chlorotic disease symptoms.

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

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

Sulfated lentinan induced mitochondrial dysfunction leads to programmed cell death of tobacco BY-2 cells

Sulphated lentinan (sLTN) is known to act as a resistance inducer by causing programmed cell death (PCD) in tobacco suspension cells. However, the underlying mechanism of this effect is largely unknown. Using tobacco BY-2 cell model, morphological and biochemical studies revealed that mitochondrial reactive oxygen species (ROS) production and mitochondrial dysfunction contribute to sLNT induced PCD. Cell viability, and HO/PI fluorescence imaging and TUNEL assays confirmed a typical cell death process caused by sLNT. Acetylsalicylic acid (an ROS scavenger), diphenylene iodonium (an inhibitor of NADPH oxidases) and protonophore carbonyl cyanide p-trifluoromethoxyphenyl hydrazone (a protonophore and an uncoupler of mitochondrial oxidative phosphorylation) inhibited sLNT-induced H2O2 generation and cell death, suggesting that ROS generation linked, at least partly, to a mitochondrial dysfunction and caspase-like activation. This conclusion was further confirmed by double-stained cells with the mitochondria-specific marker MitoTracker RedCMXRos and the ROS probe H2DCFDA. Moreover, the sLNT-induced PCD of BY-2 cells required cellular metabolism as up-regulation of the AOX family gene transcripts and induction of the SA biosynthesis, the TCA cycle, and miETC related genes were observed. It is concluded that mitochondria play an essential role in the signaling pathway of sLNT-induced ROS generation, which possibly provided new insight into the sLNT-mediated antiviral response, including PCD.

Modulation of host plant immunity by Tobamovirus proteins

BACKGROUND

To establish successful infection, plant viruses produce profound alterations of host physiology, disturbing unrelated endogenous processes and contributing to the development of disease. In tobamoviruses, emerging evidence suggests that viral-encoded proteins display a great variety of functions beyond the canonical roles required for virus structure and replication. Among these, their modulation of host immunity appears to be relevant in infection progression.

SCOPE

In this review, some recently described effects on host plant physiology of Tobacco mosaic virus (TMV)-encoded proteins, namely replicase, movement protein (MP) and coat protein (CP), are summarized. The discussion is focused on the effects of each viral component on the modulation of host defense responses, through mechanisms involving hormonal imbalance, innate immunity modulation and antiviral RNA silencing. These effects are described taking into consideration the differential spatial distribution and temporality of viral proteins during the dynamic process of replication and spread of the virus.

CONCLUSION

In discussion of these mechanisms, it is shown that both individual and combined effects of viral-encoded proteins contribute to the development of the pathogenesis process, with the host plant's ability to control infection to some extent potentially advantageous to the invading virus.

Molecular mechanisms behind the accumulation of ATP and H2O2 in citrus plants in response to 'Candidatus Liberibacter asiaticus' infection

Candidatus Liberibacter asiaticus (Las) is a fastidious, phloem-restricted pathogen with a significantly reduced genome, and attacks all citrus species with no immune cultivars documented to date. Like other plant bacterial pathogens, Las deploys effector proteins into the organelles of plant cells, such as mitochondria and chloroplasts to manipulate host immunity and physiology. These organelles are responsible for the synthesis of adenosine triphosphate (ATP) and have a critical role in plant immune signaling during hydrogen peroxide (H₂O₂) production. In this study, we investigated H₂O₂ and ATP accumulation in relation to citrus huanglongbing (HLB) in addition to revealing the expression profiles of genes critical for the production and detoxification of H₂O₂ and ATP synthesis. We also found that as ATP and H₂O₂ concentrations increased in the leaf, so did the severity of the HLB symptoms, a trend that remained consistent among the four different citrus varieties tested. Furthermore, the upregulation of ATP synthase, a key enzyme for energy conversion, may contribute to the accumulation of ATP in infected tissues, whereas downregulation of the H₂O₂ detoxification system may cause oxidative damage to plant macromolecules and cell structures. This may explain the cause of some of the HLB symptoms such as chlorosis or leaf discoloration. The findings in this study highlight important molecular and physiological mechanisms involved in the host plants' response to Las infection and provide new targets for interrupting the disease cycle.

CaRDR1, an RNA-Dependent RNA Polymerase Plays a Positive Role in Pepper Resistance against TMV

RNA silencing functions as a major natural antiviral defense mechanism in plants. RNA-dependent RNA polymerases (RDRs) that catalyze the synthesis of double-stranded RNAs, are considered as a fundamental element in RNA silencing pathways. In Arabidopsis thaliana, RDR1, 2 and 6 play important roles in anti-viral RNA silencing. Expression of RDR1 can be elevated following plant treatment with defense hormones and virus infection. RDR1 has been studied in several crop species, but not in pepper (Capsicum annuum L.). Here, a RDR1 gene was isolated from Capsicum annuum L., designated as CaRDR1. The full-length cDNA of CaRDR1 was 3,351 bp, encoding a 1,116-amino acid protein, which contains conserved regions, such as the most remarkable motif DLDGD. The transcripts of CaRDR1 could be induced by salicylic acid (SA), abscisic acid (ABA), H₂O₂, and tobacco mosaic virus (TMV). Silencing of CaRDR1 in pepper resulted in increased susceptibility to TMV as evident by severe symptom, increased of TMV-CP transcript, higher malondialdehyde (MDA) content and lower antioxidant enzymes activities compared with that of control plants. CaRDR1-overexpressing in Nicotiana benthamiana showed mild disease symptom and reduced TMV-CP transcripts than that of empty vector (EV) following TMV inoculation. The RNA silencing related genes, including NbAGO2, NbDCL2, NbDCL3, and NbDCL4 elevated expression in overexpressed plants. Alternative oxidase (AOX), the terminal oxidase of the cyanide (CN)-resistant alternative respiratory pathway, catalyze oxygen-dependent oxidation of ubiquinol in plants. It has an important function in plant defense against TMV. In addition, CaRDR1 overexpression promoted the expression of NbAOX1a and NbAOX1b. In conclusion, these results suggest that CaRDR1 plays a positive role in TMV resistance by regulating antioxidant enzymes activities and RNA silencing-related genes expression to suppress the replication and movement of TMV.

Nitric Oxide and Hydrogen Peroxide Mediate Wounding-Induced Freezing Tolerance through Modifications in Photosystem and Antioxidant System in Wheat

Mechanical wounding is a common stress caused by herbivores or manual and natural manipulations, whereas its roles in acclimation response to a wide spectrum of abiotic stresses remain unclear. The present work showed that local mechanical wounding enhanced freezing tolerance in untreated systemic leaves of wheat plants (Triticum aestivum L.), and meanwhile the signal molecules hydrogen peroxide (H₂O₂) and nitric oxide (NO) were accumulated systemically. Pharmacological study showed that wounding-induced NO synthesis was substantially arrested by pretreatment with scavengers of reactive oxygen species and an inhibitor of NADPH oxidase (respiratory burst oxidase homolog, RBOH). On the contrary, wounding-induced H₂O₂ accumulation was not sensitive to NO synthetic inhibitors or scavenger, indicating that H₂O₂ acts upstream of NO in wounding signal transduction pathways. Cytochemical and vascular tissues localizations approved that RBOH-dependent H₂O₂ acts as long-distance signal in wounding response. Transcriptome analysis revealed that 279 genes were up-regulated in plants treated with wounding and freezing, but not in plants treated with freezing alone. Importantly, freezing- and wounding-induced genes were significantly enriched in the categories of "photosynthesis" and "signaling." These results strongly supported that primary mechanical wounding can induce freezing tolerance in wheat through the systemic accumulation of NO and H₂O₂, and further modifications in photosystem and antioxidant system.

Label-free Proteomic Reveals that Cowpea Severe Mosaic Virus Transiently Suppresses the Host Leaf Protein Accumulation During the Compatible Interaction with Cowpea (Vigna unguiculata [L.] Walp.)

Viruses are important plant pathogens that threaten diverse crops worldwide. Diseases caused by Cowpea severe mosaic virus (CPSMV) have drawn attention because of the serious damages they cause to economically important crops including cowpea. This work was undertaken to quantify and identify the responsive proteins of a susceptible cowpea genotype infected with CPSMV, in comparison with mock-inoculated controls, using label-free quantitative proteomics and databanks, aiming at providing insights on the molecular basis of this compatible interaction. Cowpea leaves were mock- or CPSMV-inoculated and 2 and 6 days later proteins were extracted and analyzed. More than 3000 proteins were identified (data available via ProteomeXchange, identifier PXD005025) and 75 and 55 of them differentially accumulated in response to CPSMV, at 2 and 6 DAI, respectively. At 2 DAI, 76% of the proteins decreased in amount and 24% increased. However, at 6 DAI, 100% of the identified proteins increased. Thus, CPSMV transiently suppresses the synthesis of proteins involved particularly in the redox homeostasis, protein synthesis, defense, stress, RNA/DNA metabolism, signaling, and other functions, allowing viral invasion and spread in cowpea tissues.

Transient Expression of Candidatus Liberibacter Asiaticus Effector Induces Cell Death in Nicotiana benthamiana

Candidatus Liberibacter asiaticus "Las" is a phloem-limited bacterial plant pathogen, and the most prevalent species of Liberibacter associated with citrus huanglongbing (HLB), a devastating disease of citrus worldwide. Although, the complete sequence of the Las genome provides the basis for studying functional genomics of Las and molecular mechanisms of Las-plant interactions, the functional characterization of Las effectors remains a slow process since remains to be cultured. Like other plant pathogens, Las may deliver effector proteins into host cells and modulate a variety of host cellular functions for their infection progression. In this study, we identified 16 putative Las effectors via bioinformatics, and transiently expressed them in Nicotiana benthamiana. Diverse subcellular localization with different shapes and aggregation patterns of the effector candidates were revealed by UV- microscopy after transient expression in leaf tissue. Intriguingly, one of the 16 candidates, Las5315mp (mature protein), was localized in the chloroplast and induced cell death at 3 days post inoculation (dpi) in N. benthamiana. Moreover, Las5315mp induced strong callose deposition in plant cells. This study provides new insights into the localizations and potential roles of these Las effectors in planta.

Effect of Pyraclostrobin Application on Viral and Bacterial Diseases of Tomato

Quinone outside inhibitors (QoI) are powerful fungicides, which have been reported, additionally to their fungicide activity, to increase plant capacity to activate cellular defense responses and to promote plant growth. In this work, the effect of the QoI class fungicide pyraclostrobin was examined against Cucumber mosaic virus (CMV), Potato virus Y (PVY) and Pseudomonas syringae pv. tomato in tomato plants following artificial inoculation of the plants with the pathogens. Under controlled environmental conditions, pyraclostrobin delayed viral and bacterial disease development, even if P. syringae pv. tomato internal population levels were not affected significantly. In contrast, under field conditions in commercial greenhouses, a reduced CMV disease incidence throughout the tomato cultivation period was recorded. Gene expression analysis indicated an effect of pyraclostrobin application on tomato MAPKs transcript levels and a possible interference with plant stress responses.

RNA-dependent RNA polymerase 1 in potato (Solanum tuberosum) and its relationship to other plant RNA-dependent RNA polymerases

Cellular RNA-dependent RNA polymerases (RDRs) catalyze synthesis of double-stranded RNAs that can serve to initiate or amplify RNA silencing. Arabidopsis thaliana has six RDR genes; RDRs 1, 2 and 6 have roles in anti-viral RNA silencing. RDR6 is constitutively expressed but RDR1 expression is elevated following plant treatment with defensive phytohormones. RDR1 also contributes to basal virus resistance. RDR1 has been studied in several species including A. thaliana, tobacco (Nicotiana tabacum), N. benthamiana, N. attenuata and tomato (Solanum lycopersicum) but not to our knowledge in potato (S. tuberosum). StRDR1 was identified and shown to be salicylic acid-responsive. StRDR1 transcript accumulation decreased in transgenic potato plants constitutively expressing a hairpin construct and these plants were challenged with three viruses: potato virus Y, potato virus X, and tobacco mosaic virus. Suppression of StRDR1 gene expression did not increase the susceptibility of potato to these viruses. Phylogenetic analysis of RDR genes present in potato and in a range of other plant species identified a new RDR gene family, not present in potato and found only in Rosids (but apparently lost in the Rosid A. thaliana) for which we propose the name RDR7.

Salicylic acid treatment and expression of an RNA-dependent RNA polymerase 1 transgene inhibit lethal symptoms and meristem invasion during tobacco mosaic virus infection in Nicotiana benthamiana

BACKGROUND

Host RNA-dependent RNA polymerases (RDRs) 1 and 6 contribute to antiviral RNA silencing in plants. RDR6 is constitutively expressed and was previously shown to limit invasion of Nicotiana benthamiana meristem tissue by potato virus X and thereby inhibit disease development. RDR1 is inducible by salicylic acid (SA) and several other phytohormones. But although it contributes to basal resistance to tobacco mosaic virus (TMV) it is dispensable for SA-induced resistance in inoculated leaves. The laboratory accession of N. benthamiana is a natural rdr1 mutant and highly susceptible to TMV. However, TMV-induced symptoms are ameliorated in transgenic plants expressing Medicago truncatula RDR1.

RESULTS

In MtRDR1-transgenic N. benthamiana plants the spread of TMV expressing the green fluorescent protein (TMV.GFP) into upper, non-inoculated, leaves was not inhibited. However, in these plants exclusion of TMV.GFP from the apical meristem and adjacent stem tissue was greater than in control plants and this exclusion effect was enhanced by SA. TMV normally kills N. benthamiana plants but although MtRDR1-transgenic plants initially displayed virus-induced necrosis they subsequently recovered. Recovery from disease was markedly enhanced by SA treatment in MtRDR1-transgenic plants whereas in control plants SA delayed but did not prevent systemic necrosis and death. Following SA treatment of MtRDR1-transgenic plants, extractable RDR enzyme activity was increased and Western blot analysis of RDR extracts revealed a band cross-reacting with an antibody raised against MtRDR1. Expression of MtRDR1 in the transgenic N. benthamiana plants was driven by a constitutive 35S promoter derived from cauliflower mosaic virus, confirmed to be non-responsive to SA. This suggests that the effects of SA on MtRDR1 are exerted at a post-transcriptional level.

CONCLUSIONS

MtRDR1 inhibits severe symptom development by limiting spread of virus into the growing tips of infected plants. Thus, RDR1 may act in a similar fashion to RDR6. MtRDR1 and SA acted additively to further promote recovery from disease symptoms in MtRDR1-transgenic plants. Thus it is possible that SA promotes MtRDR1 activity and/or stability through post-transcriptional effects.

Transcriptomic Response to Nitric Oxide Treatment in Larix olgensis Henry

Larix olgensis Henry is an important coniferous species found in plantation forests in northeastern China, but it is vulnerable to pathogens. Nitric oxide (NO) is an important molecule involved in plant resistance to pathogens. To study the regulatory role of NO at the transcriptional level, we characterized the transcriptomic response of L. olgensis seedlings to sodium nitroprusside (SNP, NO donor) using Illumina sequencing and de novo transcriptome assembly. A significant number of putative metabolic pathways and functions associated with the unique sequences were identified. Genes related to plant pathogen infection (FLS2, WRKY33, MAPKKK, and PR1) were upregulated with SNP treatment. This report describes the potential contribution of NO to disease resistance in L. olgensis as induced by biotic stress. Our results provide a substantial contribution to the genomic and transcriptomic resources for L. olgensis, as well as expanding our understanding of the involvement of NO in defense responses at the transcriptional level.

Involvement of nitric oxide in the jasmonate-dependent basal defense against root-knot nematode in tomato plants

Jasmonic acid (JA) and nitric oxide (NO) are well-characterized signaling molecules in plant defense responses. However, their roles in plant defense against root-knot nematode (RKN, Meloidogyne incognita) infection are largely unknown. In this study, we found that the transcript levels of the JA- and NO-related biosynthetic and signaling component genes were induced after RKN infection. Application of exogenous JA and sodium nitroprusside (SNP; a NO donor) significantly decreased the number of egg masses in tomato roots after RKN infection and partially alleviated RKN-induced decreases in plant fresh weight and net photosynthetic rate. These molecules also alleviated RKN-induced increases in root electrolyte leakage and membrane peroxidation. Importantly, NO scavenger partially inhibited JA-induced RKN defense. The pharmacological inhibition of JA biosynthesis significantly increased the plants' susceptibility to RKNs, which was effectively alleviated by SNP application, showing that NO may be involved in the JA-dependent RKN defense pathway. Furthermore, both JA and SNP induced increases in protease inhibitor 2 (PI2) gene expression after RKN infestation. Silencing of PI2 compromised both JA- and SNP-induced RKN defense responses, suggesting that the PI2 gene mediates JA- and NO-induced defense against RKNs. This work will be important for deepening the understanding of the mechanisms involved in basal defense against RKN attack in plants.

The relationship between the plant-encoded RNA-dependent RNA polymerase 1 and alternative oxidase in tomato basal defense against Tobacco mosaic virus

Salicylic acid (SA) plays a critical role in plant defense against pathogen attack. The SA-induced viral defense in plants is distinct from the pathways mediating bacterial and fungal defense, which is pathogenesis-related protein-independent but involves an RNA-dependent RNA polymerase 1 (RDR1)-mediated RNA silencing mechanism and/or an alternative oxidase (AOX)-associated defense pathway. However, the relationship between these two viral defense-related pathways remains unclear. In this study, Tobacco mosaic virus (TMV) inoculation onto Solanum lycopersicum (tomato) leaves induced a rapid induction of the SlAOX1a transcript level as well as the total and CN-resistant respiration at 0.5 dpi, followed by an increase in SlRDR1 gene expression at 1 dpi in the upper uninoculated leaves. Silencing SlRDR1 using virus-induced gene silencing system significantly reduced SlRDR1 expression and tomato defense against TMV but had no evident effect on SlAOX1a transcription. Conversely, silencing SlAOX1a not only effectively reduced the AOX1a transcript level, but also blocked the TMV-induced SlRDR1 expression and decreased the basal defense against TMV. Furthermore, the application of an exogenous AOX activator on empty vector-silenced control plants greatly induced the accumulation of SlRDR1 and SlAOX1a transcript and reduced TMV viral RNA accumulation, but failed to have such effects on SlRDR1-silenced plants. Moreover, RDR1-overexpressed transgenic Nicotiana benthamiana plants enhanced defense against TMV than the empty vector-transformed plants, but these effects were not affected by the exogenous AOX activator or inhibitor. These results indicate that RDR1 is involved in the AOX-mediated defense pathway against TMV infection and plays a crucial role in enhancing RNA silencing to limit virus systemic spread.

Salicylic acid binding of mitochondrial alpha-ketoglutarate dehydrogenase E2 affects mitochondrial oxidative phosphorylation and electron transport chain components and plays a role in basal defense against tobacco mosaic virus in tomato

Salicylic acid (SA) plays a critical role in plant defense against pathogen invasion. SA-induced viral defense in plants is distinct from the pathways mediating bacterial and fungal defense and involves a specific pathway mediated by mitochondria; however, the underlying mechanisms remain largely unknown. The SA-binding activity of the recombinant tomato (Solanum lycopersicum) alpha-ketoglutarate dehydrogenase (Slα-kGDH) E2 subunit of the tricarboxylic acid (TCA) cycle was characterized. The biological role of this binding in plant defenses against tobacco mosaic virus (TMV) was further investigated via Slα-kGDH E2 silencing and transient overexpression in plants. Slα-kGDH E2 was found to bind SA in two independent assays. SA treatment, as well as Slα-kGDH E2 silencing, increased resistance to TMV. SA did not further enhance TMV defense in Slα-kGDH E2-silenced tomato plants but did reduce TMV susceptibility in Nicotiana benthamiana plants transiently overexpressing Slα-kGDH E2. Furthermore, Slα-kGDH E2-silencing-induced TMV resistance was fully blocked by bongkrekic acid application and alternative oxidase 1a silencing. These results indicated that binding by Slα-kGDH E2 of SA acts upstream of and affects the mitochondrial electron transport chain, which plays an important role in basal defense against TMV. The findings of this study help to elucidate the mechanisms of SA-induced viral defense.

Tomato-Pseudomonas syringae interactions under elevated CO₂ concentration: the role of stomata

Increasing atmospheric CO₂ concentrations ([CO₂]) in agricultural and natural ecosystems is known to reduce plant stomatal opening, but it is unclear whether these CO₂-induced stomatal alterations are associated with foliar pathogen infections. In this study, tomato plants were grown under ambient and elevated [CO₂] and inoculated with Pseudomonas syringae pv. tomato strain DC3000, a strain that is virulent on tomato plants. We found that elevated [CO₂] enhanced tomato defence against P. syringae. Scanning electron microscopy analysis revealed that stomatal aperture of elevated [CO₂] plants was considerably smaller than their ambient counterparts, which affected the behaviour of P. syringae bacteria on the upper surface of epidermal peels. Pharmacological experiments revealed that nitric oxide (NO) played a role in elevated [CO₂]-induced stomatal closure. Silencing key genes involved in NO generation and stomatal closing, nitrate reductase (NR) and guard cell slow-type anion channel 1 (SLAC1), blocked elevated [CO₂]-induced stomatal closure and resulted in significant increases in P. syringae infection. However, the SLAC1-silenced plants, but not the NR-silenced plants, still had significantly higher defence under elevated [CO₂] compared with plants treated with ambient [CO₂]. Similar results were obtained when the stomata-limiting factor for P. syringae entry was excluded by syringe infiltration inoculation. These results indicate that elevated [CO₂] induces defence against P. syringae in tomato plants, not only by reducing the stomata-mediated entry of P. syringae but also by invoking a stomata-independent pathway to counteract P. syringae. This information is valuable for designing proper strategies against bacterial pathogens under changing agricultural and natural ecosystems.

Early responses to cadmium of two poplar clones that differ in stress tolerance

Soil cadmium (Cd) contamination is becoming a matter of great global concern. The identification of plants differentially sensitive to Cd excess is of interest for the selection of genotype adaptive to grow and develop in polluted areas and capable of ameliorating or reducing the negative environmental effects of this toxic metal. The two poplar clones I-214 (Populus×canadensis) and Eridano (Populus deltoides×maximowiczii) are, respectively, tolerant and sensitive to ozone (O3) exposure. Because stress tolerance is mediated by an array of overlapping defence mechanisms, we tested the hypothesis that these two clones differently sensitive to O3 stress factor also exhibit different tolerance to Cd. With this purpose, an outdoor pot experiment was designed to study the responses of I-214 and Eridano to the distribution of different Cd solutions enriched with CdCl2 (0, 50 and 150μM) for 35 days. Changes in leaf area, biomass allocation and Cd uptake, photosynthesis, chlorophyll fluorescence, leaf concentration of nutrients and pigments, hydrogen peroxide (H2O2) and nitric oxide (NO) production and thiol compounds were investigated. The two poplar clones showed similar sensitivity to excess Cd in terms of biomass production, photosynthesis activity and Cd accumulation, though physiological and biochemical traits revealed different defence strategies. In particular, Eridano maintained in any Cd treatment the number of its constitutively wider blade leaves, while the number of I-214 leaves (with lower size) was reduced. H2O2 increased 4.5- and 13-fold in I-214 leaves after the lowest (L) and highest (H) Cd treatments, respectively, revealing the induction of oxidative burst. NO, constitutively higher in I-214 than Eridano, progressively increased in both clones with the enhancement of Cd concentration in the substrate. I-214 showed a more elevated antioxidative capacity (GSH/GSSG) and higher photochemical efficiency of PSII (Fv/Fm) and de-epoxidation degree of xantophylls-cycle (DEPS). The glutathione pool was not affected by Cd treatment in both clones, while non-protein thiols and phytochelatins were reduced at L Cd treatment in I-214. Overall, these two clones presented high adaptability to Cd stress and are both suitable to develop and growth in environments contaminated with this metal, thus being promising for their potential use in phytoremediation programmes.

Effects of modifying alternative respiration on nitric oxide-induced virus resistance and PR1 protein accumulation

Nitric oxide (NO) is an important defensive signal in plants but its effects on virus infection are not well understood. Administration of NO-releasing compounds immediately before inoculation of tobacco leaves with potato virus X and tobacco mosaic virus decreased the accumulation of virus, indicating that NO can induce resistance rapidly. Resistance induction was inhibited by co-administration with an NO-scavenging compound or when experiments were done in transgenic tobacco plants expressing increased alternative respiratory pathway capacity due to constitutive expression of the plant mitochondrial enzyme, alternative oxidase (AOX). These results indicate that NO, which inhibits electron transport chain activity, is triggering defensive signalling by inducing changes in mitochondrial reactive oxygen species levels that are in turn regulated by AOX. Experiments using nahG-transgenic plants, which cannot accumulate the defensive plant hormone salicylic acid (SA) showed that NO rapidly induces resistance to virus infection independently of SA. However, this initial state of resistance may be transient. Subsequently, by 5 days post-treatment, NO had caused an increase in pathogenesis-related protein 1 (PR1) expression (a proxy for increased SA biosynthesis), which correlated with a longer-term state of resistance to virus infection. The induction by NO of PR1 accumulation was modified in AOX-transgenic plants. This indicates that the influence of NO on defensive gene expression is in part mediated through its effects on mitochondria.

TMV-Cg Coat Protein stabilizes DELLA proteins and in turn negatively modulates salicylic acid-mediated defense pathway during Arabidopsis thaliana viral infection

BACKGROUND

Plant viral infections disturb defense regulatory networks during tissue invasion. Emerging evidence demonstrates that a significant proportion of these alterations are mediated by hormone imbalances. Although the DELLA proteins have been reported to be central players in hormone cross-talk, their role in the modulation of hormone signaling during virus infections remains unknown.

RESULTS

This work revealed that TMV-Cg coat protein (CgCP) suppresses the salicylic acid (SA) signaling pathway without altering defense hormone SA or jasmonic acid (JA) levels in Arabidopsis thaliana. Furthermore, it was observed that the expression of CgCP reduces plant growth and delays the timing of floral transition. Quantitative RT-qPCR analysis of DELLA target genes showed that CgCP alters relative expression of several target genes, indicating that the DELLA proteins mediate transcriptional changes produced by CgCP expression. Analyses by fluorescence confocal microscopy showed that CgCP stabilizes DELLA proteins accumulation in the presence of gibberellic acid (GA) and that the DELLA proteins are also stabilized during TMV-Cg virus infections. Moreover, DELLA proteins negatively modulated defense transcript profiles during TMV-Cg infection. As a result, TMV-Cg accumulation was significantly reduced in the quadruple-DELLA mutant Arabidopsis plants compared to wild type plants.

CONCLUSIONS

Taken together, these results demonstrate that CgCP negatively regulates the salicylic acid-mediated defense pathway by stabilizing the DELLA proteins during Arabidopsis thaliana viral infection, suggesting that CgCP alters the stability of DELLAs as a mechanism of negative modulation of antiviral defense responses.

Mutation of the RDR1 gene caused genome-wide changes in gene expression, regional variation in small RNA clusters and localized alteration in DNA methylation in rice

BACKGROUND

Endogenous small (sm) RNAs (primarily si- and miRNAs) are important trans/cis-acting regulators involved in diverse cellular functions. In plants, the RNA-dependent RNA polymerases (RDRs) are essential for smRNA biogenesis. It has been established that RDR2 is involved in the 24 nt siRNA-dependent RNA-directed DNA methylation (RdDM) pathway. Recent studies have suggested that RDR1 is involved in a second RdDM pathway that relies mostly on 21 nt smRNAs and functions to silence a subset of genomic loci that are usually refractory to the normal RdDM pathway in Arabidopsis. Whether and to what extent the homologs of RDR1 may have similar functions in other plants remained unknown.

RESULTS

We characterized a loss-of-function mutant (Osrdr1) of the OsRDR1 gene in rice (Oryza sativa L.) derived from a retrotransposon Tos17 insertion. Microarray analysis identified 1,175 differentially expressed genes (5.2% of all expressed genes in the shoot-tip tissue of rice) between Osrdr1 and WT, of which 896 and 279 genes were up- and down-regulated, respectively, in Osrdr1. smRNA sequencing revealed regional alterations in smRNA clusters across the rice genome. Some of the regions with altered smRNA clusters were associated with changes in DNA methylation. In addition, altered expression of several miRNAs was detected in Osrdr1, and at least some of which were associated with altered expression of predicted miRNA target genes. Despite these changes, no phenotypic difference was identified in Osrdr1 relative to WT under normal condition; however, ephemeral phenotypic fluctuations occurred under some abiotic stress conditions.

CONCLUSIONS

Our results showed that OsRDR1 plays a role in regulating a substantial number of endogenous genes with diverse functions in rice through smRNA-mediated pathways involving DNA methylation, and which participates in abiotic stress response.