Biocontrol activity of an alkaline serine protease from Aureobasidium pullulans expressed in Pichia pastoris against four postharvest pathogens on apple

Screening a 681-membered yeast collection for the secretion of proteins with antifungal activity

Fungal pathogens pose a threat to human health and food security. Few antifungals are available and resistance to all has been reported. Novel strategies to control plant and human pathogens as well as food spoilers are urgently required. Environmental yeasts provide a functionally diverse, yet underexploited potential for fungal control based on their natural competition via the secretion of proteins and other small molecules such as iron chelators, volatile organic compounds or biosurfactants. However, there is a lack of standardized workflows to systematically access application-relevant yeast-based compounds and understand their molecular functioning. Towards this goal, we developed a workflow to identify and characterize yeast isolates that are active against spoilage yeasts and relevant human and plant pathogens, herein focusing on discovering yeasts that secrete antifungal proteins. The workflow includes the classification of the secreted molecules and cross-comparison of their antifungal capacity using an independent synthetic calibrant. Our workflow delivered a collection of 681 yeasts of which 212 isolates (31 %) displayed antagonism against at least one target strain. While 57.5 % of the active yeasts showed iron-depended antagonism, likely due to pulcherrimin-like iron chelators, 31.7 % secreted antifungal proteins. Those yeast candidates clustered within twelve OTUs, showed narrow and broad target spectra, and several showed a broad pH and temperature activity profile. Given the tools for yeast biotechnology and protein engineering available, our collection can serve as a rich starting point for genetic and molecular characterization of the various antifungal phenotypes, their mode of action and their future exploitation.

Heterologous expression of frog antimicrobial peptide Odorranain-C1 in Pichia pastoris: Biological characteristics and its application in food preservation

To reduce food spoilage and deterioration caused by microbial contamination, antimicrobial peptides (AMPs) have gradually gained attention as a biological preservative. Odorranain-C1 is an α-helical cationic antimicrobial peptide extracted from the skin of frogs with broad-spectrum antimicrobial activity. In this study, we achieved the expression of Odorranain-C1 in Pichia pastoris (P. pastoris) (also known as Komagataella phaffii) by employing DNA recombination technology. The recombinant Odorranain-C1 showed broad-spectrum antibacterial activity and displayed a minimum inhibitory concentration within the range of 8-12 μg.mL-1. Meanwhile, Odorranain-C1 exhibited superior stability and lower hemolytic activity. Mechanistically, Odorranain-C1 disrupted the bacterial membrane's integrity, ultimately causing membrane rupture and subsequent cell death. In tilapia fillets preservation, Odorranain-C1 inhibited the total colony growth and pH variations, while also reducing the production of total volatile basic nitrogen (TVB-N) and thiobarbituric acid (TBA). In conclusion, these studies demonstrated the efficient recombinant expression of Odorranain-C1 in P. pastoris, highlighting its promising utilization in food preservation.

Mechanism of oxalate decarboxylase Oxd_S12 from Bacillus velezensis BvZ45-1 in defence against cotton verticillium wilt

Verticillium wilt, a soilborne vascular disease caused by Verticillium dahliae, strongly affects cotton yield and quality. In this study, an isolated rhizosphere bacterium, designated Bacillus velezensis BvZ45-1, exhibited >46% biocontrol efficacy against cotton verticillium wilt under greenhouse and field conditions. Moreover, through crude protein extraction and mass spectrometry analyses, we found many antifungal compounds present in the crude protein extract of BvZ45-1. The purified oxalate decarboxylase Odx_S12 from BvZ45-1 inhibited the growth of V. dahliae Vd080 by reducing the spore yield, causing mycelia to rupture, spore morphology changes, cell membrane rupture, and cell death. Subsequently, overexpression of Odx_S12 in Arabidopsis significantly improved plant resistance to V. dahliae. Through studies of the resistance mechanism of Odx_S12, V. dahliae was shown to produce oxalic acid (OA), which has a toxic effect on Arabidopsis leaves. Odx_S12 overexpression reduced Arabidopsis OA content, enhanced tolerance to OA, and improved resistance to verticillium wilt. Transcriptomics and quantitative real-time PCR analysis revealed that Odx_S12 promoted a reactive oxygen species burst and a salicylic acid- and abscisic acid-mediated defence response in Arabidopsis. In summary, this study not only identified B. velezensis BvZ45-1 as an efficient biological control agent, but also identified the resistance gene Odx_S12 as a candidate for cotton breeding against verticillium wilt.

Efficacy of Essential Oil Vapours in Reducing Postharvest Rots and Effect on the Fruit Mycobiome of Nectarines

Nectarines can be affected by many diseases, resulting in significant production losses. Natural products, such as essential oils (EOs), are promising alternatives to pesticides to control storage rots. This work aimed to test the efficacy of biofumigation with EOs in the control of nectarine postharvest diseases while also evaluating the effect on the quality parameters (firmness, total soluble solids, and titratable acidity) and on the fruit fungal microbiome. Basil, fennel, lemon, oregano, and thyme EOs were first tested in vitro at 0.1, 0.5, and 1.0% concentrations to evaluate their inhibition activity against Monilinia fructicola. Subsequently, an in vivo screening trial was performed by treating nectarines inoculated with M. fructicola, with the five EOs at 2.0% concentration by biofumigation, performed using slow-release diffusers placed inside the storage cabinets. Fennel, lemon, and basil EOs were the most effective after storage and were selected to be tested in efficacy trials using naturally infected nectarines. After 28 days of storage, all treatments showed a significant rot reduction compared to the untreated control. Additionally, no evident phytotoxic effects were observed on the treated fruits. EO vapors did not affect the overall quality of the fruits but showed a positive effect in reducing firmness loss. Metabarcoding analysis showed a significant impact of tissue, treatment, and sampling time on the fruit microbiome composition. Treatments were able to reduce the abundance of Monilinia spp., but basil EO favored a significant increase in Penicillium spp. Moreover, the abundance of other fungal genera was found to be modified.

Enhancing biological control of postharvest green mold in lemons: Synergistic efficacy of native yeasts with diverse mechanisms of action

Argentina is among the most important lemon fruit producers in the world. Penicillium digitatum is the primary lemon fungal phytopathogen, causing green mold during the postharvest. Several alternatives to the use of synthetic fungicides have been developed, being the use of biocontrol yeasts one of the most promising. Although many of the reports are based on the use of a single yeast species, it has been shown that the combination of agents with different mechanisms of action can increase control efficiency through synergistic effects. The combined use of native yeasts with different mechanisms of action had not been studied as a biological control strategy in lemons. In this work, the mechanisms of action of native yeasts (Clavispora lusitaniae AgL21, Clavispora lusitaniae AgL2 and Clavispora lusitaniae AcL2) with biocontrol activity against P. digitatum were evaluated. Isolate AgL21 was selected for its ability to form biofilm, colonize lemon wounds, and inhibit fungal spore germination. The compatibility of C. lusitaniae AgL21 with two killer yeasts of the species Kazachstania exigua (AcL4 and AcL8) was evaluated. In vivo assays were then carried out with the yeasts applied individually or mixed in equal cell concentrations. AgL21 alone was able to control green mold with 87.5% efficiency, while individual killer yeasts were significantly less efficient (43.3% and 38.3%, respectively). Inhibitory effects were increased when C. lusitaniae AgL21 and K. exigua strains were jointly applied. The most efficient treatment was the combination of AgL21 and AcL4, reaching 100% efficiency in wound protection. The combination of AgL21 with AcL8 was as well promising, with an efficiency of 97.5%. The combined application of native yeasts showed a synergistic effect considering that the multiple mechanisms of action involved could hinder the development of green mold in lemon more efficiently than using single yeasts. Therefore, this work demonstrates that the integration of native yeasts with diverse modes of action can provide new insights to formulate effective microbial consortia. This could lead to the development of tailor-made biofungicides, allowing control of postharvest fungal diseases in lemons while remaining competitive with traditionally used synthetic chemicals.

Biocontrol rhizobacteria enhances growth and yield of wheat (Triticum aestivum) under field conditions against Fusarium oxysporum

The current study aimed to identify the survival of bio-control bacteria with antifungal activity against Fusarium oxysporum and assess their growth promoting activity in wheat crop field conditions. To evaluate the fungicidal activities of isolated bacteria using the dual culture method, both qualitative and quantitative bioassays were performed. Plant Growth Promoting activities such as Indole 3-Acetic Acid (IAA), phosphate solubilization, Hydrogen cyanide (HCN), and Siderophore production were assessed for three biocontrol bacterial isolates (BCB 07, BCB16, and BCB 83) out of 180 with 70% antagonistic activity against Fusarium oxysporum. Chitinase, protease, and cellulase interaction in isolates was also tested. BCB16 was selected as it had 70% antagonist activity against F. oxysporum but also had the highest PGPR (Plant Growth Promoting Rhizobacteria) traits when compared to the other two isolates. BCB16 was also tested for survival in talc powder and in wheat crop field conditions. Even after 4 months in talc powder, the survival rate remained stable. In a wheat crop field, BCB16 reduced the disease incidence of Fusarium oxysporum by 54.38%. When compared to fungus alone treatment, BCB16 increased average yield by 57% alone and 32% in challenged conditions. BCB16 was identified molecularly using the 16s rRNA gene. Bacillus amyloliquefaciens shared 97% of the deduced sequence. The sequence was submitted to genbank and assigned the accession number OM333889. Bacillus amyloliquefaciens has the potential to be used in the field as an alternative to synthetic fungicides against Fusarium oxysporum.

Biological Control of Aspergillus parasiticus and Aspergillus ochraceus and Reductions in the Amount of Ochratoxin A and Aflatoxins in Bread by Selected Non-Conventional Yeast

Aspergillus parasiticus and Aspergillus ochraceus are important pathogenic fungi that pose a serious threat because of their ability to produce mycotoxins, including ochratoxin A (OTA) and aflatoxins (AFs). The main method of reducing these pathogens is the use of chemical fungicides, though recently there has been a focus on finding biological control agents. The obtained results from this study indicate the great potential of two wild yeast strains, Aureobasidium pullulans PP3 and Saitozyma podzolicus D10, in the biological control of A. parasiticus and A. ochraceus and reductions in the amount of OTA and AFs they produce. In vitro, the growth of the mycelium of pathogens was reduced by 41.21% to 53.64%, and spore germination was inhibited by 58.39% to 71.22%. Both yeast strains produced the enzymes chitinase, β-1,3-glucanase, and amylase, and A. pullulans PP3 additionally produced protease and cellulase. This yeast strain also had the ability to grow over a wide range of temperature (4-30 °C), salinity (0-12%) and pH (4-11) conditions. No growth of the yeast was observed at 37 °C, nor any biogenic amines or hydrogen sulfide production. Adding the tested yeast inoculum to the dough reduced OTA (within 14.55-21.80%) and AFs (within 18.10-25.02%) in the model bread.

Antimicrobial function of yeast against pathogenic and spoilage microorganisms via either antagonism or encapsulation: A review

Contaminations of pathogenic and spoilage microbes on foods are threatening food safety and quality, highlighting the importance of developing antimicrobial agents. According to different working mechanisms, the antimicrobial activities of yeast-based agents were summarized from two aspects: antagonism and encapsulation. Antagonistic yeasts are usually applied as biocontrol agents for the preservation of fruits and vegetables via inactivating spoilage microbes, usually phytopathogens. This review systematically summarized various species of antagonistic yeasts, potential combinations to improve the antimicrobial efficiency, and the antagonistic mechanisms. The wide applications of the antagonistic yeasts are significantly limited by undesirable antimicrobial efficiency, poor environmental resistance, and a narrow antimicrobial spectrum. Another strategy for achieving effective antimicrobial activity is to encapsulate various chemical antimicrobial agents into a yeast-based carrier that has been previously inactivated. This is accomplished by immersing the dead yeast cells with porous structure in an antimicrobial suspension and applying high vacuum pressure to allow the agents to diffuse inside the yeast cells. Typical antimicrobial agents encapsulated in the yeast carriers have been reviewed, including chlorine-based biocides, antimicrobial essential oils, and photosensitizers. Benefiting from the existence of the inactive yeast carrier, the antimicrobial efficiencies and functional durability of the encapsulated antimicrobial agents, such as chlorine-based agents, essential oils, and photosensitizers, are significantly improved compared with the unencapsulated ones.

Biological Control of Aspergillus flavus by the Yeast Aureobasidium pullulans In Vitro and on Tomato Fruit

Aspergillus flavus is an important pathogenic fungus affecting many crops and is one of the main sources of their aflatoxin contamination. The primary method of limiting this pathogen is using chemical fungicides, but researchers focus on searching for other effective agents for its control due to many disadvantages and limitations of these agrochemicals. The results obtained in the present study indicate the high potential of two yeast strains, Aureobasidium pullulans PP4 and A. pullulans ZD1, in the biological control of A. flavus. Under in vitro conditions, mycelial growth was reduced by 53.61% and 63.05%, and spore germination was inhibited by 68.97% and 79.66% by ZD1 and PP4 strains, respectively. Both strains produced the lytic enzymes chitinase and β-1,3-glucanase after 5 days of cultivation with cell wall preparations (CWP) of A. flavus in the medium as a carbon source. In addition, the tested yeasts showed the ability to grow over a wide range of temperatures (4-30 °C), pH (4-11), and salinity (0-12%) and showed tolerance to fungicides at concentrations corresponding to field conditions. Both isolates tested were highly tolerant to cupric oxychloride, showing biomass gains of 85.84% (ZD1) and 87.25% (PP4). Biomass growth in the presence of fungicides azoxystrobin was 78.71% (ZD1) and 82.65% (PP4), while in the presence of difenoconazole, it was 70.09% (ZD1) and 75.25% (PP4). The yeast strains were also tested for antagonistic effects against A. flavus directly on tomato fruit. Both isolates acted effectively by reducing lesion diameter from 29.13 mm (control) to 8.04 mm (PP4) and 8.83 mm (ZD1).

Isolation and characterization of extracellular vesicles from biotechnologically important fungus Aureobasidium pullulans

Extracellular vesicles (EVs) are increasingly recognized as an important mechanism for cell-cell interactions. Their role in fungi is still poorly understood and they have been isolated from only a handful of species. Here, we isolated and characterized EVs from Aureobasidium pullulans, a biotechnologically important black yeast-like fungus that is increasingly used for biocontrol of phytopathogenic fungi and bacteria. After optimization of the isolation protocol, characterization of EVs from A. pullulans by transmission electron microscopy (TEM) revealed a typical cup-shaped morphology and different subpopulations of EVs. These results were confirmed by nanoparticle tracking analysis (NTA), which revealed that A. pullulans produced 6.1 × 108 nanoparticles per milliliter of culture medium. Proteomic analysis of EVs detected 642 proteins. A small fraction of them had signal peptides for secretion and transmembrane domains. Proteins characteristic of different synthesis pathways were found, suggesting that EVs are synthesized by multiple pathways in A. pullulans. Enrichment analysis using Gene Ontology showed that most of the proteins found in the EVs were associated with primary metabolism. When sequencing the small RNA fraction of A. pullulans EVs, we found two hypothetical novel mil-RNAs. Finally, we tested the biocontrol potential of EVs from A. pullulans. The EVs did not inhibit the germination of spores of three important phytopathogenic fungi-Botrytis cinerea, Colletotrichum acutatum, and Penicillium expansum. However, exposure of grown cultures of C. acutatum and P. expansum to A. pullulans EVs resulted in visible changes in morphology of colonies. These preliminary results suggest that EVs may be part of the antagonistic activity of A. pullulans, which is so far only partially understood. Thus, the first isolation and characterization of EVs from A. pullulans provides a starting point for further studies of EVs in the biotechnologically important traits of the biocontrol black fungus A. pullulans in particular and in the biological role of fungal EVs in general.

Epiphytic Yeasts and Bacteria as Candidate Biocontrol Agents of Green and Blue Molds of Citrus Fruits

Overall, 180 yeasts and bacteria isolated from the peel of citrus fruits were screened for their in vitro antagonistic activity against Penicillium digitatum and P. italicum, causative agents of green and blue mold of citrus fruits, respectively. Two yeast and three bacterial isolates were selected for their inhibitory activity on mycelium growth. Based on the phylogenetic analysis of 16S rDNA and ITS rDNA sequences, the yeast isolates were identified as Candida oleophila and Debaryomyces hansenii while the bacterial isolates were identified as Bacillus amyloliquefaciens, B. pumilus and B. subtilis. All five selected isolates significantly reduced the incidence of decay incited by P. digitatum and P. italicum on ‘Valencia’ orange and ‘Eureka’ lemon fruits. Moreover, they were effective in preventing natural infections of green and blue mold of fruits stored at 4 °C. Treatments with antagonistic yeasts and bacteria did not negatively affect the quality and shelf life of fruits. The antagonistic efficacy of the five isolates depended on multiple modes of action, including the ability to form biofilms and produce antifungal lipopeptides, lytic enzymes and volatile compounds. The selected isolates are promising as biocontrol agents of postharvest green and blue molds of citrus fruits.

From Glaciers to Refrigerators: the Population Genomics and Biocontrol Potential of the Black Yeast Aureobasidium subglaciale

Apples are affected by numerous fungi known as storage rots, which cause significant losses before and after harvest. Concerns about increasing antimicrobial resistance, bans on various fungicides, and changing consumer preferences are motivating the search for safer means to prevent fruit rot. The use of antagonistic microbes has been shown to be an efficient and environmentally friendly alternative to conventional phytopharmaceuticals. Here, we investigate the potential of Aureobasidium subglaciale for postharvest rot control. We tested the antagonistic activity of 9 strains of A. subglaciale and 7 closely related strains against relevant phytopathogenic fungi under conditions simulating low-temperature storage: Botrytis cinerea, Penicillium expansum, and Colletotrichum acutatum. We also investigated a selection of phenotypic traits of all strains and sequenced their whole genomes. The tested strains significantly reduced postharvest rot of apples at low temperatures caused by B. cinerea, C. acutatum (over 60%), and P. expansum (about 40%). Several phenotypic traits were observed that may contribute to this biocontrol capacity: growth at low temperatures, tolerance to high temperatures and elevated solute concentrations, and strong production of several extracellular enzymes and siderophores. Population genomics revealed that 7 of the 15 strains originally identified as A. subglaciale most likely belong to other, possibly undescribed species of the same genus. In addition, the population structure and linkage disequilibrium of the species suggest that A. subglaciale is strictly clonal and therefore particularly well suited for use in biocontrol. Overall, these data suggest substantial biological control potential for A. subglaciale, which represents another promising biological agent for disease control in fresh fruit.

IMPORTANCE After harvest, fruits are often stored at low temperatures to prolong their life. However, despite the low temperatures, much of the fruit is lost to rot caused by a variety of fungi, resulting in major economic losses and food safety risks. An increasingly important environmentally friendly alternative to conventional methods of mitigating the effects of plant diseases is the use of microorganisms that act similarly to probiotics—occupying the available space, producing antimicrobial compounds, and consuming the nutrients needed by the rot-causing species. To find a new microorganism for biological control that is particularly suitable for cold storage of fruit, we tested different isolates of the cold-loving yeast Aureobasidium subglaciale and studied their phenotypic characteristics and genomes. We demonstrated that A. subglaciale can significantly reduce rotting of apples caused by three rot-causing molds at low temperatures and thus has great potential for preventing fruit rot during cold storage.

Physiological and transcriptomics analysis of the effect of recombinant serine protease on the preservation of loquat

This study aimed to explore the effects of recombinant serine protease treatment on the development of post-harvest loquat diseases, fruit quality, and disease resistance enzyme activities. It also sought to analyze differential genes expression using RNA-seq technology. Transcriptomics analysis revealed 708 and 398 differentially expressed genes (DEGs) in loquat fruits treated with serine protease for 24 and 48 h. Furthermore, 2198 DEGs were obtained between 24 and 48 h after treatment. The genes encoding JAZ, MYC2 and ERF in the plant signal transduction pathway were significantly up-regulated. The resistance-related genes, such as PPO, PAL, TLP, WRKY, and transcription factors were also significantly up-regulated. These results indicated that the recombinant serine protease can induce plant signal transduction pathway in loquat fruit. The expression of some resistance-related genes enhanced the disease resistance of loquat fruit and revealed the molecular mechanism of loquat fruit resistance induced by recombinant serine protease.

Genome, transcriptome and secretome analyses of the antagonistic, yeast-like fungus Aureobasidium pullulans to identify potential biocontrol genes

Aureobasidium pullulans is an extremotolerant, cosmopolitan yeast-like fungus that successfully colonises vastly different ecological niches. The species is widely used in biotechnology and successfully applied as a commercial biocontrol agent against postharvest diseases and fireblight. However, the exact mechanisms that are responsible for its antagonistic activity against diverse plant pathogens are not known at the molecular level. Thus, it is difficult to optimise and improve the biocontrol applications of this species. As a foundation for elucidating biocontrol mechanisms, we have de novo assembled a high-quality reference genome of a strongly antagonistic A. pullulans strain, performed dual RNA-seq experiments, and analysed proteins secreted during the interaction with the plant pathogen Fusarium oxysporum. Based on the genome annotation, potential biocontrol genes were predicted to encode secreted hydrolases or to be part of secondary metabolite clusters (e.g., NRPS-like, NRPS, T1PKS, terpene, and β-lactone clusters). Transcriptome and secretome analyses defined a subset of 79 A. pullulans genes (among the 10,925 annotated genes) that were transcriptionally upregulated or exclusively detected at the protein level during the competition with F. oxysporum. These potential biocontrol genes comprised predicted secreted hydrolases such as glycosylases, esterases, and proteases, as well as genes encoding enzymes, which are predicted to be involved in the synthesis of secondary metabolites. This study highlights the value of a sequential approach starting with genome mining and consecutive transcriptome and secretome analyses in order to identify a limited number of potential target genes for detailed, functional analyses.

Isolation, purification, gene cloning and expression of antifungal protein from Bacillus amyloliquefaciens MG-3

The antifungal protein MG-3A was isolated from Bacillus amyloliquefaciens MG-3, and was purified and identified. The results showed that antifungal protein MG-3A was likely a serine protease with a molecular weight of ~48 kDa. The serine protease exhibited a broad antifungal spectrum and effectively extended the shelf-life of loquat fruit up to 25 d. The antifungal protein MG-3A showed good stabilities to temperature, pH and protease K. Primers were designed according to the mass spectrum of antifungal protein and the comparison with proteins in the NCBI database. The serine protease gene MG-3A from B. amyloliquefaciens genome was isolated and cloned using PCR. The prokaryotic expression plasmid pET28a-MG-3A was constructed and used to express the antimicrobial protein in vitro. The SDS-PAGE results showed that the recombinant protein expressed in Escherichia coli BL21 (DE3) was highly soluble. Affinity chromatography was used to purify the recombinant protein and its antifungal activity was evaluated.

Screening antagonistic yeasts against citrus green mold and the possible biocontrol mechanisms of Pichia galeiformis (BAF03)

BACKGROUND

Penicillium digitatum is one of the most important pathogens causing citrus green mold, leading to significant economic losses. Traditionally, synthetic fungicides are used to control diseases. However, the side effects of fungicides should not be ignored. Thus, antagonistic yeasts were proposed to be safe and effective alternatives for managing diseases. Orchards are excellent sources of naturally occurring antagonists against pathogens. Therefore, in the present study, antagonistic yeasts obtained from orchards were screened, and the possible biocontrol mechanisms of the most promising yeast were investigated.

RESULTS

Seventy-eight isolates of yeasts (15 species of 10 genera) were obtained from citrus orchards. In in vitro assays, 16 strains showed antifungal activity against Pichia digitatum and 15 strains showed biocontrol potential against green mold on Olinda oranges. Pichia galeiformis (BAF03) exhibited the best antagonistic activity against P. digitatum during 6 days storage at 25 °C and a good antagonistic activity during 29 days at 4 °C. Pichia galeiformis (BAF03) could colonize and amplify quickly in wounded citrus. Scanning electron microscopy results showed that the citrus wound was colonised by the yeast. A total of eight volatile organic compounds (VOCs) were identified by gas chromatography-mass spectrometry The VOCs produced by P. galeiformis (BAF03) efficiently inhibited P. digitatum.

CONCLUSION

Pichia galeiformis (BAF03) isolated from a citrus orchard showed potential to control postharvest green mold of citrus. The possible mechanisms of action likely include competition for space and nutrients as well as production of VOCs.

Biocontrol yeasts: mechanisms and applications

Yeasts occur in all environments and have been described as potent antagonists of various plant pathogens. Due to their antagonistic ability, undemanding cultivation requirements, and limited biosafety concerns, many of these unicellular fungi have been considered for biocontrol applications. Here, we review the fundamental research on the mechanisms (e.g., competition, enzyme secretion, toxin production, volatiles, mycoparasitism, induction of resistance) by which biocontrol yeasts exert their activity as plant protection agents. In a second part, we focus on five yeast species (Candida oleophila, Aureobasidium pullulans, Metschnikowia fructicola, Cryptococcus albidus, Saccharomyces cerevisiae) that are or have been registered for the application as biocontrol products. These examples demonstrate the potential of yeasts for commercial biocontrol usage, but this review also highlights the scarcity of fundamental studies on yeast biocontrol mechanisms and of registered yeast-based biocontrol products. Yeast biocontrol mechanisms thus represent a largely unexplored field of research and plentiful opportunities for the development of commercial, yeast-based applications for plant protection exist.

Snf2 controls pulcherriminic acid biosynthesis and antifungal activity of the biocontrol yeast Metschnikowia pulcherrima

Metschnikowia pulcherrima synthesises the pigment pulcherrimin, from cyclodileucine (cyclo(Leu-Leu)) as a precursor, and exhibits strong antifungal activity against notorious plant pathogenic fungi. This yeast therefore has great potential for biocontrol applications against fungal diseases; particularly in the phyllosphere where this species is frequently found. To elucidate the molecular basis of the antifungal activity of M. pulcherrima, we compared a wild-type strain with a spontaneously occurring, pigmentless, weakly antagonistic mutant derivative. Whole genome sequencing of the wild-type and mutant strains identified a point mutation that creates a premature stop codon in the transcriptional regulator gene SNF2 in the mutant. Complementation of the mutant strain with the wild-type SNF2 gene restored pigmentation and recovered the strong antifungal activity. Mass spectrometry (UPLC HR HESI-MS) proved the presence of the pulcherrimin precursors cyclo(Leu-Leu) and pulcherriminic acid and identified new precursor and degradation products of pulcherriminic acid and/or pulcherrimin. All of these compounds were identified in the wild-type and complemented strain, but were undetectable in the pigmentless snf2 mutant strain. These results thus identify Snf2 as a regulator of antifungal activity and pulcherriminic acid biosynthesis in M. pulcherrima and provide a starting point for deciphering the molecular functions underlying the antagonistic activity of this yeast.

Characterization and overexpression of RHO1 from Cryptococcus laurentii ZJU10 activates CWI signaling pathway on enhancing the inhibition of blue mold on pears

Results from this study explored the inhibitory effect of RHO1 gene (GenBank accession number KY859864) from the antagonistic yeast, Cryptococcus laurentii ZJU10, on the control of Penicillium expansum in pear fruit and its possible mechanism involved. The RHO1 gene was successfully cloned and overexpressed in Saccharomyces cerevisiae. Sequence analysis showed high similarity with Rho family proteins, implying a primary role of Rho1 in the cell wall integrity (CWI) signaling pathway. Gene expression of RHO1 and other five CWI-related genes (including Pkc1, Rlm1, Fks1, Fks2 and Chs3) were significant up-regulated in the treatment of SC/Rho1-induced strain (Saccharomyces cerevisiae transformed with RHO1 and induced by galactose). Meanwhile, SC/Rho1-induced treatment reduced about 61.5% of disease incidence and almost 5-times lower lesion diameter compared to the control. In addition, the growth of transformed strains was slightly lower in contrast to the wild Saccharomyces cerevisiae and the induction of fruit resistance was significantly enhanced, which was tightly linked with triggering stronger host defensive responses by priming activation. This is the first study that Rho1 has a potential function of suppressing fungal disease in harvested fruit by activating CWI signaling pathway and indicates an alternative strategy for postharvest disease management.

Biocontrol activity of recombinant aspartic protease from Trichoderma harzianum against pathogenic fungi

The use of cell wall degrading enzymes of Trichoderma is a promising alternative for improving food storage. The aspartic protease P6281 secreted by the fungus Trichoderma harzianum plays an important role in mycoparasitism on phytopathogenic fungi. In this study, recombinant P6281 (rP6281) expressed in Pichia pastoris showed high activity of 321.8 U/mL. Maximum activity was observed at pH 2.5 and 40 °C, and the enzyme was stable in the pH range of 2.5-6.0. rP6281 significantly inhibited spore germination and growth of plant and animal pathogenic fungi such as Botrytis cinerea, Mucor circinelloides, Aspergillus fumigatus, Aspergillus flavus, Rhizoctonia solani, and Candida albicans. Transmission electron microscopy revealed that rP6281 efficiently damages the cell wall of Botrytis cinerea. In addition, the protease significantly inhibited the development of grey mold that causes rotting of apple, orange, and cucumber, indicating that rP6281 may be developed as an effective anti-mold agent for fruit storage.

To disinfect or not to disinfect in postharvest research on the fungal decay of apple?

Postharvest losses of fruit and vegetables can reach up to 30%, the main cause being microbial decay. For apple fruit, mostly fungal pathogens, such as Penicillium expansum, Colletotrichum spp., Neofabraea spp. and Botrytis cinerea, are important. As such losses are unsustainable in many ways, it is necessary that research is conducted to prevent them. Generally, for plants and fruit grown under non-sterile field conditions, disinfection is carried out prior to the start of a phytopathological experiment. The motivation for this practice is the removal of background contamination so that it will not affect the experimental outcome and its interpretation. In literature, a plethora of disinfection methods exists, differing in disinfectant, strength and duration. The following two disinfectants are commonly used: sodium hypochlorite (NaOCl) and ethanol. This article presents a targeted investigation into the effects of these two disinfectants on apple fruit surface and physiology. The results clearly demonstrate that both were affected by both disinfectants. NaOCl caused oxidative damage to the apple's wax layer, causing it to crack. Ethanol affected a redistribution of the wax on the fruit surface and altered the wax composition and/or metabolism. Both NaOCl and ethanol treatment resulted in an increased respiration rate. Therefore, apple and possibly other fruit should not be disinfected in phytopathological studies. A negative control, as is typically used, is not solving this issue, as we clearly demonstrate that the living tissue shows metabolic effects following disinfection, and hence the study objects are changed, hampering a clear interpretation of the experimental outcomes. Moreover, fungal inoculation during experiments is typically taking place at rather large levels in wounded tissue (as infection success is the exception), outnumbering the variable levels of background population, if present.

Molecular characterization of alkaline protease of Bacillus amyloliquefaciens SP1 involved in biocontrol of Fusarium oxysporum

An alkaline protease gene was amplified from genomic DNA of Bacillus amyloliquefaciens SP1 which was involved in effective biocontrol of Fusarium oxysporum. We investigated the antagonistic capacity of protease of B. amyloliquifaciens SP1, under in vitro conditions. The 5.62 fold purified enzyme with specific activity of 607.69U/mg reported 24.14% growth inhibition of F. oxysporum. However, no antagonistic activity was found after addition of protease inhibitor i.e. PMSF (15mM) to purified enzyme. An 1149bp nucleotide sequence of protease gene encoded 382 amino acids of 43kDa and calculated isoelectric point of 9.29. Analysis of deduced amino acid sequence revealed high homology (86%) with subtilisin E of Bacillus subtilis. The B. amyloliquefaciens SP1 protease gene was expressed in Escherichiax coli BL21. The expressed protease was secreted into culture medium by E. coli and exhibited optimum activity at pH8.0 and 60°C. The most reliable three dimensional structure of alkaline protease was determined using Phyre 2 server which was validated on the basis of Ramachandran plot and ERRAT value. The expression and structure prediction of the enzyme offers potential value for commercial application in agriculture and industry.

Attachment Capability of Antagonistic Yeast Rhodotorula glutinis to Botrytis cinerea Contributes to Biocontrol Efficacy

Rhodotorula glutinis as an antagonism show good biocontrol performance against various post-harvest diseases in fruits. In the present study, strong attachment capability of R. glutinis to spores and hyphae of Botrytis cinerea was observed. Further analysis showed that certain protein components on the yeast cell surface played critical role during the interaction between R. glutinis and B. cinerea. The components mainly distributed at the poles of yeast cells and might contain glycosylation modification, as tunicamycin treated yeast cells lost attachment capability to B. cinerea. To investigate contributions of attachment capability of R. glutinis to its biocontrol efficacy, yeast cells were mutagenized with 3% methane-sulfonic acid ethyl ester (EMS), and a mutant CE4 with stable non-attaching phenotype was obtained. No significant difference was found on colony, cell morphology, reproductive ability, and capsule formation between the mutant and wild-type. However, there was a distinct difference in India ink positive staining patterns between the two strains. Moreover, wild-type strain of R. glutinis showed better performance on inhibiting spore germination and mycelial growth of B. cinerea than CE4 strain when yeast cells and B. cinerea were co-cultured in vitro. In biocontrol assay, both wild-type and CE4 strains showed significant biocontrol efficacy against gray mold caused by B. cinerea in apple fruit, whereas, control effect of CE4 strain was lower than that of wild-type. Our findings provided new evidences that attachment capability of R. glutinis to B. cinerea contributed to its biocontrol efficacy.

Postharvest application of a novel chitinase cloned from Metschnikowia fructicola and overexpressed in Pichia pastoris to control brown rot of peaches

Metschnikowia fructicola strain AP47 is a yeast antagonist against postharvest pathogens of fruits. The yeast was able to produce chitinase enzymes in the presence of pathogen cell wall. A novel chitinase gene MfChi (GenBank accession number HQ113461) was amplified from the genomic DNA of Metschnikowia fructicola AP47. Sequence analysis showed lack of introns, an open reading frame (ORF) of 1098 bp encoding a 365 amino acid protein with a calculated molecular weight of 40.9 kDa and a predicted pI of 5.27. MfChi was highly induced in Metschnikowia fructicola after interaction with Monilinia fructicola cell wall, suggesting a primary role of MfChi chitinase in the antagonistic activity of the yeast. The MfChi gene overexpressed in the heterologous expression system of Pichia pastoris KM71 and the recombinant chitinase showed high endochitinase activity towards 4-Nitrophenyl β-d-N,N',N″-triacetylchitotriose substrate. The antifungal activity of the recombinant chitinase was investigated against Monilinia fructicola and Monilinia laxa in vitro and on peaches. The chitinase significantly controlled the spore germination and the germ tube length of the tested pathogens in PDB medium and the mycelium diameter in PDA. The enzyme, when applied on peaches cv. Redhaven, successfully reduced brown rot severity. This work shows that the chitinase MfChi could be developed as a postharvest treatment with antimicrobial activity for fruit undergoing a short shelf life, and confirms that P. pastoris KM71 is a suitable microorganism for cost-effective large-scale production of recombinant chitinases.