Structure-activity determinants in antifungal plant defensins MsDef1 and MtDef4 with different modes of action against Fusarium graminearum

Characterization and functional analysis of gerbera plant defensin (PDF) genes reveal the role of GhPDF2.4 in defense against the root rot pathogen Phytophthora cryptogea

Gerbera (Gerbera hybrida), a major fresh cut flower crop, is very susceptible to root rot disease. Although plant defensins (PDFs), a major group of plant antimicrobial peptides, display broad-spectrum antifungal and antibacterial activities, PDF genes in gerbera have not been systematically characterized. Here, we identified and cloned nine PDF genes from gerbera and divided them into two classes based on phylogenetic analysis. Most Class I GhPDF genes were highly expressed in petioles, whereas all Class II GhPDF genes were highly expressed in roots. Phytophthora cryptogea inoculation strongly upregulated all Class II GhPDF genes in roots and upregulated all Class I GhPDF genes in petioles. Transient overexpression of GhPDF1.5 and GhPDF2.4 inhibited P. cryptogea infection in tobacco (Nicotiana benthamiana) leaves. Transient overexpression of GhPDF2.4, but not GhPDF1.5, significantly upregulated ACO and LOX gene expression in tobacco leaves, indicating that overexpressing GhPDF2.4 activated the jasmonic acid/ethylene defense pathway and that the two types of GhPDFs have different modes of action. Prokaryotically expressed recombinant GhPDF2.4 inhibited mycelial growth and delayed the hyphal swelling of P. cryptogea, in vitro, indicating that GhPDF2.4 is a morphogenetic defensin. Moreover, the addition of GhPDF2.4 to plant culture medium alleviated the root rot symptoms of in vitro-grown gerbera seedlings and greatly reduced pathogen titer in P. cryptogea-inoculated gerbera roots in the early stages of treatment. Our study provides a basis for the use of GhPDFs, especially GhPDF2.4, for controlling root rot disease in gerbera.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42994-024-00146-8.

Update on the Basic Understanding of Fusarium graminearum Virulence Factors in Common Wheat Research

Wheat is one of the most important food crops, both in China and worldwide. Wheat production is facing extreme stresses posed by different diseases, including Fusarium head blight (FHB), which has recently become an increasingly serious concerns. FHB is one of the most significant and destructive diseases affecting wheat crops all over the world. Recent advancements in genomic tools provide a new avenue for the study of virulence factors in relation to the host plants. The current review focuses on recent progress in the study of different strains of Fusarium infection. The presence of genome-wide repeat-induced point (RIP) mutations causes genomic mutations, eventually leading to host plant susceptibility against Fusarium invasion. Furthermore, effector proteins disrupt the host plant resistance mechanism. In this study, we proposed systematic modification of the host genome using modern biological tools to facilitate plant resistance against foreign invasion. We also suggested a number of scientific strategies, such as gene cloning, developing more powerful functional markers, and using haplotype marker-assisted selection, to further improve FHB resistance and associated breeding methods.

Modes of action and potential as a peptide-based biofungicide of a plant defensin MtDef4

Due to rapidly emerging resistance to single-site fungicides in fungal pathogens of plants, there is a burgeoning need for safe and multisite fungicides. Plant antifungal peptides with multisite modes of action (MoA) have potential as bioinspired fungicides. Medicago truncatula defensin MtDef4 was previously reported to exhibit potent antifungal activity against fungal pathogens. Its MoA involves plasma membrane disruption and binding to intracellular targets. However, specific biochemical processes inhibited by this defensin and causing cell death have not been determined. Here, we show that MtDef4 exhibited potent antifungal activity against Botrytis cinerea. It induced severe plasma membrane and organelle irregularities in the germlings of this pathogen. It bound to fungal ribosomes and inhibited protein translation in vitro. A MtDef4 variant lacking antifungal activity exhibited greatly reduced protein translation inhibitory activity. A cation-tolerant MtDef4 variant was generated that bound to β-glucan of the fungal cell wall with higher affinity than MtDef4. It also conferred a greater reduction in the grey mould disease symptoms than MtDef4 when applied exogenously on Nicotiana benthamiana plants, tomato fruits and rose petals. Our findings revealed inhibition of protein synthesis as a likely target of MtDef4 and the potential of its cation-tolerant variant as a peptide-based fungicide.

Transcriptome-Wide Identification and Expression Analysis of Genes Encoding Defense-Related Peptides of Filipendula ulmaria in Response to Bipolaris sorokiniana Infection

Peptides play an essential role in plant development and immunity. Filipendula ulmaria, belonging to the Rosaceae family, is a medicinal plant which exhibits valuable pharmacological properties. F. ulmaria extracts in vitro inhibit the growth of a variety of plant and human pathogens. The role of peptides in defense against pathogens in F. ulmaria remains unknown. The objective of this study was to explore the repertoire of antimicrobial (AMPs) and defense-related signaling peptide genes expressed by F. ulmaria in response to infection with Bipolaris sorokiniana using RNA-seq. Transcriptomes of healthy and infected plants at two time points were sequenced on the Illumina HiSeq500 platform and de novo assembled. A total of 84 peptide genes encoding novel putative AMPs and signaling peptides were predicted in F. ulmaria transcriptomes. They belong to known, as well as new, peptide families. Transcriptional profiling in response to infection disclosed complex expression patterns of peptide genes and identified both up- and down-regulated genes in each family. Among the differentially expressed genes, the vast majority were down-regulated, suggesting suppression of the immune response by the fungus. The expression of 13 peptide genes was up-regulated, indicating their possible involvement in triggering defense response. After functional studies, the encoded peptides can be used in the development of novel biofungicides and resistance inducers.

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.

Insights into the Adsorption Mechanisms of the Antimicrobial Peptide CIDEM-501 on Membrane Models

CIDEM-501 is a hybrid antimicrobial peptide rationally designed based on the structure of panusin and panulirin template peptides. The new peptide exhibits significant antibacterial activity against multidrug-resistant pathogens (MIC = 2-4 μM) while conserving no toxicity in human cell lines. We conducted molecular dynamics (MD) simulations using the CHARMM-36 force field to explore the CIDEM-501 adsorption mechanism with different membrane compositions. Several parameters that characterize these interactions were analyzed to elucidate individual residues' structural and thermodynamic contributions. The membrane models were constructed using CHARMM-GUI, mimicking the bacterial and eukaryotic phospholipid compositions. Molecular dynamics simulations were conducted over 500 ns, showing rapid and highly stable peptide adsorption to bacterial lipids components rather than the zwitterionic eucaryotic model membrane. A predominant peptide orientation was observed in all models dominated by an electric dipole. The peptide remained parallel to the membrane surface with the center loop oriented to the lipids. Our findings shed light on the antibacterial activity of CIDEM-501 on bacterial membranes and yield insights valuable for designing potent antimicrobial peptides targeting multi- and extreme drug-resistant bacteria.

Bioinformatic analysis of wheat defensin gene family and function verification of candidate genes

Plant defensins are widely distributed in the leaves, fruits, roots, stems, seeds, and tubers. Research shows that defensin in plants play a significant role in physiological metabolism, growth and development. Plant defensins can kill and suppress a variety of pathogenic bacteria. In this study, we understand the phylogenetic relationships, protein characterization, chromosomal localization, promoter and gene structural features of the TaPDFs family through sequence alignment and conserved protein structural domain analysis. A total of 73 PDF gene members in wheat, 15 PDF genes in maize, and 11 PDF genes in rice were identified. A total of 35, 65, and 34 PDF gene members were identified in the genomes of Ae. tauschii, T. urartu, and T. dicoccoides, respectively. TaPDF4.9 and TaPDF2.15 were constructed into pART27 vector with YFP by homologous recombination for subcellular localization analysis. Subcellular localization results showed that TaPDF4.9 and TaPDF2.15 were basically located in the cell membrane and cytoplasm, and TaPDF4.9 was also located in the nucleus. TaPDF4.9 and TaPDF2.15 could inhibit the infection of Phytophthora infestans strain '88069'. The results suggest that TaPDFs may be able to improve disease resistance. The study of wheat defensins will be beneficial for improving wheat yield and provides a theoretical basis for research on resistance to wheat diseases.

Plant defensin MtDef4-derived antifungal peptide with multiple modes of action and potential as a bio-inspired fungicide

Chemical fungicides have been instrumental in protecting crops from fungal diseases. However, increasing fungal resistance to many of the single-site chemical fungicides calls for the development of new antifungal agents with novel modes of action (MoA). The sequence-divergent cysteine-rich antifungal defensins with multisite MoA are promising starting templates for design of novel peptide-based fungicides. Here, we experimentally tested such a set of 17-amino-acid peptides containing the γ-core motif of the antifungal plant defensin MtDef4. These designed peptides exhibited antifungal properties different from those of MtDef4. Focused analysis of a lead peptide, GMA4CG_V6, showed that it was a random coil in solution with little or no secondary structure elements. Additionally, it exhibited potent cation-tolerant antifungal activity against the plant fungal pathogen Botrytis cinerea, the causal agent of grey mould disease in fruits and vegetables. Its multisite MoA involved localization predominantly to the plasma membrane, permeabilization of the plasma membrane, rapid internalization into the vacuole and cytoplasm, and affinity for the bioactive phosphoinositides phosphatidylinositol 3-phosphate (PI3P), PI4P, and PI5P. The sequence motif RRRW was identified as a major determinant of the antifungal activity of this peptide. While topical spray application of GMA4CG_V6 on Nicotiana benthamiana and tomato plants provided preventive and curative suppression of grey mould disease symptoms, the peptide was not internalized into plant cells. Our findings open the possibility that truncated and modified defensin-derived peptides containing the γ-core sequence could serve as promising candidates for further development of bio-inspired fungicides.

Medicago Sativa Defensin1 as a tumor sensitizer for improving chemotherapy: translation from anti-fungal agent to a potential anti-cancer agent

Plant defensins including Medicago Sativa defensin 1 (MsDef1) are cysteine-rich antifungal peptides which are known for potent broad-spectrum antifungal activity against bacterial or fungal pathogens of plants. The antimicrobial activities of these cationic defensins are attributed to their capacity to bind to cell membranes to create potentially structural defects tin the cell membranes to interact with intracellular target (s) and mediates cytotoxic effects. Our earlier work identified Glucosylceramide (GlcCer) of fungus F. graminearum as a potential target for biological activity. Multi-drug resistant (MDR) cancer cells overexpress GlcCer on the surface of plasma membrane. Hence, MsDef1 may have a potential to bind to GlcCer of MDR cancer cells to induce cell death. We have characterized the three-dimensional structure of MsDef1 and the solution dynamics using of 15N-labeled MsDef1 nuclear magnetic resonance (NMR) spectroscopy which showed that GlcCer binds MsDef1 at two specific sites on the peptide molecule. The ability of MsDef1 to permeate MDR cancer cells was demonstrated by measuring the release of apoptotic ceramide in drug resistant MCF-7R cells. It was also shown that MsDef1 activated dual cell death pathways ceramide and Apoptosis Stimulating Kinase ASK1 by disintegrating GlcCer and oxidizing tumor specific biomarker thioredoxin (Trx) respectively. As a result, MsDef1 sensitizes MDR cancer cells to evoke a better response from Doxorubicin, a front-line chemotherapy for triple negative breast cancer (TNBC) treatment. The combination of MsDef1 and Doxorubicin induced 5 to10-fold greater apoptosis in vitro MDR cells MDA-MB-231R compared to either MsDef1 or Doxorubicin alone. Confocal microscopy revealed that MsDef1 facilitates a) influx of Doxorubicin in MDR cancer cells, b) preferential uptake by MDR cells but not by normal fibroblasts and breast epithelial cells (MCF-10A). These results suggest that MsDef1 targets MDR cancer cells and may find utility as a neoadjuvant chemotherapy. Hence, the extension of antifungal properties of MsDef1 to cancer my result in addressing the MDR problems in cancer.

Rationally designed antifungal protein chimeras reveal new insights into structure-activity relationship

Antifungal proteins (AFPs) are promising antimicrobial compounds that represent a feasible alternative to fungicides. Penicillium expansum encodes three phylogenetically distinct AFPs (PeAfpA, PeAfpB and PeAfpC) which show different antifungal profiles and fruit protection effects. To gain knowledge about the structural determinants governing their activity, we solved the crystal structure of PeAfpB and rationally designed five PeAfpA::PeAfpB chimeras (chPeAFPV1-V5). Chimeras showed significant differences in their antifungal activity. chPeAFPV1 and chPeAFPV2 improved the parental PeAfpB potency, and it was very similar to that of PeAfpA. chPeAFPV4 and chPeAFPV5 showed an intermediate profile of activity compared to the parental proteins while chPeAFPV3 was inactive towards most of the fungi tested. Structural analysis of the chimeras evidenced an identical scaffold to PeAfpB, suggesting that the differences in activity are due to the contributions of specific residues and not to induced conformational changes or structural rearrangements. Results suggest that mannoproteins determine protein interaction with the cell wall and its antifungal activity while there is not a direct correlation between binding to membrane phospholipids and activity. This work provides new insights about the relevance of sequence motifs and the feasibility of modifying protein specificity, opening the door to the rational design of chimeras with biotechnological applicability.

The γ-Core Motif Peptides of Plant AMPs as Novel Antimicrobials for Medicine and Agriculture

The γ-core motif is a structural element shared by most host antimicrobial peptides (AMPs), which is supposed to contribute to their antimicrobial properties. In this review, we summarized the available data on the γ-core peptides of plant AMPs. We describe γ-core peptides that have been shown to exhibit inhibitory activity against plant and human bacterial and fungal pathogens that make them attractive scaffolds for the development of novel anti-infective agents. Their advantages include origin from natural AMP sequences, broad-spectrum and potent inhibitory activity, and cost-effective production. In addition, some γ-core peptides combine antimicrobial and immunomodulatory functions, thus broadening the spectrum of practical applications. Some act synergistically with antimycotics and fungicides, so combinations of peptides with conventionally used antifungal agents can be suggested as an effective strategy to reduce the doses of potentially harmful chemicals. The presented information will pave the way for the design of novel antimicrobials on the basis of γ-core motif peptides, which can find application in medicine and the protection of crops from diseases.

Histidine 19 Residue Is Essential for Cell Internalization of Antifungal Peptide SmAPα1-21 Derived from the α-Core of the Silybum marianum Defensin DefSm2-D in Fusarium graminearum

The synthetic peptide SmAP_α1-21 (KLCEKPSKTWFGNCGNPRHCG) derived from DefSm2-D defensin α-core is active at micromolar concentrations against the phytopathogenic fungus Fusarium graminearum and has a multistep mechanism of action that includes alteration of the fungal cell wall and membrane permeabilization. Here, we continued the study of this peptide’s mode of action and explored the correlation between the biological activity and its primary structure. Transmission electron microscopy was used to study the ultrastructural effects of SmAP_α1-21 in conidial cells. New peptides were designed by modifying the parent peptide SmAP_α1-21 (SmAPH19R and SmAPH19A, where His19 was replaced by Arg or Ala, respectively) and synthesized by the Fmoc solid phase method. Antifungal activity was determined against F. graminearum. Membrane permeability and subcellular localization in conidia were studied by confocal laser scanning microscopy (CLSM). Reactive oxygen species (ROS) production was assessed by fluorescence spectroscopy and CLSM. SmAP_α1-21 induced peroxisome biogenesis and oxidative stress through ROS production in F. graminearum and was internalized into the conidial cells’ cytoplasm. SmAPH19R and SmAPH19A were active against F. graminearum with minimal inhibitory concentrations (MICs) of 38 and 100 µM for SmAPH19R and SmAPH19A, respectively. The replacement of His19 by Ala produced a decrease in the net charge with a significant increase in the MIC, thus evidencing the importance of the positive charge in position 19 of the antifungal peptide. Like SmAP_α1-21, SmAP2H19A and SmAP2H19R produced the permeabilization of the conidia membrane and induced oxidative stress through ROS production. However, SmAPH19R and SmAPH19A were localized in the conidia cell wall. The replacement of His19 by Ala turned all the processes slower. The extracellular localization of peptides SmAPH19R and SmAPH19A highlights the role of the His19 residue in the internalization.

Plant-Derived Antimicrobial Peptides: Novel Preservatives for the Food Industry

Food spoilage is a widespread issue brought on by the undesired growth of microbes in food products. Thousands of tons of usable food or food products are wasted every day due to rotting in different parts of the world. Several food preservation techniques are employed to prevent food from rotting, including the use of natural or manufactured chemicals or substances; however, the issue persists. One strategy for halting food deterioration is the use of plant-derived antimicrobial peptides (AMPs), which have been investigated for possible bioactivities against a range of human, plant, and food pathogens. The food industry may be able to benefit from the development of synthetic AMPs, produced from plants that have higher bioactivity, better stability, and decreased cytotoxicity as a means of food preservation. In order to exploit plant-derived AMPs in various food preservation techniques, in this review, we also outline the difficulties in developing AMPs for use as commercial food preservatives. Nevertheless, as technology advances, it will soon be possible to fully explore the promise of plant-derived AMPs as food preservatives.

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.

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.

Seed-derived defensins from Scots pine: structural and functional features

The recombinant PsDef5.1 defensin inhibits the growth of phytopathogenic fungi, Gram-positive and Gram-negative bacteria, and human pathogen Candida albicans. Expression of seed-derived Scots pine defensins is tissue-specific and developmentally regulated. Plant defensins are ubiquitous antimicrobial peptides that possess a broad spectrum of activities and multi-functionality. The genes for these antimicrobial proteins form a multigenic family in the plant genome and are expressed in every organ. Most of the known defensins have been isolated from seeds of various monocot and dicot species, but seed-derived defensins have not yet been characterized in gymnosperms. This study presents the isolation of two new 249 bp cDNA sequences from Scots pine seeds with 97.9% nucleotide homology named PsDef5.1 and PsDef5.2. Their deduced amino acid sequences have typical plant defensin features, including an endoplasmic reticulum signal sequence of 31 amino acids (aa), followed by a characteristic defensin domain of 51 aa. To elucidate the functional activity of new defensins, we expressed the mature form of PsDef5.1 in a prokaryotic system. The purified recombinant peptide exhibited activity against the phytopathogenic fungi and Gram-negative and Gram-positive bacteria with the IC₅₀ of 5-18 µM. Moreover, it inhibited the growth of the human pathogen Candida albicans with the IC₅₀ of 6.0 µM. Expression analysis showed that transcripts of PsDef5.1-2 genes were present in immature and mature pine seeds and different parts of seedlings at the early stage of germination. In addition, unlike the PsDef5.2, the PsDef5.1 gene was expressed in the reproductive organs. Our findings indicate that novel defensins are promising candidates for transgenic application and the development of new antimicrobial drugs.

Synthetic Oligopeptides Mimicking γ-Core Regions of Cysteine-Rich Peptides of Solanum lycopersicum Possess Antimicrobial Activity against Human and Plant Pathogens

Plant cysteine-rich peptides (CRPs) represent a diverse group of molecules involved in different aspects of plant physiology. Antimicrobial peptides, which directly suppress the growth of pathogens, are regarded as promising templates for the development of next-generation pharmaceuticals and ecologically friendly plant disease control agents. Their oligopeptide fragments are even more promising because of their low production costs. The goal of this work was to explore the antimicrobial activity of nine short peptides derived from the γ-core-containing regions of tomato CRPs against important plant and human pathogens. We discovered antimicrobial activity in peptides derived from the defensin-like peptides, snakins, and MEG, which demonstrates the direct involvement of these CRPs in defense reactions in tomato. The CRP-derived short peptides appeared particularly active against the gram-positive bacterium Clavibacter michiganensis, which causes bacterial wilt—opening up new possibilities for their use in agriculture to control this dangerous disease. Furthermore, high inhibitory potency of short oligopeptides was demonstrated against the yeast Cryptococcus neoformans, which causes serious diseases in humans, making these peptide molecules promising candidates for the development of next-generation pharmaceuticals. Studies of the mode of action of the two most active peptides indicate fungal membrane permeabilization as a mechanism of antimicrobial action.

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.

Identification of a crocodylian β-defensin variant from Alligator mississippiensis with antimicrobial and antibiofilm activity

β-defensin host defense peptides are important components of the innate immune system of vertebrates. Although evidence of their broad antimicrobial, antibiofilm and immunomodulatory activities in mammals have been presented, β-defensins from other vertebrate species, like crocodylians, remain largely unexplored. In this study, five new crocodylian β-defensin variants from Alligator mississippiensis and Crocodylus porosus were selected for synthesis and characterization based on their charge and hydrophobicity values. Linear peptides were synthesized, folded, purified and then evaluated for their antimicrobial and antibiofilm activities against the bacterial pathogens, Salmonella enterica serovar Typhimurium, Staphylococcus aureus, Enterobacter cloacae and Acinetobacter baumannii. The Am23SK variant (SCRFSGGYCIWNWERCRSGHFLVALCPFRKRCCK) from A. mississippiensis displayed promising activity against both planktonic cells and bacterial biofilms, outperforming the human β-defensin 3 under the experimental conditions. Moreover, Am23SK exhibited no cytotoxicity towards mammalian cells and exerted immunomodulatory effects in vitro, moderately suppressing the production of proinflammatory mediators from stimulated human bronchial epithelial cells. Overall, our results have expanded the activity landscape of crocodylian and reptilian β-defensin in general.

Molecular Insights into the Role of Cysteine-Rich Peptides in Induced Resistance to Fusarium oxysporum Infection in Tomato Based on Transcriptome Profiling

Cysteine-rich peptides (CRPs) play an important role in plant physiology. However, their role in resistance induced by biogenic elicitors remains poorly understood. Using whole-genome transcriptome sequencing and our CRP search algorithm, we analyzed the repertoire of CRPs in tomato Solanum lycopersicum L. in response to Fusarium oxysporum infection and elicitors from F. sambucinum. We revealed 106 putative CRP transcripts belonging to different families of antimicrobial peptides (AMPs), signaling peptides (RALFs), and peptides with non-defense functions (Major pollen allergen of Olea europaea (Ole e 1 and 6), Maternally Expressed Gene (MEG), Epidermal Patterning Factor (EPF)), as well as pathogenesis-related proteins of families 1 and 4 (PR-1 and 4). We discovered a novel type of 10-Cys-containing hevein-like AMPs named SlHev1, which was up-regulated both by infection and elicitors. Transcript profiling showed that F. oxysporum infection and F. sambucinum elicitors changed the expression levels of different overlapping sets of CRP genes, suggesting the diversification of functions in CRP families. We showed that non-specific lipid transfer proteins (nsLTPs) and snakins mostly contribute to the response of tomato plants to the infection and the elicitors. The involvement of CRPs with non-defense function in stress reactions was also demonstrated. The results obtained shed light on the mode of action of F. sambucinum elicitors and the role of CRP families in the immune response in tomato.

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.

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.

Preliminary investigations on the pathogenesis-related protein expression profile of the medicinal herb Macleaya cordata and anti-bacterial properties of recombinant proteins

The plant pathogenesis-related (PR) proteins play a crucial role in the defense of plants against pathogens and orchestrate the innate immune system of plants. In this paper, a non-normalized cDNA library of the leaf was constructed to obtain a comprehensive view of PR proteins of Macleaya cordata. Specifically, 511 expressed sequence tags (ESTs) were generated using Sanger sequencing. All ESTs were assembled into 364 non-redundancy sequences, including 78 clusters and 286 singlets. The PR protein expression profile of the medicinal herb M. cordata has been investigated and is represented by defensin, lipid-transfer protein, (S)-norcoclaurine synthase, and major allergen protein, suggesting that the herb contains rich active proteins against pathogens. Furthermore, two defensins were selected for recombinant expression in yeast, and the antimicrobial activities were explored. Since they both present a broad antimicrobial spectrum, they are of particular importance for agricultural and medicinal applications. Our study describes defensins in Papaveraceae for the first time and provides novel insights into the effective components. In addition to the alkaloids, PR proteins (such as defensins, lipid transfer proteins, (S) - norcoclaurine synthase, major allergen protein, and Class IV chitinases) are involved in the antibacterial and anti-inflammatory activities of M. cordata.

Cassava (Manihot esculenta) defensins: Prospection, structural analysis and tissue-specific expression under biotic/abiotic stresses

Defensins are a prominent family of antimicrobial peptides. They play sophisticated roles in the defense against pathogens in all living organisms, but few works address their expression under different conditions and plant tissues. The present work prospected defensins of Manihot esculenta Crantz, popularly known as cassava. Five defensin candidates (MeDefs) were retrieved from the genome sequences and characterized. Considering chromosome distribution, only MeDef1 and 2 occupy adjacent positions in the same chromosome arm. All 3D structures had antiparallel ß-sheets, an α-helix, and amphipathic residues distributed throughout the peptides with a predominance of cationic surface charge. MeDefs expression was validated by RT-qPCR, including two stress types (biotic: fungus Macrophomina pseudophaseolina, and abiotic: mechanical injury) and a combination of both stresses (fungus+injury) in three different tissues (root, stem, and leaf). For this purpose, ten reference genes (RGs) were tested, and three were chosen to characterize MeDef expression. MeDef3 was up-regulated at roots in all stress situations tested. MeDef1 and MeDef5 were induced in leaves under biotic and abiotic stresses, but not in both stress types simultaneously. Only MeDef2 was down-regulated in the stem tissue also with biotic/abiotic combined stresses. These results indicate that although defensins are known to be responsive to pathogen infection, they may act as preformed defense or, still, have tissue or stress specificities. Aspects of their structure, stability and evolution are also discussed.

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.

A novel defensin-like peptide contributing to antimicrobial and antioxidant capacity of the tick Dermacentor silvarum (Acari: Ixodidae)

Defensins are the most diverse groups of antimicrobial peptides in invertebrate animals. In ticks, defensins show great potential as targets for tick control, and display future prospect for therapeutic drug development. In the present study, a novel defensin-like gene (Ds-defensin) contributing to the antimicrobial and antioxidant capacity of the tick Dermacentor silvarum was characterized. The full-length of the Ds-defensin gene was 382 bp, which displayed tissue-specific expression and was highly abundant in the salivary glands and carcasses of the adults. It encodes a 71-amino acid defensin-like protein, and the protein precursor is characterized by a 22-amino acid signal peptide and a 34-amino acid mature peptide. The peptide displayed potent activity against most of the tested gram-positive bacteria, including Staphylococcus aureus, S. carnosus and Nocardia asteroides, and one tested gram-negative bacterium, Psychrobacter faecalis. Scanning electron microscopy revealed that the cell wall and surface of treated bacteria became rough and gradually formed pores after a 30-min exposure to the Ds-defensin peptide. Additionally, the peptide also showed significant antioxidant capacity. The above results implied that the defensin-like peptide may play an important role in tick defense and the interaction with microorganisms.

Design of improved synthetic antifungal peptides with targeted variations in charge, hydrophobicity and chirality based on a correlation study between biological activity and primary structure of plant defensin γ-cores

Microbial resistance to available drugs is a growing health threat imposing the need for the development of new drugs. The scaffold of plant defensins, including their γ-cores, are particularly good candidates for drug design. This work aimed to improve the antifungal activity of a previous design peptide, named A₃₆,₄₂,₄₄γ_32-46VuDef (for short DD) against yeasts by altering its biochemical parameters. We explore the correlation of the biological activity and structure of plant defensins and compared their primary structures by superimposition with VuDef₁ and DD which indicated us the favorable position and the amino acid to be changed. Three new peptides with modifications in charge, hydrophobicity (RR and WR) and chirality (D-RR) were designed and tested against pathogenic yeasts. Inhibition was determined by absorbance. Viability of mammalian cells was determined by MTT. The three designed peptides had better inhibitory activity against the yeasts with better potency and spectrum of yeast species inhibition, with low toxicity to mammalian cells. WR, the most hydrophobic and cationic, exhibited better antifungal activity and lower toxicity. Our study provides experimental evidence that targeted changes in the primary structure of peptides based on plant defensins γ-core primary structures prove to be a good tool for the synthesis of new compounds that may be useful as alternative antifungal drugs. The method described did not have the drawback of synthesis of several peptides, because alterations are guided. When compared to other methods, the design process described is efficient and viable to those with scarce resources.

Plant compounds for the potential reduction of food waste - a focus on antimicrobial peptides

A large portion of global food waste is caused by microbial spoilage. The modern approach to preserve food is to apply different hurdles for microbial pathogens to overcome. These vary from thermal processes and chemical additives, to the application of irradiation and modified atmosphere packaging. Even though such preservative techniques exist, loss of food to spoilage still prevails. Plant compounds and peptides represent an untapped source of potential novel natural food preservatives. Of these, antimicrobial peptides (AMPs) are very promising for exploitation. AMPs are a significant component of a plant's innate defense system. Numerous studies have demonstrated the potential application of these AMPs; however, more studies, particularly in the area of food preservation are warranted. This review examines the literature on the application of AMPs and other plant compounds for the purpose of reducing food losses and waste (including crop protection). A focus is placed on the plant defensins, their natural extraction and synthetic production, and their safety and application in food preservation. In addition, current challenges and impediments to their full exploitation are discussed.

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.

Hevein-Like Antimicrobial Peptides Wamps: Structure-Function Relationship in Antifungal Activity and Sensitization of Plant Pathogenic Fungi to Tebuconazole by WAMP-2-Derived Peptides

Keywords: hevein-like antimicrobial peptides; antifungal activity; antifungal determinants; synergy; chemosensitization; tebuconazole; plant pathogenic fungi.

Molecular and Biological Properties of Snakins: The Foremost Cysteine-Rich Plant Host Defense Peptides

Plant host defense peptides (HDPs), also known as antimicrobial peptides (AMPs), are regarded as one of the most prevalent barriers elaborated by plants to combat various infective agents. Among the multiple classes of HDPs, the Snakin class attracts special concern, as they carry 12 cysteine residues, being the foremost cysteine-rich peptides of the plant HDPs. Also, their cysteines are present at very highly conserved positions and arranged in an extremely similar way among different members. Like other plant HDPs, Snakins have been shown to exhibit strong antifungal and antibacterial activity against a wide range of plant pathogens. Moreover, they display diversified biological activities in many aspects of plant growth and the development process. This review is devoted to present the general characters of the Snakin class of plant HDPs, as well as the individual features of different Snakin family members. Specifically, the sequence properties, spatial structures, distributions, expression patterns and biological activities of Snakins are described. In addition, further detailed classification of the Snakin family members, along with their possible mode of action and potential applications in the field of agronomy and pathology are discussed.

Developmentally regulated activation of defense allows for rapid inhibition of infection in age-related resistance to Phytophthora capsici in cucumber fruit

Background

Age-related resistance (ARR) is a developmentally regulated phenomenon conferring resistance to pathogens or pests. Although ARR has been observed in several host-pathogen systems, the underlying mechanisms are largely uncharacterized. In cucumber, rapidly growing fruit are highly susceptible to Phytophthora capsici but become resistant as they complete exponential growth. We previously demonstrated that ARR is associated with the fruit peel and identified gene expression and metabolomic changes potentially functioning as preformed defenses.

Results

Here, we compare the response to infection in fruit at resistant and susceptible ages using microscopy, quantitative bioassays, and weighted gene co-expression analyses. We observed strong transcriptional changes unique to resistant aged fruit 2–4 h post inoculation (hpi). Microscopy and bioassays confirmed this early response, with evidence of pathogen death and infection failure as early as 4 hpi and cessation of pathogen growth by 8–10 hpi. Expression analyses identified candidate genes involved in conferring the rapid response including those encoding transcription factors, hormone signaling pathways, and defenses such as reactive oxygen species metabolism and phenylpropanoid biosynthesis.

Conclusion

The early pathogen death and rapid defense response in resistant-aged fruit provide insight into potential mechanisms for ARR, implicating both pre-formed biochemical defenses and developmentally regulated capacity for pathogen recognition as key factors shaping age-related resistance.

The potential use of the Penicillium chrysogenum antifungal protein PAF, the designed variant PAFopt and its γ-core peptide Pγopt in plant protection

The prevention of enormous crop losses caused by pesticide-resistant fungi is a serious challenge in agriculture. Application of alternative fungicides, such as antifungal proteins and peptides, provides a promising basis to overcome this problem; however, their direct use in fields suffers limitations, such as high cost of production, low stability, narrow antifungal spectrum and toxicity on plant or mammalian cells. Recently, we demonstrated that a Penicillium chrysogenum-based expression system provides a feasible tool for economic production of P. chrysogenum antifungal protein (PAF) and a rational designed variant (PAFopt ), in which the evolutionary conserved γ-core motif was modified to increase antifungal activity. In the present study, we report for the first time that γ-core modulation influences the antifungal spectrum and efficacy of PAF against important plant pathogenic ascomycetes, and the synthetic γ-core peptide Pγopt , a derivative of PAFopt , is antifungal active against these pathogens in vitro. Finally, we proved the protective potential of PAF against Botrytis cinerea infection in tomato plant leaves. The lack of any toxic effects on mammalian cells and plant seedlings, as well as the high tolerance to harsh environmental conditions and proteolytic degradation further strengthen our concept for applicability of these proteins and peptide in agriculture.

Plant Defensins from a Structural Perspective

Plant defensins form a family of proteins with a broad spectrum of protective activities against fungi, bacteria, and insects. Furthermore, some plant defensins have revealed anticancer activity. In general, plant defensins are non-toxic to plant and mammalian cells, and interest in using them for biotechnological and medicinal purposes is growing. Recent studies provided significant insights into the mechanisms of action of plant defensins. In this review, we focus on structural and dynamics aspects and discuss structure-dynamics-function relations of plant defensins.

Defensins of Grasses: A Systematic Review

The grass family (Poaceae) is one of the largest families of flowering plants, growing in all climatic zones of all continents, which includes species of exceptional economic importance. The high adaptability of grasses to adverse environmental factors implies the existence of efficient resistance mechanisms that involve the production of antimicrobial peptides (AMPs). Of plant AMPs, defensins represent one of the largest and best-studied families. Although wheat and barley seed γ-thionins were the first defensins isolated from plants, the functional characterization of grass defensins is still in its infancy. In this review, we summarize the current knowledge of the characterized defensins from cultivated and selected wild-growing grasses. For each species, isolation of defensins or production by heterologous expression, peptide structure, biological activity, and structure-function relationship are described, along with the gene expression data. We also provide our results on in silico mining of defensin-like sequences in the genomes of all described grass species and discuss their potential functions. The data presented will form the basis for elucidation of the mode of action of grass defensins and high adaptability of grasses to environmental stress and will provide novel potent molecules for practical use in medicine and agriculture.

Plant defensin antibacterial mode of action against Pseudomonas species

BACKGROUND

Though many plant defensins exhibit antibacterial activity, little is known about their antibacterial mode of action (MOA). Antimicrobial peptides with a characterized MOA induce the expression of multiple bacterial outer membrane modifications, which are required for resistance to these membrane-targeting peptides. Mini-Tn5-lux mutant strains of Pseudomonas aeruginosa with Tn insertions disrupting outer membrane protective modifications were assessed for sensitivity against plant defensin peptides. These transcriptional lux reporter strains were also evaluated for lux gene expression in response to sublethal plant defensin exposure. Also, a plant pathogen, Pseudomonas syringae pv. syringae was modified through transposon mutagenesis to create mutants that are resistant to in vitro MtDef4 treatments.

RESULTS

Plant defensins displayed specific and potent antibacterial activity against strains of P. aeruginosa. A defensin from Medicago truncatula, MtDef4, induced dose-dependent gene expression of the aminoarabinose modification of LPS and surface polycation spermidine production operons. The ability for MtDef4 to damage bacterial outer membranes was also verified visually through fluorescent microscopy. Another defensin from M. truncatula, MtDef5, failed to induce lux gene expression and limited outer membrane damage was detected with fluorescent microscopy. The transposon insertion site on MtDef4 resistant P. syringae pv. syringae mutants was sequenced, and modifications of ribosomal genes were identified to contribute to enhanced resistance to plant defensin treatments.

CONCLUSIONS

MtDef4 damages the outer membrane similar to polymyxin B, which stimulates antimicrobial peptide resistance mechanisms to plant defensins. MtDef5, appears to have a different antibacterial MOA. Additionally, the MtDef4 antibacterial mode of action may also involve inhibition of translation.

Biofungicidal Potential of Neosartorya (Aspergillus) Fischeri Antifungal Protein NFAP and Novel Synthetic γ-Core Peptides

Because of enormous crop losses worldwide due to pesticide-resistant plant pathogenic fungi, there is an increasing demand for the development of novel antifungal strategies in agriculture. Antifungal proteins (APs) and peptides are considered potential biofungicides; however, several factors limit their direct agricultural application, such as the high cost of production, narrow antifungal spectrum, and detrimental effects to plant development and human/animal health. This study evaluated the safety of the application of APs and peptides from the ascomycete Neosartorya fischeri as crop preservatives. The full-length N. fischeri AP (NFAP) and novel rationally designed γ-core peptide derivatives (PDs) γNFAP-opt and γNFAP-optGZ exhibited efficacy by inhibiting the growth of the agriculturally relevant filamentous ascomycetes in vitro. A high positive net charge, however, neither the hydrophilicity nor the primary structure supported the antifungal efficacy of these PDs. Further testing demonstrated that the antifungal activity did not require a conformational change of the β-pleated NFAP or the canonically ordered conformation of the synthetic PDs. Neither hemolysis nor cytotoxicity was observed when the NFAP and γNFAP-opt were applied at antifungally effective concentrations in human cell lines. Similarly, the Medicago truncatula plants that served as toxicity model and were grown from seedlings that were treated with NFAP, γNFAP-opt, or γNFAP-optGZ failed to exhibit morphological aberrations, reduction in primary root length, or the number of lateral roots. Crop protection experiments demonstrated that NFAP and associated antifungal active γ-core PDs were able to protect tomato fruits against the postharvest fungal pathogen Cladosporium herbarum.

Crystal structure of rice defensin OsAFP1 and molecular insight into lipid-binding

Defensins are antibacterial peptides that function in the innate immune system. OsAFP1, a defensin identified from Oryza sativa (rice), exhibits antimicrobial activity against rice pathogens. Intriguingly, OsAFP1 was also shown to demonstrate potent antifungal activity against the human pathogenic fungus Candida albicans by inducing apoptosis in target cells, suggesting that OsAFP1 represents a potential new antibiotic candidate; however, further analyses, particularly at the structural level, are required to elucidate the mechanistic underpinnings of OsAFP1 antifungal activity. Here, we determined the three-dimensional structure of OsAFP1 using X-ray crystallography. OsAFP1 features the cysteine-stabilized αβ structure highly conserved in plant defensins and presents a dimeric structure that appears necessary for antifungal activity. Superimposition of the OsAFP1 structure with that of Nicotiana alata NaD1 complexed with phosphatidic acid indicated that the target molecule is likely trapped between the S2-S3 loops of each OsAFP1 dimer. In lipid-binding analyses performed using nitrocellulose membranes immobilized with various membrane lipid components, OsAFP1 was found to bind to phosphatidylinositols (PIPs) harboring phosphate groups, particularly PI(3)P. These results indicate that OsAFP1 exerts antifungal activity by binding to PI(3)P contained in the C. albicans cell membrane, thereby applying cellular stress and inducing apoptosis. Furthermore, the OsAFP1 structure and site-specific-mutation analyses revealed that Arg1, His2, Leu4, Arg9, and Phe10 play critical roles in OsAFP1 dimer formation. Thus, our study provides novel insights into the antifungal mechanism of OsAFP1.

Two small, cysteine-rich and cationic antifungal proteins from Penicillium chrysogenum: A comparative study of PAF and PAFB

The filamentous fungus Penicillium chrysogenum Q176 secretes the antimicrobial proteins (AMPs) PAF and PAFB, which share a compact disulfide-bond mediated, β-fold structure rendering them highly stable. These two AMPs effectively inhibit the growth of human pathogenic fungi in micromolar concentrations and exhibit antiviral potential without causing cytotoxic effects on mammalian cells in vitro and in vivo.

The antifungal mechanism of action of both AMPs is closely linked to - but not solely dependent on - the lipid composition of the fungal cell membrane and requires a strictly regulated protein uptake into the cell, indicating that PAF and PAFB are not canonical membrane active proteins. Variations in their antifungal spectrum and their killing dynamics point towards a divergent mode of action related to their physicochemical properties and surface charge distribution.

In this review, we relate characteristic features of PAF and PAFB to the current knowledge about other AMPs of different sources. In addition, we present original data that have never been published before to substantiate our assumptions and provide evidences that help to explain and understand better the mechanistic function of PAF and PAFB. Finally, we underline the promising potential of PAF and PAFB as future antifungal therapeutics.

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Penicillium chrysogenum secretes the small, cysteine-rich proteins PAF and PAFB.

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Both exhibit antifungal activity, but with differences in their mode of action.

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Structure, membrane interaction and cellular uptake determine their function.

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PAF and PAFB are well tolerated by mammalian cells.

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They promise applicability in medicine, plant protection and food industry.

Antifungal Potency and Modes of Action of a Novel Olive Tree Defensin Against Closely Related Ascomycete Fungal Pathogens

Antimicrobial peptides play a pivotal role in the innate immunity of plants. Defensins are cysteine-rich antifungal peptides with multiple modes of action. A novel Oleaceae-specific defensin gene family has been discovered in the genome sequences of wild and cultivated species of a perennial olive tree, Olea europaea. OefDef1.1, a member of this defensin family, potently inhibits the in-vitro growth of ascomycete fungal pathogens Botrytis cinerea and three Fusarium spp. OefDef1.1 rapidly permeabilizes the plasma membrane of the conidial and germling cells of B. cinerea. Interestingly, it induces reactive oxygen species and translocates to the cytoplasm only in the germlings but not in the conidia. In medium containing a high concentration of Na1+, antifungal activity of OefDef1.1 is significantly reduced. Surprisingly, a chimeric OefDef1.1 peptide containing the γ-core motif of a Medicago truncatula defensin, MtDef4, displays Na1+-tolerant antifungal activity. In a phospholipid-protein overlay assay, the chimeric peptide exhibits stronger binding to its phosphoinositide partners than OefDef1.1 and is also more potent in inhibiting gray mold disease on the surface of Nicotiana benthamiana and lettuce leaves than OefDef1.1. Significant differences are observed among the four ascomycete pathogens in their responses to OefDef1.1 in growth medium with or without the elevated concentration of Na1+. The varied responses of closely related ascomycete pathogens to this defensin have implications for engineering disease resistance in plants.

A synthetic peptide derived of the β2-β3 loop of the plant defensin from Vigna unguiculata seeds induces Leishmania amazonensis apoptosis-like cell death

For medical use of proteins and peptide-based drugs, it is desirable to have small biologically active sequences because they improve stability, reduce side effects, and production costs. Several plant defensins have their biological activities imparted by a sequence named γ-core. Vu-Def, a Vigna unguiculata defensin, has activity against Leishmania amazonensis, which is one etiological agent of leishmaniasis and for which new drugs are needed. Our intention was to understand if the region comprising the Vu-Def γ-core is responsible for the biological activity against L. amazonensis and to unveil its mechanism of action. Different microbiological assays with L. amazonensis in the presence of the synthetic peptide A₃₆,₄₂,₄₄γ_32-46Vu-Def were done, as well as ultrastructural and fluorescent analyses. A₃₆,₄₂,₄₄γ_32-46Vu-Def showed biological activity similar to Vu-Def. A₃₆,₄₂,₄₄γ_32-46Vu-Def (74 µM) caused 97% inhibition of L. amazonensis culture and parasites were unable to regrow in fresh medium. The cells of the treated parasites showed morphological alterations by ultrastructural analysis and fluorescent labelings that corroborate with the data of the organelles alterations. The general significance of our work is based on the description of a small synthetic peptide, A₃₆,₄₂,₄₄γ_32-46Vu-Def, which has activity on L. amazonensis and that the interaction between A₃₆,₄₂,₄₄γ_32-46Vu-Def-L. amazonensis results in parasite inhibition by the activation of an apoptotic-like cell death pathway.

Screening the Saccharomyces cerevisiae Nonessential Gene Deletion Library Reveals Diverse Mechanisms of Action for Antifungal Plant Defensins

Plant defensins are a large family of proteins, most of which have antifungal activity against a broad spectrum of fungi. However, little is known about how they exert their activity. The mechanisms of action of only a few members of the family have been investigated and, in most cases, there are still a number of unknowns. To gain a better understanding of the antifungal mechanisms of a set of four defensins, NaD1, DmAMP1, NbD6, and SBI6, we screened a pooled collection of the nonessential gene deletion set of Saccharomyces cerevisiae Strains with increased or decreased ability to survive defensin treatment were identified based on the relative abundance of the strain-specific barcode as determined by MiSeq next-generation sequencing. Analysis of the functions of genes that are deleted in strains with differential growth in the presence of defensin provides insight into the mechanism of action. The screen identified a novel role for the vacuole in the mechanisms of action for defensins NbD6 and SBI6. The effect of these defensins on vacuoles was further confirmed by using confocal microscopy in both S. cerevisiae and the cereal pathogen Fusarium graminearum These results demonstrate the utility of this screening method to identify novel mechanisms of action for plant defensins.

Antibacterial Activity of Plant Defensins

Plant defensins are antimicrobial host defense peptides expressed in all higher plants. Performing a significant role in plant innate immunity, plant defensins display potent activity against a wide range of pathogens. Vertebrate and invertebrate defensins have well-characterized antibacterial activity, but plant defensins are commonly considered to display antimicrobial activity against only fungi. In this review, we highlight the often-overlooked antibacterial activity of plant defensins. Also, we illustrate methods to evaluate defensins for antibacterial activity and describe the current advances in uncovering their antibacterial modes of action.

Structural and functional characterization of the membrane-permeabilizing activity of Nicotiana occidentalis defensin NoD173 and protein engineering to enhance oncolysis

Defensins are an extensive family of host defense peptides found ubiquitously across plant and animal species. In addition to protecting against infection by pathogenic microorganisms, some defensins are selectively cytotoxic toward tumor cells. As such, defensins have attracted interest as potential antimicrobial and anticancer therapeutics. The mechanism of defensin action against microbes and tumor cells appears to be conserved and involves the targeting and disruption of cellular membranes. This has been best defined for plant defensins, which upon binding specific phospholipids, such as phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidic acid, form defensin-lipid oligomeric complexes that destabilize membranes, leading to cell lysis. In this study, to further define the anticancer and therapeutic properties of plant defensins, we have characterized a novel plant defensin, Nicotiana occidentalis defensin 173 (NoD173), from N. occidentalis. NoD173 at low micromolar concentrations selectively killed a panel of tumor cell lines over normal primary cells. To improve the anticancer activity of NoD173, we explored increasing cationicity by mutation, with NoD173 with the substitution of Q22 with lysine [NoD173(Q22K)], increasing the antitumor cell activity by 2-fold. NoD173 and the NoD173(Q22K) mutant exhibited only low levels of hemolytic activity, and both maintained activity against tumor cells in serum. The ability of NoD173 to inhibit solid tumor growth in vivo was tested in a mouse B16-F1 model, whereby injection of NoD173 into established subcutaneous tumors significantly inhibited tumor growth. Finally, we showed that NoD173 specifically targets PIP2 and determined by X-ray crystallography that a high-resolution structure of NoD173, which forms a conserved family-defining cysteine-stabilized-αβ motif with a dimeric lipid-binding conformation, configured into an arch-shaped oligomer of 4 dimers. These data provide insights into the mechanism of how defensins target membranes to kill tumor cells and provide proof of concept that defensins are able to inhibit tumor growth in vivo.-Lay, F. T., Ryan, G. F., Caria, S., Phan, T. K., Veneer, P. K., White, J. A., Kvansakul, M., Hulett M. D. Structural and functional characterization of the membrane-permeabilizing activity of Nicotiana occidentalis defensin NoD173 and protein engineering to enhance oncolysis.