Differential antifungal and calcium channel-blocking activity among structurally related plant defensins

Structure and bioactivity of an insecticidal trans-defensin from assassin bug venom

Disulfide-rich peptides such as defensins play diverse roles in immunity and ion channel modulation, as well as constituting the bioactive components of many animal venoms. We investigated the structure and bioactivity of U-RDTX-Pp19, a peptide previously discovered in venom of the assassin bug Pristhesancus plagipennis. Recombinant Pp19 (rPp19) was found to possess insecticidal activity when injected into Drosophila melanogaster. A bioinformatic search revealed that domains homologous to Pp19 are produced by assassin bugs and diverse other arthropods. rPp19 co-eluted with native Pp19 isolated from P. plagipennis, which we found is more abundant in hemolymph than venom. We solved the three-dimensional structure of rPp19 using 2D 1H NMR spectroscopy, finding that it adopts a disulfide-stabilized structure highly similar to known trans-defensins, with the same cystine connectivity as human α-defensin (I-VI, II-IV, and III-V). The structure of Pp19 is unique among reported structures of arthropod peptides.

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.

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.

Genome-wide identification and analysis of a cotton secretome reveals its role in resistance against Verticillium dahliae

BACKGROUND

The extracellular space between the cell wall and plasma membrane is a battlefield in plant-pathogen interactions. Within this space, the pathogen employs its secretome to attack the host in a variety of ways, including immunity manipulation. However, the role of the plant secretome is rarely studied for its role in disease resistance.

RESULTS

Here, we examined the secretome of Verticillium wilt-resistant Gossypium hirsutum cultivar Zhongzhimian No.2 (ZZM2, encoding 95,327 predicted coding sequences) to determine its role in disease resistance against the wilt causal agent, Verticillium dahliae. Bioinformatics-driven analyses showed that the ZZM2 genome encodes 2085 secreted proteins and that these display disequilibrium in their distribution among the chromosomes. The cotton secretome displayed differences in the abundance of certain amino acid residues as compared to the remaining encoded proteins due to the localization of these putative proteins in the extracellular space. The secretome analysis revealed conservation for an allotetraploid genome, which nevertheless exhibited variation among orthologs and comparable unique genes between the two sub-genomes. Secretome annotation strongly suggested its involvement in extracellular stress responses (hydrolase activity, oxidoreductase activity, and extracellular region, etc.), thus contributing to resistance against the V. dahliae infection. Furthermore, the defense response genes (immunity marker NbHIN1, salicylic acid marker NbPR1, and jasmonic acid marker NbLOX4) were activated to varying degrees when Nicotina benthamiana leaves were agro-infiltrated with 28 randomly selected members, suggesting that the secretome plays an important role in the immunity response. Finally, gene silencing assays of 11 members from 13 selected candidates in ZZM2 displayed higher susceptibility to V. dahliae, suggesting that the secretome members confer the Verticillium wilt resistance in cotton.

CONCLUSIONS

Our data demonstrate that the cotton secretome plays an important role in Verticillium wilt resistance, facilitating the development of the resistance gene markers and increasing the understanding of the mechanisms regulating disease resistance.

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.

In silico characterization of cysteine-stabilized αβ defensins from neglected unicellular microeukaryotes

BACKGROUND

The emergence of multi-resistant pathogens have increased dramatically in recent years, becoming a major public-health concern. Among other promising antimicrobial molecules with potential to assist in this worldwide struggle, cysteine-stabilized αβ (CS-αβ) defensins are attracting attention due their efficacy, stability, and broad spectrum against viruses, bacteria, fungi, and protists, including many known human pathogens.

RESULTS

Here, 23 genomes of ciliated protists were screened and two CS-αβ defensins with a likely antifungal activity were identified and characterized, using bioinformatics, from a culturable freshwater species, Laurentiella sp. (LsAMP-1 and LsAMP-2). Although any potential cellular ligand could be predicted for LsAMP-2; evidences from structural, molecular dynamics, and docking analyses suggest that LsAMP-1 may form stably associations with phosphatidylinositol 4,5-bisphosphates (PIP2), a phospholipid found on many eukaryotic cells, which could, in turn, represent an anchorage mechanism within plasma membrane of targeted cells.

CONCLUSION

These data stress that more biotechnology-oriented studies should be conducted on neglected protists, such ciliates, which could become valuable sources of novel bioactive molecules for therapeutic uses.

In silico docking analysis of beta-defensin 20 against cation channel sperm-associated protein 1-4 to predict its role in the sperm maturation

Beta-defensin 20 (DEFB20) is widely expressed in the epididymis with gene features involved in epididymal sperm maturation. However, the action mechanism and function of DEFB20 in sperm maturation are still unclear. One of the important roles of beta-defensin is the ion channel activity. The cation channel sperm-associated protein (CatSper) alpha is an ion channel protein found on the sperm surface. This study aimed to investigate the interaction between DEFB20 and CatSper1-4 protein in relation to the sperm maturation process. Protein sequences were obtained from the National Center for Biotechnology Information (NCBI). Protein modeling and validation were carried out by using the Robetta modeling server and the Ramachandran plot method. Rosetta web server was used for the docking analysis. The results revealed a natural interaction between DEFB20 and CatSper1-4. The interaction occurred at the cation channel (close to the casein kinase II), ion transport protein, and kinase c phosphorylation of the CatSper1-4 active site. The DEFB20 region interacting with CatSper2-4 was the beta-defensin domain, while with CatSper1 was the non-beta-defensin domain. Based on the analysis, DEFB20 may interact with CatSper α subunits, particularly CatsSper1, to affect ion channel activity during sperm maturation.

The potential of plant proteins as antifungal agents for agricultural applications

Fungal pathogens induce a variety of diseases in both plants and post-harvest food crops, resulting in significant crop losses for the agricultural industry. Although the usage of chemical-based fungicides is the most common way to control these diseases, they damage the environment, have the potential to harm human and animal life, and may lead to resistant fungal strains. Accordingly, there is an urgent need for diverse and effective agricultural fungicides that are environmentally- and eco-friendly. Plants have evolved various mechanisms in their innate immune system to defend against fungal pathogens, including soluble proteins secreted from plants with antifungal activities. These proteins can inhibit fungal growth and infection through a variety of mechanisms while exhibiting diverse functionality in addition to antifungal activity. In this mini review, we summarize and discuss the potential of using plant antifungal proteins for future agricultural applications from the perspective of bioengineering and biotechnology.

Novel Polyhydroquinoline-Hydrazide-Linked Schiff's Base Derivatives: Multistep Synthesis, Antimicrobial, and Calcium-Channel-Blocking Activities

Polyhydroquinoline (PHQ) are the unsymmetrical Hantzsch derivatives of 1,4-dihydropyridines with several biological applications. In this work, twenty-five (3-27) new Schiff's base derivatives of polyhydroquinoline hydrazide were synthesized in excellent to good yields by a multi-component reaction. The structures of the synthesized products (1-27) were deduced with the help of spectroscopic techniques, such as 1H-, 13C -NMR, and HR-ESI-MS. The synthesized products (1-27) were tested for their antibacterial and in vitro calcium -channel-blocking (CCB) potentials using the agar-well diffusion method, and isolated rat aortic ring preparations, respectively. Among the series, sixteen compounds were found to inhibit the growth of Escherichia coli and Enterococcus faecalis. Among them, compound 17 was observed to be the most potent one at a dose 2 µg/mL, with an 18 mm zone of inhibition against both bacteria when it was compared with the standard drug amoxicillin. Eight compounds showed CCB activity of variable potency; in particular, compound 27 was more potent, with an EC50 value of 0.7 (0.3-1.1) µg/mL, indicating their CCB effect.

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.

Defensin-lipid interactions in membrane targeting: mechanisms of action and opportunities for the development of antimicrobial and anticancer therapeutics

Defensins are a class of host defence peptides (HDPs) that often harbour antimicrobial and anticancer activities, making them attractive candidates as novel therapeutics. In comparison with current antimicrobial and cancer treatments, defensins uniquely target specific membrane lipids via mechanisms distinct from other HDPs. Therefore, defensins could be potentially developed as therapeutics with increased selectivity and reduced susceptibility to the resistance mechanisms of tumour cells and infectious pathogens. In this review, we highlight recent advances in defensin research with a particular focus on membrane lipid-targeting in cancer and infection settings. In doing so, we discuss strategies to harness lipid-binding defensins for anticancer and anti-infective therapies.

Calcium nutrition nanoagent rescues tomatoes from mosaic virus disease by accelerating calcium transport and activating antiviral immunity

As an essential structural, metabolic and signaling element, calcium shows low remobilization from old to young tissues in plants, restricting the nutrient-use efficiency and control efficacy against mosaic virus disease. Nanotechnology has been applied to prevent/minimize nutrient losses and improve the accessibility of poorly-available nutrients. Herein, the current study applied a star polycation (SPc) to prepare a calcium nutrition nanoagent. The SPc could assemble with calcium glycinate through hydrogen bond and Van der Waals force, forming stable spherical particles with nanoscale size (17.72 nm). Transcriptomic results revealed that the calcium glycinate/SPc complex could activate the expression of many transport-related genes and disease resistance genes in tomatoes, suggesting the enhanced transport and antiviral immunity of SPc-loaded calcium glycinate. Reasonably, the calcium transport was accelerated by 3.17 times into tomato leaves with the help of SPc, and the protective effect of calcium glycinate was remarkably improved to 77.40% and 67.31% toward tomato mosaic virus with the help of SPc after the third and fifth applications. Furthermore, SPc-loaded calcium glycinate could be applied to increase the leaf photosynthetic rate and control the unusual fast growth of tomatoes. The current study is the first success to apply nano-delivery system for enhanced calcium transport and antiviral immunity, which is beneficial for increasing nutrient-use efficiency and shows good prospects for field application.

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.

Genome-scale identification of plant defensin (PDF) family genes and molecular characterization of their responses to diverse nutrient stresses in allotetraploid rapeseed

Plant defensins (PDFs), short peptides with strong antibacterial activity, play important roles in plant growth, development, and stress resistance. However, there are few systematic analyses on PDFs in Brassica napus. Here, bioinformatics methods were used to identify genome-wide PDFs in Brassica napus, and systematically analyze physicochemical properties, expansion pattern, phylogeny, and expression profiling of BnaPDFs under diverse nutrient stresses. A total of 37 full-length PDF homologs, divided into two subgroups (PDF1s and PDF2s), were identified in the rapeseed genome. A total of two distinct clades were identified in the BnaPDF phylogeny. Clade specific conserved motifs were identified within each clade respectively. Most BnaPDFs were proved to undergo powerful purified selection. The PDF members had enriched cis-elements related to growth and development, hormone response, environmental stress response in their promoter regions. GO annotations indicate that the functional pathways of BnaPDFs are mainly involved in cells killing and plant defense responses. In addition, bna-miRNA164 and bna-miRNA172 respectively regulate the expression of their targets BnaA2.PDF2.5 and BnaC7.PDF2.6. The expression patterns of BnaPDFs were analyzed in different tissues. BnaPDF1.2bs was mainly expressed in the roots, whereas BnaPDF2.2s and BnaPDF2.3s were both expressed in stamen, pericarp, silique, and stem. However, the other BnaPDF members showed low expression levels in various tissues. Differential expression of BnaPDFs under nitrate limitation, ammonium excess, phosphorus starvation, potassium deficiency, cadmium toxicity, and salt stress indicated that they might participate in different nutrient stress resistance. The genome-wide identification and characterization of BnaPDFs will enrich understanding of their molecular characteristics and provide elite gene resources for genetic improvement of rapeseed resistance to nutrient stresses.

Game-changing alternatives to conventional fungicides: small RNAs and short peptides

Fungicide use is one of the core elements of intensive agriculture because it is necessary to fight pathogens that would otherwise cause large production losses. Oomycete and fungal pathogens are kept under control using several active compounds, some of which are predicted to be banned in the near future owing to serious concerns about their impact on the environment, non-targeted organisms, and human health. To avoid detrimental repercussions for food security, it is essential to develop new biomolecules that control existing and emerging pathogens but are innocuous to human health and the environment. This review presents and discusses the use of novel low-risk biological compounds based on small RNAs and short peptides that are attractive alternatives to current contentious fungicides.

An Apoplastic Defensin of Wheat Elicits the Production of Extracellular Polysaccharides in Snow Mold

TAD1 (Triticum aestivum defensin 1) is a plant defensin specifically induced by low temperature in winter wheat. In this study, we demonstrated that TAD1 accumulated in the apoplast during cold acclimation and displayed antifungal activity against the pink snow mold fungi Microdochium nivale. When M. nivale was treated with TAD1, Congo red-stainable extracellular polysaccharides (EPS) were produced. The EPS were degradable by cellulase treatment, suggesting the involvement of β-1,4 glucans. Interestingly, when the fungus was treated with FITC-labeled TAD1, fluorescent signals were observed within the EPS layer. Taken together, these results support the hypothesis that the EPS plays a role as a physical barrier against antimicrobial proteins secreted by plants. We anticipate that the findings from our study will have broad impact and will increase our understanding of plant–snow mold interactions under snow.

Tick defensin γ-core reduces Fusarium graminearum growth and abrogates mycotoxins production with high efficiency

Fusarium graminearum is a major fungal pathogen affecting crops of worldwide importance. F. graminearum produces type B trichothecene mycotoxins (TCTB), which are not fully eliminated during food and feed processing. Therefore, the best way to minimize TCTB contamination is to develop prevention strategies. Herein we show that treatment with the reduced form of the γ-core of the tick defensin DefMT3, referred to as TickCore3 (TC3), decreases F. graminearum growth and abrogates TCTB production. The oxidized form of TC3 loses antifungal activity, but retains anti-mycotoxin activity. Molecular dynamics show that TC3 is recruited by specific membrane phospholipids in F. graminearum and that membrane binding of the oxidized form of TC3 is unstable. Capping each of the three cysteine residues of TC3 with methyl groups reduces its inhibitory efficacy. Substitutions of the positively-charged residues lysine (Lys) 6 or arginine 7 by threonine had the highest and the lesser impact, respectively, on the anti-mycotoxin activity of TC3. We conclude that the binding of linear TC3 to F. graminearum membrane phospholipids is required for the antifungal activity of the reduced peptide. Besides, Lys6 appears essential for the anti-mycotoxin activity of the reduced peptide. Our results provide foundation for developing novel and environment-friendly strategies for controlling F. graminearum.

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.

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.

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.

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.

Structural Diversity and Highly Specific Host-Pathogen Transcriptional Regulation of Defensin Genes Is Revealed in Tomato

Defensins are small and rather ubiquitous cysteine-rich anti-microbial peptides. These proteins may act against pathogenic microorganisms either directly (by binding and disrupting membranes) or indirectly (as signaling molecules that participate in the organization of the cellular defense). Even though defensins are widespread across eukaryotes, still, extensive nucleotide and amino acid dissimilarities hamper the elucidation of their response to stimuli and mode of function. In the current study, we screened the Solanum lycopersicum genome for the identification of defensin genes, predicted the relating protein structures, and further studied their transcriptional responses to biotic (Verticillium dahliae, Meloidogyne javanica, Cucumber Mosaic Virus, and Potato Virus Y infections) and abiotic (cold stress) stimuli. Tomato defensin sequences were classified into two groups (C8 and C12). Our data indicate that the transcription of defensin coding genes primarily depends on the specific pathogen recognition patterns of V. dahliae and M. javanica. The immunodetection of plant defensin 1 protein was achieved only in the roots of plants inoculated with V. dahliae. In contrast, the almost null effects of viral infections and cold stress, and the failure to substantially induce the gene transcription suggest that these factors are probably not primarily targeted by the tomato defensin network.

Toward the Discovery of Host-Defense Peptides in Plants

Defense peptides protect multicellular eukaryotes from infections. In biomedical sciences, a dominant conceptual framework refers to defense peptides as host-defense peptides (HDPs), which are bifunctional peptides with both direct antimicrobial and immunomodulatory activities. No HDP has been reported in plants so far, and the very concept of HDP has not been captured yet by the plant science community. Plant science thus lacks the conceptual framework that would coordinate research efforts aimed at discovering plant HDPs. In this perspective article, I used bibliometric and literature survey approaches to raise awareness about the HDP concept among plant scientists, and to encourage research efforts aimed at discovering plant HDPs. Such discovery would enrich our comprehension of the function and evolution of the plant immune system, and provide us with novel molecular tools to develop innovative strategies to control crop diseases.

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.

Biosynthesis and Antimicrobial Activity of Pseudodesmin and Viscosinamide Cyclic Lipopeptides Produced by Pseudomonads Associated with the Cocoyam Rhizosphere

Pseudomonas cyclic lipopeptides (CLPs) are encoded non-ribosomally by biosynthetic gene clusters (BGCs) and possess diverse biological activities. In this study, we conducted chemical structure and BGC analyses with antimicrobial activity assays for two CLPs produced by Pseudomonas strains isolated from the cocoyam rhizosphere in Cameroon and Nigeria. LC-MS and NMR analyses showed that the Pseudomonas sp. COR52 and A2W4.9 produce pseudodesmin and viscosinamide, respectively. These CLPs belong to the Viscosin group characterized by a nonapeptidic moiety with a 7-membered macrocycle. Similar to other Viscosin-group CLPs, the initiatory non-ribosomal peptide synthetase (NRPS) gene of the viscosinamide BGC is situated remotely from the other two NRPS genes. In contrast, the pseudodesmin genes are all clustered in a single genomic locus. Nano- to micromolar levels of pseudodesmin and viscosinamide led to the hyphal distortion and/or disintegration of Rhizoctonia solani AG2-2 and Pythium myriotylum CMR1, whereas similar levels of White Line-Inducing Principle (WLIP), another member of the Viscosin group, resulted in complete lysis of both soil-borne phytopathogens. In addition to the identification of the biosynthetic genes of these two CLPs and the demonstration of their interaction with soil-borne pathogens, this study provides further insights regarding evolutionary divergence within the Viscosin group.

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.

Modulation of Lymphocyte Potassium Channel KV1.3 by Membrane-Penetrating, Joint-Targeting Immunomodulatory Plant Defensin

We describe a cysteine-rich, membrane-penetrating, joint-targeting, and remarkably stable peptide, EgK5, that modulates voltage-gated K_V1.3 potassium channels in T lymphocytes by a distinctive mechanism. EgK5 enters plasma membranes and binds to K_V1.3, causing current run-down by a phosphatidylinositol 4,5-bisphosphate-dependent mechanism. EgK5 exhibits selectivity for K_V1.3 over other channels, receptors, transporters, and enzymes. EgK5 suppresses antigen-triggered proliferation of effector memory T cells, a subset enriched among pathogenic autoreactive T cells in autoimmune disease. PET-CT imaging with ¹⁸F-labeled EgK5 shows accumulation of the peptide in large and small joints of rodents. In keeping with its arthrotropism, EgK5 treats disease in a rat model of rheumatoid arthritis. It was also effective in treating disease in a rat model of atopic dermatitis. No signs of toxicity are observed at 10-100 times the in vivo dose. EgK5 shows promise for clinical development as a therapeutic for autoimmune diseases.

CATION-CHLORIDE CO-TRANSPORTER 1 (CCC1) Mediates Plant Resistance against Pseudomonas syringae

Plasma membrane (PM) depolarization functions as an initial step in plant defense signaling pathways. However, only a few ion channels/transporters have been characterized in the context of plant immunity. Here, we show that the Arabidopsis (Arabidopsis thaliana) Na+:K+:2Cl- (NKCC) cotransporter CCC1 has a dual function in plant immunity. CCC1 functions independently of PM depolarization and negatively regulates pathogen-associated molecular pattern-triggered immunity. However, CCC1 positively regulates plant basal and effector-triggered resistance to Pseudomonas syringae pv. tomato (Pst) DC3000. In line with the compromised immunity to Pst DC3000, ccc1 mutants show reduced expression of genes encoding enzymes involved in the biosynthesis of antimicrobial peptides, camalexin, and 4-OH-ICN, as well as pathogenesis-related proteins. Moreover, genes involved in cell wall and cuticle biosynthesis are constitutively down-regulated in ccc1 mutants, and the cell walls of these mutants exhibit major changes in monosaccharide composition. The role of CCC1 ion transporter activity in the regulation of plant immunity is corroborated by experiments using the specific NKCC inhibitor bumetanide. These results reveal a function for ion transporters in immunity-related cell wall fortification and antimicrobial biosynthesis.

Ecotopic Expression of the Antimicrobial Peptide DmAMP1W Improves Resistance of Transgenic Wheat to Two Diseases: Sharp Eyespot and Common Root Rot

Wheat (Triticum aestivum L.) is an important staple crop. Sharp eyespot and common root rot are destructive diseases of wheat. Antimicrobial peptides (AMPs) are small peptides with broad-spectrum antimicrobial activity. In this study, we synthesized the DmAMP1W gene, encoding Dahlia merckii DmAMP1, and investigated the antifungal role of DmAMP1W in vitro and in transgenic wheat. Protein electrophoresis analysis and in vitro inhibition results demonstrated that the synthesized DmAMP1W correctly translated to the expected peptide DmAMP1W, and the purified peptide inhibited growths of the fungi Rhizoctonia cerealis and Bipolaris sorokiniana, the pathogenic causes of wheat sharp eyespot and common root rot. DmAMP1W was introduced into a wheat variety Zhoumai18 via Agrobacterium-mediated transformation. The molecular characteristics indicated that DmAMP1W could be heritable and expressed in five transgenic wheat lines in T₁–T₂ generations. Average sharp eyespot infection types of these five DmAMP1W transgenic wheat lines in T₁–T₂ generations decreased 0.69–1.54 and 0.40–0.82 compared with non-transformed Zhoumai18, respectively. Average common root rot infection types of these transgenic lines and non-transformed Zhoumai18 were 1.23–1.48 and 2.27, respectively. These results indicated that DmAMP1W-expressing transgenic wheat lines displayed enhanced-resistance to both sharp eyespot and common root rot. This study provides new broad-spectrum antifungal resources for wheat breeding.

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.

Characterization of Composition and Antifungal Properties of Leaf Secondary Metabolites from Thirteen Cultivars of Chrysanthemum morifolium Ramat

Chrysanthemum morifolium Ramat is an ornamental plant of worldwide cultivation. Like many other species in the family Asteraceae, C. morifolium is a rich producer of secondary metabolites. There are two objectives in this study: (I) to determine and compare the diversity of apolar secondary metabolites among different cultivars of C. morifolium and (II) to compare their properties as antifungal agents. To attain these objectives, we selected 13 cultivars of C. morifolium that are commonly used for making chrysanthemum tea as experimental materials. Leaves at the same developmental stage were collected from respective mature plants and subjected to organic extraction. The extracts were analyzed using gas chromatography–mass spectrometry. A total of 37 apolar secondary metabolites including 26 terpenoids were detected from the 13 cultivars. These 13 cultivars can be largely divided into three chemotypes based on chemical principal components analysis. Next, the extracts from the 13 cultivars were examined in in vitro assays for their antifungal properties against three species of pathogenic fungi: Fusarium oxysporum, Magnaporthe oryzae, and Verticillium dahliae. Significant variability in antifungal activity of the leaf extracts among different cultivars was observed. The 13 cultivars can be divided into four groups based on their antifungal activities, which could be partly correlated to the contents of terpenoids. In short, this study reveals large variations in chemical composition, particularly of terpenoids, of leaf secondary metabolites among different cultivars of C. morifolium and their different abilities in functioning as antifungal agents.

Gene expression and evidence of coregulation of the production of some metabolites of chilli pepper inoculated with Pectobacterium carotovorum ssp. carotovorum

Chilli pepper (Capsicum annuum L.) is susceptible to Pectobacterium carotovorum subsp. carotovorum (Pcc), the causal agent of soft rot disease in crops. Understanding the molecular principles of systemic acquired resistance, which is poorly understood in chilli pepper, represents an important step towards understanding inducible defence responses and can assist in designing appropriate intervention strategies for crop disease management. Accordingly, we investigated (via real-time PCR and metabolomics profiling) the molecular response of chilli pepper to Pcc by characterisation of the crucial metabolic regulators involved in the establishment of defence response. We profiled 13 key inducible defence response genes, which included MYB transcriptor factor, ethylene response element-binding protein, suppressor of the G2 allele of Skp1, cytochrome P450, small Sar1 (GTPase), hydroxycinnamoyl-CoA:quinate hydroxycinnamoyl transferase, pathogenesis-related protein 1a, endo-1,3-β-glucanase, chitinase, proteinase inhibitor, defensin, coiled-coil-nucleotide-binding site-leucine-rich repeat (CC-NBS-LRR) resistance and phenylalanine ammonia lyase. In addition, we determined metabolomic shifts induced by Pcc in pepper. The PCR results revealed a significant induction of the selected plant defence-related genes in response to Pcc inoculation; the metabolomic profiling showed that of 99 primary metabolites profiled the quantities of acetylcarnitine, adenosine, adenosine 3',5' cyclic monophosphate, guanosine 3',5' cyclic monophosphate and inosine decreased in pepper leaves inoculated with Pcc.

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.

Plant defensins: types, mechanism of action and prospects of genetic engineering for enhanced disease resistance in plants

Natural antimicrobial peptides have been shown as one of the important tools to combat certain pathogens and play important role as a part of innate immune system in plants and, also adaptive immunity in animals. Defensin is one of the antimicrobial peptides with a diverse nature of mechanism against different pathogens like viruses, bacteria and fungi. They have a broad function in humans, vertebrates, invertebrates, insects, and plants. Plant defensins primarily interact with membrane lipids for their biological activity. Several antimicrobial peptides (AMPs) have been overexpressed in plants for enhanced disease protection. The plants defensin peptides have been efficiently employed as an effective strategy for control of diseases in plants. They can be successfully integrated in plants genome along with some other peptide genes in order to produce transgenic crops for enhanced disease resistance. This review summarizes plant defensins, their expression in plants and enhanced disease resistance potential against phytopathogens.

Plant Defensin Peptides have Antifungal and Antibacterial Activity Against Human and Plant Pathogens

Plant defensins are small, cysteine-rich antimicrobial peptides. These peptides have previously been shown to primarily inhibit the growth of fungal plant pathogens. Plant defensins have a γ-core motif, defined as GXCX_3-9C, which is required for their antifungal activity. To evaluate plant defensins as a potential control for a problematic agricultural disease (alfalfa crown rot), short, chemically synthesized peptides containing γ-core motif sequences were screened for activity against numerous crown rot pathogens. These peptides showed both antifungal and, surprisingly, antibacterial activity. Core motif peptides from Medicago truncatula defensins (MtDef4 and MtDef5) displayed high activity against both plant and human bacterial pathogens in vitro. Full-length defensins had higher antimicrobial activity compared with the peptides containing their predictive γ-core motifs. These results show the future promise for controlling a wide array of economically important fungal and bacterial plant pathogens through the transgenic expression of a plant defensin. They also suggest that plant defensins may be an untapped reservoir for development of therapeutic compounds for combating human and animal pathogens.

Molecular cloning and characterization of six defensin genes from lentil plant (Lens culinaris L.)

Six full-length gene and cDNA sequences of defensin were identified from Lens culinaris L. plant. The identified genes and cDNAs were different in length and their coding sequences contained Knot1 functional domain. Phylogenetic analysis classified the identified defensins into two subfamilies. All defensin genes contained only one intron and had extracellular signal peptides. Secondary structures of identified defensins were completely composed of alpha helix and beta strand. Presence of conserved Cys amino acids and disulfide bridges, interaction with defense and signaling proteins and antimicrobial activity were other common features of these peptides. The identified defensins displayed differential expression pattern in the various tissues. The highest expression level of defensins was observed in seed, pod, and root tissues. Defensin 4 was significantly expressed in all examined tissues, whereas the other defensins were only expressed in some tissues. Also, in the fungal and wounding treatments, lentil defensins showed different expression pattern. Defensin 1 was up-regulated in both fungal and wounding treatments. Defensin 4 showed decreased expression level in both fungal and wounding treatments. Defensins 2 and 6 were up-regulated in wounding and fungal treatments, respectively. In this study, for the first time, six defensin genes were isolated and characterized from lentil. Our results highlighted the role of defensins in lentil plant that can be used for future studies.

Phytohormone treatment induces generation of cryptic peptides with antimicrobial activity in the Moss Physcomitrella patens

BACKGROUND

Cryptic peptides (cryptides) are small bioactive molecules generated via degradation of functionally active proteins. Only a few examples of plant cryptides playing an important role in plant defense have been reported to date, hence our knowledge about cryptic signals hidden in protein structure remains very limited. Moreover, little is known about how stress conditions influence the size of endogenous peptide pools, and which of these peptides themselves have biological functions is currently unclear.

RESULTS

Here, we used mass spectrometry to comprehensively analyze the endogenous peptide pools generated from functionally active proteins inside the cell and in the secretome from the model plant Physcomitrella patens. Overall, we identified approximately 4,000 intracellular and approximately 500 secreted peptides. We found that the secretome and cellular peptidomes did not show significant overlap and that respective protein precursors have very different protein degradation patterns. We showed that treatment with the plant stress hormone methyl jasmonate induced specific proteolysis of new functional proteins and the release of bioactive peptides having an antimicrobial activity and capable to elicit the expression of plant defense genes. Finally, we showed that the inhibition of protease activity during methyl jasmonate treatment decreased the secretome antimicrobial potential, suggesting an important role of peptides released from proteins in immune response.

CONCLUSIONS

Using mass-spectrometry, in vitro experiments and bioinformatics analysis, we found that methyl jasmonate acid induces significant changes in the peptide pools and that some of the resulting peptides possess antimicrobial and regulatory activities. Moreover, our study provides a list of peptides for further study of potential plant cryptides.

Antimicrobial Activity of NCR Plant Peptides Strongly Depends on the Test Assays

The symbiosis specific NCR247 and NCR335 cationic plant peptides of Medicago truncatula have been shown to exert antimicrobial activity against a wide range of microbes. However, their antimicrobial efficiency is clearly limited by divalent cations. Here, the antibacterial and antifungal activities of NCR247 and NCR335 peptides were compared to those of the well-characterized peptide antibiotics polymyxin B and the aminoglycoside streptomycin on three model microbes, Escherichia coli, Bacillus subtilis and Saccharomyces cerevisiae as representatives of Gram-negative and Gram-positive bacteria as well as eukaryotic fungi. The aim of the study was to assess how the killing efficiency of these peptides depends on various, widely used antimicrobial susceptibility assays. Validated resazurin microdilution assay was used to determine minimal growth inhibitory concentrations in three general test media (MHB, MHBII and low-salt medium LSM). Bactericidal/fungicidal activities were determined by the commonly used drop plate assay. The natural plant peptides showed distinct characteristics, NCR247 had a generally high sensitivity for Ca²⁺ and Mg²⁺ in the medium, while NCR335 proved to be a robust and strong antimicrobial agent with comparable efficiency values to polymyxin B. Activity data were confirmed visually, both NCR247 and NCR335 treatments at minimal bactericidal concentrations induced complete disruption of the membranes and provoked cell lysis on all tested microorganisms as observed by scanning electron microscopy.

The role of antimicrobial peptides in plant immunity

Selective pressure imposed by millions of years of relentless biological attack has led to the development of an extraordinary array of defense strategies in plants. Among these, antimicrobial peptides (AMPs) stand out as one of the most prominent components of the plant immune system. These small and usually basic peptides are deployed as a generalist defense strategy that grants direct and durable resistance against biotic stress. Even though their name implies a function against microbes, the range of plant-associated organisms affected by these peptides is much broader. In this review, we highlight the advances in our understanding on the role of AMPs in plant immunity. We demonstrate that the capacity of plant AMPs to act against a large spectrum of enemies relies on their diverse mechanism of action and remarkable structural stability. The efficacy of AMPs as a defense strategy is evidenced by their widespread occurrence in the plant kingdom, an astonishing heterogeneity in host peptide composition, and the extent to which plant enemies have evolved effective counter-measures to evade AMP action. Plant AMPs are becoming an important topic of research due to their significance in allowing plants to thrive and for their enormous potential in agronomical and pharmaceutical fields.

Fusion of a chitin-binding domain to an antibacterial peptide to enhance resistance to Fusarium solani in tobacco (Nicotiana tabacum)

An antibacterial peptide-encoding gene from alfalfa seeds, alfAFP, was fused to the C-terminal part of chitin-binding domain (CBD) of the rice chitinase-encoding gene (CBD-alfAFP) and introduced to tobacco by Agrobacterium-mediated transformation. Polymerase chain reaction (PCR) technique was used to confirm the integration of the recombinant CBD-alfAFP encoding gene in transgenic tobacco plants. A number of transgenic lines and a non-transgenic control plant were selected for further molecular analyses. The result of analyzing the transgenic plants by semi-quantitative RT-PCR showed that the recombinant gene is expressed in transgenic plants and there is a difference between the transgenic plants in terms of the level of CBD-alfAFP expression. The total protein was extracted from a few selected transgenic plants and used to evaluate the antibacterial/antifungal of recombinant protein activity against some important plant and human pathogens. The results of this experiment showed that the total protein extract obtained from transgenic lines significantly (P < 0.05) inhibited the growth of various bacteria and fungi compared to the non-transgenic plants. Transgenic lines showed a significant (P < 0.01) difference considering their ability to inhibit bacterial and fungal pathogens growth. The recombinant CBD-alfAFP protein significantly (P < 0.01) increased the resistance of the transgenic plants against Fusarium solani. Transgenic lines showed no significant wilting symptoms and obvious wilting symptoms were not observed even 30 days post-inoculation (dpi) with Fusarium solani spores. These results suggest that transgenic tobacco plants are resistant to Fusarium solani wilt and fusion of CBD to the alfAFP antimicrobial peptide is an efficient approach to control fungal diseases.

Disease Resistance Mechanisms in Plants

Plants have developed a complex defense system against diverse pests and pathogens. Once pathogens overcome mechanical barriers to infection, plant receptors initiate signaling pathways driving the expression of defense response genes. Plant immune systems rely on their ability to recognize enemy molecules, carry out signal transduction, and respond defensively through pathways involving many genes and their products. Pathogens actively attempt to evade and interfere with response pathways, selecting for a decentralized, multicomponent immune system. Recent advances in molecular techniques have greatly expanded our understanding of plant immunity, largely driven by potential application to agricultural systems. Here, we review the major plant immune system components, state of the art knowledge, and future direction of research on plant⁻pathogen interactions. In our review, we will discuss how the decentralization of plant immune systems have provided both increased evolutionary opportunity for pathogen resistance, as well as additional mechanisms for pathogen inhibition of such defense responses. We conclude that the rapid advances in bioinformatics and molecular biology are driving an explosion of information that will advance agricultural production and illustrate how complex molecular interactions evolve.

X-ray structure of a carpet-like antimicrobial defensin-phospholipid membrane disruption complex

Defensins are cationic antimicrobial peptides expressed throughout the plant and animal kingdoms as a first line of defense against pathogens. Membrane targeting and disruption is a crucial function of many defensins, however the precise mechanism remains unclear. Certain plant defensins form dimers that specifically bind the membrane phospholipids phosphatidic acid (PA) and phosphatidylinositol 4,5-bisphosphate, thereby triggering the assembly of defensin-lipid oligomers that permeabilize cell membranes. To understand this permeabilization mechanism, here we determine the crystal structure of the plant defensin NaD1 bound to PA. The structure reveals a 20-mer that adopts a concave sheet- or carpet-like topology where NaD1 dimers form one face and PA acyl chains form the other face of the sheet. Furthermore, we show that Arg39 is critical for PA binding, oligomerization and fungal cell killing. These findings identify a putative defensin-phospholipid membrane attack configuration that supports a longstanding proposed carpet mode of membrane disruption.

Antimicrobial peptides: biochemical determinants of activity and biophysical techniques of elucidating their functionality

Antimicrobial peptides (AMPs) have been established over millennia as powerful components of the innate immune system of many organisms. Due to their broad spectrum of activity and the development of host resistance against them being unlikely, AMPs are strong candidates for controlling drug-resistant pathogenic microbial pathogens. AMPs cause cell death through several independent or cooperative mechanisms involving membrane lysis, non-lytic activity, and/or intracellular mechanisms. Biochemical determinants such as peptide length, primary sequence, charge, secondary structure, hydrophobicity, amphipathicity and host cell membrane composition together influence the biological activities of peptides. A number of biophysical techniques have been used in recent years to study the mechanisms of action of AMPs. This work appraises the molecular parameters that determine the biocidal activity of AMPs and overviews their mechanisms of actions and the diverse biochemical, biophysical and microscopy techniques utilised to elucidate these.

The evolution, function and mechanisms of action for plant defensins

Plant defensins are an extensive family of small cysteine rich proteins characterised by a conserved cysteine stabilised alpha beta protein fold which resembles the structure of insect and vertebrate defensins. However, secondary structure and disulphide topology indicates two independent superfamilies of defensins with similar structures that have arisen via an extreme case of convergent evolution. Defensins from plants and insects belong to the cis-defensin superfamily whereas mammalian defensins belong to the trans-defensin superfamily. Plant defensins are produced by all species of plants and although the structure is highly conserved, the amino acid sequences are highly variable with the exception of the cysteine residues that form the stabilising disulphide bonds and a few other conserved residues. The majority of plant defensins are components of the plant innate immune system but others have evolved additional functions ranging from roles in sexual reproduction and development to metal tolerance. This review focuses on the antifungal mechanisms of plant defensins. The activity of plant defensins is not limited to plant pathogens and many of the described mechanisms have been elucidated using yeast models. These mechanisms are more complex than simple membrane permeabilisation induced by many small antimicrobial peptides. Common themes that run through the characterised mechanisms are interactions with specific lipids, production of reactive oxygen species and induction of cell wall stress. Links between sequence motifs and functions are highlighted where appropriate. The complexity of the interactions between plant defensins and fungi helps explain why this protein superfamily is ubiquitous in plant innate immunity.

Paralithocins, Antimicrobial Peptides with Unusual Disulfide Connectivity from the Red King Crab, Paralithodes camtschaticus

As part of an ongoing exploration of marine invertebrates as a source of new antimicrobial peptides, hemocyte extracts from the red king crab, Paralithodes camtschaticus, were studied. Three cationic cysteine (Cys)-rich peptides, named paralithocins 1-3, were isolated by bioassay-guided purification, and their amino acid sequences determined by Edman degradation and expressed sequences tag analysis. Disulfide bond mapping was performed by high-resolution tandem mass spectrometry. The peptides (38-51 amino acids in length) share a unique Cys motif composed of eight Cys, forming four disulfide bridges with a bond connectivity of (Cys relative position) Cys1-Cys8, Cys2-Cys6, Cys3-Cys5, and Cys4-Cys7, a disulfide arrangement that has not been previously reported among antimicrobial peptides. Thus, paralithocins 1-3 may be assigned to a previously unknown family of antimicrobial peptides within the group of Cys-rich antimicrobial peptides. Although none of the isolated peptides displayed antimicrobial activity against the target strains Escherichia coli, Pseudomonas aeruginosa, or Staphylococcus aureus, they inhibited the growth of several marine bacterial strains with minimal inhibitory concentrations in the 12.5-100 μM range. These findings corroborate the hypothesis that marine organisms are a valuable source for discovering bioactive peptides with new structural motifs.

Recombinant Promoter (MUASCsV8CP) Driven Totiviral Killer Protein 4 (KP4) Imparts Resistance Against Fungal Pathogens in Transgenic Tobacco

Development of disease-resistant plant varieties achieved by engineering anti-microbial transgenes under the control of strong promoters can suffice the inhibition of pathogen growth and simultaneously ensure enhanced crop production. For evaluating the prospect of such strong promoters, we comprehensively characterized the full-length transcript promoter of Cassava Vein Mosaic Virus (CsVMV; -565 to +166) and identified CsVMV8 (-215 to +166) as the highest expressing fragment in both transient and transgenic assays. Further, we designed a new chimeric promoter 'MUASCsV8CP' through inter-molecular hybridization among the upstream activation sequence (UAS) of Mirabilis Mosaic Virus (MMV; -297 to -38) and CsVMV8, as the core promoter (CP). The MUASCsV8CP was found to be ∼2.2 and ∼2.4 times stronger than the CsVMV8 and CaMV35S promoters, respectively, while its activity was found to be equivalent to that of the CaMV35S² promoter. Furthermore, we generated transgenic tobacco plants expressing the totiviral 'Killer protein KP4' (KP4) under the control of the MUASCsV8CP promoter. Recombinant KP4 was found to accumulate both in the cytoplasm and apoplast of plant cells. The agar-based killing zone assays revealed enhanced resistance of plant-derived KP4 against two deuteromycetous foliar pathogenic fungi viz. Alternaria alternata and Phoma exigua var. exigua. Also, transgenic plants expressing KP4 inhibited the growth progression of these fungi and conferred significant fungal resistance in detached-leaf and whole plant assays. Taken together, we establish the potential of engineering "in-built" fungal stress-tolerance in plants by expressing KP4 under a novel chimeric caulimoviral promoter in a transgenic approach.