Induction of antiviral responses by acibenzolar-s-methyl against cucurbit chlorotic yellows virus in Melon

Begomovirus Transmission to Tomato Plants Is Not Hampered by Plant Defenses Induced by Dicyphus hesperus Knight

Plants can respond to insect infestation and virus infection by inducing plant defenses, generally mediated by phytohormones. Moreover, plant defenses alter host quality for insect vectors with consequences for the spread of viruses. In agricultural settings, other organisms commonly interact with plants, thereby inducing plant defenses that could affect plant-virus-vector interactions. For example, plant defenses induced by omnivorous insects can modulate insect behavior. This study focused on tomato yellow leaf curl virus (TYLCV), a plant virus of the family Geminiviridae and genus Begomovirus. It is transmitted in a persistent circulative manner by the whitefly Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae), posing a global threat to tomato production. Mirids (Hemiptera: Miridae) are effective biological control agents of B. tabaci, but there is a possibility that their omnivorous nature could also interfere with the process of virus transmission. To test this hypothesis, this study first addressed to what extent the mirid bug Dicyphus hesperus Knight induces plant defenses in tomato. Subsequently, the impact of this plant-omnivore interaction on the transmission of TYLCV was evaluated. Controlled cage experiments were performed in a greenhouse setting to evaluate the impact of mirids on virus transmission and vector acquisition by B. tabaci. While we observed a reduced number of whiteflies settling on plants exposed to D. hesperus, the plant defenses induced by the mirid bug did not affect TYLCV transmission and accumulation. Additionally, whiteflies were able to acquire comparable amounts of TYLCV on mirid-exposed plants and control plants. Overall, the induction of plant defenses by D. hesperus did not influence TYLCV transmission by whiteflies on tomato.

Economic versus technical efficiency in using ASM combined with fungicides to elicit wheat varieties with different disease susceptibilities

Despite the positive results of using elicitors to induce resistance against plant diseases, some factors have inhibited the popularization of their use in agriculture. There is an energetic cost related to the elicitors' induced response which can cause undesired effects on growth under low-pressure disease conditions. Elicitors can create phytotoxicity and show high variation in their efficiency between different genotypes within the same crop; in addition, the positive results related to the induced resistance may not repeat in field treatments, adding to the possibility that they are not economically viable. Thus, we carried out two experiments to investigate the technical and economic efficiency of acibenzolar-S-methyl (ASM) and its association with fungicides in the control of leaf diseases of susceptible and resistant wheat varieties, and as how it reflects on the photosynthetic and production performance of wheat. This study showed the limitations of incorporating ASM into foliar fungal disease control in economic terms. However, it was evident that ASM effectively induced plant resistance against Leaf Rust and Powdery Mildew in the field and can be considered a sustainable option for wheat cultivation. Even though its association with chemical control was not the best economic strategy the use of ASM is a tool that can be incorporated into wheat cultivation to minimize the emergence of fungicide-resistant pathogens due to the diversification of modes of action employed and reduce the toxic residue deposition to the environment and human health.

Impact of Foliar Application of Acibenzolar-S-Methyl on Rose Rosette Disease and Rose Plant Quality

Rose rosette disease (RRD) caused by rose rosette emaravirus (RRV) is a major issue in the U.S. rose industry with no effective method for its management. This study evaluated the effect of foliar application of acibenzolar-S-methyl (ASM), a plant systemic acquired resistance inducer, in reducing RRD disease severity on Rosa species cv. Radtkopink ('Pink Double Knock Out') under greenhouse conditions, and the effect of ASM on plant growth under commercial nursery production conditions. ASM at 50- or 100-mg/liter concentrations at weekly intervals significantly reduced RRD severity compared with the untreated control in two of the three greenhouse trials (P < 0.05). The plants in these trials were subsequently pruned and observed for symptoms, which further indicated that application of ASM at 50- or 100-mg/liter concentrations lowered disease severity compared with the untreated control (P < 0.05) in these two trials. Plants treated with ASM at 50- or 100-mg/liter concentrations had delayed incidence of RRD compared with the nontreated controls. Plants treated with ASM at the 50- or 100-mg/liter rate in all three trials either did not have RRV present or the virus was present in fewer leaf samples than untreated controls as indicated by quantitative reverse transcription PCR analysis. Overall, plants treated with ASM at the 50-mg/liter concentration had 36 to 43% reduced RRD incidence compared with the water control. The treatment of two cultivars of rose, 'Radtkopink' and 'Meijocos' ('Pink Drift'), with weekly foliar applications of ASM at the three rates (0.5, 0.75, and 1.0 oz/A) indicated that ASM had no negative effect on flowering or plant growth at even the highest rate of application.

Acibenzolar-S-methyl-mediated restriction of loading of plantago asiatica mosaic virus into vascular tissues of Nicotiana benthamiana

Long-distance movement via vascular tissues is an essential step for systemic infection by plant viruses. We previously reported that pre-treatment of Nicotiana benthamiana with acibenzolar-S-methyl (ASM) both suppressed the accumulation of plantago asiatica mosaic virus (PlAMV) in inoculated leaves and delayed the long-distance movement to uninoculated upper leaves. These two effects occurred independently of each other. However, it remained unclear where and when the viral long-distance movement is inhibited upon ASM treatment. In this study, we found that ASM treatment restricted the loading of GFP-expressing PlAMV (PlAMV-GFP) into vascular tissues in the inoculated leaves. This led to delays in viral translocation to the petiole and the main stem, and to untreated upper leaves. We used cryohistological fluorescence imaging to show that ASM treatment affected the viral localization and reduced its accumulation in the phloem, xylem, and mesophyll tissues. A stem girdling experiment, which blocked viral movement downward through phloem tissues, demonstrated that ASM treatment could inhibit viral systemic infection to upper leaves, which occurred even with viral downward movement restricted. Taken together, our results showed that ASM treatment affects the loading of PlAMV-GFP into the vascular system in the inoculated leaf, and that this plays a key role in the ASM-mediated delay of viral long-distance movement.

Role of salicylic acid glucosyltransferase in balancing growth and defence for optimum plant fitness

Salicylic acid (SA), an essential secondary messenger for plant defence responses, plays a role in maintaining a balance (trade-off) between plant growth and resistance induction, but the detailed mechanism has not been explored. Because the SA mimic benzothiadiazole (BTH) is a more stable inducer of plant defence than SA after exogenous application, we analysed expression profiles of defence genes after BTH treatment to better understand SA-mediated immune induction. Transcript levels of the salicylic acid glucosyltransferase (SAGT) gene were significantly lower in BTH-treated Nicotiana tabacum (Nt) plants than in SA-treated Nt control plants, suggesting that SAGT may play an important role in SA-related host defence responses. Treatment with BTH followed by SA suppressed SAGT transcription, indicating that the inhibitory effect of BTH is not reversible. In addition, in BTH-treated Nt and Nicotiana benthamiana (Nb) plants, an early high accumulation of SA and SA 2-O-β-d-glucoside was only transient compared to the control. This observation agreed well with the finding that SAGT-overexpressing (OE) Nb lines contained less SA and jasmonic acid (JA) than in the Nb plants. When inoculated with a virus, the OE Nb plants showed more severe symptoms and accumulated higher levels of virus, while resistance increased in SAGT-silenced (IR) Nb plants. In addition, the IR plants restricted bacterial spread to the inoculated leaves. After the BTH treatment, OE Nb plants were slightly larger than the Nb plants. These results together indicate that SAGT has a pivotal role in the balance between plant growth and SA/JA-mediated defence for optimum plant fitness.

Priming Melon Defenses with Acibenzolar-S-methyl Attenuates Infections by Phylogenetically Distinct Viruses and Diminishes Vector Preferences for Infected Hosts

Plant virus management is mostly achieved through control of insect vectors using insecticides. However, insecticides are only marginally effective for preventing virus transmission. Furthermore, it is well established that symptoms of virus infections often encourage vector visitation to infected hosts, which exacerbates secondary spread. Plant defense elicitors, phytohormone analogs that prime the plant immune system against attack, may be a viable approach for virus control that complements insecticide use by disrupting pathologies that attract vectors. To explore this, we tested the effect of a commercial plant elicitor, acibenzolar-S-methyl (ASM), on infection rates, virus titers, and symptom development in melon plants inoculated with one of two virus species, Cucumber mosaic virus (CMV) and Cucurbit yellow stunting disorder virus (CYSDV). We also conducted behavioral assays to assess the effect of ASM treatment and virus inoculation on vector behavior. For both pathogens, ASM treatment reduced symptom severity and delayed disease progression. For CYSDV, this resulted in the attenuation of symptoms that encourage vector visitation and virion uptake. We did observe slight trade-offs in growth vs. defense following ASM treatment, but these effects did not translate into reduced yields or plant performance in the field. Our results suggest that immunity priming may be a valuable tool for improving management of insect-transmitted plant viruses.

Acibenzolar-S-Methyl Restricts Infection of Nicotiana benthamiana by Plantago Asiatica Mosaic Virus at Two Distinct Stages

Plant activators, including acibenzolar-S-methyl (ASM), are chemical compounds that stimulate plant defense responses to pathogens. ASM treatment inhibits infection by a variety of plant viruses, however, the mechanisms of this broad-spectrum and strong effect remain poorly understood. We employed green fluorescent protein (GFP)-expressing viruses and Nicotiana benthamiana plants to identify the infection stages that are restricted by ASM. ASM suppressed infection by three viral species, plantago asiatica mosaic virus (PlAMV), potato virus X (PVX), and turnip mosaic virus (TuMV), in inoculated cells. Furthermore, ASM delayed the long-distance movement of PlAMV and PVX, and the cell-to-cell (short range) movement of TuMV. The ASM-mediated delay of long-distance movement of PlAMV was not due to the suppression of viral accumulation in the inoculated leaves, indicating that ASM restricts PlAMV infection in at least two independent steps. We used Arabidopsis thaliana mutants to show that the ASM-mediated restriction of PlAMV infection requires the NPR1 gene but was independent of the dicer-like genes essential for RNA silencing. Furthermore, experiments using protoplasts showed that ASM treatment inhibited PlAMV replication without cell death. Our approach, using GFP-expressing viruses, will be useful for the analysis of mechanisms underlying plant activator-mediated virus restriction.

Use of Systemic Acquired Resistance and Whitefly Optical Barriers to Reduce Tomato Yellow Leaf Curl Disease Damage to Tomato Crops

Epidemics of tomato yellow leaf curl disease (TYLCD) caused by tomato yellow leaf curl-like begomoviruses (genus Begomovirus, family Geminiviridae) severely damage open field and protected tomato crops worldwide. Intensive application of insecticides against the whitefly vector Bemisia tabaci is generally used as control strategy to reduce TYLCD impact. This practice, however, is frequently ineffective and has a negative impact on the environment and human health. TYLCD-resistant varieties are commercially available, but cultivation of susceptible traditional tasting ones is also requested if possible. For susceptible tomatoes, here we show that using whitefly optical barriers by means of UV-blocking plastics in protected crops can contribute to reducing TYLCD damage and increasing commercial fruit yield. Moreover, induction of systemic acquired resistance by application of the elicitor of plant defense acibenzolar-S-methyl was effective to reduce yield losses when viral pressure was moderate. Interestingly, combining both practices in protected tomato crops can result in a significant TYLCD control. Therefore, these control practices are proposed to be used commercially as management alternatives to include in integrated management of TYLCD.

Integrating transcriptome and microRNA analysis identifies genes and microRNAs for AHO-induced systemic acquired resistance in N. tabacum

3-Acetonyl-3-hydroxyoxindole (AHO) induces systemic acquired resistance (SAR) in Nicotiana. However, the underlying molecular mechanism is not well understood. To understand the molecular regulation during SAR induction, we examined mRNA levels, microRNA (miRNA) expression, and their regulatory mechanisms in control and AHO-treated tobacco leaves. Using RNA-seq analysis, we identified 1,445 significantly differentially expressed genes (DEGs) at least 2 folds with AHO treatment. The DEGs significantly enriched in six metabolism pathways including phenylpropanoid biosynthesis, sesquiterpenoid and triterpenoid biosynthesis for protective cuticle and wax. Key DEGs including PALs and PR-10 in salicylic acid pathway involved in SAR were significantly regulated. In addition, we identified 403 miRNAs belonging to 200 miRNA families by miRNA sequencing. In total, AHO treatment led to 17 up- and 6 down-regulated at least 2 folds (Wald test, P < 0.05) miRNAs (DEMs), respectively. Targeting analysis implicated four DEMs regulating three DEGs involved in disease resistance, including miR156, miR172f, miR172g, miR408a, SPL6 and AP2. We concluded that both mRNA and miRNA regulation enhances AHO-induced SAR. These data regarding DEGs, miRNAs, and their regulatory mechanisms provide molecular evidence for the mechanisms involved in tobacco SAR, which are likely to be present in other plants.

Manipulation of induced resistance to viruses

Induced resistance against plant viruses has been studied for many years. However, with the exception of RNA silencing, induced resistance to viruses remains mechanistically less well understood than for other plant pathogens. In contrast, the induction processes involved in induced resistance, comprising basal resistance signaling, effector-triggered immunity, and phytohormone pathways, have been increasingly well characterized in recent years. This has allowed induced resistance to viruses to be placed in a broader conceptual framework linking it to other defense systems, which we discuss in this review. We also discuss the range of agents, including chemicals and beneficial microorganisms and application methods that can be used to induce resistance to viruses.

Is modulating virus virulence by induced systemic resistance realistic?

Induction of plant resistance, either achieved by chemicals (systemic acquired resistance, SAR) or by rhizobacteria (induced systemic resistance, ISR) is a possible and/or complementary alternative to manage virus infections in crops. SAR mechanisms operating against viruses are diverse, depending on the pathosystem, and may inhibit virus replication as well as cell-to-cell and long-distance movement. Inhibition is often mediated by salicylic acid with the involvement of alternative oxidase and reactive oxygen species. However, salicylate may also stimulate a separate downstream pathway, leading to the induction of an additional mechanism, based on RNA-dependent RNA polymerase 1-mediated RNA silencing. Thus, SAR and RNA silencing would closely cooperate in the defence against virus infection. Despite tremendous recent progress in the knowledge of SAR mechanisms, only a few compounds, including benzothiadiazole and chitosan have been shown to reduce the severity of systemic virus disease in controlled environment and, more modestly, in open field. Finally, ISR induction, has proved to be a promising strategy to control virus disease, particularly by seed bacterization with a mixture of plant growth-promoting rhizobacteria. However, the use of any of these treatments should be integrated with cultivation practices that reduce vector pressure by the use of insecticides, or by Bt crops.

A single amino acid at N-terminal region of the 2b protein of cucumber mosaic virus strain m1 has a pivotal role in virus attenuation

Host responses to infection by a mild strain of cucumber mosaic virus, termed CMV-m1, were re-examined in several plant species in comparison with those by a severe strain CMV-Y. Mild systemic symptoms were developed on the six plant species inoculated with CMV-m1. Virus titer in the Nicotiana benthamiana plants infected with CMV-m1 was significantly lower than those infected with CMV-Y, although infection by CMV-m1 interfered with further infection by CMV-Y in the plants. Subsequently, the attenuated virulence of CMV-m1 was analyzed by reassortment and recombination analyses between CMV-m1 and CMV-Y RNAs. The results suggested that the 2b protein of CMV-m1 (m1-2b) is involved in the formation of mild symptoms in N. benthamiana. Furthermore, site-directed mutagenesis demonstrated that Thr18 of m1-2b is responsible for formation of mild symptoms. Local RNA silencing suppressor activity of m1-2b was a little lower than that of severe strain CMV-Y. We discuss the relationship between attenuation of CMV-m1 and the features of m1-2b.

Crop immunity against viruses: outcomes and future challenges

Viruses cause epidemics on all major cultures of agronomic importance, representing a serious threat to global food security. As strict intracellular pathogens, they cannot be controlled chemically and prophylactic measures consist mainly in the destruction of infected plants and excessive pesticide applications to limit the population of vector organisms. A powerful alternative frequently employed in agriculture relies on the use of crop genetic resistances, approach that depends on mechanisms governing plant-virus interactions. Hence, knowledge related to the molecular bases of viral infections and crop resistances is key to face viral attacks in fields. Over the past 80 years, great advances have been made on our understanding of plant immunity against viruses. Although most of the known natural resistance genes have long been dominant R genes (encoding NBS-LRR proteins), a vast number of crop recessive resistance genes were cloned in the last decade, emphasizing another evolutive strategy to block viruses. In addition, the discovery of RNA interference pathways highlighted a very efficient antiviral system targeting the infectious agent at the nucleic acid level. Insidiously, plant viruses evolve and often acquire the ability to overcome the resistances employed by breeders. The development of efficient and durable resistances able to withstand the extreme genetic plasticity of viruses therefore represents a major challenge for the coming years. This review aims at describing some of the most devastating diseases caused by viruses on crops and summarizes current knowledge about plant-virus interactions, focusing on resistance mechanisms that prevent or limit viral infection in plants. In addition, I will discuss the current outcomes of the actions employed to control viral diseases in fields and the future investigations that need to be undertaken to develop sustainable broad-spectrum crop resistances against viruses.

Control of cucurbit viruses

More than 70 well-characterized virus species transmitted by a diversity of vectors may infect cucurbit crops worldwide. Twenty of those cause severe epidemics in major production areas, occasionally leading to complete crop failures. Cucurbit viruses' control is based on three major axes: (i) planting healthy seeds or seedlings in a clean environment, (ii) interfering with vectors activity, and (iii) using resistant cultivars. Seed disinfection and seed or seedling quality controls guarantee growers on the sanitary status of their planting material. Removal of virus or vector sources in the crop environment can significantly delay the onset of viral epidemics. Insecticide or oil application may reduce virus spread in some situations. Diverse cultural practices interfere with or prevent vector reaching the crop. Resistance can be obtained by grafting for soil-borne viruses, by cross-protection, or generally by conventional breeding or genetic engineering. The diversity of the actions that may be taken to limit virus spread in cucurbit crops and their limits will be discussed. The ultimate goal is to provide farmers with technical packages that combine these methods within an integrated disease management program and are adapted to different countries and cropping systems.