Control of the phytopathogen Botrytis cinerea using adipic acid monoethyl ester

Chitosan-Decorated Copper Oxide Nanocomposite: Investigation of Its Antifungal Activity against Tomato Gray Mold Caused by Botrytis cinerea

Owing to the remarkable antimicrobial potential of these materials, research into the possible use of nanomaterials as alternatives to fungicides in sustainable agriculture is increasingly progressing. Here, we investigated the potential antifungal properties of chitosan-decorated copper oxide nanocomposite (CH@CuO NPs) to control gray mold diseases of tomato caused by Botrytis cinerea throughout in vitro and in vivo trials. The nanocomposite CH@CuO NPs were chemically prepared, and size and shape were determined using Transmission Electron Microscope (TEM). The chemical functional groups responsible for the interaction of the CH NPs with the CuO NPs were detected using the Fourier Transform Infrared (FTIR) spectrophotometry. The TEM images confirmed that CH NPs have a thin and semitransparent network shape, while CuO NPs were spherically shaped. Furthermore, the nanocomposite CH@CuO NPs ex-habited an irregular shape. The size of CH NPs, CuO NPs and CH@CuO NPs as measured through TEM, were approximately 18.28 ± 2.4 nm, 19.34 ± 2.1 nm, and 32.74 ± 2.3 nm, respectively. The antifungal activity of CH@CuO NPs was tested at three concentrations of 50, 100 and 250 mg/L and the fungicide Teldor 50% SC was applied at recommended dose 1.5 mL/L. In vitro experiments revealed that CH@CuO NPs at different concentrations significantly inhibited the reproductive growth process of B. cinerea by suppressing the development of hyphae, spore germination and formation of sclerotia. Interestingly, a significant control efficacy of CH@CuO NPs against tomato gray mold was observed particularly at concentrations 100 and 250 mg/L on both detached leaves (100%) as well as the whole tomato plants (100%) when compared to the conventional chemical fungicide Teldor 50% SC (97%). In addition, the tested concentration 100 mg/L improved to be sufficient to guarantee a complete reduction in the disease's severity (100%) to tomato fruits from gray mold without any morphological toxicity. In comparison, tomato plants treated with the recommended dose 1.5 mL/L of Teldor 50% SC ensured disease reduction up to 80%. Conclusively, this research enhances the concept of agro-nanotechnology by presenting how a nano materials-based fungicide could be used to protect tomato plants from gray mold under greenhouse conditions and during the postharvest stage.

Cutinase ACut2 from Blastobotrysraffinosifermentans for the Selective Desymmetrization of the Symmetric Diester Diethyl Adipate to the Monoester Monoethyl Adipate

Monoethyl adipate (MEA) is a highly valuable monoester for activating resistance mechanisms and improving protective effects in pathogen-attacked plants. The cutinase ACut2 from the non-conventional yeast Blastobotrys (Arxula) raffinosifermentans (adeninivorans) was used for its synthesis by the desymmetrization of dicarboxylic acid diester diethyl adipate (DEA). Up to 78% MEA with 19% diacid adipic acid (AA) as by-product could be synthesized by the unpurified ACut2 culture supernatant from the B. raffinosifermentans overexpression strain. By adjusting pH and enzyme concentration, the selectivity of the free ACut2 culture supernatant was increased, yielding 95% MEA with 5% AA. Selectivity of the carrier immobilized ACut2 culture supernatant was also improved by pH adjustment during immobilization, as well as carrier enzyme loading, ultimately yielding 93% MEA with an even lower AA concentration of 3-4%. Thus, optimizations enabled the selective hydrolysis of DEA into MEA with only a minor AA impurity. In the up-scaling, a maximum of 98% chemical and 87.8% isolated MEA yield were obtained by the adsorbed enzyme preparation with a space time yield of 2.6 g L-1 h-1. The high monoester yields establish the ACut2-catalyzed biosynthesis as an alternative to existing methods.

Hexanoic acid: a new potential substitute for copper-based agrochemicals against citrus canker

AIMS

The aim of the study is to evaluate hexanoic acid (HA) as an alternative to manage citrus canker.

METHODS AND RESULTS

The minimal growth inhibitory concentration of HA against Xanthomonas citri subsp. citri was determined at 2·15 mmol l^-1 using a respiratory activity assay. Growth curves at different pH values showed that growth inhibition was not due to media acidification induced by HA. The germination rate and root elongation of Lactuca sativa seeds exposed to different concentrations of HA (varying from 0·86 to 5·16 mmol l^-1 ) were assessed to screen for phytotoxicity. The acid exhibited low phytotoxicity for L. sativa at 1·29 and 2·58 mmol l^-1 . To evaluate the ability of HA to protect citrus against X. citri infection, leaves of Citrus sinensis were sprayed with the acid and subsequently challenged with X. citri. HA at 3·44 mmol l^-1 was able to protect citrus against infection, showing a reduction of three orders of magnitude in the number of citrus canker lesions per cm² when compared to the untreated negative control.

CONCLUSION

HA is a potential alternative to copper for citrus canker management.

SIGNIFICANCE AND IMPACT OF THE STUDY

HA inhibits X. citri growth, exhibits low phytotoxicity and is an alternative to copper for the protection of citrus plants against bacterial infection.

Biological Control of Tomato Gray Mold Caused by Botrytis Cinerea with the Entomopathogenic Fungus Metarhizium Anisopliae

Gray mold disease caused by Botrytis cinerea is a devastating disease that leads to serious financial loss. In this study, the entomopathogenic fungus Metarhizium anisopliae that acts against the gray mold pathogen B. cinerea was evaluated. M. anisopliae produced a significant inhibition zone in front of the B. cinerea colony in the dual culture test. In addition, volatile organic compounds generated by M. anisopliae were shown to have an inhibitory effect on B. cinerea mycelia growth and reduced 41% of gray mold severity of postharvest tomatoes. The 10% concentration of the culture filtrate of M. anisopliae inhibited 88.62% of colony radial growth as well as 63.85% of sclerotia germination and all conidia germination of B. cinerea. Furthermore, the culture filtrate of M. anisopliae retained its inhibitory effect against the radial growth of B. cinerea even after heating for 15 min at 100 °C. Feasible mechanisms of M. anisopliae involved in the control of B. cinerea were explored, and it was demonstrated that the plasma membrane of B. cinerea conidia was damaged by the product of metabolism of M. anisopliae. In addition, after treating with culture filtrate of M. anisopliae, the B. cinerea phenotype was shown to be abnormal, and cell organelles of B. cinerea mycelia were damaged significantly. A significant control efficacy of M. anisopliae against tomato gray mold was detected on both the detached leaf assay (84.24%) as well as the whole plant (72.38%). In addition, a 78% reduction in tomato fruit mold was detected at a 10% treated concentration of M. anisopliae. These findings suggest that M. anisopliae possesses potential as a biocontrol agent against tomato gray mold in the greenhouse and during the postharvest stage.

Increased phenylalanine levels in plant leaves reduces susceptibility to Botrytis cinerea

Botrytis cinerea is a major plant pathogen, causing losses in crops during growth and storage. Here we show that increased accumulation of phenylalanine (Phe) and Phe-derived metabolites in plant leaves significantly reduces their susceptibility to B. cinerea. Arabidopsis, petunia and tomato plants were enriched with Phe by either overexpressing a feedback-insensitive E.coli DAHP synthase (AroG*), or by spraying or drenching detached leaves or whole plants with external Phe, prior to infection with B. cinerea. Metabolic analysis of Arabidopsis and petunia plants overexpressing AroG* as well as wt petunia plants treated externally with Phe, revealed an increase in Phe-derived phenylpropanoids accumulated in their leaves, and specifically in those inhibiting B. cinerea germination and growth, suggesting that different compounds reduce susceptibility to B. cinerea in different plants. Phe itself had no inhibitory effect on germination or growth of B. cinerea, and inhibition of Phe metabolism in petunia plants treated with external Phe prevented decreased susceptibility to the fungus. Thus, Phe metabolism into an array of metabolites, unique to each plant and plant organ, is the most probable cause for increased resistance to Botrytis. This mechanism may provide a basis for ecologically friendly control of a wide range of plant pathogens.

Organic acids as antimicrobial food agents: applications and microbial productions

Food safety is a global health and socioeconomic concern since many people still suffer from various acute and life-long diseases, which are caused by consumption of unsafe food. Therefore, ensuring safety of the food is one of the most essential issues in the food industry, which needs to be considered during not only food composition formulation but also handling and storage. For safety purpose, various chemical preservatives have been used so far in the foods. Recently, there has been renewed interest in replacing chemically originated food safety compounds with natural ones in the industry, which can also serve as antimicrobial agents. Among these natural compounds, organic acids possess the major portion. Therefore, in this paper, it is aimed to review and compile the applications, effectiveness, and microbial productions of various widely used organic acids as antimicrobial agents in the food industry.

Inhibitory effect and possible mechanism of a Pseudomonas strain QBA5 against gray mold on tomato leaves and fruits caused by Botrytis cinerea

The fungal pathogen Botrytis cinerea causes gray mold disease on various hosts, which results in serious economic losses. Over the past several decades, many kinds of fungicides have been used to successfully control the disease. Meanwhile, the uses of fungicides lead to environmental pollution as well as a potential threat to the human health by the chemical residues in tomato fruit. Also, the gray mold disease is difficult to control with fungicides. Therefore, exploring alternative measures such as biological controls could be the best choice to control the disease and alleviate damages caused by fungicides. In this study, we isolated and identified a novel Pseudomonas strain termed as QBA5 from healthy tomato plant based on the morphological, biochemical characteristics and molecular detection. The antifungal activity assays revealed that, in the presence of QBA5, conidia germination, germ tube elongation and mycelial growth of B. cinerea were significantly inhibited. Most importantly, QBA5 exerted a significant preventive effectiveness against gray mold on tomato fruits and plants. The possible mechanism of QBA5 involved in the inhibition of B. cinerea was investigated. It revealed that the conidia plasma membrane of B. cinerea was severely damaged by QBA5. Further, four different antifungal compounds in the supernatant of QBA5 were separated by preparative high performance liquid chromatography (PHPLC). Overall, the data indicate that there is a considerable potential for QBA5 to reduce the damage caused by gray mold disease on tomato.

Priming by Hexanoic Acid Induce Activation of Mevalonic and Linolenic Pathways and Promotes the Emission of Plant Volatiles

Hexanoic acid (Hx) is a short natural monocarboxylic acid present in some fruits and plants. Previous studies reported that soil drench application of this acid induces effective resistance in tomato plants against Botrytis cinerea and Pseudomonas syringae and in citrus against Alternaria alternata and Xanthomonas citri. In this work, we performed an in deep study of the metabolic changes produced in citrus by the application of Hx in response to the challenge pathogen A. alternata, focusing on the response of the plant. Moreover, we used (13)C labeled hexanoic to analyze its behavior inside the plants. Finally, we studied the volatile emission of the treated plants after the challenge inoculation. Drench application of (13)C labeled hexanoic demonstrated that this molecule stays in the roots and is not mobilized to the leaves, suggesting long distance induction of resistance. Moreover, the study of the metabolic profile showed an alteration of more than 200 molecules differentially induced by the application of the compound and the inoculation with the fungus. Bioinformatics analysis of data showed that most of these altered molecules could be related with the mevalonic and linolenic pathways suggesting the implication of these pathways in the induced resistance mediated by Hx. Finally, the application of this compound showed an enhancement of the emission of 17 volatile metabolites. Taken together, this study indicates that after the application of Hx this compound remains in the roots, provoking molecular changes that may trigger the defensive response in the rest of the plant mediated by changes in the mevalonic and linolenic pathways and enhancing the emission of volatile compounds, suggesting for the first time the implication of mevalonic pathway in response to hexanoic application.

Unraveling the Function of the Response Regulator BcSkn7 in the Stress Signaling Network of Botrytis cinerea

Important for the lifestyle and survival of every organism is the ability to respond to changing environmental conditions. The necrotrophic plant pathogen Botrytis cinerea triggers an oxidative burst in the course of plant infection and therefore needs efficient signal transduction to cope with this stress. The factors involved in this process and their precise roles are still not well known. Here, we show that the transcription factor Bap1 and the response regulator (RR) B. cinerea Skn7 (BcSkn7) are two key players in the oxidative stress response (OSR) of B. cinerea; both have a major influence on the regulation of classical OSR genes. A yeast-one-hybrid (Y1H) approach proved direct binding to the promoters of gsh1 and grx1 by Bap1 and of glr1 by BcSkn7. While the function of Bap1 is restricted to the regulation of oxidative stress, analyses of Δbcskn7 mutants revealed functions beyond the OSR. Involvement of BcSkn7 in development and virulence could be demonstrated, indicated by reduced vegetative growth, impaired formation of reproductive structures, and reduced infection cushion-mediated penetration of the host by the mutants. Furthermore, Δbcskn7 mutants were highly sensitive to oxidative, osmotic, and cell wall stress. Analyses of Δbap1 bcskn7 double mutants indicated that loss of BcSkn7 uncovers an underlying phenotype of Bap1. In contrast to Saccharomyces cerevisiae, the ortholog of the glutathione peroxidase Gpx3p is not required for nuclear translocation of Bap1. The presented results contribute to the understanding of the OSR in B. cinerea and prove that it differs substantially from that of yeast, demonstrating the complexity and versatility of components involved in signaling pathways.

Redox systems in Botrytis cinerea: impact on development and virulence

The thioredoxin system is of great importance for maintenance of cellular redox homeostasis. Here, we show that it has a severe influence on virulence of Botrytis cinerea, demonstrating that redox processes are important for host-pathogen interactions in this necrotrophic plant pathogen. The thioredoxin system is composed of two enzymes, the thioredoxin and the thioredoxin reductase. We identified two genes encoding for thioredoxins (bctrx1, bctrx2) and one gene encoding for a thioredoxin reductase (bctrr1) in the genome of B. cinerea. Knockout mutants of bctrx1 and bctrr1 were severely impaired in virulence and more sensitive to oxidative stress. Additionally, Δbctrr1 showed enhanced H2O2 production and retarded growth. To investigate the impact of the second major cellular redox system, glutathione, we generated deletion mutants for two glutathione reductase genes. The effects were only marginal; deletion of bcglr1 resulted in reduced germination and, correspondingly, to retarded infection as well as reduced growth on minimal medium, whereas bcglr2 deletion had no distinctive phenotype. In summary, we showed that the balanced redox status maintained by the thioredoxin system is essential for development and pathogenesis of B. cinerea, whereas the second major cellular redox system, the glutathione system, seems to have only minor impact on these processes.

Do strigolactones contribute to plant defence?

Strigolactones are multifunctional molecules involved in several processes outside and within the plant. As signalling molecules in the rhizosphere, they favour the establishment of arbuscular mycorrhizal symbiosis, but they also act as host detection cues for root parasitic plants. As phytohormones, they are involved in the regulation of plant architecture, adventitious rooting, secondary growth and reproductive development, and novel roles are emerging continuously. In the present study, the possible involvement of strigolactones in plant defence responses was investigated. For this purpose, the resistance/susceptibility of the strigolactone-deficient tomato mutant Slccd8 against the foliar fungal pathogens Botrytis cinerea and Alternaria alternata was assessed. Slccd8 was more susceptible to both pathogens, pointing to a new role for strigolactones in plant defence. A reduction in the content of the defence-related hormones jasmonic acid, salicylic acid and abscisic acid was detected by high-performance liquid chromatography coupled to tandem mass spectrometry in the Slccd8 mutant, suggesting that hormone homeostasis is altered in the mutant. Moreover, the expression level of the jasmonate-dependent gene PinII, involved in the resistance of tomato to B. cinerea, was lower than in the corresponding wild-type. We propose here that strigolactones play a role in the regulation of plant defences through their interaction with other defence-related hormones, especially with the jasmonic acid signalling pathway.

Priming of plant resistance by natural compounds. Hexanoic acid as a model

Some alternative control strategies of currently emerging plant diseases are based on the use of resistance inducers. This review highlights the recent advances made in the characterization of natural compounds that induce resistance by a priming mechanism. These include vitamins, chitosans, oligogalacturonides, volatile organic compounds, azelaic and pipecolic acid, among others. Overall, other than providing novel disease control strategies that meet environmental regulations, natural priming agents are valuable tools to help unravel the complex mechanisms underlying the induced resistance (IR) phenomenon. The data presented in this review reflect the novel contributions made from studying these natural plant inducers, with special emphasis placed on hexanoic acid (Hx), proposed herein as a model tool for this research field. Hx is a potent natural priming agent of proven efficiency in a wide range of host plants and pathogens. It can early activate broad-spectrum defenses by inducing callose deposition and the salicylic acid (SA) and jasmonic acid (JA) pathways. Later it can prime pathogen-specific responses according to the pathogen's lifestyle. Interestingly, Hx primes redox-related genes to produce an anti-oxidant protective effect, which might be critical for limiting the infection of necrotrophs. Our Hx-IR findings also strongly suggest that it is an attractive tool for the molecular characterization of the plant alarmed state, with the added advantage of it being a natural compound.

Enhancement of the citrus immune system provides effective resistance against Alternaria brown spot disease

In addition to basal defense mechanisms, plants are able to develop enhanced defense mechanisms such as induced resistance (IR) upon appropriate stimulation. We recently described the means by which several carboxylic acids protect Arabidopsis and tomato plants against fungi. In this work, we demonstrate the effectiveness of hexanoic acid (Hx) in the control of Alternaria brown spot (ABS) disease via enhancement of the immune system of Fortune mandarin. The application of 1mM Hx in irrigation water to 2-year-old Fortune plants clearly reduced the incidence of the disease and led to smaller lesions. We observed that several of the most important mechanisms involved in induced resistance were affected by Hx application. Our results demonstrate enhanced callose deposition in infected plants treated with Hx, which suggests an Hx priming mechanism. Plants treated with the callose inhibitor 2-DDG were more susceptible to the fungus. Moreover, polygalacturonase-inhibiting protein (PGIP) gene expression was rapidly and significantly upregulated in treated plants. However, treatment with Hx decreased the levels of reactive oxygen species (ROS) in infected plants. Hormonal and gene analyses revealed that the jasmonic acid (JA) pathway was activated due to a greater accumulation of 12-oxo-phytodienoic acid (OPDA) and JA along with a rapid accumulation of JA-isoleucine (JA-Ile). Furthermore, we observed a more rapid accumulation of abscisic acid (ABA), which could act as a positive regulator of callose deposition. Thus, our results support the hypothesis that both enhanced physical barriers and the JA signaling pathway are involved in hexanoic acid-induced resistance (Hx-IR) to Alternaria alternata.

Hexanoic acid-induced resistance against Botrytis cinerea in tomato plants

We have demonstrated that root treatment with hexanoic acid protects tomato plants against Botrytis cinerea. Hexanoic acid-induced resistance (Hx-IR) was blocked in the jasmonic acid (JA)-insensitive mutant jai1 (a coi1 homolog) and in the abscisic acid (ABA)-deficient mutant flacca (flc). Upon infection, the LoxD gene as well as the oxylipin 12-oxo-phytodienoic acid and the bioactive molecule JA-Ile were clearly induced in treated plants. However, the basal ABA levels were not altered. Hexanoic acid primed callose deposition against B. cinerea in a cultivar-dependent manner. Treated plants from Ailsa Craig, Moneymaker, and Rheinlands Ruhm showed increased callose deposition but not from Castlemart. Hexanoic acid did not prime callose accumulation in flc plants upon B. cinerea infection; therefore, ABA could act as a positive regulator of Hx-IR by enhancing callose deposition. Furthermore, although hexanoic acid protected the JA-deficient mutant defensless1 (def1), the priming for callose was higher than in the wild type. This suggests a link between JA and callose deposition in tomato. Hence, the obtained results support the idea that callose, oxylipins, and the JA-signaling pathway are involved in Hx-IR against B. cinerea. Moreover our data support the relevance of JA-signaling for basal defense against this necrotroph in tomato. Hexanoic acid also protected against Pseudomonas syringae, indicating a broad-spectrum effect for this new inducer.

Delineation of an in vivo inhibitor for Aspergillus glutamate dehydrogenase

NADP-glutamate dehydrogenase (NADP-GDH) along with glutamine synthetase plays a pivotal role in ammonium assimilation. Specific inhibitors were valuable in defining the importance of glutamine synthetase in nitrogen metabolism. Selective in vivo inhibition of NADP-GDH has so far been an elusive desideratum. Isophthalate, a potent in vitro inhibitor of Aspergillus niger NADP-GDH [Noor S, Punekar NS. Allosteric NADP-glutamate dehydrogenase from aspergilli: purification, characterization and implications for metabolic regulation at the carbon-nitrogen interface. Microbiology 2005;151:1409-19], was evaluated for its efficacy in vivo. Dimethyl ester of isophthalate (DMIP), but not isophthalate, inhibited A. niger growth on agar as well as in liquid culture. This was ascribed to the inability of isophthalate to enter fungal mycelia. Subsequent to DMIP addition however, intracellular isophthalate could be demonstrated. Apart from NAD-GDH, no other enzyme including NAD-glutamate synthase was inhibited by isophthalate. A cross-over at NADP-GDH step of metabolism was observed as a direct consequence of isophthalate (formed in vivo from DMIP) inhibiting this enzyme. Addition of ammonium to DMIP-treated A. niger mycelia resulted in intensive vacuolation, retraction of cytoplasm and autolysis. Taken together, these results implicate glutamate dehydrogenase and NADP-GDH in particular, as a key target of in vivo isophthalate inhibition during ammonium assimilation.

Inhibition of conidia germination and mycelial growth of Botrytis cinerea by some alternative chemicals

Fungal activities of food additives, potassium sorbate, methylparaben, sodium benzoate, propylparaben, sorbic acid and plant activators harpin-protein and potassium dioxide were comparatively examined with ipradion in vitro conditions on two isolates of Botrytis cinerea obtained from strawberry fruits. Food additives and plant activators showed inhibitory effect at different levels on the mycelial growth of fungus isolates. According to the Minimum Inhibition Concentration (MIC), sorbic acid (300 microg mL(-1) for each isolate) and comparison fungicide ipradion (10 microg mL(-1) for each isolate) displayed the highest inhibition effect on the radial mycelial growth of fungal isolates. Similar efficacy was obtained at the highest dose (1000 microg mL(-1)) from other additives propylparaben (for both isolates), potassium sorbet and methylparaben (for isolate 1) and from plant activator potassium oxide. It was detected that all alternative substances has high inhibition activity according to ED50 values. Methylparaben, harpin-protein and potassium oxide revealed efficacies similar to ipradion. While harpin-protein did not have any effect on B. Cinerea germination, other alternative substances excluding potassium sorbate inhibited germination completely at differing doses. Potassium oxide and food additives except potassium sorbate inhibited the spore germination of isolates by 50% at doses between 25-345 microg mL(-1). MIC value of ipradion on spore germination of isolate-1 and isolate-2 was determined as 10 and 30 microg mL(-1), while ED50 values were determined as 2.9 and 12.5 microg mL(-1), respectively. Data obtained from the research verified that food additives may be used in traditional and organic agriculture individually, or in appropriate combination with each other or with fungicides. Moreover, the study put forth that harpin-protein and potassium oxide not only activate plants against pathogens, but they may also have toxic effect on different growth periods of fungus. Although ipradion works only with two isolates, its MIC and ED50 values were considered noteworthy in terms of the resistance risk of B. cinerea.

A Tolerant Behavior in Salt-Sensitive Tomato Plants can be Mimicked by Chemical Stimuli

Lycopersicon esculentum plants exhibit increased salt stress tolerance following treatment with adipic acid monoethylester and 1,3-diaminepropane (DAAME), known as an inducer of resistance against biotic stress in tomato and pepper. For an efficient water and nutrient uptake, plants should adapt their water potential to compensate a decrease in water soil potential produced by salt stress. DAAME-treated plants showed a faster and stronger water potential reduction and an enhanced proline accumulation. Salinity-induced oxidative stress was also ameliorated by DAAME treatments. Oxidative membrane damage and ethylene emission were both reduced in DAAME-treated plants. This effect is probably a consequence of an increase of both non-enzymatic antioxidant activity as well as peroxidase activity. DAAME-mediated tolerance resulted in an unaltered photosynthetic rate and a stimulation of the decrease in transpiration under stress conditions without a cost in growth due to salt stress. The reduction in transpiration rate was concomitant with a reduction in phytotoxic Na(+) and Cl(-) accumulation under saline stress. Interestingly, the ABA deficient tomato mutant sitiens was insensitive to DAAME-induced tolerance following NaCl stress exposure. Additionally, DAAME treatments increased the ABA content of leaves, therefore, an intact ABA signalling pathway seems to be important to express DAAME-induced salt tolerance. Here, we show a possibility of enhance tomato stress tolerance by chemical induction of the major plant defences against salt stress. DAAME-induced tolerance against salt stress could be complementary to or share elements with induced resistance against biotic stress. This might be the reason for the observed wide spectrum of effectiveness of this compound.