Expression of giant silkmoth cecropin B genes in tobacco

High-yield, plant-based production of an antimicrobial peptide with potent activity in a mouse model

Plants offer a promising chassis for the large-scale, cost-effective production of diverse therapeutics, including antimicrobial peptides (AMPs). However, key advances will reduce production costs, including simplifying the downstream processing and purification steps. Here, using Nicotiana benthamiana plants, we present an improved modular design that enables AMPs to be secreted via the endomembrane system and sequestered in an extracellular compartment, the apoplast. Additionally, we translationally fused an AMP to a mutated small ubiquitin-like modifier sequence, thereby enhancing peptide yield and solubilizing the peptide with minimal aggregation and reduced occurrence of necrotic lesions in the plant. This strategy resulted in substantial peptide accumulation, reaching around 2.9 mg AMP per 20 g fresh weight of leaf tissue. Furthermore, the purified AMP demonstrated low collateral toxicity in primary human skin cells, killed pathogenic bacteria by permeabilizing the membrane and exhibited anti-infective efficacy in a preclinical mouse (Mus musculus) model system, reducing bacterial loads by up to three orders of magnitude. A base-case techno-economic analysis demonstrated the economic advantages and scalability of our plant-based platform. We envision that our work can establish plants as efficient bioreactors for producing preclinical-grade AMPs at a commercial scale, with the potential for clinical applications.

Recombinant production of antimicrobial peptides in plants

Classical plant breeding methods are limited in their ability to confer disease resistance on plants. However, in recent years, advancements in molecular breeding and biotechnological have provided new approaches to overcome these limitations and protect plants from disease. Antimicrobial peptides (AMPs) constitute promising agents that may be able to protect against infectious agents. Recently, peptides have been recombinantly produced in plants at scale and low cost. Because AMPs are less likely than conventional antimicrobials to elicit resistance of pathogenic bacteria, they open up exciting new avenues for agricultural applications. Here, we review recent advances in the design and production of bioactive recombinant AMPs that can effectively protect crop plants from diseases.

Enhancing disease resistance in poplar through modification of its natural defense pathway

Modification of the poplar defense pathway through pathogen-induced expression of an amphibian host defense peptide modulates plant innate immunity and confers robust and reliable resistance against a major poplar pathogen, Septoria musiva. Host defense peptides (HDPs), also known as cationic antimicrobial peptides, represent a diverse group of small membrane-active molecules that are part of the innate defense system of their hosts against pathogen invasion. Here we describe a strategy for development of poplar plants with enhanced HDP production and resistance to the commercially significant fungal pathogen Septoria musiva. The naturally occurring linear amphipathic α-helical HDP dermaseptin B1, which has 31 residues and originated from the skin secretion of arboreal frogs, was N-terminally modified (MsrA2) and evaluated in vitro for antifungal activity and phytotoxicity. The MsrA2 peptide inhibited germination of S. musiva conidia at physiologically relevant low micromolar concentrations that were non-toxic to poplar protoplasts. The nucleotide sequence of MsrA2, optimized for expression in plants, was introduced into the commercial hybrid poplar Populus nigra L. × P. maximowiczii A. Henry (NM6) via Agrobacterium-mediated transformation. Transgene expression was regulated by the pathogen-inducible poplar promoter win3.12T, a part of the poplar innate defense system. Most importantly, the induced accumulation of MsrA2 peptide in poplar leaves was sufficient to confer resistance against S. musiva. The antifungal resistance of plants with high MsrA2 expression and MsrA2 accumulation was strong and reproducible, and without deleterious effects on plant growth and development. These results provide an insight into development of new technologies for engineering durable disease resistance against major pathogens of poplar and other plants.

Enhanced vascular activity of a new chimeric promoter containing the full CaMV 35S promoter and the plant XYLOGEN PROTEIN 1 promoter

To develop a new strategy that controls vascular pathogen infections in economic crops, we examined a possible enhancer of the vascular activity of XYLOGEN PROTEIN 1 promoter (Px). This protein is specifically expressed in the vascular tissues of Arabidopsis thaliana and plays an important role in xylem development. Although Px is predicted as vascular-specific, its activity is hard to detect and highly susceptible to plant and environmental conditions. The cauliflower mosaic virus 35S promoter (35S) is highly active in directing transgene expression. To test if 35S could enhance Px activity, while vascular specificity of the promoter is retained, we examined the expression of the uidA reporter gene, which encodes β-glucuronidase (GUS), under the control of a chimeric promoter (35S-Px) or Px by generating 35S-Px-GUS and Px-GUS constructs, which were transformed into tobacco seedlings. Both 35S-Px and Px regulated gene expression in vascular tissues. However, GUS expression driven by 35S-Px was not detected in 30- and 60-day-old plants. Quantitative real-time PCR analysis showed that GUS gene expression regulated by 35S-Px was 6.2-14.9-fold higher in vascular tissues than in leaves. Histochemical GUS staining demonstrated that 35S-Px was strongly active in the xylem and phloem. Thus, fusion of 35S and Px might considerably enhance the strength of Px and increase its vascular specificity. In addition to confirming that 35S enhances the activity of a low-level tissue-specific promoter, these findings provide information for further improving the activity of such promoters, which might be useful for engineering new types of resistant genes against vascular infections.

Transgenic expression of plant-specific insert of potato aspartic proteases (StAP-PSI) confers enhanced resistance to Botrytis cinerea in Arabidopsis thaliana

The plant-specific insert of Solanum tuberosum aspartic proteases (StAP-PSI) has high structural similarity with NK-lysin and granulysin, two saposin-like proteins (SAPLIPs) with antimicrobial activity. Recombinant StAP-PSI and some SAPLIPs show antimicrobial activity against pathogens that affect human and plants. In this work, we transformed Arabidopsis thaliana plants with StAP-PSI encoding sequence with its corresponding signal peptide under the control of the cauliflower mosaic virus (CaMV) 35S promoter. Results obtained show that StAP-PSI significantly enhances Arabidopsis resistance against Botrytis cinerea infection. StAP-PSI is secreted into the leaf apoplast and acts directly against pathogens; thereby complementing plant innate immune responses. Data obtained from real-time PCR assays show that the constitutive expression of StAP-PSI induces the expression of genes that regulate jasmonic acid signalling pathway, such as PDF1.2, in response to infection due to necrotrophic pathogens. On the other hand, according to the data described for other antimicrobial peptides, the presence of the StAP-PSI protein in the apoplast of A. thaliana leaves is responsible for the expression of salicylic acid-associated genes, such as PR-1, irrespective of infection with B. cinerea. These results indicate that the increased resistance demonstrated by A. thaliana plants that constitutively express StAP-PSI owing to B. cinerea infection compared to the wild-type plants is a consequence of two factors, i.e., the antifungal activity of StAP-PSI and the overexpression of A. thaliana defense genes induced by the constitutive expression of StAP-PSI. We suggest that the use of this protein would help in minimizing the ecological and health risks that arise from the use of pesticides. We suggest that the use of this protein would help in minimizing the ecological and health risks that arise from the spreading of resistance of agriculturally important pathogens.

SP-LL-37, human antimicrobial peptide, enhances disease resistance in transgenic rice

Human LL-37 is a multifunctional antimicrobial peptide of cathelicidin family. It has been shown in recent studies that it can serve as a host's defense against influenza A virus. We now demonstrate in this study how signal peptide LL-37 (SP-LL-37) can be used in rice resistance against bacterial leaf blight and blast. We synthesized LL-37 peptide and subcloned in a recombinant pPZP vector with pGD1 as promoter. SP-LL-37 was introduced into rice plants by Agrobacterium mediated transformation. Stable expression of SP-LL-37 in transgenic rice plants was confirmed by RT-PCR and ELISA analyses. Subcellular localization of SP-LL-37-GFP fusion protein showed evidently in intercellular space. Our data on testing for resistance to bacterial leaf blight and blast revealed that the transgenic lines are highly resistant compared to its wildtype. Our results suggest that LL-37 can be further explored to improve wide-spectrum resistance to biotic stress in rice.

Transgenic citrus expressing synthesized cecropin B genes in the phloem exhibits decreased susceptibility to Huanglongbing

Expression of synthesized cecropin B genes in the citrus phloem, where Candidatus Liberibacter asiaticus resides, significantly decreased host susceptibility to Huanglongbing. Huanglongbing (HLB), associated with Candidatus Liberibacter asiaticus bacteria, is the most destructive disease of citrus worldwide. All of the commercial sweet orange cultivars lack resistance to this disease. The cationic lytic peptide cecropin B, isolated from the Chinese tasar moth (Antheraea pernyi), has been shown to effectively eliminate bacteria. In this study, we demonstrated that transgenic citrus (Citrus sinensis Osbeck) expressing the cecropin B gene specifically in the phloem had a decreased susceptibility to HLB. Three plant codon-optimized synthetic cecropin B genes, which were designed to secrete the cecropin B peptide into three specific sites, the extracellular space, the cytoplasm, and the endoplasmic reticulum, were constructed. Under the control of the selected phloem-specific promoter GRP1.8, these constructs were transferred into the citrus genome. All of the cecropin B genes were efficiently expressed in the phloem of transgenic plants. Over more than a year of evaluation, the transgenic lines exhibited reduced disease severity. Bacterial populations in transgenic lines were significantly lower than in the controls. Two lines, in which bacterial populations were significantly lower than in others, showed no visible symptoms. Thus, we demonstrated the potential application of the phloem-specific expression of an antimicrobial peptide gene to protect citrus plants from HLB.

Inducible Expression of the De-Novo Designed Antimicrobial Peptide SP1-1 in Tomato Confers Resistance to Xanthomonas campestris pv. vesicatoria

Antimicrobial peptides (AMPs) are small peptides with less than 50 amino acids and are part of the innate immune response in almost all organisms, including bacteria, vertebrates, invertebrates and plants. AMPs are active against a broad-spectrum of pathogens. The inducible expression of AMPs in plants is a promising approach to combat plant pathogens with minimal negative side effects, such as phytotoxicity or infertility. In this study, inducible expression of the de-novo designed AMP SP1-1 in Micro Tom tomato protected tomato fruits against bacterial spot disease caused by Xanthomonas campestris pv. vesicatoria. The peptide SP1-1 was targeted to the apoplast which is the primary infection site for plant pathogens, by fusing SP1-1 peptide to the signal peptide RsAFP1 of radish (Raphanus sativus). The pathogen inducibility of the expression was enabled by using an optimized inducible 4XW2/4XS promoter. As a result, the tomato fruits of independently generated SP1-1 transgenic lines were significantly more resistant to X. campestris pv. vesicatoria than WT tomato fruits. In transgenic lines, bacterial infection was reduced up to 65% in comparison to the infection of WT plants. Our study demonstrates that the combination of the 4XW2/4XS cis-element from parsley with the synthetic antimicrobial peptide SP1-1 is a good alternative to protect tomato fruits against infections with X. campestris pv. vesicatoria.

Production of Biologically Active Cecropin A Peptide in Rice Seed Oil Bodies

Cecropin A is a natural antimicrobial peptide that exhibits fast and potent activity against a broad spectrum of pathogens and neoplastic cells, and that has important biotechnological applications. However, cecropin A exploitation, as for other antimicrobial peptides, is limited by their production and purification costs. Here, we report the efficient production of this bioactive peptide in rice bran using the rice oleosin 18 as a carrier protein. High cecropin A levels were reached in rice seeds driving the expression of the chimeric gene by the strong embryo-specific oleosin 18 own promoter, and targeting the peptide to the oil body organelle as an oleosin 18-cecropin A fusion protein. The accumulation of cecropin A in oil bodies had no deleterious effects on seed viability and seedling growth, as well as on seed yield. We also show that biologically active cecropin A can be easily purified from the transgenic rice seeds by homogenization and simple flotation centrifugation methods. Our results demonstrate that the oleosin fusion technology is suitable for the production of cecropin A in rice seeds, which can potentially be extended to other antimicrobial peptides to assist their exploitation.

Overexpression of antimicrobial lytic peptides protects grapevine from Pierce's disease under greenhouse but not field conditions

Pierce's disease (PD) caused by Xylella fastidiosa prevents cultivation of grapevine (Vitis vinifera) and susceptible hybrids in the southeastern United States and poses a major threat to the grape industry of California and Texas. Genetic resistance is the only proven control of X. fastidiosa. Genetic engineering offers an alternative to heretofore ineffective conventional breeding in order to transfer only PD resistance traits into elite cultivars. A synthetic gene encoding lytic peptide LIMA-A was introduced into V. vinifera and a Vitis hybrid to assess in planta inhibition of X. fastidiosa. Over 1050 independent transgenic plant lines were evaluated in the greenhouse, among which nine lines were selected and tested under naturally-inoculated field conditions. These selected plant lines in the greenhouse remain disease-free for 10 years, to date, even with multiple manual pathogen inoculations. However, all these lines in the field, including a grafted transgenic rootstock, succumbed to PD within 7 years. We conclude that in planta production of antimicrobial lytic peptides does not provide durable PD resistance to grapevine under field conditions.

Label-free nanoUPLC-MSE based quantification of antimicrobial peptides from the leaf apoplast of Nicotiana attenuata

BACKGROUND

Overexpressing novel antimicrobial peptides (AMPs) in plants is a promising approach for crop disease resistance engineering. However, the in planta stability and subcellular localization of each AMP should be validated for the respective plant species, which can be challenging due to the small sizes and extreme pI ranges of AMPs which limits the utility of standard proteomic gel-based methods. Despite recent advances in quantitative shotgun proteomics, its potential for AMP analysis has not been utilized and high throughput methods are still lacking.

RESULTS

We created transgenic Nicotiana attenuata plants that independently express 10 different AMPs under a constitutive 35S promoter and compared the extracellular accumulation of each AMP using a universal and versatile protein quantification method. We coupled a rapid apoplastic peptide extraction with label-free protein quantification by nanoUPLC-MSE analysis using Hi3 method and identified/quantified 7 of 10 expressed AMPs in the transgenic plants ranging from 37 to 91 amino acids in length. The quantitative comparison among the transgenic plant lines showed that three particular peptides, belonging to the defensin, knottin and lipid-transfer protein families, attained the highest concentrations of 91 to 254 pmol per g leaf fresh mass, which identified them as best suited for ectopic expression in N. attenuata. The chosen mass spectrometric approach proved to be highly sensitive in the detection of different AMP types and exhibited the high level of analytical reproducibility required for label-free quantitative measurements along with a simple protocol required for the sample preparation.

CONCLUSIONS

Heterologous expression of AMPs in plants can result in highly variable and non-predictable peptide amounts and we present a universal quantitative method to confirm peptide stability and extracellular deposition. The method allows for the rapid quantification of apoplastic peptides without cumbersome and time-consuming purification or chromatographic steps and can be easily adapted to other plant species.

Multitasking antimicrobial peptides in plant development and host defense against biotic/abiotic stress

Crop losses due to pathogens are a major threat to global food security. Plants employ a multilayer defense against a pathogen including the use of physical barriers (cell wall), induction of hypersensitive defense response (HR), resistance (R) proteins, and synthesis of antimicrobial peptides (AMPs). Unlike a complex R gene-mediated immunity, AMPs directly target diverse microbial pathogens. Many a times, R-mediated immunity breaks down and plant defense is compromised. Although R-gene dependent pathogen resistance has been well studied, comparatively little is known about the interactions of AMPs with host defense and physiology. AMPs are ubiquitous, low molecular weight peptides that display broad spectrum resistance against bacteria, fungi and viruses. In plants, AMPs are mainly classified into cyclotides, defensins, thionins, lipid transfer proteins, snakins, and hevein-like vicilin-like and knottins. Genetic distance lineages suggest their conservation with minimal effect of speciation events during evolution. AMPs provide durable resistance in plants through a combination of membrane lysis and cellular toxicity of the pathogen. Plant hormones - gibberellins, ethylene, jasmonates, and salicylic acid, are among the physiological regulators that regulate the expression of AMPs. Transgenically produced AMP-plants have become a means showing that AMPs are able to mitigate host defense responses while providing durable resistance against pathogens.

Production of cecropin A antimicrobial peptide in rice seed endosperm

BACKGROUND

Cecropin A is a natural antimicrobial peptide that exhibits rapid, potent and long-lasting lytic activity against a broad spectrum of pathogens, thus having great biotechnological potential. Here, we report a system for producing bioactive cecropin A in rice seeds.

RESULTS

Transgenic rice plants expressing a codon-optimized synthetic cecropin A gene drived by an endosperm-specific promoter, either the glutelin B1 or glutelin B4 promoter, were generated. The signal peptide sequence from either the glutelin B1 or the glutelin B4 were N-terminally fused to the coding sequence of the cecropin A. We also studied whether the presence of the KDEL endoplasmic reticulum retention signal at the C-terminal has an effect on cecropin A subcellular localization and accumulation. The transgenic rice plants showed stable transgene integration and inheritance. We show that cecropin A accumulates in protein storage bodies in the rice endosperm, particularly in type II protein bodies, supporting that the glutelin N-terminal signal peptides play a crucial role in directing the cecropin A to this organelle, independently of being tagged with the KDEL endoplasmic reticulum retention signal. The production of cecropin A in transgenic rice seeds did not affect seed viability or seedling growth. Furthermore, transgenic cecropin A seeds exhibited resistance to infection by fungal and bacterial pathogens (Fusarium verticillioides and Dickeya dadantii, respectively) indicating that the in planta-produced cecropin A is biologically active.

CONCLUSIONS

Rice seeds can sustain bioactive cecropin A production and accumulation in protein bodies. The system might benefit the production of this antimicrobial agent for subsequent applications in crop protection and food preservation.

Expression of antimicrobial peptides thanatin(S) in transgenic Arabidopsis enhanced resistance to phytopathogenic fungi and bacteria

Thanatin(S) is an analog of thanatin, an insect antimicrobial peptide possessing strong and broad spectrum of antimicrobial activity. In order to investigate if the thanatin could be used in engineering transgenic plants for increased resistance against phytopathogens, the synthetic thanatin(S) was introduced into Arabidopsis thaliana plants. To increase the expression level of thanatin(S) in plants, the coding sequence was optimized by plant-preference codon. To avoid cellular protease degradation, signal peptide of rice Cht1 was fused to N terminal of thanatin(S) for secreting the expressed thanatin(S) into intercellular spaces. To evaluate the application value of thanatin(S) in plant disease control, the synthesized coding sequence of Cht1 signal peptide (Cht1SP)-thanatin(S) was ligated to plant gateway destination binary vectors pGWB11 (with FLAG tag). Meanwhile, in order to observe the subcellular localization of Cht1SP-thanatin(S)-GFP and thanatin(S)-GFP, the sequences of Cht1SP-thanatin(S) and thanatin(S) were respectively linked to pGWB5 (with GFP tag). The constructs were transformed into Arabidopsis ecotype Col-0 and mutant pad4-1 via Agrobacterium-mediated transformation. The transformants with Cht1SP-thanatin(S)-FLAG fusion gene were analyzed by genomic PCR, real-time PCR, and western blots and the transgenic Arabidopsis plants introduced respectively Cht1SP-thanatin(S)-GFP and thanatin(S)-GFP were observed by confocal microscopy. Transgenic plants expressing Cht1SP-thanatin(S)-FLAG fusion protein showed antifungal activity against Botrytis cinerea and powdery mildew, as well as antibacterial activity against Pseudomonas syringae pv. tomato. And the results from confocal observation showed that the GFP signal from Cht1SP-thanatin(S)-GFP transgenic Arabidopsis plants occurred mainly in intercellular space, while that from thanatin(S)-GFP transgenic plants was mainly detected in the cytoplasm and that from empty vector transgenic plants was distributed uniformly throughout the cell, demonstrating that Cht1 signal peptide functioned. In addition, thanatin(S) and thanatin(S)-FLAG chemically synthesized have both in vitro antimicrobial activities against P. syringae pv. tomato and B. cinerea. So, thanatin(S) is an ideal candidate AMPs for the construction of transgenic crops endowed with a broad-spectrum resistance to phytopathogens and the strategy is feasible to link a signal peptide to the target gene.

Constitutive expression of transgenes encoding derivatives of the synthetic antimicrobial peptide BP100: impact on rice host plant fitness

BACKGROUND

The Biopeptide BP100 is a synthetic and strongly cationic α-helical undecapeptide with high, specific antibacterial activity against economically important plant-pathogenic bacteria, and very low toxicity. It was selected from a library of synthetic peptides, along with other peptides with activities against relevant bacterial and fungal species. Expression of the BP100 series of peptides in plants is of major interest to establish disease-resistant plants and facilitate molecular farming. Specific challenges were the small length, peptide degradation by plant proteases and toxicity to the host plant. Here we approached the expression of the BP100 peptide series in plants using BP100 as a proof-of-concept.

RESULTS

Our design considered up to three tandemly arranged BP100 units and peptide accumulation in the endoplasmic reticulum (ER), analyzing five BP100 derivatives. The ER retention sequence did not reduce the antimicrobial activity of chemically synthesized BP100 derivatives, making this strategy possible. Transformation with sequences encoding BP100 derivatives (bp100der) was over ten-fold less efficient than that of the hygromycin phosphotransferase (hptII) transgene. The BP100 direct tandems did not show higher antimicrobial activity than BP100, and genetically modified (GM) plants constitutively expressing them were not viable. In contrast, inverted repeats of BP100, whether or not elongated with a portion of a natural antimicrobial peptide (AMP), had higher antimicrobial activity, and fertile GM rice lines constitutively expressing bp100der were produced. These GM lines had increased resistance to the pathogens Dickeya chrysanthemi and Fusarium verticillioides, and tolerance to oxidative stress, with agronomic performance comparable to untransformed lines.

CONCLUSIONS

Constitutive expression of transgenes encoding short cationic α-helical synthetic peptides can have a strong negative impact on rice fitness. However, GM plants expressing, for example, BP100 based on inverted repeats, have adequate agronomic performance and resistant phenotypes as a result of a complex equilibrium between bp100der toxicity to plant cells, antimicrobial activity and transgene-derived plant stress response. It is likely that these results can be extended to other peptides with similar characteristics.

Control of apple blue mold by Pichia pastoris recombinant strains expressing cecropin A

Recombinant Pichia pastoris yeasts expressing cecropin A (GS115/CEC), was evaluated for the control of the blue mold of apple caused by Penicillium expansum due to cecropin A peptide's effective antimicrobial effects on P. expansum spores by the thiazolyl blue (MTT) assay. Then, the protein concentration was determined and it was expressed at high levels up to 14.2 mg/L in the culture medium. Meanwhile, the population growth was assayed in vivo. The population growth of recombinant strain GS115/CEC was higher than that of non-transformed strain GS115 in red Fuji apples wounds. Recombinant yeast strains GS115/CEC significantly inhibited growth of germinated P. expansum spores in vitro and inhibited decay development caused by P. expansum in apple fruits in vivo when compared with apple fruits inoculated with sterile water or the yeast strain GS115/pPIC (plasmid pPIC9k transformed in GS115). This study demonstrated the potential of expression of the antifungal peptide in yeast for the control of postharvest blue mold infections on pome fruits.

Expression of a novel antimicrobial peptide Penaeidin4-1 in creeping bentgrass (Agrostis stolonifera L.) enhances plant fungal disease resistance

Background

Turfgrass species are agriculturally and economically important perennial crops. Turfgrass species are highly susceptible to a wide range of fungal pathogens. Dollar spot and brown patch, two important diseases caused by fungal pathogens Sclerotinia homoecarpa and Rhizoctonia solani, respectively, are among the most severe turfgrass diseases. Currently, turf fungal disease control mainly relies on fungicide treatments, which raises many concerns for human health and the environment. Antimicrobial peptides found in various organisms play an important role in innate immune response.

Methodology/Principal Findings

The antimicrobial peptide - Penaeidin4-1 (Pen4-1) from the shrimp, Litopenaeus setiferus has been reported to possess in vitro antifungal and antibacterial activities against various economically important fungal and bacterial pathogens. In this study, we have studied the feasibility of using this novel peptide for engineering enhanced disease resistance into creeping bentgrass plants (Agrostis stolonifera L., cv. Penn A-4). Two DNA constructs were prepared containing either the coding sequence of a single peptide, Pen4-1 or the DNA sequence coding for the transit signal peptide of the secreted tobacco AP24 protein translationally fused to the Pen4-1 coding sequence. A maize ubiquitin promoter was used in both constructs to drive gene expression. Transgenic turfgrass plants containing different DNA constructs were generated by Agrobacterium-mediated transformation and analyzed for transgene insertion and expression. In replicated in vitro and in vivo experiments under controlled environments, transgenic plants exhibited significantly enhanced resistance to dollar spot and brown patch, the two major fungal diseases in turfgrass. The targeting of Pen4-1 to endoplasmic reticulum by the transit peptide of AP24 protein did not significantly impact disease resistance in transgenic plants.

Conclusion/Significance

Our results demonstrate the effectiveness of Pen4-1 in a perennial species against fungal pathogens and suggest a potential strategy for engineering broad-spectrum fungal disease resistance in crop species.

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.

Stable production of peptide antigens in transgenic tobacco chloroplasts by fusion to the p53 tetramerisation domain

The production of short peptides as single molecules in recombinant systems is often limited by the low stability of the foreign peptide. In the plant expression system this problem has been solved by translational fusions to recombinant proteins that are highly stable or are able to form complex structures. Previously, we demonstrated that the highly immunogenic 21 amino acid peptide 2L21, which is derived from the canine parvovirus (CPV) VP2 protein, did not accumulate in transgenic tobacco chloroplasts. In this report, we translationally fused the 2L21 peptide to the 42 amino acid tetramerisation domain (TD) from the human transcription factor p53. The chimaeric 2L21-TD protein was expressed in tobacco chloroplasts. Leaves accumulated high levels of the recombinant protein (up to 0.4 mg/g fresh weight of leaf material, equivalent to ~6% of total soluble protein; 2% considering only the 2L21 peptide). The 2L21-TD protein was able to form tetramers in the stroma of the chloroplast. Mice immunised intraperitoneally with partially purified leaf extracts containing the 2L21-TD protein developed specific antibodies with titres similar to those elicited by a previously reported fusion between 2L21 and the B subunit of the cholera toxin. Mouse sera were able to detect both the 2L21 synthetic peptide and the CPV VP2 protein, showing that the antigenicity of the 2L21 epitope was preserved in the chimaeric protein. These results demonstrate that the p53 TD can be used as a carrier molecule for the accumulation of short peptides (such as 2L21) in the chloroplast without altering the immunogenic properties of the peptide.

Expression of a synthesized gene encoding cationic peptide cecropin B in transgenic tomato plants protects against bacterial diseases

The cationic lytic peptide cecropin B (CB), isolated from the giant silk moth (Hyalophora cecropia), has been shown to effectively eliminate Gram-negative and some Gram-positive bacteria. In this study, the effects of chemically synthesized CB on plant pathogens were investigated. The S(50)s (the peptide concentrations causing 50% survival of a pathogenic bacterium) of CB against two major pathogens of the tomato, Ralstonia solanacearum and Xanthomonas campestris pv. vesicatoria, were 529.6 microg/ml and 0.29 microg/ml, respectively. The CB gene was then fused to the secretory signal peptide (sp) sequence from the barley alpha-amylase gene, and the new construct, pBI121-spCB, was used for the transformation of tomato plants. Integration of the CB gene into the tomato genome was confirmed by PCR, and its expression was confirmed by Western blot analyses. In vivo studies of the transgenic tomato plant demonstrated significant resistance to bacterial wilt and bacterial spot. The levels of CB expressed in transgenic tomato plants ( approximately 0.05 microg in 50 mg of leaves) were far lower than the S(50) determined in vitro. CB transgenic tomatoes could therefore be a new mode of bioprotection against these two plant diseases with significant agricultural applications.

Identification and rational design of novel antimicrobial peptides for plant protection

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

Comparison of pathogen-induced expression and efficacy of two amphibian antimicrobial peptides, MsrA2 and temporin A, for engineering wide-spectrum disease resistance in tobacco

The rapid accumulation of defensive transgene products in plants only on pathogen invasion has clear advantages over their constitutive synthesis. In this study, two antimicrobial peptides from the skin secretions of frogs, MsrA2 (N-methionine-dermaseptin B1) and temporin A, were evaluated for engineering pathogen-induced disease resistance in plants. Both peptides inhibited plant-specific pathogens in vitro at micromolar concentrations that were not toxic to plant protoplasts. The plant-optimized nucleotide sequences encoding MsrA2 and temporin A were transcriptionally fused to the inducible win3.12T poplar promoter, which had strong systemic activity in response to fungal infection, and introduced into tobacco (Nicotiana tabacum L. cv. Xanthi). Transgene expression was very low in the leaves of unstressed plants; however, it was strongly increased after pathogen challenge or wounding. The pathogen responsiveness of the win3.12T promoter was found to be universal rather than species specific, with high activity in response to all pathogens tested. On induction, the amount of MsrA2 was up to 6-7 microg per gram of fresh leaf tissue. Most importantly, the induced accumulation of MsrA2 and temporin A in transgenic tobacco was sufficient to confer resistance to a variety of phytopathogenic fungi, such as Fusarium solani, F. oxysporum, Alternaria alternata, Botrytis cinerea, Sclerotinia sclerotiorum, the oomycete Pythium aphanidermatum and the bacterium Pectobacterium carotovorum. The accumulation of these peptides in transgenic plants did not alter the normal phenotype of tobacco. Thus, the expression of MsrA2 and temporin A in a pathogen-inducible manner enables the development of tobacco, and possibly other plant species, with wide-spectrum disease resistance, which can reduce the use of pesticides and the associated environmental risks.

Increased pathogen resistance and yield in transgenic plants expressing combinations of the modified antimicrobial peptides based on indolicidin and magainin

Reverse peptide of indolicidin (Rev4), a 13-residue peptide based on the sequence of indolicidin, has been shown to possess both strong antimicrobial and protease inhibitory activities in vitro. To evaluate its efficacy in vivo, we produced and evaluated transgenic tobacco (Nicotiana tabacum L.) and Arabidopsis thaliana [(L.) Heynh.] plants expressing Rev4 with different signal peptide sequences for pathogen resistance. All transgenic plants showed normal growth and development, an indication of no or low cytotoxicity of the peptide. Furthermore, the transgenic plants exhibited elevated resistance to three bacterial and two oomycete pathogens. Interestingly, tobacco plants expressing Rev4 displayed enhanced yield compared to the control as indicated by an increased biomass production by as much as 34% in two field trials. When Rev4 was coexpressed with another antimicrobial peptide, Myp30, the disease resistance levels in the transgenic Arabidopsis were enhanced. These findings suggest the potential of using these peptides to protect plants from microbial pathogens and to enhance yield.

Enhanced resistance to the rice blast fungus Magnaporthe grisea conferred by expression of a cecropin A gene in transgenic rice

Cecropins are a family of antimicrobial peptides, which constitute an important key component of the immune response in insects. Here, we demonstrate that transgenic rice (Oryza sativa L.) plants expressing the cecropin A gene from the giant silk moth Hyalophora cecropia show enhanced resistance to Magnaporthe grisea, the causal agent of the rice blast disease. Two plant codon-optimized synthetic cecropin A genes, which were designed either to retain the cecropin A peptide in the endoplasmic reticulum, the ER-CecA gene, or to secrete cecropin A to the extracellular space, the Ap-CecA gene, were prepared. Both cecropin A genes were efficiently expressed in transgenic rice. The inhibitory activity of protein extracts prepared from leaves of cecropin A-expressing plants on the in vitro growth of M. grisea indicated that the cecropin A protein produced by the transgenic rice plants was biologically active. Whereas no effect on plant phenotype was observed in ER-CecA plants, most of the rice lines expressing the Ap-CecA gene were non-fertile. Cecropin A rice plants exhibited resistance to rice blast at various levels. Transgene expression of cecropin A genes was not accompanied by an induction of pathogenesis-related (PR) gene expression supporting that the transgene product itself is directly active against the pathogen. Taken together, the results presented in this study suggest that the cecropin A gene, when designed for retention of cecropin A into the endoplasmic reticulum, could be a useful candidate for protection of rice plants against the rice blast fungus M. grisea.

Effects of cecropin B transgene expression on Mannheimia haemolytica serotype 1 colonization of the nasal mucosa of calves

OBJECTIVE

To express a cecropin B transgene on bovine nasal mucosa and determine the effect on Mannheimia haemolytica serotype 1 (S1) colonization.

ANIMALS

27 crossbred beef calves.

PROCEDURE

The antibacterial efficacy of cecropin B against M. haemolytica S1 was first determined by measuring its minimum inhibitory concentration (MIC). The peptide was also diluted in pooled bovine nasal secretions, and its antibacterial activity was evaluated. The nasal passages of 16 calves were aerosolized with 25, 50, or 100 microg of plasmid DNA/nostril, whereas 11 control calves were aerosolized with only the transfection reagent. In 2 of the experiments, 12 treated and 8 control calves were exposed intranasally with an aerosol of M. haemolytica S1. Nasal swab specimens and secretions were collected and analyzed by use of polymerase chain reaction (PCR), real-time PCR, real-time reverse-transcriptase PCR, ELISA, and bacterial culture.

RESULTS

In vitro, cecropin B inhibited M. haemolytica S1 at an MIC of 2 microg/mL and its antibacterial activity was not affected by proteolytic activity in nasal secretions. Cecropin B transgene expression was detected in calves transfected with 50 or 100 microg of DNA/nostril. Antibacterial activity against M. haemolytica S1 was observed in all calves transfected with 100 microg of DNA/nostril but in only 2 of the 4 calves transfected with 50 microg of DNA/nostril.

CONCLUSIONS AND CLINICAL RELEVANCE

In vitro, cecropin B has an effective antibacterial activity against M. haemolytica S1 and can prevent colonization of the nasal mucosa after transfection of a vector expressing cecropin B in vivo.

Disease resistance conferred by the expression of a gene encoding a synthetic peptide in transgenic cotton (Gossypium hirsutum L.) plants

Fertile, transgenic cotton plants expressing the synthetic antimicrobial peptide, D4E1, were produced through Agrobacterium-mediated transformation. PCR products and Southern blots confirmed integration of the D4E1 gene, while RT-PCR of cotton RNA confirmed the presence of D4E1 transcripts. In vitro assays with crude leaf protein extracts from T0 and T1 plants confirmed that D4E1 was expressed at sufficient levels to inhibit the growth of Fusarium verticillioides and Verticillium dahliae compared to extracts from negative control plants transformed with pBI-d35S(Omega)-uidA-nos (CGUS). Although in vitro assays did not show control of pre-germinated spores of Aspergillus flavus, bioassays with cotton seeds in situ or in planta, inoculated with a GFP-expressing A. flavus, indicated that the transgenic cotton seeds inhibited extensive colonization and spread by the fungus in cotyledons and seed coats. In planta assays with the fungal pathogen, Thielaviopsis basicola, which causes black root rot in cotton, showed typical symptoms such as black discoloration and constriction on hypocotyls, reduced branching of roots in CGUS negative control T1 seedlings, while transgenic T1 seedlings showed a significant reduction in disease symptoms and increased seedling fresh weight, demonstrating tolerance to the fungal pathogen. Significant advantages of synthetic peptides in developing transgenic crop plants that are resistant to diseases and mycotoxin-causing fungal pathogens are highlighted in this report.

Production of an engineered killer peptide in Nicotiana benthamiana by using a potato virus X expression system

The decapeptide killer peptide (KP) derived from the sequence of a single-chain, anti-idiotypic antibody acting as a functional internal image of a microbicidal, broad-spectrum yeast killer toxin (KT) was shown to exert a strong microbicidal activity against human pathogens. With the aim to exploit this peptide to confer resistance to plant pathogens, we assayed its antimicrobial activity against a broad spectrum of phytopathogenic bacteria and fungi. Synthetic KP exhibited antimicrobial activity in vitro towards Pseudomonas syringae, Erwinia carotovora, Botrytis cinerea, and Fusarium oxysporum. KP was also expressed in plants by using a Potato virus X (PVX)-derived vector as a fusion to the viral coat protein, yielding chimeric virus particles (CVPs) displaying the heterologous peptide. Purified CVPs showed enhanced antimicrobial activity against the above-mentioned plant pathogens and human pathogens such as Staphylococcus aureus and Candida albicans. Moreover, in vivo assays designed to challenge KP-expressing plants (as CVPs) with Pseudomonas syringae pv. tabaci showed enhanced resistance to bacterial attack. The results indicate that the PVX-based display system is a high-yield, rapid, and efficient method to produce and evaluate antimicrobial peptides in plants, representing a milestone for the large-scale production of high-added-value peptides through molecular farming. Moreover, KP is a promising molecule to be stably engineered in plants to confer broad-spectrum resistance to phytopathogens.

Pathogen-induced expression of a cecropin A-melittin antimicrobial peptide gene confers antifungal resistance in transgenic tobacco

Expression of defensive genes from a promoter that is specifically activated in response to pathogen invasion is highly desirable for engineering disease-resistant plants. A plant transformation vector was constructed with transcriptional fusion between the pathogen-responsive win3.12T promoter from poplar and the gene encoding the novel cecropin A-melittin hybrid peptide (CEMA) with strong antimicrobial activity. This promoter-transgene combination was evaluated in transgenic tobacco (Nicotiana tabacum L. cv. Xanthi) for enhanced plant resistance against a highly virulent pathogenic fungus Fusarium solani. Transgene expression in leaves was strongly increased after fungal infection or mechanical wounding, and the accumulation of CEMA transcripts was found to be systemic and positively correlated with the number of transgene insertions. A simple and efficient in vitro regeneration bioassay for preliminary screening of transgenic lines against pathogenic fungi was developed. CEMA had strong antifungal activity in vitro, inhibiting conidia germination at concentrations that were non-toxic to tobacco protoplasts. Most importantly, the expression level of the CEMA peptide in vivo, regulated by the win3.12T promoter, was sufficient to confer resistance against F. solani in transgenic tobacco. The antifungal resistance of plants with high CEMA expression was strong and reproducible. In addition, leaf tissue extracts from transgenic plants significantly reduced the number of fungal colonies arising from germinated conidia. Accumulation of CEMA peptide in transgenic tobacco had no deleterious effect on plant growth and development. This is the first report showing the application of a heterologous pathogen-inducible promoter to direct the expression of an antimicrobial peptide in plants, and the feasibility of this approach to provide disease resistance in tobacco and, possibly, other crops.

Recent advances in rice biotechnology--towards genetically superior transgenic rice

Rice biotechnology has made rapid advances since the first transgenic rice plants were produced 15 years ago. Over the past decade, this progress has resulted in the development of high frequency, routine and reproducible genetic transformation protocols for rice. This technology has been applied to produce rice plants that withstand several abiotic stresses, as well as to gain tolerance against various pests and diseases. In addition, quality improving and increased nutritional value traits have also been introduced into rice. Most of these gains were not possible through conventional breeding technologies. Transgenic rice system has been used to understand the process of transformation itself, the integration pattern of transgene as well as to modulate gene expression. Field trials of transgenic rice, especially insect-resistant rice, have recently been performed and several other studies that are prerequisite for safe release of transgenic crops have been initiated. New molecular improvisations such as inducible expression of transgene and selectable marker-free technology will help in producing superior transgenic product. It is also a step towards alleviating public concerns relating to issues of transgenic technology and to gain regulatory approval. Knowledge gained from rice can also be applied to improve other cereals. The completion of the rice genome sequencing together with a rich collection of full-length cDNA resources has opened up a plethora of opportunities, paving the way to integrate data from the large-scale projects to solve specific biological problems.

Plant defense gene promoter enhances the reliability of shiva-1 gene-induced resistance to soft rot disease in potato

PAL5, a tomato (Lycopersicon esculentum Mill.) plant defense gene that encodes phenylalanine ammonia-lyase, is known to respond to a variety of environmental stresses including pathogen infection and wounding. A shiva-1 gene recombinant that encodes a small synthetic antibacterial peptide under the PAL5 gene promoter was transformed into potato (Solanum tuberosum L.) and its ability to induce resistance to Erwinia carotovora was compared with a construct under the control of the constitutive and widely used cauliflower mosaic virus (CaMV) 35S promoter. The shiva-1 peptide, an analog of natural cecropin B, was shown previously to have high bactericidal activity in vitro, but when expressed in vivo under the control of the CaMV 35S promoter, the effects were very inconsistent. As observed previously, in the present studies a few transformants with the CaMV 35S promoter were highly resistant when assayed for susceptibility to soft rot disease. In marked contrast the majority of transformants with the PAL5 gene promoter were highly resistant. More-detailed analyses of the incorporated DNA indicated that most of the transformants with the CaMV 35S promoter contained multiple copies of the transforming DNA while all of the PAL5 recombinants contained single copies. The highly resistant CaMV 35S recombinant also was present as a single copy. The results indicate that, at least in this instance, a constitutive promoter may not be ideal for the effective expression of a foreign gene and suggest that multiple insertions may have negative consequences.

Sensitivity of bacterial and fungal plant pathogens to the lytic peptides, MSI-99, magainin II, and cecropin B

In vitro and leaf disk assays of bacterial and fungal plant pathogens were conducted using three cationic lytic peptides, MSI-99, magainin II (MII), and cecropin B (CB). Growth of bacterial organisms was retarded or completely inhibited by low concentrations of these lytic peptides. The peptides also significantly reduced germination of fungal spores and growth of mycelia; however, higher concentrations of peptides were needed to inhibit fungal growth compared with those needed to inhibit bacteria. The relative efficacy of the peptides depended on the microorganism tested, but CB was the most inhibitory to the majority of the bacteria and fungi assayed. MSI-99, a synthetic derivative of MII with increased positive charge, showed equal or two- to fivefold higher antibacterial activity compared to MII in the in vitro assays. MSI-99 was also superior to MII against the oomycete, Phytophthora infestans but was slightly inferior to MII in assays with the true fungi, Penicillium digitatum and Alternaria solani. In the leaf disk assays, pretreating spores of Alternaria solani and Phytophthora infestans with the peptides at concentrations as low as 10 microg per ml led to significant reductions in the size of early blight lesions and prevented development of any late blight lesions on tomato leaf disks. Our results from in vitro and leaf disk assays suggest that MSI-99 can be used as a transgene to generate tomato lines with enhanced resistance to bacterial and fungal diseases of this crop.

Expression of an Antifungal Peptide in Saccharomyces: A New Approach for Biological Control of the Postharvest Disease Caused by Colletotrichum coccodes

ABSTRACT A cecropin A-based peptide inhibited germination of Colletotrichum coccodes at 50 muM. The DNA sequence encoding the peptide was cloned in pRS413, using the Saccharomyces cerevisiae invertase leader sequence for secretion of the peptide, and expressed in yeast. Yeast transformants inhibited the growth of germinated C. coccodes spores and inhibited decay development caused by C. coccodes in tomato fruits. Expression of the antifungal peptide in yeast therefore represents a new approach for the biological control of postharvest diseases.

Effects of synthetic cecropin analogs on in vitro growth of Acholeplasma laidlawii

Four synthetic peptides (Peptidyl MIMs; Demeter Biotechnologies, Inc.) were evaluated for their in vitro activity against Acholeplasma laidlawii. Fifty percent effective concentration values ranged from 1 to 15 microM. Three of these compounds are more lethal than cecropin B against A. laidlawii.

Broad-spectrum antimicrobial activity in vitro of the synthetic peptide D4E1

Broad-spectrum antimicrobial activity of a synthetic peptide, D4E1, is documented in this paper. D4E1 inhibited the growth of several fungal phytopathogens belonging to four classes-Ascomycetes, Basidiomycetes, Deuteromycetes, and Oomycetes, and two bacterial pathogens, Pseudomonas syringae pv. tabaci and Xanthomonas campestris pv. malvacearum race 18. The minimum inhibitory concentration (MIC) of D4E1 required to completely inhibit the growth of all fungi studied ranged from 4.67 to 25 microM. Fungal pathogens highly sensitive to D4E1 include Thielaviopsis basicola, Verticillium dahliae, Fusarium moniliforme, Phytophthora cinnamomi, and Phytophthora parasitica. Comparatively, the least sensitive fungal pathogens were Alternaria alternata, Colletotrichum destructivum, and Rhizoctonia solani. The two bacterial pathogens, P. syringae pv. tabaci and X. campestris pv. malvacearum race 18, were most sensitive to D4E1 with MIC values of 2.25 and 1.25 microM, respectively. Microscopic analysis of D4E1 effects on fungal morphology of Aspergillus flavus and R. solani revealed abnormal hyphal growth and discontinuous cytoplasm. After 8 h of exposure to 25 microM D4E1, A. flavus spore germination was reduced by 75%. The suitability of peptide D4E1 to enhance disease resistance in transgenic crop plants is discussed.

Transgenic plants expressing cationic peptide chimeras exhibit broad-spectrum resistance to phytopathogens

Here we describe a strategy for engineering transgenic plants with broad-spectrum resistance to bacterial and fungal phytopathogens. We expressed a synthetic gene encoding a N terminus-modified, cecropin-melittin cationic peptide chimera (MsrA1), with broad-spectrum antimicrobial activity. The synthetic gene was introduced into two potato (Solanum tuberosum L.) cultivars, Desiree and Russet Burbank, stable incorporation was confirmed by PCR and DNA sequencing, and expression confirmed by reverse transcription (RT)-PCR and recovery of the biologically active peptide. The morphology and yield of transgenic Desiree plants and tubers was unaffected. Highly stringent challenges with bacterial or fungal phytopathogens demonstrated powerful resistance. Tubers retained their resistance to infectious challenge for more than a year, and did not appear to be harmful when fed to mice. Expression of msrA1 in the cultivar Russet Burbank caused a striking lesion-mimic phenotype during leaf and tuber development, indicating its utility may be cultivar specific. Given the ubiquity of antimicrobial cationic peptides as well as their inherent capacity for recombinant and combinatorial variants, this approach may potentially be used to engineer a range of disease-resistant plants.

Transgenic expression of cecropin B, an antibacterial peptide from Bombyx mori, confers enhanced resistance to bacterial leaf blight in rice

The short persistence of cecropin B peptide in plants, due to post-translational degradation, is a serious impediment in its effective utilization for developing bacterial resistance transgenic plants. Two DNA constructs encoding the full-length precursor of cecropin B peptide and the mature sequence of cecropin B peptide preceded by a signal peptide derived from rice chitinase gene were transformed in rice. The differences in the transcriptional levels in independent transgenic lines showed moderate to high expression of cecropin B gene that correlated well with the differences in cecropin B accumulation observed by Western blot analysis. The development of lesions resulting from infection by Xanthomonas oryzae pv. oryzae was significantly confined in the infected leaflet of transgenic lines, when compared with the control plants.

Inhibition of fungal and bacterial plant pathogens by synthetic peptides: in vitro growth inhibition, interaction between peptides and inhibition of disease progression

Four synthetic cationic peptides, pep6, pep7, pep11 and pep20, were tested alone and in combinations for their antimicrobial activities against economically important plant pathogenic fungi (Phytophthora infestans and Alternaria solani) and bacteria (Erwinia carotovora subsp. carotovora and E. carotovora subsp. atroseptica). In in vitro studies, P. infestans and A. solani were inhibited by all four peptides, while E. carotovora subsp. carotovora and E. carotovora subsp. atroseptica were inhibited only by pep11 and pep20. All peptides completely inhibited P. infestans and A. solani on potato leaves and P. infestans on tubers at concentrations comparable to the in vitro IC50 (effective concentration for 50% growth inhibition) values, suggesting that these peptides are more potent in preventing infection than in inhibiting hyphal growth in vitro. Microscopic observations of P. infestans and A. solani when treated with these peptides revealed hyphal anomalies. In tuber-infectivity assays, pep11 and pep20 reduced bacterial softrot symptoms by 50% at 2.0 to 2.30 microM and by 100% at 20 microM. In assays involving two-way combinations of these peptides, growth inhibitions of fungi and bacteria by the combinations were no more than the sum of growth inhibitions by each peptide when used alone, indicating that they act additively. pep11 and pep20 are not phytotoxic to potato plants at 200 microM. With strong and broad-spectrum antimicrobial activities of pep11 and pep20 against fungi and bacteria, and with no antagonistic activities, the expression of these peptides in transgenic potato plants could lead to enhanced disease resistance against these pathogens.

Transgenic expression of a gene encoding a synthetic antimicrobial peptide results in inhibition of fungal growth in vitro and in planta

Transgenic tobacco plants producing the synthetic antimicrobial peptide D4E1, encoded by a gene under the control of an enhanced cauliflower mosaic virus 35S RNA promoter, were obtained by Agrobacterium-mediated transformation. Successful transformation was demonstrated by PCR and Southern hybridization analysis of tobacco DNAs. Expression of the synthetic D4E1 gene was shown by RT-PCR of tobacco mRNA. Crude protein extracts from leaf tissue of transformed plants significantly reduced the number of fungal colonies arising from germinating conidia of Aspergillus flavus and Verticillium dahliae by up to 75 and 99%, respectively, compared to extracts from plants transformed with pBI121. Compared to negative controls, tobacco plants expressing the D4E1 gene showed greater levels of disease resistance in planta to the fungal pathogen, Colletotrichum destructivum, which causes anthracnose.

Fungicidal properties, sterol binding, and proteolytic resistance of the synthetic peptide D4E1

The fungicidal properties of the synthetic peptide D4E1 were studied with nongerminated and germinating conidia of Aspergillus flavus, Aspergillus fumigatus, Aspergillus niger, Fusarium moniliforme, and Fusarium oxysporum. The minimal lethal concentrations (MLC) needed to kill 100% of germinating conidia of A. fumigatus, A. flavus, and A. niger were 12.5, 12.5, and 25 microM, respectively. The MLC value for nongerminated and germinating conidia of both Fusarium spp. was 3.0 microM. Except for A. fumigatus, D4E1 was inactive against the nongerminated conidia of the Aspergillus spp. Physicochemical studies showed D4E1 complexed with ergosterol, a sterol present in conidial walls. Cholesterol, present in nongerminated conidia of F. moniliforme, had a greater affinity for D4E1 than did ergosterol. D4E1 was more resistant to fungal and plant protease degradation than the natural peptide, cecropin A. These in vitro results suggest D4E1 is a candidate for transgenic expression in plants to enhance host resistance to fungal infection.

Strategies to improve plant resistance to bacterial diseases through genetic engineering

Many different genetic strategies have been proposed to engineer plant resistance to bacterial diseases, including producing antibacterial proteins of non-plant origin, inhibiting bacterial pathogenicity or virulence factors, enhancing natural plant defenses and artificially inducing programmed cell death at the site of infection. These are based on our knowledge of the mechanisms of action of antibacterial compounds and of the successive steps in plant-bacterial interactions. This article presents the different approaches and demonstrates that, even though several of these ideas have already been applied, no commercial applications have yet been achieved.

Cecropin A-derived peptides are potent inhibitors of fungal plant pathogens

Cecropins are naturally occurring peptides that play an important role in the immune response of insects. Cecropin A-derived and cecropin A-melittin hybrid peptides, all smaller than the natural compound cecropin A, were synthesized and tested for their ability to inhibit growth of several agronomically important fungal pathogens. We found that an 11-amino-acid sequence, corresponding to the N-terminal amphipathic alpha-helix domain of cecropin A, exhibited antifungal activity. Differences in susceptibility of the various pathogens were observed, Phytophthora infestans being particularly sensitive to the shortened cecropin A peptides (IC50 = 2 x 10(-6) M). Biotoxicity of the shortest cecropin A-derived peptide was variously affected by the presence of proteins extracted from leaves of tobacco and tomato plants, either total extracts or intercellular fluids (ICFs). Overall, there was a greater tolerance to tomato protein extracts than to tobacco extracts. These findings suggest that tobacco should not be used as a model for testing the possible protective effects of transgenically expressed, cecropin-based genes. The feasibility of tailoring cecropin A genes to enhance crop protection in particular plant/fungus combinations is discussed.

Cationic peptides: a new source of antibiotics

Antimicrobial cationic peptides are an important component of the innate defenses of all species of life. Different peptides may have antibacterial, antiendotoxic, antibiotic-potentiating or antifungal properties, and so they are being developed for use as a novel class of antimicrobial agents and as the basis for making transgenic disease-resistant plants and animals.

A single amino acid substitution in the antimicrobial defense protein cecropin B is associated with diminished degradation by leaf intercellular fluid

Degradation is one of several factors that may affect the level of accumulation of transgene products in plants. In plants engineered to secrete antimicrobial proteins to the intercellular compartment of leaves, the degenerative activity of proteases residing in leaf intercellular fluid (IF) could be critical to achieving the expected transgene function. We synthesized a structural analogue (MB39) of the antibacterial protein cecropin B and compared the susceptibility of both proteins to degradation in vitro by IF extracted from leaves of various crops. The half-life of the two proteins in the various IF extracts ranged from 3 min to 25.5 h, with the analogue MB39 displaying the longer half-life in IF from nine of 10 species. Overall, the half-life of MB39 averaged 2.9 times greater than that of cecropin B. Analysis of the peptides produced by endopeptidase activity in potato iF indicated that the 5.7-fold lower degradation rate of MB39 was associated with the substitution of valine for methionine at residue 11 of cecropin B. These findings point to the possibility of tailoring antimicrobial protein genes to reduce the rate of protein degradation in a particular target crop.