Antifungal plant defensins: mechanisms of action and production

Proline-Modified RWn Peptides: Enhanced Antifungal Efficacy and Synergy with Conventional Antibiotics for Combating Resistant Fungal Infections

Candida albicans (Ca) and Cryptococcus neoformans (Cn) infections pose a growing threat due to rising antifungal resistance. This study explores a new class of antifungal agents, the RWn series (n = 4, 6, 8) peptides. These synthetic peptides were evaluated for their ability to inhibit Ca and Cn growth. All peptides except RW4 displayed antifungal activity, with RW6 exhibiting exceptional potency against Cn. Importantly, the incorporation of a proline residue significantly reduced cytotoxicity while maintaining antifungal activity against Cn for all RWnP peptides. Notably, RW6P demonstrated broad-spectrum activity against both Ca and Cn with low minimum inhibitory concentrations (MICs) and minimal toxicity. Furthermore, combining RW6P with trace amounts of traditional antibiotics (penicillin, vancomycin, and ampicillin) achieved synergistic effects, significantly reducing MICs against both fungi. These findings suggest that RWnP peptides, particularly RW6P, have promising potential as novel antifungal agents due to their high potency, broad-spectrum activity, and ability to resensitize fungi to existing antibiotics.

Dynamics of interaction and internalisation of the antifungal protein PeAfpA into Penicillium digitatum morphotypes

Antifungal proteins (AFPs) as the highly active PeAfpA from Penicillium expansum or PdAfpB from Penicillium digitatum exert promising antifungal activity, but their mode of action is not fully understood. We characterised the interaction of PeAfpA against P. digitatum, comparing it to the less active PdAfpB. Despite similar effect on conidia germination, PeAfpA did not induce a burst of reactive oxygen species as PdAfpB. Live-cell fluorescence microscopy revealed complex dynamics of interaction and internalisation of both proteins with distinct P. digitatum morphotypes (quiescent conidia, swollen conidia, germlings and hyphae). Labelled PeAfpA co-localised at the cell wall of quiescent conidia, where its localisation was punctate and not uniformly distributed. This pattern changed during germination to a uniform distribution with increased intensity. Conidia from mutants of genes involved in melanin biosynthesis (pksP/alb1 or arp2) showed an altered distribution of PeAfpA but later mimicked the wild type trend of changes during germination. In swollen conidia and germlings, PeAfpA remained attached to the cell wall. In hyphae, PeAfpA was internalised through the growing hyphal tip after binding to the cell wall, in a non-endocytic but energy-dependent process that caused vacuolisation, which preceded cell death. These results may help the development of biofungicides based on AFPs.

Anti-fungal peptides: an emerging category with enthralling therapeutic prospects in the treatment of candidiasis

Candida infections, particularly invasive candidiasis, pose a serious global health threat. Candida albicans is the most prevalent species causing candidiasis, and resistance to key antifungal drugs, such as azoles, echinocandins, polyenes, and fluoropyrimidines, has emerged. This growing multidrug resistance (MDR) complicates treatment options, highlighting the need for novel therapeutic approaches. Antifungal peptides (AFPs) are gaining recognition for their potential as new antifungal agents due to their diverse structures and functions. These natural or recombinant peptides can effectively target fungal virulence and viability, making them promising candidates for future antifungal development. This review examines infections caused by Candida species, the limitations of current antifungal treatments, and the therapeutic potential of AFPs. It emphasizes the importance of identifying novel AFP targets and their production for advancing treatment strategies. By discussing the therapeutic development of AFPs, the review aims to draw researchers' attention to this promising field. The integration of knowledge about AFPs could pave the way for novel antifungal agents with broad-spectrum activity, reduced toxicity, targeted action, and mechanisms that limit resistance in pathogenic fungi, offering significant advancements in antifungal therapeutics.

Plant Defensin PgD1 a Biotechnological Alternative Against Plant Pathogens

Plant defensins are small antimicrobial proteins (AMP) that participate in the immune defense of plants through their antibacterial, antiviral and antifungal activities. PgD1 is a defensin from Picea glauca (Canadian Pine) and has antifungal activity against plant pathogens. This activity positions it as an alternative biotechnological agent to pesticides commonly used against these plant fungi diseases. The present study aimed to recombinantly produce PgD1 in Escherichia coli to characterize its in vitro antifungal potential against different phytopathogens. To achieve this, the coding gene was amplified and cloned into pET30a( +). Recombinant plasmid was subsequently introduced into E. coli for the soluble expression of defensin PgD1. To evaluate the antifungal activity of the expressed protein, the growth inhibition test was used in solid and liquid media for approximately 7 days against significant plant pathogens, that cause significant crop damage including: Botrytis cinerea, Colletotrichum gloeosporioides, Colletotrichum musae, Colletotrichum graminicola and Fusarium oxysporum. Additionally, stability assessments included temperature variation experiments and inhibition tests using dithiothreitol (DTT). The results showed that there was significant inhibition of the fungal species tested when in the presence of PgD1. Furthermore, defensin proved to be resistant to temperature variations and demonstrated that part of its stability is due to its primary structure rich in cysteine ​​residues through the denaturation test with dithiothreitol (DTT) where the antifungal activity of PgD1 defensin was inhibited. These data indicate that recombinant PgD1 could be utilized as a plant protection technology in agriculture.

Metagenomic Analysis of Rhizospheric Bacterial Community of Citrus Trees Expressing Phloem-Directed Antimicrobials

Huanglongbing, also known as citrus greening, is currently the most devastating citrus disease with limited success in prevention and mitigation. A promising strategy for Huanglongbing control is the use of antimicrobials fused to a carrier protein (phloem protein of 16 kDa or PP16) that targets vascular tissues. This study investigated the effects of genetically modified citrus trees expressing Citrus sinensis PP16 (CsPP16) fused to human lysozyme and β-defensin-2 on the soil microbiome diversity using 16S amplicon analysis. The results indicated that there were no significant alterations in alpha diversity, beta diversity, phylogenetic diversity, differential abundance, or functional prediction between the antimicrobial phloem-overexpressing plants and the control group, suggesting minimal impact on microbial community structure. However, microbiota diversity analysis revealed distinct bacterial assemblages between the rhizosphere soil and root environments. This study helps to understand the ecological implications of crops expressing phloem-targeted antimicrobials for vascular disease management, with minimal impact on soil microbiota.

Detailed analysis of agro-industrial byproducts/wastes to enable efficient sorting for various agro-industrial applications

Agriculture-based industries generate huge amounts of byproducts/wastes every year, which are not exploited or disposed efficiently posing an environmental problem with implications to human and animal health. Finding strategies to increase the recycling of agro-industrial byproducts/wastes (AIBWs) is a primary objective of the current study. A thorough examination of AIBWs in conjunction with experimental research is proposed to facilitate sorting for various agro-industrial applications and consequently increasing byproduct/waste utilization. Accordingly, two sustainable, locally available sources of AIBWs, namely, wheat bran (WB) and garlic straw and peels (GSP) were studied in detail including content and composition of proteins, phytohormones and nutritional elements, as well as the effect of AIBW extracts on plant and microbial growth. Hundreds of proteins were recovered from AIBW mainly from WBs, including chaperons, metabolite and protein modifying enzymes, and antimicrobial proteins. In-gel assays showed that WB and GSP possess high protease and nuclease activities. Conspicuously, phytohormone analysis of AIBWs revealed the presence of high levels of strigolactones, stimulants of seed germination of root parasitic weeds, as well as indole acetic acid (IAA) and abscisic acid (ABA). Garlic straw extract strongly inhibited germination of the weed Amaranthus palmeri but not of Abutilon theophrasti and all examined AIBWs significantly affected post-germination growth. Bacterial growth was strongly inhibited by garlic straw, but enhanced by WBs, which can be used at least partly as a bacterial growth medium. Thus, an in-depth examination of AIBW characteristics will enable appropriate sorting for diverse agro-industrial applications, which will increase their utilization and consequently their economic value.

Nature-inspired peptide of MtDef4 C-terminus tail enables protein delivery in mammalian cells

Cell-penetrating peptides show promise as versatile tools for intracellular delivery of therapeutic agents. Various peptides have originated from natural proteins with antimicrobial activity. We investigated the mammalian cell-penetrating properties of a 16-residue peptide with the sequence GRCRGFRRRCFCTTHC from the C-terminus tail of the Medicago truncatula defensin MtDef4. We evaluated the peptide's ability to penetrate multiple cell types. Our results demonstrate that the peptide efficiently penetrates mammalian cells within minutes and at a micromolar concentration. Moreover, upon N-terminal fusion to the fluorescent protein GFP, the peptide efficiently delivers GFP into the cells. Despite its remarkable cellular permeability, the peptide has only a minor effect on cellular viability, making it a promising candidate for developing a cell-penetrating peptide with potential therapeutic applications.

Potential Regulatory Gene Network Associated with the Ameliorative Effect of Oat Antibacterial Peptides on Rat Colitis

Oat protein is unstable in intestinal fluid digestion, and it is easily degraded by trypsin and chymotrypsin, producing low molecular weight peptides. Endopeptidase hydrolysis can improve the bioavailability of active peptides and avoid further digestion in the gastrointestinal tract. Antimicrobial peptides (AMPs) can effectively improve host immunity, but most related studies focus on physiology and ecology, and there are few reports on their molecular level. Therefore, in this article, oat peptides were prepared via the simulated digestion method in vitro, and the main metabolites and action factors affecting colitis were screened by using the multi-omics methods in a high-throughput mode to analyze the effect and mechanism of colitis. Firstly, oat antimicrobial peptides were prepared from cationic resin combined with HPLC, and the anti-inflammatory effects of antimicrobial peptides were analyzed in vitro through the use of human colon epithelial (HCoEpiC) anti-inflammatory cells. In vivo experiments using rats have verified that AMPs can effectively prevent colitis caused by dextran sodium sulfate (DSS), reduce intestinal inflammatory cell infiltration and glandular disappearance in the colon, and reduce the apoptosis rate of colon cells. Secondly, metabolomics and transcriptomics were combined to analyze the mechanism of preventing enteritis, and it was found that oat antimicrobial peptides can promote DAG diglycerol production and inhibit the activation of T helper cells (TH), resulting in the down-regulation of key factors in the main downstream pathways of TH1, TH2 and TH17, and inhibit the production of inflammatory cells. At the same time, AMP can activate the wnt pathway, improve the expression of key genes of wnt and frizzled, promote the generation of intestinal stem cells, facilitate the differentiation and repair of intestinal epithelial cells, and prevent the generation of enteritis. Finally, the underlying genetic regulatory network of the important pathway was constructed from the effect of AMP on rat colitis.

Hyperpolarisation of Mitochondrial Membranes Is a Critical Component of the Antifungal Mechanism of the Plant Defensin, Ppdef1

Plant defensins are a large family of small cationic proteins with diverse functions and mechanisms of action, most of which assert antifungal activity against a broad spectrum of fungi. The partial mechanism of action has been resolved for a small number of members of plant defensins, and studies have revealed that many act by more than one mechanism. The plant defensin Ppdef1 has a unique sequence and long loop 5 with fungicidal activity against a range of human fungal pathogens, but little is known about its mechanism of action. We screened the S. cerevisiae non-essential gene deletion library and identified the involvement of the mitochondria in the mechanism of action of Ppdef1. Further analysis revealed that the hyperpolarisation of the mitochondrial membrane potential (MMP) activates ROS production, vacuolar fusion and cell death and is an important step in the mechanism of action of Ppdef1, and it is likely that a similar mechanism acts in Trichophyton rubrum.

The Plant Defensin Ppdef1 Is a Novel Topical Treatment for Onychomycosis

Onychomycosis, or fungal nail infection, causes not only pain and discomfort but can also have psychological and social consequences for the patient. Treatment of onychomycosis is complicated by the location of the infection under the nail plate, meaning that antifungal molecules must either penetrate the nail or be applied systemically. Currently, available treatments are limited by their poor nail penetration for topical products or their potential toxicity for systemic products. Plant defensins with potent antifungal activity have the potential to be safe and effective treatments for fungal infections in humans. The cystine-stabilized structure of plant defensins makes them stable to the extremes of pH and temperature as well as digestion by proteases. Here, we describe a novel plant defensin, Ppdef1, as a peptide for the treatment of fungal nail infections. Ppdef1 has potent, fungicidal activity against a range of human fungal pathogens, including Candida spp., Cryptococcus spp., dermatophytes, and non-dermatophytic moulds. In particular, Ppdef1 has excellent activity against dermatophytes that infect skin and nails, including the major etiological agent of onychomycosis Trichophyton rubrum. Ppdef1 also penetrates human nails rapidly and efficiently, making it an excellent candidate for a novel topical treatment of onychomycosis.

Antimicrobial Peptides: The Production of Novel Peptide-Based Therapeutics in Plant Systems

The increased prevalence of antibiotic resistance is alarming and has a significant impact on the economies of emerging and underdeveloped nations. The redundancy of antibiotic discovery platforms (ADPs) and injudicious use of conventional antibiotics has severely impacted millions, across the globe. Potent antimicrobials from biological sources have been extensively explored as a ray of hope to counter the growing menace of antibiotic resistance in the population. Antimicrobial peptides (AMPs) are gaining momentum as powerful antimicrobial therapies to combat drug-resistant bacterial strains. The tremendous therapeutic potential of natural and synthesized AMPs as novel and potent antimicrobials is highlighted by their unique mode of action, as exemplified by multiple research initiatives. Recent advances and developments in antimicrobial discovery and research have increased our understanding of the structure, characteristics, and function of AMPs; nevertheless, knowledge gaps still need to be addressed before these therapeutic options can be fully exploited. This thematic article provides a comprehensive insight into the potential of AMPs as potent arsenals to counter drug-resistant pathogens, a historical overview and recent advances, and their efficient production in plants, defining novel upcoming trends in drug discovery and research. The advances in synthetic biology and plant-based expression systems for AMP production have defined new paradigms in the efficient production of potent antimicrobials in plant systems, a prospective approach to countering drug-resistant pathogens.

Expression of Mn-sod, PAL1, aos1 and HPL genes in soybean plants overexpressing the NmDef02 defensin

Plant defensins are a potential tool in crop improvement programs through biotechnology. Their antifungal action makes them attractive molecules for the production of transgenic plants. Information is currently lacking on what happens to the expression of defense genes in transgenic plants that overexpress a defensin. Here we show the relative expression of four defense-related genes: Mn-sod, PAL1, aos1 and HPL evaluated in two transgenic soybean events (Def1 and Def17) constitutively expressing the NmDef02 defensin gene from Nicotiana megalosiphon. The expression of these defense genes showed a differential profile in the transgenic events, with the increased expression of the aos1 gene and the repression of the Mn-sod gene in both events, when compared to the non-transgenic control. Furthermore, the expression of the PAL1 gene only increased in the Def17 event. The results indicate that although there were some changes in the expression of defense genes in transgenic plants overexpressing the defensin NmDef02; the morphoagronomic parameters evaluated were similar to the non-transgenic control. Understanding the molecular changes that occur in these transgenic plants could be of interest in the short, medium and long term.

Membrane binding properties of plant defensins

The membrane interaction characteristics of five antifungal plant defensin peptides: NaD1, and the related HXP4 and L5, as well as NaD2 and the related ZmD32 were studied. These peptides were chosen to cover a broad range of cationic charges with little structural variations, allowing for assessment of the role of charge in their membrane interactions. Membrane permeabilizing activity against C. albicans was confirmed and quantified for benchmarking purposes. Viscoelastic characteristics of the membrane interactions were studied in typical neutral and charged model membranes using quartz crystal microbalance with dissipation (QCM-D. Frequency-dissipation fingerprinting analysis of the QCM-D results revealed that all of the peptides were able to bind to all studied model membranes albeit with slightly different viscoelastic character for each membrane type. However, characteristic disruption patterns were not observed suggesting that the membrane disrupting activity of these defensins is mostly specific to fungal membranes, and that increasing the peptide charge does not enhance their action. The results also show that the presence of specific sterols has a profound effect on the ability of the peptides to disrupt the membrane.

Cecropin D-derived synthetic peptides in the fight against Candida albicans cell filamentation and biofilm formation

The recent progressive increase in the incidence of invasive fungal infections, especially in immunocompromised patients, makes the search for new therapies crucial in the face of the growing drug resistance of prevalent nosocomial yeast strains. The latest research focuses on the active compounds of natural origin, inhibiting fungal growth, and preventing the formation of fungal biofilms. Antimicrobial peptides are currently the subject of numerous studies concerning effective antifungal therapy. In the present study, the antifungal properties of two synthetic peptides (ΔM3, ΔM4) derived from an insect antimicrobial peptide - cecropin D - were investigated. The fungicidal activity of both compounds was demonstrated against the yeast forms of Candida albicans, Candida tropicalis, and Candida parapsilosis, reaching a MFC_99.9 in the micromolar range, while Candida glabrata showed greater resistance to these peptides. The scanning electron microscopy revealed a destabilization of the yeast cell walls upon treatment with both peptides; however, their effectiveness was strongly modified by the presence of salt or plasma in the yeast environment. The transition of C. albicans cells from yeast to filamentous form, as well as the formation of biofilms, was effectively reduced by ΔM4. Mature biofilm viability was inhibited by a higher concentration of this peptide and was accompanied by increased ROS production, activation of the GPX3 and SOD5 genes, and finally, increased membrane permeability. Furthermore, both peptides showed a synergistic effect with caspofungin in inhibiting the metabolic activity of C. albicans cells, and an additive effect was also observed for the mixtures of peptides with amphotericin B. The results indicate the possible potential of the tested peptides in the prevention and treatment of candidiasis.

Non-targeted metabolomics and microbial analyses of the impact of oat antimicrobial peptides on rats with dextran sulfate sodium-induced enteritis

To study the prevention and mechanism of oat antimicrobial peptides (AMPs) on enteritis. Oat protein was hydrolyzed by alkaline protease and isolated to obtain oat antimicrobial peptides. Rat enteritis models were constructed using dextran sodium sulfate (DSS), and a blank group, a negative control group, a positive control group, and an experimental group (low dose, medium dose, and high dose) were established. Through pathological test, antioxidant test, intestinal microbial and metabolite determination, it was found that AMPS can improve the antioxidant capacity of colon, reduce the production of inflammatory cells, and have the effect of preventing enteritis. In addition, the AMPS group is able to change and reduce the abundance of Bacteroides-eggerthii-DSM-20697 and Desulfovibrionaceae, increase the abundance of probiotics such as roboutsia and Ruminococcus and optimize the diversity of intestinal microorganisms. Then, the combined analysis of microorganism and metabolites showed that Romboutsia and Ruminococcus reduced the contents of amino acid and glucose and promoted the production of phospholipid, while Bacteroides promoted the synthesis of amino acid in the body. From the above, it can be seen that DSS causes damage to the mechanical barrier of the gut. Oat antimicrobial peptides provide a microbial barrier for the gut microbes, which produce acetic acid and succinic acid with small amounts of isobutyric acid, isovaleric acid, and lactic acid. The acidic metabolites produced reduce the pH of the gut and produce substances with antibacterial effects (such as lipophilic molecules, antibiotics, and hydroperoxides). Inhibit the growth and reproduction of other harmful bacteria, Vibrio desulphuris, from adhering to and colonizing the intestinal mucosa. Secreted short-chain fatty acids, such as acetate and butyric acid, maintain tight connections between the epithelial cells of the intestinal mucosa, thus protecting the mechanical barrier of the intestinal mucosa. Moreover, amino acids are converted into phospholipid metabolism through protein digestion and absorption to promote the production of phospholipid in the intestine and repair damaged cell membranes.

Characterization of Defensin-like Protein 1 for Its Anti-Biofilm and Anti-Virulence Properties for the Development of Novel Antifungal Drug against Candida auris

Candida auris has emerged as a pan-resistant pathogenic yeast among immunocompromised patients worldwide. As this pathogen is involved in biofilm-associated infections with serious medical manifestations due to the collective expression of pathogenic attributes and factors associated with drug resistance, successful treatment becomes a major concern. In the present study, we investigated the candidicidal activity of a plant defensin peptide named defensin-like protein 1 (D-lp1) against twenty-five clinical strains of C. auris. Furthermore, following the standard protocols, the D-lp1 was analyzed for its anti-biofilm and anti-virulence properties. The impact of these peptides on membrane integrity was also evaluated. For cytotoxicity determination, a hemolytic assay was conducted using horse blood. The minimum inhibitory concentration (MIC) and minimum fungicidal concentration (MFC) values ranged from 0.047-0.78 mg/mL and 0.095-1.56 mg/mL, respectively. D-lp1 at sub-inhibitory concentrations potentially abrogated both biofilm formation and 24-h mature biofilms. Similarly, the peptide severely impacted virulence attributes in the clinical strain of C. auris. For the insight mechanism, D-lp1 displayed a strong impact on the cell membrane integrity of the test pathogen. It is important to note that D-lp1 at sub-inhibitory concentrations displayed minimal hemolytic activity against horse blood cells. Therefore, it is highly useful to correlate the anti-Candida property of D-lp1 along with anti-biofilm and anti-virulent properties against C. auris, with the aim of discovering an alternative strategy for combating serious biofilm-associated infections caused by C. auris.

Amaranthus hypochondriacus seeds as a rich source of cysteine rich bioactive peptides

Amaranthus hypochondriacus is a nutritious alternative grain native to Central and South America. Increased interest in the impact of A. hypochondriacus on the human body has driven characterization of bioactive secondary metabolites. The seeds are known to contain bioactive small molecules but little is known regarding endogenous peptides. Cysteine-rich peptides (CRPs) in foodstuffs are particularly relevant because they are stabilized by disulfide bonds enhancing resistance to digestion. Here, in silico predictions, proteomics, and simulated gastrointestinal digestions are leveraged to identify digestion resistant CRPs within A. hypochondriacus seeds. Thirteen in silico predicted CRPs were detected in a seed extract providing evidence for the translation of five CRP families. Mature forms of six CRPs were characterized via top-down proteomics revealing multiple post-translational modifications. All six peptides demonstrated resistance to simulated gastrointestinal digestion, suggesting that A. hypochondriacus CRPs may exhibit bioactivity after consumption and should be prioritized for further characterization.

Biological Functions and Applications of Antimicrobial Peptides

Abstract:

Despite antimicrobial resistance, which is attributed to the misuse of broad-spectrum antibiotics, antibiotics can indiscriminately kill pathogenic and beneficial microorganisms. These events disrupt the delicate microbial balance in both humans and animals, leading to secondary infections and other negative effects. Antimicrobial peptides (AMPs) are functional natural biopolymers in plants and animals. Due to their excellent antimicrobial activities and absence of microbial resistance, AMPs have attracted enormous research attention. We reviewed the antibacterial, antifungal, antiviral, antiparasitic, as well as antitumor properties of AMPs and research progress on AMPs. In addition, we highlighted various recommendations and potential research areas for their progress and challenges in practical applications.

Structure, dynamics, and function of PsDef2 defensin from Pinus sylvestris

Plant defensins demonstrate high structural stability at extreme temperatures and pH values and, in general, are non-toxic to mammalian cells. These properties make them attractive candidates for use in biotechnology and biomedicine. Knowing the structure-function relationship is desirable to guide the design of plant defensin-based applications. Thus far, the broad range of biological activities was described only for one defensin from gymnosperms, the defensin PsDef1 from Scots pine. Here, we report that closely related defensin from the same taxonomy group, PsDef2, differing from PsDef1 by six amino acids, also possesses antimicrobial, antibacterial, and insect α-amylase inhibitory activities. We also report the solution structure and dynamics properties of PsDef2 assessed using a combination of experimental nuclear magnetic resonance (NMR) techniques. Lastly, we perform a comparative analysis of PsDef2 and PsDef1 gaining a molecular-level insight into their structure-dynamics-function relationship.

Use of Defensins to Develop Eco-Friendly Alternatives to Synthetic Fungicides to Control Phytopathogenic Fungi and Their Mycotoxins

Crops are threatened by numerous fungal diseases that can adversely affect the availability and quality of agricultural commodities. In addition, some of these fungal phytopathogens have the capacity to produce mycotoxins that pose a serious health threat to humans and livestock. To facilitate the transition towards sustainable environmentally friendly agriculture, there is an urgent need to develop innovative methods allowing a reduced use of synthetic fungicides while guaranteeing optimal yields and the safety of the harvests. Several defensins have been reported to display antifungal and even-despite being under-studied-antimycotoxin activities and could be promising natural molecules for the development of control strategies. This review analyses pioneering and recent work addressing the bioactivity of defensins towards fungal phytopathogens; the details of approximately 100 active defensins and defensin-like peptides occurring in plants, mammals, fungi and invertebrates are listed. Moreover, the multi-faceted mechanism of action employed by defensins, the opportunity to optimize large-scale production procedures such as their solubility, stability and toxicity to plants and mammals are discussed. Overall, the knowledge gathered within the present review strongly supports the bright future held by defensin-based plant protection solutions while pointing out the obstacles that still need to be overcome to translate defensin-based in vitro research findings into commercial products.

Efficacy of defensins as neutralizing agents against the deadly SARS-CoV-2

SARS-CoV-2 infection causes asymptomatic to severe human respiratory diseases. Vaccinations are effective only to a certain extent, and the disease recurs with milder symptoms even after booster doses. Hence, we hypothesize that antiviral therapy in conjunction with vaccination is the need of the hour for containing the disease. SARS-CoV-2 enters the host cell through interaction between viral spike (S) protein and human Angiotensin II converting enzyme2 (ACE2). So, any S-protein neutralizing molecule could be a potential antiviral moiety. The interaction-interface architecture indicates that cationic peptides effectively bind to anionic interface residues of S protein-receptor binding domain (S-RBD). Subsequently, we adopted molecular docking and simulation approaches to examine the binding affinity of cationic human α and β defensins, HNP1 and HBD2 with S-RBD. We observed strong hydrogen bonds, electrostatic, salt bridge, and hydrophobic interactions between these defensins and S-RBD with binding energy (BE) of -10.7 kcal/mol. Interestingly, defensins from Zea mays (ZmD32), Solanum lycopersicum (TPP3), and Sorghum bicolor (DEF1_SORBI) exhibited approximately similar BE of -11.1 kcal/mol, -11.9 kcal/mol, and -12.6 kcal/mol respectively, comparable to ACE2 (BE= -11.9 kcal/mol). Molecular dynamics simulation of S-RBD complexes formed with HBD2, ZmD32 and TPP3, showed stable associations for 100 ns. Results of in-silico studies demonstrated higher binding affinity of more positively-charged peptides with S-RBD, suggesting the potential of plant defensins to block ACE2 binding of S-RBD. These results warrant experimental validation. However these findings indicate the usefulness of plant defensin homologues as neutralizing antiviral agents for use as ideal prophylactic and therapeutic drugs for COVID-19.Communicated by Ramaswamy H. Sarma.

Characterization of Aspergillus nidulans Biofilm Formation and Structure and Their Inhibition by Pea Defensin Psd2

Approximately four million people contract fungal infections every year in Brazil, primarily caused by Aspergillus spp. The ability of these fungi to form biofilms in tissues and medical devices complicates treatment and contributes to high rates of morbidity and mortality in immunocompromised patients. Psd2 is a pea defensin of 5.4 kDa that possesses good antifungal activity against planktonic cells of representative pathogenic fungi. Its function depends on interactions with membrane and cell wall lipid components such as glucosylceramide and ergosterol. In the present study, we characterized Aspergillus nidulans biofilm formation and determined the effect of Psd2 on A. nidulans biofilms. After 4 hours, A. nidulans conidia adhered to polystyrene surfaces and formed a robust extracellular matrix-producing biofilm at 24 h, increasing thickness until 48 h Psd2 inhibited A. nidulans biofilm formation in a dose-dependent manner. Most notably, at 10 μM Psd2 inhibited 50% of biofilm viability and biomass and 40% of extracellular matrix production. Psd2 significantly decreased the colonized surface area by the biofilm and changed its level of organization, causing a shortening of length and diameter of hyphae and inhibition of conidiophore formation. This activity against A. nidulans biofilm suggests a potential use of Psd2 as a prototype to design new antifungal agents to prevent biofilm formation by A. nidulans and related species.

Natural Peptides Inducing Cancer Cell Death: Mechanisms and Properties of Specific Candidates for Cancer Therapeutics

Nowadays, cancer has become the second highest leading cause of death, and it is expected to continue to affect the population in forthcoming years. Additionally, treatment options will become less accessible to the public as cases continue to grow and disease mechanisms expand. Hence, specific candidates with confirmed anticancer effects are required to develop new drugs. Among the novel therapeutic options, proteins are considered a relevant source, given that they have bioactive peptides encrypted within their sequences. These bioactive peptides, which are molecules consisting of 2–50 amino acids, have specific activities when administered, producing anticancer effects. Current databases report the effects of peptides. However, uncertainty is found when their molecular mechanisms are investigated. Furthermore, analyses addressing their interaction networks or their directly implicated mechanisms are needed to elucidate their effects on cancer cells entirely. Therefore, relevant peptides considered as candidates for cancer therapeutics with specific sequences and known anticancer mechanisms were accurately reviewed. Likewise, those features which turn certain peptides into candidates and the mechanisms by which peptides mediate tumor cell death were highlighted. This information will make robust the knowledge of these candidate peptides with recognized mechanisms and enhance their non-toxic capacity in relation to healthy cells and further avoid cell resistance.

Natural Peptides Inducing Cancer Cell Death: Mechanisms and Properties of Specific Candidates for Cancer Therapeutics

Nowadays, cancer has become the second highest leading cause of death, and it is expected to continue to affect the population in forthcoming years. Additionally, treatment options will become less accessible to the public as cases continue to grow and disease mechanisms expand. Hence, specific candidates with confirmed anticancer effects are required to develop new drugs. Among the novel therapeutic options, proteins are considered a relevant source, given that they have bioactive peptides encrypted within their sequences. These bioactive peptides, which are molecules consisting of 2–50 amino acids, have specific activities when administered, producing anticancer effects. Current databases report the effects of peptides. However, uncertainty is found when their molecular mechanisms are investigated. Furthermore, analyses addressing their interaction networks or their directly implicated mechanisms are needed to elucidate their effects on cancer cells entirely. Therefore, relevant peptides considered as candidates for cancer therapeutics with specific sequences and known anticancer mechanisms were accurately reviewed. Likewise, those features which turn certain peptides into candidates and the mechanisms by which peptides mediate tumor cell death were highlighted. This information will make robust the knowledge of these candidate peptides with recognized mechanisms and enhance their non-toxic capacity in relation to healthy cells and further avoid cell resistance.

Mass Spectrometric Identification of Antimicrobial Peptides from Medicinal Seeds

Traditional medicinal plants contain a variety of bioactive natural products including cysteine-rich (Cys-rich) antimicrobial peptides (AMPs). Cys-rich AMPs are often crosslinked by multiple disulfide bonds which increase their resistance to chemical and enzymatic degradation. However, this class of molecules is relatively underexplored. Herein, in silico analysis predicted 80–100 Cys-rich AMPs per species from three edible traditional medicinal plants: Linum usitatissimum (flax), Trifolium pratense (red clover), and Sesamum indicum (sesame). Bottom-up proteomic analysis of seed peptide extracts revealed direct evidence for the translation of 3–10 Cys-rich AMPs per species, including lipid transfer proteins, defensins, α-hairpinins, and snakins. Negative activity revealed by antibacterial screening highlights the importance of employing a multi-pronged approach for AMP discovery. Further, this study demonstrates that flax, red clover, and sesame are promising sources for further AMP discovery and characterization.

Antimicrobial Peptides: An Update on Classifications and Databases

Antimicrobial peptides (AMPs) are distributed across all kingdoms of life and are an indispensable component of host defenses. They consist of predominantly short cationic peptides with a wide variety of structures and targets. Given the ever-emerging resistance of various pathogens to existing antimicrobial therapies, AMPs have recently attracted extensive interest as potential therapeutic agents. As the discovery of new AMPs has increased, many databases specializing in AMPs have been developed to collect both fundamental and pharmacological information. In this review, we summarize the sources, structures, modes of action, and classifications of AMPs. Additionally, we examine current AMP databases, compare valuable computational tools used to predict antimicrobial activity and mechanisms of action, and highlight new machine learning approaches that can be employed to improve AMP activity to combat global antimicrobial resistance.

Short Peptides Make a Big Difference: The Role of Botany-Derived AMPs in Disease Control and Protection of Human Health

Botany-derived antimicrobial peptides (BAMPs), a class of small, cysteine-rich peptides produced in plants, are an important component of the plant immune system. Both in vivo and in vitro experiments have demonstrated their powerful antimicrobial activity. Besides in plants, BAMPs have cross-kingdom applications in human health, with toxic and/or inhibitory effects against a variety of tumor cells and viruses. With their diverse molecular structures, broad-spectrum antimicrobial activity, multiple mechanisms of action, and low cytotoxicity, BAMPs provide ideal backbones for drug design, and are potential candidates for plant protection and disease treatment. Lots of original research has elucidated the properties and antimicrobial mechanisms of BAMPs, and characterized their surface receptors and in vivo targets in pathogens. In this paper, we review and introduce five kinds of representative BAMPs belonging to the pathogenesis-related protein family, dissect their antifungal, antiviral, and anticancer mechanisms, and forecast their prospects in agriculture and global human health. Through the deeper understanding of BAMPs, we provide novel insights for their applications in broad-spectrum and durable plant disease prevention and control, and an outlook on the use of BAMPs in anticancer and antiviral drug design.

Analysis of a gene family for PDF-like peptides from Arabidopsis

Plant defensins are small, basic peptides that have a characteristic three-dimensional folding pattern which is stabilized by four disulfide bridges. We show here that Arabidopsis contains in addition to the proper plant defensins a group of 9 plant defensin-like (PdfL) genes. They are all expressed at low levels while GUS fusions of the promoters showed expression in most tissues with only minor differences. We produced two of the encoded peptides in E. coli and tested the antimicrobial activity in vitro. Both were highly active against fungi but had lower activity against bacteria. At higher concentrations hyperbranching and swollen tips, which are indicative of antimicrobial activity, were induced in Fusarium graminearum by both peptides. Overexpression lines for most PdfL genes were produced using the 35S CaMV promoter to study their possible in planta function. With the exception of PdfL4.1 these lines had enhanced resistance against F. oxysporum. All PDFL peptides were also transiently expressed in Nicotiana benthamiana leaves with agroinfiltration using the pPZP3425 vector. In case of PDFL1.4 this resulted in complete death of the infiltrated tissues after 7 days. All other PDFLs resulted only in various degrees of small necrotic lesions. In conclusion, our results show that at least some of the PdfL genes could function in plant resistance.

Study of Triticum aestivum Resistome in Response to Wheat dwarf India Virus Infection

Susceptible and resistant germplasm respond differently to pathogenic attack, including virus infections. We compared the transcriptome changes between a resistant wheat cultivar, Sonalika, and a susceptible cultivar, WL711, to understand this process in wheat against wheat dwarf India virus (WDIV) infection. A total of 2760 and 1853 genes were differentially expressed in virus-infected and mock-inoculated Sonalika, respectively, compared to WL711. The overrepresentation of genes involved in signaling, hormone metabolism, enzymes, secondary metabolites, proteolysis, and transcription factors was documented, including the overexpression of multiple PR proteins. We hypothesize that the virus resistance in Sonalika is likely due to strong intracellular surveillance via the action of multiple PR proteins (PR1, RAR1, and RPM1) and ChiB. Other genes such as PIP1, LIP1, DnaJ, defensins, oxalate oxidase, ankyrin repeat protein, serine-threonine kinase, SR proteins, beta-1,3-glucanases, and O-methyltransferases had a significant differential expression and play roles in stress tolerance, may also be contributing towards the virus resistance in Sonalika. In addition, we identified putative genes with unknown functions, which are only expressed in response to WDIV infection in Sonalika. The role of these genes could be further validated and utilized in engineering resistance in wheat and other crops.

Antimicrobial Peptides from Plants: A cDNA-Library Based Isolation, Purification, Characterization Approach and Elucidating Their Modes of Action

Even in a natural ecosystem, plants are continuously threatened by various microbial diseases. To save themselves from these diverse infections, plants build a robust, multilayered immune system through their natural chemical compounds. Among the several crucial bioactive compounds possessed by plants' immune systems, antimicrobial peptides (AMPs) rank in the first tier. These AMPs are environmentally friendly, anti-pathogenic, and do not bring harm to humans. Antimicrobial peptides can be isolated in several ways, but recombinant protein production has become increasingly popular in recent years, with the Escherichia coli expression system being the most widely used. However, the efficacy of this expression system is compromised due to the difficulty of removing endotoxin from its system. Therefore, this review suggests a high-throughput cDNA library-based plant-derived AMP isolation technique using the Bacillus subtilis expression system. This method can be performed for large-scale screening of plant sources to classify unique or homologous AMPs for the agronomic and applied field of plant studies. Furthermore, this review also focuses on the efficacy of plant AMPs, which are dependent on their numerous modes of action and exceptional structural stability to function against a wide range of invaders. To conclude, the findings from this study will be useful in investigating how novel AMPs are distributed among plants and provide detailed guidelines for an effective screening strategy of AMPs.

A Fungal Defensin Targets the SARS-CoV-2 Spike Receptor-Binding Domain

Coronavirus Disease 2019 (COVID-19) elicited by the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) is calling for novel targeted drugs. Since the viral entry into host cells depends on specific interactions between the receptor-binding domain (RBD) of the viral Spike protein and the membrane-bound monocarboxypeptidase angiotensin converting enzyme 2 (ACE2), the development of high affinity RBD binders to compete with human ACE2 represents a promising strategy for the design of therapeutics to prevent viral entry. Here, we report the discovery of such a binder and its improvement via a combination of computational and experimental approaches. The binder micasin, a known fungal defensin from the dermatophytic fungus Microsporum canis with antibacterial activity, can dock to the crevice formed by the receptor-binding motif (RBM) of RBD via an extensive shape complementarity interface (855.9 Å2 in area) with numerous hydrophobic and hydrogen-bonding interactions. Using microscale thermophoresis (MST) technique, we confirmed that micasin and its C-terminal γ-core derivative with multiple predicted interacting residues exhibited a low micromolar affinity to RBD. Expanding the interface area of micasin through a single point mutation to 970.5 Å2 accompanying an enhanced hydrogen bond network significantly improved its binding affinity by six-fold. Our work highlights the naturally occurring fungal defensins as an emerging resource that may be suitable for the development into antiviral agents for COVID-19.

Identification and Characterization of Two Defensins from Capsicum annuum Fruits that Exhibit Antimicrobial Activity

Scientific advances have not been enough to combat the growing resistance to antimicrobial medicines. Antimicrobial peptides (AMPs) are effector molecules of the innate immune defense system in plants and could provide an important source of new antimicrobial drugs. The aim of this work was to extract, purify, characterize, and evaluate the antifungal activities present in fractions obtained from Capsicum annum fruits through reversed-phase chromatography. The fractions named F2 and F3 presented the highest inhibitory activity against Candida and Mycobacterium tuberculosis species. In addition, we identified two sequences of AMPs in the F2 and F3 fractions through mass spectrometry that showed similarity to an already well-characterized family of plant defensins. A plasma membrane permeabilization assay demonstrated that the peptides present in F2, F3, and F4 fractions induced changes in the membrane of some yeast strains, culminating in permeabilization. The production of reactive oxygen species was induced by the fractions in some yeast strains. Fractions F2, F3, and F4 also did not show toxicity in macrophage or monocyte cultures. In conclusion, the obtained data demonstrate that the AMPs, especially those present in the fractions F2 and F3, are promising antimicrobial agents that may be useful to enhance the development of new therapeutic agents for the treatment of diseases.

The Influence of Cellulose-Type Formulants on Anti-Candida Activity of the Tyrocidines

Candida species are highly adaptable to environmental changes with their phenotypic flexibility allowing for the evasion of most host defence mechanisms. Moreover, increasing resistance of human pathogenic Candida strains has been reported against all four classes of available antifungal drugs, which highlights the need for combinational therapies. Tyrocidines are cyclic antimicrobial peptides that have shown synergistic activity with antifungal drugs such as caspofungin and amphotericin B. However, these cyclodecapeptides have haemolytic activity and cytotoxicity, but they have been used for decades in the clinic for topical applications. The tyrocidines tend to form higher-order structures in aqueous solutions and excessive aggregation can result in variable or diminished activity. Previous studies have shown that the tyrocidines prefer ordered association to celluloses. Therefore, a formulation with soluble cellulose was used to control the oligomer stability and size, thereby increasing the activity against Candida spp. Of the formulants tested, it was found that commercial hydroxy-propyl-methyl cellulose, E10M, yielded the best results with increased stability, increased anti-Candida activity, and improved selectivity. This formulation holds promise in topical applications against Candida spp. infections.

Design, Dimerization, and Recombinant Production of MCh-AMP1-Derived Peptide in Escherichia coli and Evaluation of Its Antifungal Activity and Cytotoxicity

Fungal species resistant to current antifungal agents are considered as a serious threat to human health, the dilemma that has dragged attentions toward other sources of antifungals such as antimicrobial peptides (AMPs). In order to improve biological activity of a recently described antifungal peptide MCh-AMP1 from Matricaria chamomilla flowers, MCh-AMP1dimer (DiMCh-AMP1), containing 61 amino acid residues connected by flexible linker (GPDGSGPDESGPDES), was designed and expressed in Escherichia coli, and its structure was analyzed using bioinformatics tools. DiMCh-AMP1 synthetic gene was cloned into pET-28a expression vector, which was then used to transform E. coli BL21 (DE3) strain. His-tag purification was achieved using metal-chelate affinity chromatography. Because there is no methionine residue in the DiMCh-AMP1 sequence, cyanogen bromide was successfully used to separate the target product from the tag. Reverse-phase high-performance liquid chromatography was used as the final step of purification. Results showed that recombinant peptide was produced in considerable amounts (0.9 mg/L) with improved antifungal activity toward both yeasts and molds compared to its monomeric counterpart. The minimum inhibition concentration and minimum fungicidal concentration values of DiMCh-AMP1 against Candida and Aspergillus species were reported in the range of 1.67-6.66 μM and 3.33-26.64 μM, respectively. Our results showed that while antifungal activity of dimerized peptide was improved considerably, its cytotoxicity was decreased, implying that DiMCh-AMP1 could be a potential candidate to design an effective antifungal agent against pathogenic yeasts and molds.

Bifunctional Inhibitors from Capsicum chinense Seeds with Antimicrobial Activity and Specific Mechanism of Action Against Phytopathogenic Fungi

BACKGROUND

Antimicrobial peptides (AMPs) are found in the defense system in virtually all life forms, being present in many, if not all, plant species.

OBJECTIVE

The present work evaluated the antimicrobial, enzymatic activity and mechanism of action of the PEF2 fraction from Capsicum chinense Jack. seeds against phytopathogenic fungi.

METHODS

Peptides were extracted from C. chinense seeds and subjected to reverse-phase chromatography on an HPLC system using a C18 column coupled to a C8 guard column, then the obtained PEF2 fraction was rechromatographed using a C2/C18 column. Two fractions, named PEF2A and PEF2B, were obtained. The fractions were tested for antimicrobial activity on Colletotrichum gloeosporioides, Colletotrichum lindemuthianum, Fusarium oxysporum and Fusarium solani. Trypsin inhibition assays, reverse zymographic detection of protease inhibition and α-amylase activity assays were also performed. The mechanism of action by which PEF2 acts on filamentous fungi was studied through analysis of membrane permeability and production of reactive oxygen species (ROS). Additionally, we investigated mitochondrial functionality and caspase activation in fungal cells.

RESULTS

It is possible to observe that PEF2 significantly inhibited trypsin activity and T. molitor larval α-amylase activity. The PEF2 fraction was able to inhibit the growth of C. gloeosporioides, C. lindemuthianum and F. oxysporum. PEF2A inhibited the growth of C. lindemuthianum (75%) and F. solani (43%). PEF2B inhibited C. lindemuthianum growth (66%) and F. solani (94%). PEF2 permeabilized F. solani cell membranes and induced ROS in F. oxysporum and F. solani. PEF2 could dissipate mitochondrial membrane potential but did not cause the activation of caspases in all studied fungi.

CONCLUSION

The results may contribute to the biotechnological application of these AMPs in the control of pathogenic microorganisms in plants of agronomic importance.

Proteomic response of Escherichia coli to a membrane lytic and iron chelating truncated Amaranthus tricolor defensin

BACKGROUND

Plant defensins are a broadly distributed family of antimicrobial peptides which have been primarily studied for agriculturally relevant antifungal activity. Recent studies have probed defensins against Gram-negative bacteria revealing evidence for multiple mechanisms of action including membrane lysis and ribosomal inhibition. Herein, a truncated synthetic analog containing the γ-core motif of Amaranthus tricolor DEF2 (Atr-DEF2) reveals Gram-negative antibacterial activity and its mechanism of action is probed via proteomics, outer membrane permeability studies, and iron reduction/chelation assays.

RESULTS

Atr-DEF2(G39-C54) demonstrated activity against two Gram-negative human bacterial pathogens, Escherichia coli and Klebsiella pneumoniae. Quantitative proteomics revealed changes in the E. coli proteome in response to treatment of sub-lethal concentrations of the truncated defensin, including bacterial outer membrane (OM) and iron acquisition/processing related proteins. Modification of OM charge is a common response of Gram-negative bacteria to membrane lytic antimicrobial peptides (AMPs) to reduce electrostatic interactions, and this mechanism of action was confirmed for Atr-DEF2(G39-C54) via an N-phenylnaphthalen-1-amine uptake assay. Additionally, in vitro assays confirmed the capacity of Atr-DEF2(G39-C54) to reduce Fe3+ and chelate Fe2+ at cell culture relevant concentrations, thus limiting the availability of essential enzymatic cofactors.

CONCLUSIONS

This study highlights the utility of plant defensin γ-core motif synthetic analogs for characterization of novel defensin activity. Proteomic changes in E. coli after treatment with Atr-DEF2(G39-C54) supported the hypothesis that membrane lysis is an important component of γ-core motif mediated antibacterial activity but also emphasized that other properties, such as metal sequestration, may contribute to a multifaceted mechanism of action.

Transcriptome-based analysis of resistance mechanism to black point caused by Bipolaris sorokiniana in wheat

Black point is a cereal disease caused by complex pathogens, of which the pathogenicity of Bipolaris sorokiniana is the most serious in wheat. Resistance to black point is quantitative in nature, and thus the mechanism is poorly understood. We conducted a comparative transcriptome analysis to identify differentially expressed genes (DEGs) in black point-slightly susceptible and -highly susceptible wheat lines at different timepoints following B. sorokiniana inoculation. DEGs associated with photosynthesis were upregulated in black point-slightly susceptible lines. The top Gene Ontology enrichment terms for biological processes were oxidation–reduction, response to cold, salt stress, oxidative stress, and cadmium ion; terms for cellular component genes were mainly involved in plasma membrane and cytoplasmic membrane-bounded vesicle, whereas those for molecular function were heme binding and peroxidase activity. Moreover, activities of antioxidant enzymes superoxide dismutase, catalase, and peroxidase were higher in slightly susceptible lines than those in highly susceptible lines (except peroxidase 12–24 days post-inoculation). Thus, resistance to B. sorokiniana-caused black point in wheat was mainly related to counteracting oxidative stress, although the specific metabolic pathways require further study. This study presents new insights for understanding resistance mechanisms of selected wheat lines to black point.

Promising Drug Candidates and New Strategies for Fighting against the Emerging Superbug Candida auris

Invasive fungal infections represent an expanding threat to public health. During the past decade, a paradigm shift of candidiasis from Candida albicans to non-albicans Candida species has fundamentally increased with the advent of Candida auris. C. auris was identified in 2009 and is now recognized as an emerging species of concern and underscores the urgent need for novel drug development strategies. In this review, we discuss the genomic epidemiology and the main virulence factors of C. auris. We also focus on the different new strategies and results obtained during the past decade in the field of antifungal design against this emerging C. auris pathogen yeast, based on a medicinal chemist point of view. Critical analyses of chemical features and physicochemical descriptors will be carried out along with the description of reported strategies.

Multiple Classes of Antimicrobial Peptides in Amaranthus tricolor Revealed by Prediction, Proteomics, and Mass Spectrometric Characterization

Traditional medicinal plants are rich reservoirs of antimicrobial agents, including antimicrobial peptides (AMPs). Advances in genomic sequencing, in silico AMP predictions, and mass spectrometry-based peptidomics facilitate increasingly high-throughput bioactive peptide discovery. Herein, Amaranthus tricolor aerial tissue was profiled via MS-based proteomics/peptidomics, identifying AMPs predicted in silico. Bottom-up proteomics identified seven novel peptides spanning three AMP classes including lipid transfer proteins, snakins, and a defensin. Characterization via top-down peptidomic analysis of Atr-SN1, Atr-DEF1, and Atr-LTP1 revealed unexpected proteolytic processing and enumerated disulfide bonds. Bioactivity screening of isolated Atr-LTP1 showed activity against the high-risk ESKAPE bacterial pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, and Enterobacter cloacae). These results highlight the potential for integrating AMP prediction algorithms with complementary -omics approaches to accelerate characterization of biologically relevant AMP peptidoforms.

Peptides Derived From the α-Core and γ-Core Regions of a Putative Silybum marianum Flower Defensin Show Antifungal Activity Against Fusarium graminearum

Fusarium graminearum is the etiological agent of Fusarium head blight (FHB), a disease that produces a significant decrease in wheat crop yield and it is further aggravated by the presence of mycotoxins in the affected grains that may cause health problems to humans and animals. Plant defensins and defensin-like proteins are antimicrobial peptides (AMPs); they are small basic, cysteine-rich peptides (CRPs) ubiquitously expressed in the plant kingdom and mostly involved in host defence. They present a highly variable sequence but a conserved structure. The γ-core located in the C-terminal region of plant defensins has a conserved β-hairpin structure and is a well-known determinant of the antimicrobial activity among disulphide-containing AMPs. Another conserved motif of plant defensins is the α-core located in the N-terminal region, not conserved among the disulphide-containing AMPs, it has not been yet extensively studied. In this report, we have cloned the putative antimicrobial protein DefSm2, expressed in flowers of the wild plant Silybum marianum. The cDNA encodes a protein with two fused basic domains of an N-terminal defensin domain (DefSm2-D) and a C-terminal Arg-rich and Lys-rich domain. To further characterize the DefSm2-D domain, we built a 3D template-based model that will serve to support the design of novel antifungal peptides. We have designed four potential antifungal peptides: two from the DefSm2-D α-core region (SmAP_α1-21 and SmAP_α10-21) and two from the γ-core region (SmAP_γ27-44 and SmAP_γ29-35). We have chemically synthesized and purified the peptides and further characterized them by electrospray ionization mass spectrometry (ESI-MS) and Circular dichroism (CD) spectroscopy. SmAP_α1-21, SmAP_α10-21, and SmAP_γ27-44 inhibited the growth of the phytopathogen F. graminearum at low micromolar concentrations. Conidia exposure to the fungicidal concentration of the peptides caused membrane permeabilization to the fluorescent probe propidium iodide (PI), suggesting that this is one of the main contributing factors in fungal cell killing. Furthermore, conidia treated for 0.5h showed cytoplasmic disorganization as observed by transmission electron microscopy (TEM). Remarkably, the peptides derived from the α-core induced morphological changes on the conidia cell wall, which is a promising target since its distinctive biochemical and structural organization is absent in plant and mammalian cells.

Analysis of structures, functions, and transgenicity of phytopeptides defensin and thionin: a review

Background

Antimicrobial peptides are very primitive innate defense molecules of almost all organisms, from microbes to mammalians and vascular seed-bearing plants. Antimicrobial peptides of plants categorized into cysteine-rich peptides (CRPs) and others and most of the antimicrobial peptides belong to CRPs group. These peptides reported showing the great extent of protecting property against bacteria, fungi, viruses, insect, nematode, and another kind of microbes. To develop a resistant plant against pathogenic fungi, there have been several studies executed to understand the efficiency of transgenicity of these antimicrobial peptides.

Main text

Apart from the intrinsic property of the higher organism for identifying and activating microbial attack defense device, it also involves innate defense mechanism and molecules. In the current review article, apart from the structural and functional characterization of peptides defensin and thionin, we have attempted to provide a succinct overview of the transgenic development of these defense peptides, that are expressed in a constitutive and or over-expressive manner when biotic and abiotic stress inflicted. Transgenic of different peptides show different competence in plants. Most of the transgenic studies made for defensin and thionin revealed the effective transgenic capacity of these peptides.

Conclusion

There have been several studies reported successful development of transgenic plants based on peptides defensin and thionin and observed diverse level of resistance-conferring potency in different plants against phytopathogenic fungi. But due to long regulatory process, there has not been marketed any antimicrobial peptides based transgenic plants yet. However, success report state that possibly in near future transgenic plants of AMPs would be released with devoid of harmful effect, with good efficiency, reproducibility, stability, and least production cost.

Molecular characterization and evaluation of the antibacterial activity of a plant defensin peptide derived from a gene of oat (Avena sativa L.)

Plant defensins are a group of small disulfide-rich cationic peptides that exhibit a broad spectrum of antimicrobial activities. In the present study, an antibacterial plant defensin peptide was successfully identified and characterized from the transcriptome of the oat (Avena sativa L.), and called AsDef1. The complete nucleotide sequence of AsDef1 was determined (321 bp) and found to contain an open reading frame (ORF) encoding a peptide of 77 aa with a putative 22 aa signal peptide sequence that addresses the mature defensin to the apoplast. Further in silico analyses revealed that the structure of the identified defensin (AsDef1) consists of the Knot1 functional domain with eight conserved cysteine residues and four disulfide bonds. The highest expression of AsDef1 was observed in the developing seeds of the A. sativa plant. AsDef1 also showed antibacterial activity against both Gram-positive and Gram-negative bacteria. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values ranged from 0.15625 μM to 0.625 μM. In this study, we identified and characterized an antibacterial defensin from A. sativa for the first time. The findings of the present study offer insights that can be used in producing pathogen-resistant transgenic plants and in developing potential antibacterial agents in the future using AsDef1 from A. sativa.

Analysis of mutations of defensin protein using accelerated molecular dynamics simulations

Plant defensins possess diverse biological functions that include antifungal and antibacterial activities and α-amylase and trypsin inhibitory properties. Two mutations, G9R and V39R, were confirmed to increase the antifungal activity of Raphanus sativus antifungal protein 2 (RsAFP2). Accelerated Molecular Dynamics (aMD) were carried out to examine the conformational changes present in these RsAFP2 mutants, and its two closest homologs compared to the wild-type protein. Specifically, the root mean square fluctuation values for the eight cysteine amino acids involved in the four disulfide bonds were low in the V39R mutant compared to the wild-type. Additionally, analysis of the free energy change revealed that G9R and V39R mutations exert a neutral and stabilizing effect on RsAFP2 conformation, and this is supported by the observed lower total energy of mutants compared to the wild-type, suggesting that enhanced stability of the mutants. However, MD simulations to a longer time scale would aid in capturing more conformational state of the wild-type and mutants defensin protein. Furthermore, the aMD simulations on fungal mimic membranes with RsAFP2 and its mutants and homologs showed that the mutant proteins caused higher deformation and water diffusion than the native RsAFP2, especially the V39R mutant. The mutant variants seem to interact by specifically targeting the POPC and POPI lipids amongst others. This work highlights the stabilizing effect of mutations at the 9^th and 39^th positions of RsAFP2 and their increased membrane deformation activity.

Structure-Function Insights of Jaburetox and Soyuretox: Novel Intrinsically Disordered Polypeptides Derived from Plant Ureases

Intrinsically disordered proteins (IDPs) and intrinsically disordered regions (IDRs) do not have a stable 3D structure but still have important biological activities. Jaburetox is a recombinant peptide derived from the jack bean (Canavalia ensiformis) urease and presents entomotoxic and antimicrobial actions. The structure of Jaburetox was elucidated using nuclear magnetic resonance which reveals it is an IDP with small amounts of secondary structure. Different approaches have demonstrated that Jaburetox acquires certain folding upon interaction with lipid membranes, a characteristic commonly found in other IDPs and usually important for their biological functions. Soyuretox, a recombinant peptide derived from the soybean (Glycine max) ubiquitous urease and homologous to Jaburetox, was also characterized for its biological activities and structural properties. Soyuretox is also an IDP, presenting more secondary structure in comparison with Jaburetox and similar entomotoxic and fungitoxic effects. Moreover, Soyuretox was found to be nontoxic to zebra fish, while Jaburetox was innocuous to mice and rats. This profile of toxicity affecting detrimental species without damaging mammals or the environment qualified them to be used in biotechnological applications. Both peptides were employed to develop transgenic crops and these plants were active against insects and nematodes, unveiling their immense potentiality for field applications.

Dead but Not Dead End: Multifunctional Role of Dead Organs Enclosing Embryos in Seed Biology

Dry fruits consist of two types, dehiscent and indehiscent, whereby the fruit is splitting open or remains closed at maturity, respectively. The seed, the dispersal unit (DU) of dehiscent fruits, is composed of three major parts, the embryo and the food reserve, encapsulated by the maternally-derived organ, the seed coat. Indehiscent fruit constitutes the DU in which the embryo is covered by two protective layers (PLs), the seed coat and the fruit coat. In grasses, the caryopsis, a one-seeded fruit, can be further enclosed by the floral bracts to generate two types of DUs, florets and spikelets. All protective layers enclosing the embryo undergo programmed cell death (PCD) at maturation and are thought to provide mainly a physical shield for embryo protection and a means for dispersal. In this review article, I wish to highlight the elaborate function of these dead organs enclosing the embryo as unique storage structures for beneficial substances and discuss their potential role in seed biology and ecology.

Identification of New Antimicrobial Peptides from Mediterranean Medical Plant Charybdis pancration (Steinh.) Speta

The present work was designed to identify and characterize novel antimicrobial peptides (AMPs) from Charybdis pancration (Steinh.) Speta, previously named Urginea maritima, is a Mediterranean plant, well-known for its biological properties in traditional medicine. Polypeptide-enriched extracts from different parts of the plant (roots, leaves and bulb), never studied before, were tested against two relevant pathogens, Staphylococcus aureus and Pseudomonas aeruginosa. With the aim of identifying novel natural AMPs, peptide fraction displaying antimicrobial activity (the bulb) that showed minimum inhibitory concentration (MICs) equal to 30 µg/mL against the above mentioned strains, was analysed by high-resolution mass spectrometry and database search. Seventeen peptides, related to seven proteins present in the investigated database, were described. Furthermore, we focused on three peptides, which due to their net positive charge, have a better chance to be AMPs and they were investigated by molecular modelling approaches, in order to shed light on the solution properties of their equilibrium structures. Some of new detected peptides could represent a good platform for the development of new antimicrobials in the fight against antibiotic resistance phenomenon.

Characterization and antifungal activity of a plant peptide expressed in the interaction between Capsicum annuum fruits and the anthracnose fungus

Plant defensins are low molecular weight basic peptides ranging from 5 to 7 kDa, with capacity of inhibiting various pathogens, including fungi. They are present in different tissues of plants, including floral parts and fruits of Capsicum sp. The IIF48 extract, present in immature fruits of Capsicum annuum inoculated with C. gloeosporioides, was able to inhibit up to 100% growth ‘in vitro’ of the fungus Colletotrichum gloeosporioides. The main objective of this work was the purification and antifungal activity characterization of a defense-related plant defensin-like isolated of the IIF48 immature fruits extract. The IIF48 extract was subjected to HPLC purification and 13 fractions were obtained, followed by a tricine gel electrophoresis to obtain the protein profile. The different fractions were submitted to a growth inhibition assay against C. gloeosporioides fungus. Fraction 7 (F7) was the most active causing 73% inhibition. Because of the higher F7 activity and the presence of only a peptide of approximately 5 kDa this fraction was subjected to N-terminal sequencing. F7 fraction was carried out plasma membrane permeabilization assays, induction of intracellular ROS production analysis and investigated mitochondrial membrane potential. The F7 fraction showed significant inhibitory activity on the tested fungus, besides promoting membrane permeabilization, induction of endogenous ROS production in Colletotrichum cells and impairing mitochondrial functionality. The first 18 amino acid sequence of the F7 fraction peptide suggests homology to plant-like defensin and was named IIFF7Ca. We also concluded that IIFF7Ca peptide has an effective antimicrobial action against the fungus C. gloeosporioides.

Cysteine-rich antimicrobial peptides from plants: The future of antimicrobial therapy

There has been a spurt in the spread of microbial resistance to antibiotics due to indiscriminate use of antimicrobial agents in human medicine, agriculture, and animal husbandry. It has been realized that conventional antibiotic therapy would be less effective in the coming decades and more emphasis should be given for the development of novel antiinfective therapies. Cysteine rich peptides (CRPs) are broad-spectrum antimicrobial agents that modulate the innate immune system of different life forms such as bacteria, protozoans, fungi, plants, insects, and animals. These are also expressed in several plant tissues in response to invasion by pathogens, and play a crucial role in the regulation of plant growth and development. The present work explores the importance of CRPs as potent antimicrobial agents, which can supplement and/or replace the conventional antibiotics. Different plant parts of diverse plant species showed the presence of antimicrobial peptides (AMPs), which had significant structural and functional diversity. The plant-derived AMPs exhibited potent activity toward a range of plant and animal pathogens, protozoans, insects, and even against cancer cells. The cysteine-rich AMPs have opened new avenues for the use of plants as biofactories for the production of antimicrobials and can be considered as promising antimicrobial drugs in biotherapeutics.

Synthesis, Structure, and Activity of the Antifungal Plant Defensin PvD1

Available treatments for invasive fungal infections have limitations, including toxicity and the emergence of resistant strains. Therefore, there is an urgent need for alternative solutions. Because of their unique mode of action and high selectivity, plant defensins (PDs) are worthy therapeutic candidates. Chemical synthesis remains a preferred method for the production of many peptide-based therapeutics. Given the relatively long sequence of PDs, as well as their complicated posttranslational modifications, the synthetic route can be considered challenging. Here, we describe a total synthesis of PvD₁, the defensin from the common bean Phaseolus vulgaris. Analytical, structural, and functional characterization revealed that both natural and synthetic peptides fold into a canonical CSαβ motif stabilized by conserved disulfide bonds. Moreover, synthetic PvD₁ retained the biological activity against four different Candida species and showed no toxicity in vivo. Adding the high resistance of synthetic PvD₁ to proteolytic degradation, we claim that conditions are now met to consider PDs druggable biologicals.