Mechanisms of action for 2-phenylethanol isolated from Kloeckera apiculata in control of Penicillium molds of citrus fruits

CYP79D73 Participates in Biosynthesis of Floral Scent Compound 2-Phenylethanol in Plumeria rubra

Plumeria (Plumeria rubra), well known for its brightly colored and fragrant flowers, emits a number of floral volatile organic compounds (VOCs). Plumeria flowers emit a total of 43 VOCs including nine phenylpropanoids/benzenoids, such as 2-phenylethanol (2PE), benzaldehyde, 2-phenylacetaldehyde (PAld), (E/Z)-phenylacetaldoxime (PAOx), benzyl nitrile (BN), and 2-phenylnitroethane (PN). To identify genes and pathways involved in the production of the major compound 2PE, we analyzed the plumeria floral transcriptome and found a highly expressed, flower-specific gene encoding a cytochrome P450 family 79D protein (PrCYP79D73), which catalyzed the formation of (E/Z)-PAOx. Feeding experiments with deuterated phenylalanine or deuterated (E/Z)-PAOx showed that (E/Z)-PAOx is an intermediate in the biosynthesis of 2PE, as are two nitrogen-containing volatiles, BN and PN, in plumeria flowers. Crude enzyme extracts from plumeria flowers converted l-phenylalanine to (E/Z)-PAOx, PAld, 2PE, BN, and PN. The biosynthesis of these compounds increased with addition of PrCYP79D73-enriched microsomes but was blocked by pretreatment with 4-phenylimidazole, an inhibitor of cytochrome P450 enzymes. Moreover, overexpression of PrCYP79D73 in Nicotiana benthamiana resulted in the emission of (E/Z)-PAOx as well as PAld, 2PE, BN, and PN, all of which were also found among plumeria floral VOCs. Taken together, our results demonstrate that PrCYP79D73 is a crucial player in the biosynthesis of the major floral VOC 2PE and other nitrogen-containing volatiles. These volatiles may be required for plant defense as well as to attract pollinators for the successful reproduction of plumeria.

Natural Compounds That Modulate the Development of the Fungus Botrytis cinerea and Protect Solanum lycopersicum

Botrytis cinerea is the causal agent of gray mold disease and is responsible for the loss of millions of dollars in crops in worldwide. Currently, this pathogen exhibits increasing resistance to conventional fungicides; therefore, better control methods and novel compounds with a more specific mechanism of action but without biocidal effects, are required. In this work, several natural compounds to control B. cinerea were analyzed in vitro. Detected effects were dependent on the stage of fungus development, and 3-phenyl-1-propanol displayed the most potent inhibition of in vitro germination, germ tube development, and sporulation. However, it had lower protection of leaves and postharvest fruit in plant infection. Isoeugenol and 1-phenylethanol exhibited lower inhibition of in vitro germination and sporulation, but at the highest concentrations, they inhibited germ tube elongation. Although the lowest rates of foliage infection were recorded using isoeugenol and 3-phenyl-1-propanol, 1-phenylethanol significantly decreased the disease in postharvest tomato fruit, with an efficacy like Mancozeb, but at 18 times lower micromolar concentration. All compounds resulted in high cell viability after spores were removed from the treatment solution exhibited high cell viability, suggesting a non-biocidal effect. The diversity of in vitro and in-plant effects seems to indicate a different mechanism of action.

Volatile Organic Compounds Produced by Pseudomonas chlororaphis subsp. aureofaciens SPS-41 as Biological Fumigants To Control Ceratocystis fimbriata in Postharvest Sweet Potatoes

The biocontrol activity and chemical composition of the volatile organic compounds (VOCs) produced by Pseudomonas chlororaphis subsp. aureofaciens SPS-41 were investigated. The VOCs inhibited mycelial growth and spore germination in Ceratocystis fimbriata, which causes black rot disease in sweet potato tuber roots (TRs) and showed wide-spectrum antifungal activity against several plant pathogenic fungi. A microscopic examination of C. fimbriata cells suggested morphological changes and a loss of cellular contents. Different inoculation strategies significantly affected the antifungal activity of the VOCs. In the volatile profile of SPS-41, the most abundant compound, 3-methyl-1-butanol, followed by phenylethyl alcohol and 2-methyl-1-butanol showed strong inhibition toward C. fimbriata. The weight loss rate and disease severity of the TRs were significantly reduced in response to the VOCs emitted by SPS-41. The results suggest that the VOCs produced by P. chlororaphis subsp. aureofaciens SPS-41 might constitute an attractive biological fumigant for controlling black rot disease in sweet potato TRs.

Cytosporone B as a Biological Preservative: Purification, Fungicidal Activity and Mechanism of Action against Geotrichum citri-aurantii

To prevent citrus decay caused by Geotrichum citri-aurantii, 12 natural products were isolated from two endophytic fungi, in which cytosporone B was shown to have excellent bioactivity for control of G. citri-aurantii with median effect concentration (EC₅₀) of 26.11 μg/mL and minimum inhibitory concentration (MIC) of 105 μg/mL, and also significantly reduced the decay of sugar orange during the in vivo trials. In addition, cytosporone B could alter the morphology of G. citri-aurantii by causing distortion of the mycelia and loss of membrane integrity. Differentially expressed genes (DEGs) between cytosporone B-treated and -untreated samples were revealed by Illumina sequencing, including 3540 unigenes. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses showed that most DEGs were related to metabolic production and cell membrane. These findings suggest cytosporone B is a promising biological preservative to control citrus decay and reveal the action mechanism of cytosporone B in relation to the destruction of the fungal cell membrane at both morphological and molecular levels.

The occurrence and formation of monoterpenes in herbivore-damaged poplar roots

Volatiles are often released upon herbivory as plant defense compounds. While the formation of volatiles above-ground has been intensively studied, little is known about herbivore-induced root volatiles. Here, we show that cockchafer larvae-damaged roots of Populus trichocarpa and P. nigra release a mixture of monoterpenes, including (−)-α-pinene, (−)-camphene, (−)-β-pinene, p-cymene, and 1,8-cineole. Three terpene synthases, PtTPS16 and PtTPS21 from P. trichocarpa and PnTPS4 from P. nigra, could be identified and characterized in vitro. PnTPS4 was found to produce 1,8-cineole as sole product. PtTPS16 and PtTPS21, although highly similar to each other, showed different product specificities and produced γ-terpinene and a mixture of (−)-camphene, (−)-α-pinene, (−)-β-pinene, and (−)-limonene, respectively. Four active site residues were found to determine the different product specificities of the two enzymes. The expression profiles of PtTPS16, PtTPS21, and PnTPS4 in undamaged and herbivore-damaged poplar roots generally matched the emission pattern of monoterpenes, indicating that monoterpene emission in roots is mainly determined at the gene transcript level. Bioassays with Phytophtora cactorum (Oomycetes) revealed inhibitory effects of vapor-phase 1,8-cineole and (−)-β-pinene on the growth of this important plant pathogen. Thus herbivore-induced volatile monoterpenes may have a role in defense against pathogens that cause secondary infections after root wounding.

Antagonistic Activity and the Mechanism of Bacillus amyloliquefaciens DH-4 Against Citrus Green Mold

Citrus fruit usually suffer significant losses during the storage and transportation stages. Green mold, a postharvest rot of citrus fruit caused by Penicillium digitatum, is one of the most serious fungal diseases. In this study, the antagonist strain DH-4 was identified as Bacillus amyloliquefaciens according to morphological observation and 16S ribosomal DNA analysis. In addition, it showed broad antifungal activity, especially the suppression of Penicillium spp. The culture filtrate of strain DH-4 exhibited apparent activity against P. digitatum in vitro and in vivo. In storage, the culture filtrate with DH-4 in it showed a better antiseptic effect. The antifungal substances in the culture filtrate, produced by strain DH-4, displayed stable activity in various extreme conditions. In addition, the antifungal substances in the culture filtrate were identified as macrolactin, bacillaene, iturins, fengycin, and surfactin by ultraperformance liquid chromatography (UPLC) electrospray ionization mass spectrometry analysis. The UPLC fractions containing these antifungal compounds were basically heat tolerant and all responsible for the antagonistic activity against P. digitatum. Transmission electron microscope observation indicated that the antifungal substances might cause abnormalities in the P. digitatum cellular ultrastructure, which could be the possible mode of action of B. amyloliquefaciens against P. digitatum. In addition, it was confirmed via scanning electron microscope analysis that the main way it inhibited P. digitatum was by secreting antimicrobial compounds without direct interaction. This study contributes to the understanding of the mechanism of B. amyloliquefaciens against citrus green mold as well as providing a potential application for the biocontrol of postharvest rot diseases in citrus fruit.

A Transcriptomic Analysis of Saccharomyces cerevisiae Under the Stress of 2-Phenylethanol

2-Phenylethanol (2-PE) is a kind of advanced aromatic alcohol with rose fragrance, which is wildly used for the deployment of flavors and fragrances. Microbial transformation is the most feasible method for the production of natural 2-PE. But a bottleneck problem is the toxicity of 2-PE on the cells. The molecular mechanisms of the toxic effect of 2-PE to Saccharomyces cerevisiae are not well studied. In this study, we analyzed the transcriptomes of S. cerevisiae in the media with and without 2-PE, respectively, using Illumina RNA-Seq technology. We identified 580 differentially expressed genes between S. cerevisiae in two different treatments. GO and KEGG enrichment analyses of these genes suggested that most genes encoding mitochondrial proteins, cytoplasmic, and plasma membrane proteins were significantly up-regulated, whereas the enzymes related to amino acid metabolism were down-regulated. These results indicated that 2-PE suppressed the synthesis of plasma membrane proteins, which suppressed the transport of nutrients required for growth. The findings in this study will provide insight into the inhibitory mechanism of 2-PE to yeast and other microbes.

Characterization and Synergistic Effect of Antifungal Volatile Organic Compounds Emitted by the Geotrichum candidum PF005, an Endophytic Fungus from the Eggplant

Plant-associated endophytes are recognized as sources of novel bioactive molecules having diverse applications. In this study, an endophytic yeast-like fungal strain was isolated from the fruit of eggplant (Solanum melongena) and identified as Geotrichum candidum through phenotypic and genotypic characterizations. This endophytic G. candidum isolate PF005 was found to emit fruity scented volatiles. The compositional profiling of volatile organic compounds (VOCs) revealed the presence of 3-methyl-1-butanol, ethyl 3-methylbutanoate, 2-phenylethanol, isopentyl acetate, naphthalene, and isobutyl acetate in significant proportion when analyzed on a time-course basis. The VOCs from G. candidum exhibited significant mycelial growth inhibition (54%) of phytopathogen Rhizoctonia solani, besides having mild antifungal activity against a few other fungi. The source of carbon as a nutrient was found to be an important factor for the enhanced biosynthesis of antifungal VOCs. The antifungal activity against phytopathogen R. solani was improved up to 91% by feeding the G. candidum with selective precursors of alcohol and ester volatiles. Furthermore, the antifungal activity of VOCs was enhanced synergistically up to 92% upon the exogenous addition of naphthalene (1.0 mg/plate). This is the first report of G. candidum as an endophyte emitting antifungal VOCs, wherein 2-penylethanol, isopentyl acetate, and naphthalene were identified as important contributors to its antifungal activity. Possible utilization of G. candidum PF005 as a mycofumigant has been discussed based upon its antifungal activity and the qualified presumption of safety status.

Plant growth-promoting rhizobacteria associated with avocado display antagonistic activity against Phytophthora cinnamomi through volatile emissions

Rhizobacteria associated with crops constitute an important source of potentially beneficial microorganisms with plant growth promoting activity or antagonistic effects against phytopathogens. In this study, we evaluated the plant growth promoting activity of 11 bacterial isolates that were obtained from the rhizosphere of healthy avocado trees and from that of avocado trees having survived root rot infestations. Seven bacterial isolates, belonging to the genera Bacillus, Pseudomonas and Arthrobacter, promoted in vitro growth of Arabidopsis thaliana. These isolates were then tested for antagonistic activity against Phytophthora cinnamomi, in direct dual culture assays. Two of those rhizobacterial isolates, obtained from symptomatic-declining trees, displayed antagonistic activity. Isolate A8a, which is closely related to Bacillus acidiceler, was also able to inhibit P. cinnamomi growth in vitro by 76% through the production of volatile compounds. Solid phase microextraction (SPME) and analysis by gas chromatography coupled with mass spectrometry (GC-MS) allowed to tentatively identify the main volatiles emitted by isolate A8a as 2,3,5-trimethylpyrazine, 6,10-dimethyl-5,9-undecadien-2-one and 3-amino-1,3-oxazolidin-2-one. These volatile compounds have been reported to show antifungal activity when produced by other bacterial isolates. These results confirm the significance of rhizobacteria and suggest that these bacteria could be used for biocontrol of soil borne oomycetes through their volatiles emissions.

Downy mildew symptoms on grapevines can be reduced by volatile organic compounds of resistant genotypes

Volatile organic compounds (VOCs) play a crucial role in the communication of plants with other organisms and are possible mediators of plant defence against phytopathogens. Although the role of non-volatile secondary metabolites has been largely characterised in resistant genotypes, the contribution of VOCs to grapevine defence mechanisms against downy mildew (caused by Plasmopara viticola) has not yet been investigated. In this study, more than 50 VOCs from grapevine leaves were annotated/identified by headspace-solid-phase microextraction gas chromatography-mass spectrometry analysis. Following P. viticola inoculation, the abundance of most of these VOCs was higher in resistant (BC4, Kober 5BB, SO4 and Solaris) than in susceptible (Pinot noir) genotypes. The post-inoculation mechanism included the accumulation of 2-ethylfuran, 2-phenylethanol, β-caryophyllene, β-cyclocitral, β-selinene and trans-2-pentenal, which all demonstrated inhibitory activities against downy mildew infections in water suspensions. Moreover, the development of downy mildew symptoms was reduced on leaf disks of susceptible grapevines exposed to air treated with 2-ethylfuran, 2-phenylethanol, β-cyclocitral or trans-2-pentenal, indicating the efficacy of these VOCs against P. viticola in receiver plant tissues. Our data suggest that VOCs contribute to the defence mechanisms of resistant grapevines and that they may inhibit the development of downy mildew symptoms on both emitting and receiving tissues.

Physiology, ecology and industrial applications of aroma formation in yeast

Yeast cells are often employed in industrial fermentation processes for their ability to efficiently convert relatively high concentrations of sugars into ethanol and carbon dioxide. Additionally, fermenting yeast cells produce a wide range of other compounds, including various higher alcohols, carbonyl compounds, phenolic compounds, fatty acid derivatives and sulfur compounds. Interestingly, many of these secondary metabolites are volatile and have pungent aromas that are often vital for product quality. In this review, we summarize the different biochemical pathways underlying aroma production in yeast as well as the relevance of these compounds for industrial applications and the factors that influence their production during fermentation. Additionally, we discuss the different physiological and ecological roles of aroma-active metabolites, including recent findings that point at their role as signaling molecules and attractants for insect vectors.

Transcriptional profiling analysis of Penicillium digitatum, the causal agent of citrus green mold, unravels an inhibited ergosterol biosynthesis pathway in response to citral

Background

Green mold caused by Penicillium digitatum is the most damaging postharvest diseases of citrus fruit. Previously, we have observed that citral dose-dependently inhibited the mycelial growth of P. digitatum, with the minimum inhibitory concentration (MIC) of 1.78 mg/mL, but the underlying molecular mechanism is barely understood.

Results

In this study, the transcriptional profiling of the control and 1/2MIC-citral treated P. digitatum mycelia after 30 min of exposure were analyzed by RNA-Seq. A total of 6355 genes, including 2322 up-regulated and 4033 down-regulated genes, were found to be responsive to citral. These genes were mapped to 155 KEGG pathways, mainly concerning mRNA surveillance, RNA polymerase, RNA transport, aminoacyl-tRNA biosynthesis, ABC transporter, glycolysis/gluconeogenesis, citrate cycle, oxidative phosphorylation, sulfur metabolism, nitrogen metabolism, inositol phosphate metabolism, fatty acid biosynthesis, unsaturated fatty acids biosynthesis, fatty acid metabolism, and steroid biosynthesis. Particularly, citral exposure affected the expression levels of five ergosterol biosynthetic genes (e.g. ERG7, ERG11, ERG6, ERG3 and ERG5), which corresponds well with the GC-MS results, the reduction in ergosterol content, and accumulation of massive lanosterol. In addition, ERG11, the gene responsible for lanosterol 14α-demethylase, was observed to be the key down-regulated gene in response to citral.

Conclusion

Our present finding suggests that citral could exhibit its antifungal activity against P. digitatum by the down-regulation of ergosterol biosynthesis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2943-4) contains supplementary material, which is available to authorized users.

Hanseniaspora uvarum from Winemaking Environments Show Spatial and Temporal Genetic Clustering

Hanseniaspora uvarum is one of the most abundant yeast species found on grapes and in grape must, at least before the onset of alcoholic fermentation (AF) which is usually performed by Saccharomyces species. The aim of this study was to characterize the genetic and phenotypic variability within the H. uvarum species. One hundred and fifteen strains isolated from winemaking environments in different geographical origins were analyzed using 11 microsatellite markers and a subset of 47 strains were analyzed by AFLP. H. uvarum isolates clustered mainly on the basis of their geographical localization as revealed by microsatellites. In addition, a strong clustering based on year of isolation was evidenced, indicating that the genetic diversity of H. uvarum isolates was related to both spatial and temporal variations. Conversely, clustering analysis based on AFLP data provided a different picture with groups showing no particular characteristics, but provided higher strain discrimination. This result indicated that AFLP approaches are inadequate to establish the genetic relationship between individuals, but allowed good strain discrimination. At the phenotypic level, several extracellular enzymatic activities of enological relevance (pectinase, chitinase, protease, β-glucosidase) were measured but showed low diversity. The impact of environmental factors of enological interest (temperature, anaerobia, and copper addition) on growth was also assessed and showed poor variation. Altogether, this work provided both new analytical tool (microsatellites) and new insights into the genetic and phenotypic diversity of H. uvarum, a yeast species that has previously been identified as a potential candidate for co-inoculation in grape must, but whose intraspecific variability had never been fully assessed.

Comparative transcriptional profiling of orange fruit in response to the biocontrol yeast Kloeckera apiculata and its active compounds

Background

The yeast Kloeckera apiculata strain 34–9 is an antagonist that shows biological control activity against the postharvest fungal pathogens of citrus. An antifungal compound, 2-phenylethanol (PEA), has been identified from the extract of K. apiculata. To better understand the molecular processes underlying the response of citrus fruit tissue to K. apiculata, the extract and PEA, microarray analyses were performed on navel oranges using an Affymetrix Citrus GeneChip.

Results

As many as 801, 339 and 608 differentially expressed genes (DEGs) were identified after the application of K. apiculata, the extract and PEA, respectively. In general, K. apiculata induced the expression of defence-related genes. In addition to chitinase and β-1,3-glucanase, genes involved in ethylene (ET), jasmonic acid (JA), calcium signalling, MAPK signalling and phenylalanine metabolism were induced. In contrast, monodehydroascorbate reductase, superoxide dismutase (SOD), catalase (CAT), peroxidase (POD) and carotenoid biosynthesis genes were down-regulated. The expression profiles for the extract- and PEA-treated samples were similar to that found for yeast (sharing 57.4 % DEGs), with a significant increase in the transcript levels of defence-related genes.

Conclusion

This study provides a global picture of the gene expression changes in navel oranges after the application of the antagonist yeast K. apiculata, its extract and PEA. The interpretation of the DEGs revealed new insight into the molecular processes that regulate the defence responses in orange tissue.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2333-3) contains supplementary material, which is available to authorized users.

The importance of chorismate mutase in the biocontrol potential of Trichoderma parareesei

Species of Trichoderma exert direct biocontrol activity against soil-borne plant pathogens due to their ability to compete for nutrients and to inhibit or kill their targets through the production of antibiotics and/or hydrolytic enzymes. In addition to these abilities, Trichoderma spp. have beneficial effects for plants, including the stimulation of defenses and the promotion of growth. Here we study the role in biocontrol of the T. parareesei Tparo7 gene, encoding a chorismate mutase (CM), a shikimate pathway branch point leading to the production of aromatic amino acids, which are not only essential components of protein synthesis but also the precursors of a wide range of secondary metabolites. We isolated T. parareesei transformants with the Tparo7 gene silenced. Compared with the wild-type, decreased levels of Tparo7 expression in the silenced transformants were accompanied by reduced CM activity, lower growth rates on different culture media, and reduced mycoparasitic behavior against the phytopathogenic fungi Rhizoctonia solani, Fusarium oxysporum and Botrytis cinerea in dual cultures. By contrast, higher amounts of the aromatic metabolites tyrosol, 2-phenylethanol and salicylic acid were detected in supernatants from the silenced transformants, which were able to inhibit the growth of F. oxysporum and B. cinerea. In in vitro plant assays, Tparo7-silenced transformants also showed a reduced capacity to colonize tomato roots. The effect of Tparo7-silencing on tomato plant responses was examined in greenhouse assays. The growth of plants colonized by the silenced transformants was reduced and the plants exhibited an increased susceptibility to B. cinerea in comparison with the responses observed for control plants. In addition, the plants turned yellowish and were defective in jasmonic acid- and ethylene-regulated signaling pathways which was seen by expression analysis of lipoxygenase 1 (LOX1), ethylene-insensitive protein 2 (EIN2) and pathogenesis-related protein 1 (PR-1) genes.

Suppression of Aflatoxin Biosynthesis in Aspergillus flavus by 2-Phenylethanol Is Associated with Stimulated Growth and Decreased Degradation of Branched-Chain Amino Acids

The saprophytic soil fungus Aspergillus flavus infects crops and produces aflatoxin. Pichia anomala, which is a biocontrol yeast and produces the major volatile 2-phenylethanol (2-PE), is able to reduce growth of A. flavus and aflatoxin production when applied onto pistachio trees. High levels of 2-PE are lethal to A. flavus and other fungi. However, at low levels, the underlying mechanism of 2-PE to inhibit aflatoxin production remains unclear. In this study, we characterized the temporal transcriptome response of A. flavus to 2-PE at a subinhibitory level (1 µL/mL) using RNA-Seq technology and bioinformatics tools. The treatment during the entire 72 h experimental period resulted in 131 of the total A. flavus 13,485 genes to be significantly impacted, of which 82 genes exhibited decreased expression. They included those encoding conidiation proteins and involved in cyclopiazonic acid biosynthesis. All genes in the aflatoxin gene cluster were also significantly decreased during the first 48 h treatment. Gene Ontology (GO) analyses showed that biological processes with GO terms related to catabolism of propionate and branched-chain amino acids (valine, leucine and isoleucine) were significantly enriched in the down-regulated gene group, while those associated with ribosome biogenesis, translation, and biosynthesis of α-amino acids were over-represented among the up-regulated genes. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis revealed that metabolic pathways negatively impacted among the down-regulated genes parallel to those active at 30 °C, a condition conducive to aflatoxin biosynthesis. In contrast, metabolic pathways positively related to the up-regulated gene group resembled those at 37 °C, which favors rapid fungal growth and is inhibitory to aflatoxin biosynthesis. The results showed that 2-PE at a low level stimulated active growth of A. flavus but concomitantly rendered decreased activities in branched-chain amino acid degradation. Since secondary metabolism occurs after active growth has ceased, this growth stimulation resulted in suppression of expression of aflatoxin biosynthesis genes. On the other hand, increased activities in degradation pathways for branched-chain amino acids probably are required for the activation of the aflatoxin pathway by providing building blocks and energy regeneration. Metabolic flux in primary metabolism apparently has an important role in the expression of genes of secondary metabolism.