Synthetic ultrashort cationic lipopeptides induce systemic plant defense responses against bacterial and fungal pathogens

Functional Peptides for Plant Disease Control

Plant disease control requires novel approaches to mitigate the spread of and losses caused by current, emerging, and re-emerging diseases and to adapt plant protection to global climate change and the restrictions on the use of conventional pesticides. Currently, disease management relies mainly on biopesticides, which are required for the sustainable use of plant-protection products. Functional peptides are candidate biopesticides because they originate from living organisms or are synthetic analogs and provide novel mechanisms of action against plant pathogens. Hundreds of compounds exist that cover an extensive range of activities against viruses, bacteria and phytoplasmas, fungi and oomycetes, and nematodes. Natural sources, chemical synthesis, and biotechnological platforms may provide peptides at large scale for the industry and growers. The main challenges for their use in plant disease protection are (a) the requirement of stability in the plant environment and counteracting resistance in pathogen populations, (b) the need to develop suitable formulations to increase their shelf life and methods of application, (c) the selection of compounds with acceptable toxicological profiles, and (d) the high cost of production for agricultural purposes. In the near future, it is expected that several functional peptides will be commercially available for plant disease control, but more effort is needed to validate their efficacy at the field level and fulfill the requirements of the regulatory framework.

Membrane interactions of Ocellatins. Where do antimicrobial gaps stem from?

The antimicrobial peptides Ocellatin-LB1, -LB2 and -F1, isolated from frogs, are identical from residue 1 to 22, which correspond to the -LB1 sequence, whereas -LB2 carries an extra N and -F1 additional NKL residues at their C-termini. Despite the similar sequences, previous investigations showed different spectra of activities and biophysical investigations indicated a direct correlation between both membrane-disruptive properties and activities, i.e., ocellatin-F1 > ocellatin-LB1 > ocellatin-LB2. This study presents experimental evidence as well as results from theoretical studies that contribute to a deeper understanding on how these peptides exert their antimicrobial activities and how small differences in the amino acid composition and their secondary structure can be correlated to these activity gaps. Solid-state NMR experiments allied to the simulation of anisotropic NMR parameters allowed the determination of the membrane topologies of these ocellatins. Interestingly, the extra Asn residue at the Ocellatin-LB2 C-terminus results in increased topological flexibility, which is mainly related to wobbling of the helix main axis as noticed by molecular dynamics simulations. Binding kinetics and thermodynamics of the interactions have also been assessed by Surface Plasmon Resonance and Isothermal Titration Calorimetry. Therefore, these investigations allowed to understand in atomic detail the relationships between peptide structure and membrane topology, which are in tune within the series -F1 >  > -LB1 ≥ -LB2, as well as how peptide dynamics can affect membrane topology, insertion and binding.

A Bifunctional Synthetic Peptide With Antimicrobial and Plant Elicitation Properties That Protect Tomato Plants From Bacterial and Fungal Infections

The hybrid peptide BP178 (KKLFKKILKYLAGPAGIGKFLHSAKKDEL-OH), derived from BP100 (KKLFKKILKYL) and magainin (1-10), and engineered for plant expression, had a strong bactericidal activity but not fungicidal. Moreover, the preventive spray of tomato plants with BP178 controlled infections by the plant pathogenic bacteria Pseudomonas syringae pv. tomato and Xanthomonas campestris pv. vesicatoria, as well as the fungus Botrytis cinerea. The treatment of tomato plants with BP178 induced the expression of several genes according to microarray and RT-qPCR analysis. Upregulated genes coded for several pathogenesis-related proteins, including PR1, PR2, PR3, PR4, PR5, PR6, PR7, PR9, PR10, and PR14, as well as transcription factors like ethylene transcription factors, WRKY, NAC and MYB, involved in the salicylic acid, jasmonic acid, and ethylene-signaling pathways. BP178 induced a similar gene expression pattern to flg15 according to RT-qPCR analysis, whereas the parent peptide BP100 did not trigger such as a strong plant defense response. It was concluded that BP178 was a bifunctional peptide protecting the plant against pathogen infection through a dual mechanism of action consisting of antimicrobial activity against bacterial pathogens and plant defense elicitation on plant host.

Impact of Supplements on Enhanced Activity of Bacillus amyloliquefaciens BmB1 Against Pythium aphanidermatum Through Lipopeptide Modulation

The present study has been designed to improve the activity of endophytic Bacillus amyloliquefaciens BmB1 against Pythium aphanidermatum through the culture supplementation with carbon sources, nitrogen sources and zinc oxide nanoparticles (ZnONPs). From the results of the study, supplementation with glucose (45 g/L), yeast extract (7.5 g/L) and ZnONPs (5 mg/L) were found to enhance the antifungal activity of B. amyloliquefaciens BmB1. This was also confirmed by comparative statistical analysis with experimental control. Further LC-Q-TOF-MS analysis of extracts of B. amyloliquefaciens BmB1 cultured with supplements showed a remarkable modulation of its lipopeptide profile. The blend of lipopeptides enhanced during the culture supplementation of B. amyloliquefaciens BmB1 as evidenced by the mass spectrometric analysis can consider to be the basis of its increased activity against P. aphanidermatum. As Bacillus spp. are well known for their biocontrol activities, the results of the study offer ways to improve its agricultural applications.

Antibacterial lipo-random peptide mixtures exhibit high selectivity and synergistic interactions

Random peptide mixtures (RPMs) have been recently proposed as powerful antimicrobial compounds. These are unique mixtures of peptides synthesized by random combination of a cationic and a hydrophobic amino acid. Here, we introduce a new type of antimicrobial compounds, short lipo-RPMs, which result from N-palmitoylation of RPMs. We report the characterization of 5-mer lipo-RPMs containing l-phenylalanine and d-lysine, named p-FdK5. p-FdK5 had high antibacterial activity against several bacterial strains and was able to reduce disease severity caused by a plant pathogen. We further synthesized and studied all 32 (2⁵) possible lipopeptides that compose the p-FdK5 mixture. We showed that the antibacterial activity of specific lipopeptides depends on the peptide hydrophobicity and on the location of the hydrophobic amino acids relative to the palmitic acid. Interestingly, synergism assays revealed positive interactions between different sequence-specific lipopeptides in terms of antimicrobial activity.

Synthetic Mono-Rhamnolipids Display Direct Antifungal Effects and Trigger an Innate Immune Response in Tomato against Botrytis Cinerea

Natural rhamnolipids are potential biocontrol agents for plant protection against bacterial and fungal diseases. In this work, we synthetized new synthetic mono-rhamnolipids (smRLs) consisting in a rhamnose connected to a simple acyl chain and differing by the nature of the link and the length of the lipid tail. We then investigated the effects of these ether, ester, carbamate or succinate smRL derivatives on Botrytis cinerea development, symptoms spreading on tomato leaves and immune responses in tomato plants. Our results demonstrate that synthetic smRLs are able to trigger early and late immunity-related plant defense responses in tomato and increase plant resistance against B. cinerea in controlled conditions. Structure-function analysis showed that chain length of the lipidic part and type of acyl chain were critical to smRLs immune activity and to the extent of symptoms caused by the fungus on tomato leaves.

Plant chemical genetics reveals colistin sulphate as a SA and NPR1-independent PR1 inducer functioning via a p38-like kinase pathway

In plants, low-dose of exogenous bacterial cyclic lipopeptides (CLPs) trigger transient membrane changes leading to activation of early and late defence responses. Here, a forward chemical genetics approach identifies colistin sulphate (CS) CLP as a novel plant defence inducer. CS uniquely triggers activation of the PATHOGENESIS-RELATED 1 (PR1) gene and resistance against Pseudomonas syringae pv. tomato DC3000 (Pst DC3000) in Arabidopsis thaliana (Arabidopsis) independently of the PR1 classical inducer, salicylic acid (SA) and the key SA-signalling protein, NON-EXPRESSOR OF PR1 (NPR1). Low bioactive concentration of CS does not trigger activation of early defence markers such as reactive oxygen species (ROS) and mitogen activated protein kinase (MAPK). However, it strongly suppresses primary root length elongation. Structure activity relationship (SAR) assays and mode-of-action (MoA) studies show the acyl chain and activation of a ∼46 kDa p38-like kinase pathway to be crucial for CS' bioactivity. Selective pharmacological inhibition of the active p38-like kinase pathway by SB203580 reverses CS' effects on PR1 activation and root length suppression. Our results with CS as a chemical probe highlight the existence of a novel SA- and NPR1-independent branch of PR1 activation functioning via a membrane-sensitive p38-like kinase pathway.

Bacillomycin D inhibits growth of Rhizopus stolonifer and induces defense-related mechanism in cherry tomato

The inhibitory effect of Bacillomycin D, a cyclic lipopeptide, on Rhizopus stolonifer colonization of cherry tomato was studied, and its possible mechanism of action was explored. Bacillomycin D showed a direct inhibitory effect on R. stolonifer spore germination and mycelial growth in vitro. It conferred both a direct inhibitory effect on R. stolonifer growth in cherry tomato in vivo and induced host resistance in cherry tomato. Moreover, Bacillomycin D treatment significantly increased the activities of plant defense-related enzymes, including chitinase (CHI), β-1,3-glucanase (GLU), phenylalanine ammonia-lyase (PAL), and peroxidase (POD). Real-time PCR (RT-PCR) showed that defense-related genes involved in the salicylic acid defense signaling pathway and genes encoding pathogenesis-related proteins were up-regulated in Bacillomycin D treatment. Furthermore, Bacillomycin D-C16 resulted in direct inhibition and a remarkable induced resistance to R. stolonifer which was higher than as induced by Bacillomycin D-C14. Together, the data indicated that Bacillomycin D can control the growth of R. stolonifer through both the direct inhibition of the fungus and the activation of defense-related genes and enzymes in cherry tomato.

DOTAP, a lipidic transfection reagent, triggers Arabidopsis plant defense responses

DOTAP triggers Arabidopsis thaliana immunity and by priming the defense response is able to reduce bacterial pathogen attack. DOTAP is a cationic lipid widely used as a liposomal transfection reagent and it has recently been identified as a strong activator of the innate immune system in animal cells. Plants are sessile organisms and unlike mammals, that have innate and acquired immunity, plants possess only innate immunity. A key feature of plant immunity is the ability to sense potentially dangerous signals, as it is the case for microbe-associated, pathogen-associated or damage-associated molecular patterns and by doing so, trigger an active defense response to cope with the perturbing stimulus. Here, we evaluated the effect of DOTAP in plant basal innate immunity. An initial plant defense response was induced by the cationic lipid DOTAP in the model plant Arabidopsis thaliana, assessed by callose deposition, reactive oxygen species production, and plant cell death. In addition, a proteomic analysis revealed that these responses are mirrored by changes in the plant proteome, such as up-regulation of proteins related to defense responses, including proteins involved in photorespiration, cysteine and oxylipin synthesis, and oxidative stress response; and down-regulation of enzymes related to photosynthesis. Furthermore, DOTAP was able to prime the defense response for later pathogenic challenges as in the case of the virulent bacterial pathogen Pseudomonas syringae pv. tomato. Disease outcome was diminished in DOTAP-pre-treated leaves and bacterial growth was reduced 100 times compared to mock leaves. Therefore, DOTAP may be considered a good candidate as an elicitor for the study of plant immunity.

Antimicrobial activity of linear lipopeptides derived from BP100 towards plant pathogens

A collection of 36 lipopeptides were designed from the cecropin A-melittin hybrid peptide BP100 (H-Lys-Lys-Leu-Phe-Lys-Lys-Ile-Leu-Lys-Tyr-Leu-NH2) previously described with activity against phytopathogenic bacteria. These lipopeptides were synthesized on solid-phase and screened for their antimicrobial activity, toxicity and proteolytic stability. They incorporated a butanoyl, a hexanoyl or a lauroyl group at the N-terminus or at the side chain of a lysine residue placed at each position of the sequence. Their antimicrobial activity and hemolysis depended on the fatty acid length and its position. In particular, lipopeptides containing a butanoyl or a hexanoyl chain exhibited the best biological activity profile. In addition, we observed that the incorporation of the acyl group did not induce the overexpression of defense-related genes in tomato. Best lipopeptides were BP370, BP378, BP381, BP387 and BP389, which were highly active against all the pathogens tested (minimum inhibitory concentration of 0.8 to 12.5 μM), low hemolytic, low phytotoxic and significantly stable to protease degradation. This family of lipopeptides might be promising functional peptides useful for plant protection.

Synthetic Rhamnolipid Bolaforms trigger an innate immune response in Arabidopsis thaliana

Stimulation of plant innate immunity by natural and synthetic elicitors is a promising alternative to conventional pesticides for a more sustainable agriculture. Sugar-based bolaamphiphiles are known for their biocompatibility, biodegradability and low toxicity. In this work, we show that Synthetic Rhamnolipid Bolaforms (SRBs) that have been synthesized by green chemistry trigger Arabidopsis innate immunity. Using structure-function analysis, we demonstrate that SRBs, depending on the acyl chain length, differentially activate early and late immunity-related plant defense responses and provide local increase in resistance to plant pathogenic bacteria. Our biophysical data suggest that SRBs can interact with plant biomimetic plasma membrane and open the possibility of a lipid driven process for plant-triggered immunity by SRBs.

Extracellular Alkalinization as a Defense Response in Potato Cells

A quantitative and robust bioassay to assess plant defense response is important for studies of disease resistance and also for the early identification of disease during pre- or non-symptomatic phases. An increase in extracellular pH is known to be an early defense response in plants. In this study, we demonstrate extracellular alkalinization as a defense response in potatoes. Using potato suspension cell cultures, we observed an alkalinization response against various pathogen- and plant-derived elicitors in a dose- and time-dependent manner. We also assessed the defense response against a variety of potato pathogens, such as protists (Phytophthora infestans and Spongospora subterranea) and fungi (Verticillium dahliae and Colletotrichum coccodes). Our results show that extracellular pH increases within 30 min in proportion to the number of pathogen spores added. Consistently with the alkalinization effect, the higher transcription level of several defense-related genes and production of reactive oxygen species was observed. Our results demonstrate that the alkalinization response is an effective marker to study early stages of defense response in potatoes.

Roles of d-Amino Acids on the Bioactivity of Host Defense Peptides

Host defense peptides (HDPs) are positively-charged and amphipathic components of the innate immune system that have demonstrated great potential to become the next generation of broad spectrum therapeutic agents effective against a vast array of pathogens and tumor. As such, many approaches have been taken to improve the therapeutic efficacy of HDPs. Amongst these methods, the incorporation of d-amino acids (d-AA) is an approach that has demonstrated consistent success in improving HDPs. Although, virtually all HDP review articles briefly mentioned about the role of d-AA, however it is rather surprising that no systematic review specifically dedicated to this topic exists. Given the impact that d-AA incorporation has on HDPs, this review aims to fill that void with a systematic discussion of the impact of d-AA on HDPs.

The synthetic cationic lipid diC14 activates a sector of the Arabidopsis defence network requiring endogenous signalling components

Natural and synthetic elicitors have contributed significantly to the study of plant immunity. Pathogen-derived proteins and carbohydrates that bind to immune receptors, allow the fine dissection of certain defence pathways. Lipids of a different nature that act as defence elicitors, have also been studied, but their specific effects have been less well characterized, and their receptors have not been identified. In animal cells, nanoliposomes of the synthetic cationic lipid 3-tetradecylamino-tert-butyl-N-tetradecylpropionamidine (diC14) activate the TLR4-dependent immune cascade. Here, we have investigated whether this lipid induces Arabidopsis defence responses. At the local level, diC14 activated early and late defence gene markers (FRK1, WRKY29, ICS1 and PR1), acting in a dose-dependent manner. This lipid induced the salicylic acid (SA)-dependent, but not jasmonic acid (JA)-dependent, pathway and protected plants against Pseudomonas syringae pv. tomato (Pst), but not Botrytis cinerea. diC14 was not toxic to plant or pathogen, and potentiated pathogen-induced callose deposition. At the systemic level, diC14 induced PR1 expression and conferred resistance against Pst. diC14-induced defence responses required the signalling protein EDS1, but not NDR1. Curiously, the lipid-induced defence gene expression was lower in the fls2/efr/cerk1 triple mutant, but still unchanged in the single mutants. The amidine headgroup and chain length were important for its activity. Given the robustness of the responses triggered by diC14, its specific action on a defence pathway and the requirement for well-known defence components, this synthetic lipid is emerging as a useful tool to investigate the initial events involved in plant innate immunity.

Effect of acyl chain length on therapeutic activity and mode of action of the CX-KYR-NH2 antimicrobial lipopeptide

Peptide lipidation has proven to be an inexpensive and effective strategy for designing next-generation peptide-based drug compounds. In this study, the effect of the acyl chain length of ultrashort LiPs (CX-KYR-NH2; X=10, 12, 14 and 16) on their bacterial killing and membrane disruption kinetics was investigated. The geometric mean of the minimum inhibitory concentration (MIC) values for 4 pathogenic bacterial strains was 25 μM, with a selectivity index of 10.24 for C14-KYR-NH2. LiPs at all concentrations exhibited no cytotoxicity towards human erythrocytes, but towards Vero cells at 80 μM. All the LiPs adopted secondary structure in a membrane mimicking environment. C14-KYR-NH2 aggregated above 256 μM, while C16-KYR-NH2 did above 80 μM. All LiPs showed outer membrane permeabilization within 3 min after treatment, yet the extent and kinetics of inner membrane penetration and depolarization were dependent on the acyl chain length. Cell death subsequently occurred within 10 min, and killing activity appeared to correlate most with depolarization activity but not with outer or inner membrane permeability. AFM imaging of cells treated with C14-KYR-NH2 revealed rupture of the cell surface and cytosolic leakage depending on the length of incubation. This study highlights and follows the progression of events that occur during the membrane disintegration process over time, and determines the optimal amphipathicity of ultrashort LiPs with 12-14 carbon atoms for this membrane disrupting activity. The fast acting bactericidal properties of ultrashort LiPs with optimal chain lengths make them promising candidates for drug lead compounds.

Membrane interactions of phylloseptin-1, -2, and -3 peptides by oriented solid-state NMR spectroscopy

Phylloseptin-1, -2, and -3 are three members of the family of linear cationic antimicrobial peptides found in tree frogs. The highly homologous peptides encompass 19 amino acids, and only differ in the amino acid composition and charge at the six most carboxy-terminal residues. Here, we investigated how such subtle changes are reflected in their membrane interactions and how these can be correlated to their biological activities. To this end, the three peptides were labeled with stable isotopes, reconstituted into oriented phospholipid bilayers, and their detailed topology determined by a combined approach using (2)H and (15)N solid-state NMR spectroscopy. Although phylloseptin-2 and -3 adopt perfect in-plane alignments, the tilt angle of phylloseptin-1 deviates by 8° probably to assure a more water exposed localization of the lysine-17 side chain. Furthermore, different azimuthal angles are observed, positioning the amphipathic helices of all three peptides with the charged residues well exposed to the water phase. Interestingly, our studies also reveal that two orientation-dependent (2)H quadrupolar splittings from methyl-deuterated alanines and one (15)N amide chemical shift are sufficient to unambiguously determine the topology of phylloseptin-1, where quadrupolar splittings close to the maximum impose the most stringent angular restraints. As a result of these studies, a strategy is proposed where the topology of a peptide structure can be determined accurately from the labeling with (15)N and (2)H isotopes of only a few amino acid residues.

Cyclic lipopeptides from Bacillus subtilis activate distinct patterns of defence responses in grapevine

Non-self-recognition of microorganisms partly relies on the perception of microbe-associated molecular patterns (MAMPs) and leads to the activation of an innate immune response. Bacillus subtilis produces three main families of cyclic lipopeptides (LPs), namely surfactins, iturins and fengycins. Although LPs are involved in induced systemic resistance (ISR) activation, little is known about defence responses induced by these molecules and their involvement in local resistance to fungi. Here, we showed that purified surfactin, mycosubtilin (iturin family) and plipastatin (fengycin family) are perceived by grapevine plant cells. Although surfactin and mycosubtilin stimulated grapevine innate immune responses, they differentially activated early signalling pathways and defence gene expression. By contrast, plipastatin perception by grapevine cells only resulted in early signalling activation. Gene expression analysis suggested that mycosubtilin activated salicylic acid (SA) and jasmonic acid (JA) signalling pathways, whereas surfactin mainly induced an SA-regulated response. Although mycosubtilin and plipastatin displayed direct antifungal activity, only surfactin and mycosubtilin treatments resulted in a local long-lasting enhanced tolerance to the necrotrophic fungus Botrytis cinerea in grapevine leaves. Moreover, challenge with specific strains overproducing surfactin and mycosubtilin led to a slightly enhanced stimulation of the defence response compared with the LP-non-producing strain of B. subtilis. Altogether, our results provide the first comprehensive view of the involvement of LPs from B. subtilis in grapevine plant defence and local resistance against the necrotrophic pathogen Bo. cinerea. Moreover, this work is the first to highlight the ability of mycosubtilin to trigger an immune response in plants.

Cyclic LIPopeptides from Bacillus subtilis ABS-S14 elicit defense-related gene expression in citrus fruit

Effects of cyclic lipopeptides (CLPs) obtained from Bacillus subtilis ABS-S14 on eliciting defense-related gene transcription and activity of defense-related enzymes; glucanase (GLU), chitinase (CHI), peroxidase (POX) and lipoxygenase (LOX) in Citrus sinensis cv. Valencia fruit were determined. The maximum level of GLU transcripts induced in fruit treated with fengycin was significantly greatest among treatments at 48 h. Surfactin enhanced the LOX and POX transcripts. In parallel, corresponding enzyme activities were correlated with changes in gene expression observed in fruit inoculated with Penicillium digitatum following treatment with individual CLPs. Synergistic effects of fengycin and iturin A, fengycin and surfactin were shown in gene transcript of GLU and CHI, respectively, and surfactin induced POX and LOX gene expression of citrus flavedo without pathogen infection. These results suggest that fengycin and surfactin act as elicitors of defense-related gene expression in “Valencia” fruit following infection.

The bacterial lipopeptide iturins induce Verticillium dahliae cell death by affecting fungal signalling pathways and mediate plant defence responses involved in pathogen-associated molecular pattern-triggered immunity

Verticillium wilt in cotton caused by Verticillium dahliae is one of the most serious plant diseases worldwide. Because no known fungicides or cotton cultivars provide sufficient protection against this pathogen, V. dahliae causes major crop yield losses. Here, an isolated cotton endophytic bacterium, designated Bacillus amyloliquefaciens 41B-1, exhibited greater than 50% biocontrol efficacy against V. dahliae in cotton plants under greenhouse conditions. Through high-performance liquid chromatography and mass analysis of the filtrate, we found that the antifungal compounds present in the strain 41B-1 culture filtrate were a series of isoforms of iturins. The purified iturins suppressed V. dahliae microsclerotial germination in the absence or presence of cotton. Treatment with the iturins induced reactive oxygen species bursts, Hog1 mitogen-activated protein kinase (MAPK) activation and defects in cell wall integrity. The oxidative stress response and high-osmolarity glycerol pathway contribute to iturins resistance in V. dahliae. In contrast, the Slt2 MAPK pathway may be involved in iturins sensitivity in this fungus. In addition to antagonism, iturins could induce plant defence responses as activators and mediate pathogen-associated molecular pattern-triggered immunity. These findings suggest that iturins may affect fungal signalling pathways and mediate plant defence responses against V. dahliae.

Small cationic antimicrobial peptides delocalize peripheral membrane proteins

Short antimicrobial peptides rich in arginine (R) and tryptophan (W) interact with membranes. To learn how this interaction leads to bacterial death, we characterized the effects of the minimal pharmacophore RWRWRW-NH2. A ruthenium-substituted derivative of this peptide localized to the membrane in vivo, and the peptide also integrated readily into mixed phospholipid bilayers that resemble Gram-positive membranes. Proteome and Western blot analyses showed that integration of the peptide caused delocalization of peripheral membrane proteins essential for respiration and cell-wall biosynthesis, limiting cellular energy and undermining cell-wall integrity. This delocalization phenomenon also was observed with the cyclic peptide gramicidin S, indicating the generality of the mechanism. Exogenous glutamate increases tolerance to the peptide, indicating that osmotic destabilization also contributes to antibacterial efficacy. Bacillus subtilis responds to peptide stress by releasing osmoprotective amino acids, in part via mechanosensitive channels. This response is triggered by membrane-targeting bacteriolytic peptides of different structural classes as well as by hypoosmotic conditions.

Novel properties of antimicrobial peptide anoplin

Antimicrobial decapeptide anoplin was tested for its antifungal activity against plant pathogen Leptosphaeria maculans and protection of Brassica napus plants from disease. To reveal the mode of action of the peptide, a natural form of anoplin amidated on C-terminus (ANP-NH2), and its carboxylated analog (ANP-OH) were used in the study. We demonstrated strong antifungal activity of anoplin in vitro regardless C-terminus modification. In addition we show that both ANP-NH2 and ANP-OH induce expression of defence genes in B. napus and protects plants from L. maculans infection. The results indicate that the amidation of anoplin is not essential for its antifungal and plant defence stimulating activities.

Role of the 4-phosphopantetheinyl transferase of Trichoderma virens in secondary metabolism and induction of plant defense responses

Trichoderma virens is a ubiquitous soil fungus successfully used in biological control due to its efficient colonization of plant roots. In fungi, 4-phosphopantetheinyl transferases (PPTases) activate enzymes involved in primary and secondary metabolism. Therefore, we cloned the PPTase gene ppt1 from T. virens and generated PPTase-deficient (?ppt1) and overexpressing strains to investigate the role of this enzyme in biocontrol and induction of plant defense responses. The ?ppt1 mutants were auxotrophic for lysine, produced nonpigmented conidia, and were unable to synthesize nonribosomal peptides. Although spore germination was severely compromised under both low and high iron availability, mycelial growth occurred faster than the wild type, and the mutants were able to efficiently colonize plant roots. The ?ppt1 mutants were unable of inhibiting growth of phytopathogenic fungi in vitro. Arabidopsis thaliana seedlings co-cultivated with wild-type T. virens showed increased expression of pPr1a:uidA and pLox2:uidA markers, which correlated with enhanced accumulation of salicylic acid (SA), jasmonic acid, camalexin, and resistance to Botrytis cinerea. Co-cultivation of A. thaliana seedlings with ?ppt1 mutants compromised the SA and camalexin responses, resulting in decreased protection against the pathogen. Our data reveal an important role of T. virens PPT1 in antibiosis and induction of SA and camalexin-dependent plant defense responses.

All-or-none membrane permeabilization by fengycin-type lipopeptides from Bacillus subtilis QST713

The fungicidal activity of Bacillus subtilis QST713 has been utilized for the highly effective and environmentally safe protection of crops against a variety of pathogens. It is based mainly on the production of cyclic lipopeptides of the fengycin (FEs), surfactin, and iturin families. The mixed population of native FEs forms micelles which solubilize individual FEs such as agrastatin 1 (AS1) that are otherwise rather insoluble on their own. Fluorescence lifetime-based calcein efflux measurements and cryo transmission electron microscopy show that these FEs show a unique scenario of membrane permeabilization. Poor miscibility of FEs with lipid probably promotes the formation of pores in 10% of the vesicles at only≈1μM free FE and in 15% of the vesicles at 10 μM. We explain why this limited, all-or-none leakage could nevertheless account for the killing of virtually all fungi whereas the same extent of graded vesicle leakage may be biologically irrelevant. Then, crystallization of AS1 and micellization of plipastatins cause a cut-off in leakage at 15% that might regulate the biological activity of FEs, protecting Bacillus and plant membranes. The fact that FE micelles solubilize only about 10 mol-% fluid lipid resembles the behavior of detergent resistance.

Improvement of the efficacy of linear undecapeptides against plant-pathogenic bacteria by incorporation of D-amino acids

A set of 31 undecapeptides, incorporating 1 to 11 d-amino acids and derived from the antimicrobial peptide BP100 (KKLFKKILKYL-NH(2)), was designed and synthesized. This set was evaluated for inhibition of growth of the plant-pathogenic bacteria Erwinia amylovora, Pseudomonas syringae pv. syringae, and Xanthomonas axonopodis pv. vesicatoria, hemolysis, and protease degradation. Two derivatives were as active as BP100, and 10 peptides displayed improved activity, with the all-d isomer being the most active. Twenty-six peptides were less hemolytic than BP100, and all peptides were more stable against protease degradation. Plant extracts inhibited the activity of BP100 as well as that of the d-isomers. Ten derivatives incorporating one d-amino acid each were tested in an infectivity inhibition assay with the three plant-pathogenic bacteria by using detached pear and pepper leaves and pear fruits. All 10 peptides studied were active against E. amylovora, 6 displayed activity against P. syringae pv. syringae, and 2 displayed activity against X. axonopodis pv. vesicatoria. Peptides BP143 (KKLFKKILKYL-NH(2)) and BP145 (KKLFKKILKYL-NH(2)), containing one d-amino acid at positions 4 and 2 (underlined), respectively, were evaluated in whole-plant assays for the control of bacterial blight of pepper and pear and fire blight of pear. Peptide BP143 was as effective as streptomycin in the three pathosystems, was more effective than BP100 against bacterial blight of pepper and pear, and equally effective against fire blight of pear.