Comparative study of antimicrobial peptides to control citrus postharvest decay caused by Penicillium digitatum

Applications and evolution of melittin, the quintessential membrane active peptide

Melittin, the main venom component of the European Honeybee, is a cationic linear peptide-amide of 26 amino acid residues with the sequence: GIGAVLKVLTTGLPALISWIKRKRQQ-NH₂. Melittin binds to lipid bilayer membranes, folds into amphipathic α-helical secondary structure and disrupts the permeability barrier. Since melittin was first described, a remarkable array of activities and potential applications in biology and medicine have been described. Melittin is also a favorite model system for biophysicists to study the structure, folding and function of peptides and proteins in membranes. Melittin has also been used as a template for the evolution of new activities in membranes. Here we overview the rich history of scientific research into the many activities of melittin and outline exciting future applications.

Inhibition of Three Citrus Pathogenic Fungi by Peptide PAF56 Involves Cell Membrane Damage

The peptide PAF56 (GHRKKWFW) was reported to be an effective control for the main diseases of citrus fruit during postharvest storage. However, the mechanism of action of PAF56 is still unknown. In this paper, PAF56 might not induce defense resistance of citrus fruit. The SEM results visually indicated that the fungi mycelia became shrunken and distorted after being treated with PAF56. The destructive effects of PAF56 on the mycelial cell membrane of three kinds of pathogenic fungi (Penicillium digitatum, Penicillium italicum, and Geotrichum citri-aurantii) were verified by the K⁺ leakage and the release of nucleic acid. Furthermore, the interaction between peptide PAF56 and the pathogen spores was investigated, including the changes in cell membrane permeability and dynamic observation of the interaction of fluorescein labeled TMR-PAF56 and Geotrichum candidum spores. The results indicated that the antifungal activity of PAF56 on spores was time-dependent and directly related to the membrane damage. This research provided useful references for further research and practical application of peptides.

Synthetic antimicrobial peptides control Penicillium digitatum infection in orange fruits

Fungal contamination is among the main reasons for food spoilage, affecting food safety and the economy. Among fungi, Penicillium digitatum is a major agent of this problem. Here, the in vitro activity of eight synthetic antimicrobial peptides was assessed against P. digitatum, and their action mechanisms were evaluated. All peptides were able to inhibit fungal growth. Furthermore, atomic force and fluorescence microscopies revealed that all peptides targeted the fungal membrane leading to pore formation, loss of internal content, and death. The induction of high levels of reactive oxygen species (ROS) was also a mechanism employed by some peptides. Interestingly, only three peptides (PepGAT, PepKAA, and Mo-CBP₃-PepI) effectively control P. digitatum colonization in orange fruits, at a concentration (50 µg mL^-1) 20-fold lower than the commercial food preservative (sodium propionate). Altogether, PepGAT, PepKAA, and Mo-CBP₃-PepI showed high biotechnological potential as new food preservatives to control food infection by P. digitatum.

Membrane-Interacting Antifungal Peptides

The incidence of invasive fungal infections is increasing worldwide, resulting in more than 1.6 million deaths every year. Due to growing antifungal drug resistance and the limited number of currently used antimycotics, there is a clear need for novel antifungal strategies. In this context, great potential is attributed to antimicrobial peptides (AMPs) that are part of the innate immune system of organisms. These peptides are known for their broad-spectrum activity that can be directed toward bacteria, fungi, viruses, and/or even cancer cells. Some AMPs act via rapid physical disruption of microbial cell membranes at high concentrations causing cell leakage and cell death. However, more complex mechanisms are also observed, such as interaction with specific lipids, production of reactive oxygen species, programmed cell death, and autophagy. This review summarizes the structure and mode of action of antifungal AMPs, thereby focusing on their interaction with fungal membranes.

Effects of Peptide Thanatin on the Growth and Transcriptome of Penicillium digitatum

Penicillium digitatum is the most damaging pathogen provoking green mold in citrus fruit during storage, and there is an urgent need for novel antifungal agents with high efficiency. The aim of this study was to investigate the antifungal effects of peptide thanatin against P. digitatum and the molecular mechanisms. Results showed that peptide thanatin had a prominent inhibitory effect on P. digitatum by in vitro and in vivo test. A total of 938 genes, including 556 downregulated and 382 upregulated genes, were differentially expressed, as revealed by RNA-seq of whole P. digitatum genomes analysis with or without thanatin treatment. The downregulated genes mainly encoded RNA polymerase, ribosome biogenesis, amino acid metabolism, and major facilitator superfamily. The genes associated with heat shock proteins and antioxidative systems were widely expressed in thanatin-treated group. DNA, RNA, and the protein content of P. digitatum were significantly decreased after thanatin treatment. In conclusion, thanatin could inhibit the growth of P. digitatum, and the underlying mechanism might be the genetic information processing and stress response were affected. The research will provide more precise and directional clues to explore the inhibitory mechanism of thanatin on growth of P. digitatum.

Expression of Melittin in Fusion with GST in Escherichia coli and Its Purification as a Pure Peptide with Good Bacteriostatic Efficacy

The expression and purification of melittin (MET) in microbials are difficult because of its antibacterial activities. In this work, MET was fused with a glutathione-S-transferase (GST) tag and expressed in Escherichia coli to overcome its lethality to host cells. The fusion protein GST-MET was highly expressed and then purified by glutathione sepharose high-performance affinity chromatography, digested with prescission protease, and further purified by Superdex Peptide 10/300 GL chromatography. Finally, 3.5 mg/L recombinant melittin (rMET) with a purity of >90% was obtained; its antibacterial activities against Gram-positive Bacillus pumilus and Staphylococcus pasteuri were similar to those of commercial MET. A circular dichroism spectroscopic assay showed that the rMET peptide secondary structure was similar to those of the commercial form. To our knowledge, this is the report of the preparation of active pure rMET with no tags. The successful expression and purification of rMET will enable large-scale, industrial biosynthesis of MET.

Control of Citrus Post-harvest Green Molds, Blue Molds, and Sour Rot by the Cecropin A-Melittin Hybrid Peptide BP21

In this study, the activity of the cecropin A-melittin hybrid peptide BP21 (Ac-FKLFKKILKVL-NH₂) in controlling of citrus post-harvest green and blue molds and sour rot and its involved mechanism was studied. The minimum inhibitory concentrations of BP21 against Penicillium digitatum, Penicillium italicum, and Geotrichum candidum were 8, 8, and 4 μmol L^-1, respectively. BP21 could inhibit the growth of mycelia, the scanning electron microscopy results clearly showed that the mycelia treated with BP21 shrank, formed a rough surface, became distorted and collapsed. Fluorescent staining with SYTOX Green (SG) indicated that BP21 could disintegrate membranes. Membrane permeability parameters, including extracellular conductivity, the leakage of potassium ions, and the release of cellular constituents, visibly increased as the BP21 concentration increased. Gross and irreversible damage to the cytoplasm and membranes was observed. There was a positive correlation between hemolytic activity and the concentration of BP21. These results suggest peptide BP21 could be used to control citrus post-harvest diseases.

Three Antifungal Proteins From Penicillium expansum: Different Patterns of Production and Antifungal Activity

Antifungal proteins of fungal origin (AFPs) are small, secreted, cationic, and cysteine-rich proteins. Filamentous fungi encode a wide repertoire of AFPs belonging to different phylogenetic classes, which offer a great potential to develop new antifungals for the control of pathogenic fungi. The fungus Penicillium expansum is one of the few reported to encode three AFPs each belonging to a different phylogenetic class (A, B, and C). In this work, the production of the putative AFPs from P. expansum was evaluated, but only the representative of class A, PeAfpA, was identified in culture supernatants of the native fungus. The biotechnological production of PeAfpB and PeAfpC was achieved in Penicillium chrysogenum with the P. chrysogenum-based expression cassette, which had been proved to work efficiently for the production of other related AFPs in filamentous fungi. Western blot analyses confirmed that P. expansum only produces PeAfpA naturally, whereas PeAfpB and PeAfpC could not be detected. From the three AFPs from P. expansum, PeAfpA showed the highest antifungal activity against all fungi tested, including plant and human pathogens. P. expansum was also sensitive to its self-AFPs PeAfpA and PeAfpB. PeAfpB showed moderate antifungal activity against filamentous fungi, whereas no activity could be attributed to PeAfpC at the conditions tested. Importantly, none of the PeAFPs showed hemolytic activity. Finally, PeAfpA was demonstrated to efficiently protect against fungal infections caused by Botrytis cinerea in tomato leaves and Penicillium digitatum in oranges. The strong antifungal potency of PeAfpA, together with the lack of cytotoxicity, and significant in vivo protection against phytopathogenic fungi that cause postharvest decay and plant diseases, make PeAfpA a promising alternative compound for application in agriculture, but also in medicine or food preservation.

The Penicillium digitatum protein O-mannosyltransferase Pmt2 is required for cell wall integrity, conidiogenesis, virulence and sensitivity to the antifungal peptide PAF26

The activity of protein O-mannosyltransferases (Pmts) affects the morphogenesis and virulence of fungal pathogens. Recently, PMT genes have been shown to determine the sensitivity of Saccharomyces cerevisiae to the antifungal peptide PAF26. This study reports the identification and characterization of the three Pdpmt genes in the citrus post-harvest pathogen Penicillium digitatum. The Pdpmt genes are expressed during fungal growth and fruit infection, with the highest induction for Pdpmt2. Pdpmt2 complemented the growth defect of the S. cerevisiae Δpmt2 strain. The Pdpmt2 gene mutation in P. digitatum caused pleiotropic effects, including a reduction in fungal growth and virulence, whereas its constitutive expression had no phenotypic effect. The Pdpmt2 null mutants also showed a distinctive colourless phenotype with a strong reduction in the number of conidia, which was associated with severe alterations in the development of conidiophores. Additional effects of the Pdpmt2 mutation were hyphal morphological alterations, increased sensitivity to cell wall-interfering compounds and a blockage of invasive growth. In contrast, the Pdpmt2 mutation increased tolerance to oxidative stress and to the antifungal activity of PAF26. These data confirm the role of protein O-glycosylation in the PAF26-mediated antifungal mechanism present in distantly related fungal species. Important to future crop protection strategies, this study demonstrates that a mutation rendering fungi more resistant to an antifungal peptide results in severe deleterious effects on fungal growth and virulence.

Concatemerization increases the inhibitory activity of short, cell-penetrating, cationic and tryptophan-rich antifungal peptides

There are short cationic and tryptophan-rich antifungal peptides such as the hexapeptide PAF26 (RKKWFW) that have selective toxicity and cell penetration properties against fungal cells. This study demonstrates that concatemeric peptides with tandem repeats of the heptapeptide PAF54 (which is an elongated PAF26 sequence) show increased fungistatic and bacteriostatic activities while maintaining the absence of hemolytic activity of the monomer. The increase in antimicrobial activity of the double-repeated PAF sequences (diPAFs), compared to the nonrepeated PAF, was higher (4-8-fold) than that seen for the triple-repeated sequences (triPAFs) versus the diPAFs (2-fold). However, concatemerization diminished the fungicidal activity against quiescent spores of the filamentous fungus Penicillium digitatum. Peptide solubility and sensitivity to proteolytic degradation were affected by the design of the concatemers: incorporation of the AGPA sequence hinge to separate PAF54 repeats increased solubility while the C-terminal addition of the KDEL sequence decreased in vitro stability. These results led to the design of the triPAF sequence PAF102 of 30 amino acid residues, with increased antimicrobial activity and minimal inhibitory concentration (MIC) value of 1-5 μM depending on the fungus. Further characterization of the mode-of-action of PAF102 demonstrated that it colocalizes first with the fungal cell wall, it is thereafter internalized in an energy dependent manner into hyphal cells of the filamentous fungus Fusarium proliferatum, and finally kills hyphal cells intracellularly. Therefore, PAF102 showed mechanistic properties against fungi similar to the parental PAF26. These observations are of high interest in the future development of PAF-based antimicrobial molecules optimized for their production in biofactories.

Inhibition of agronomically relevant fungal phytopathogens by tyrocidines, cyclic antimicrobial peptides isolated from Bacillus aneurinolyticus

The tyrocidines, a complex of analogous cyclic decapeptides produced by Bacillus aneurinolyticus, exhibited noteworthy activity against a range of phytopathogenic fungi, including Fusarium verticillioides, Fusarium solani and Botrytis cinerea. The activity of the tyrocidine peptide complex (Trc mixture) and purified tyrocidines exhibited minimum inhibition concentrations below 13 µg ml−1 (~10 µM) and was significantly more potent than that of the commercial imidazole fungicide, bifonazole. Although the tyrocidines’ activity was negatively influenced by the presence of Ca2+, it remained unaffected by the presence of Mg2+, Na+ and K+. Microscopic analysis revealed significant impact on the morphology of F. solani and Bot. cinerea including retarded germination and hyperbranching of hyphae. Studies with membrane-impermeable dyes, SYTOX green and propidium iodide suggested that the main mode of action of tyrocidines involves the disruption of fungal membrane integrity. Because of the tyrocidines’ broad spectrum and potent antifungal activity, possible multiple targets reducing the risk of overt resistance and general salt tolerance, they are promising candidates that warrant further investigation as bio-fungicides.

Antifungal activity of (KW)n or (RW)n peptide against Fusarium solani and Fusarium oxysporum

The presence of lysine (Lys) or arginine (Arg) and tryptophan (Trp) are important for the antimicrobial effects of cationic peptides. Therefore, we designed and synthesized a series of antimicrobial peptides with various numbers of Lys (or Arg) and Trp repeats [(KW and RW)(n)-NH(2), where n equals 2, 3, 4, or 5]. Antifungal activities of these peptides increased with chain length. Light microscopy demonstrated that longer peptides (n = 4, 5) strongly inhibited in vitro growth of Fusarium solani, and Fusarium oxysporum, at 4-32 μM. Furthermore, longer peptides displayed potent fungicidal activities against a variety of agronomical important filamentous fungi, including F. solani and F. oxysporum, at their minimal inhibitory concentrations (MICs). However, RW series peptides showed slightly higher fungicidal activities than KW peptides against the two strains. Taken together, the results of this study indicate that these short peptides would be good candidates for use as synthetic or transgenic antifungal agents.

Sporicidal activity of synthetic antifungal undecapeptides and control of Penicillium rot of apples

The antifungal activity of cecropin A(2-8)-melittin(6-9) hybrid undecapeptides, previously reported as active against plant pathogenic bacteria, was studied. A set of 15 sequences was screened in vitro against Fusarium oxysporum, Penicillium expansum, Aspergillus niger, and Rhizopus stolonifer. Most compounds were highly active against F. oxysporum (MIC < 2.5 microM) but were less active against the other fungi. The best peptides were studied for their sporicidal activity and for Sytox green uptake in F. oxysporum microconidia. A significant inverse linear relationship was observed between survival and fluorescence, indicating membrane disruption. Next, we evaluated the in vitro activity against P. expansum of a 125-member peptide library with the general structure R-X(1)KLFKKILKX(10)L-NH(2), where X(1) and X(10) corresponded to amino acids with various degrees of hydrophobicity and hydrophilicity and R included different N-terminal derivatizations. Fifteen sequences with MICs below 12.5 muM were identified. The most active compounds were BP21 {Ac,F,V} and BP34 {Ac,L,V} (MIC < 6.25 microM), where the braces denote R, X(1), and X(10) positions and where Ac is an acetyl group. The peptides had sporicidal activity against P. expansum conidia. Seven of these peptides were tested in vivo by evaluating their preventative effect of inhibition of P. expansum infection in apple fruits. The peptide Ts-FKLFKKILKVL-NH(2) (BP22), where Ts is a tosyl group, was the most active with an average efficacy of 56% disease reduction, which was slightly lower than that of a commercial formulation of the fungicide imazalil.

Identification and rational design of novel antimicrobial peptides for plant protection

Peptides and small proteins exhibiting antimicrobial activity have been isolated from many organisms ranging from insects to humans, including plants. Their role in defense is established, and their use in agriculture was already being proposed shortly after their discovery. However, some natural peptides have undesirable properties that complicate their application. Advances in peptide synthesis and high-throughput activity screening have made possible the de novo and rational design of novel peptides with improved properties. This review summarizes findings in the identification and design of short antimicrobial peptides with activity against plant pathogens, and will discuss alternatives for their heterologous production suited to plant disease control. Recent studies suggest that peptide antimicrobial action is not due solely to microbe permeation as previously described, but that more subtle factors might account for the specificity and absence of toxicity of some peptides. The elucidation of the mode of action and interaction with microbes will assist the improvement of peptide design with a view to targeting specific problems in agriculture and providing new tools for plant protection.