Enhanced disease resistance conferred by expression of an antimicrobial magainin analog in transgenic tobacco

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.

Plant synthetic biology for producing potent phyto-antimicrobials to combat antimicrobial resistance

Inappropriate and injudicious use of antimicrobial drugs in human health, hygiene, agriculture, animal husbandry and food industries has contributed significantly to rapid emergence and persistence of antimicrobial resistance (AMR), one of the serious global public health threats. The crisis of AMR versus slower discovery of newer antibiotics put forth a daunting task to control these drug-resistant superbugs. Several phyto-antimicrobials have been identified in recent years with direct-killing (bactericidal) and/or drug-resistance reversal (re-sensitization of AMR phenotypes) potencies. Phyto-antimicrobials may hold the key in combating AMR owing to their abilities to target major microbial drug-resistance determinants including cell membrane, drug-efflux pumps, cell communication and biofilms. However, limited distribution, low intracellular concentrations, eco-geographical variations, beside other considerations like dynamic environments, climate change and over-exploitation of plant-resources are major blockades in full potential exploration phyto-antimicrobials. Synthetic biology (SynBio) strategies integrating metabolic engineering, RNA-interference, genome editing/engineering and/or systems biology approaches using plant chassis (as engineerable platforms) offer prospective tools for production of phyto-antimicrobials. With expanding SynBio toolkit, successful attempts towards introduction of entire gene cluster, reconstituting the metabolic pathway or transferring an entire metabolic (or synthetic) pathway into heterologous plant systems highlight the potential of this field. Through this perspective review, we are presenting herein the current situation and options for addressing AMR, emphasizing on the significance of phyto-antimicrobials in this apparently post-antibiotic era, and effective use of plant chassis for phyto-antimicrobial production at industrial scales along with major SynBio tools and useful databases. Current knowledge, recent success stories, associated challenges and prospects of translational success are also discussed.

Multiple Disease Resistance in Plants

Many plants, both in nature and in agriculture, are resistant to multiple diseases. Although much of the plant innate immunity system provides highly specific resistance, there is emerging evidence to support the hypothesis that some components of plant defense are relatively nonspecific, providing multiple disease resistance (MDR). Understanding MDR is of fundamental and practical interest to plant biologists, pathologists, and breeders. This review takes stock of the available evidence related to the MDR hypothesis. Questions about MDR are considered primarily through the lens of forward genetics, starting at the organismal level and proceeding to the locus level and, finally, to the gene level. At the organismal level, MDR may be controlled by clusters of R genes that evolve under diversifying selection, by dispersed, pathogen-specific genes, and/or by individual genes providing MDR. Based on the few MDR loci that are well-understood, MDR is conditioned by diverse mechanisms at the locus and gene levels.

Antifungal Activity of a Synthetic Cationic Peptide against the Plant Pathogens Colletotrichum graminicola and Three Fusarium Species

A small cationic peptide (JH8944) was tested for activity against a number of pathogens of agricultural crops. JH8944 inhibited conidium growth in most of the tested plant pathogens with a dose of 50 μg/ml, although one isolate of Fusarium oxysporum was inhibited at 5 μg/ml of JH8944. Most conidia of Fusarium graminearum were killed within 6 hours of treatment with 50 μg/ml of JH8944. Germinating F. graminearum conidia required 238 μg/ml of JH8944 for 90% growth inhibition. The peptide did not cause any damage to tissues surrounding maize leaf punctures when tested at a higher concentration of 250 μg/ml even after 3 days. Liposomes consisting of phosphatidylglycerol were susceptible to leakage after treatment with 25 and 50 μg/ml of JH8944. These experiments suggest this peptide destroys fungal membrane integrity and could be utilized for control of crop fungal pathogens.

Chloroplast-expressed MSI-99 in tobacco improves disease resistance and displays inhibitory effect against rice blast fungus

Rice blast is a major destructive fungal disease that poses a serious threat to rice production and the improvement of blast resistance is critical to rice breeding. The antimicrobial peptide MSI-99 has been suggested as an antimicrobial peptide conferring resistance to bacterial and fungal diseases. Here, a vector harboring the MSI-99 gene was constructed and introduced into the tobacco chloroplast genome via particle bombardment. Transformed plants were obtained and verified to be homoplastomic by PCR and Southern hybridization. In planta assays demonstrated that the transgenic tobacco plants displayed an enhanced resistance to the fungal disease. The evaluation of the antimicrobial activity revealed that the crude protein extracts from the transgenic plants manifested an antimicrobial activity against E. coli, even after incubation at 120 °C for 20 min, indicating significant heat stability of MSI-99. More importantly, the MSI-99-containing protein extracts were firstly proved in vitro and in vivo to display significant suppressive effects on two rice blast isolates. These findings provide a strong basis for the development of new biopesticides to combat rice blast.

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.

Combined expression of antimicrobial genes (Bbchit1 and LJAMP2) in transgenic poplar enhances resistance to fungal pathogens

Populus species are susceptible to infection by microbial pathogens that severely affect their growth and substantially decrease their economic value. In this study, two pathogenesis-related protein genes consisting of Beauveria bassiana chitinase (Bbchit1) and motherwort lipid-transfer protein (LJAMP2) were introduced into Chinese white poplar (Populus tomentosa Carr.) via Agrobacterium-mediated transformation using the hygromycin (hyg) and neomycin phosphotransferase (NPTII) genes as selectable markers, respectively. Polymerase chain reaction analysis confirmed the stable integration of transgenes in the genome of transgenic plants. In vitro assays showed that inhibitory activity against the fungal pathogen Alternaria alternata (Fr.) Keissler was evident from the crude leaf extracts from transgenic plants. Importantly, the double-transgenic plants exhibited significantly higher resistance to the pathogen than either of the single-gene transformants and wild-type plants when inoculated with A. alternata. The level of disease reduction in double-transgenic lines was between 82 and 95%, whereas that of single-gene transformants carrying either LJAMP2 or Bbchit1 was between 65 and 89%. These results indicated that the combined expression of the LJAMP2 and Bbchit-1 genes could significantly enhance resistance to necrotrophic fungal pathogens in poplar.

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.

A cysteine-rich antimicrobial peptide from Pinus monticola (PmAMP1) confers resistance to multiple fungal pathogens in canola (Brassica napus)

Canola (Brassica napus), an agriculturally important oilseed crop, can be significantly affected by diseases such as sclerotinia stem rot, blackleg, and alternaria black spot resulting in significant loss of crop productivity and quality. Cysteine-rich antimicrobial peptides isolated from plants have emerged as a potential resource for protection of plants against phytopathogens. Here we report the significance of an antimicrobial peptide, PmAMP1, isolated from western white pine (Pinus monticola), in providing canola with resistance against multiple phytopathogenic fungi. The cDNA encoding PmAMP1 was successfully incorporated into the genome of B. napus, and it's in planta expression conferred greater protection against Alternaria brassicae, Leptosphaeria maculans and Sclerotinia sclerotiorum. In vitro experiments with proteins extracted from transgenic canola expressing Pm-AMP1 demonstrated its inhibitory activity by reducing growth of fungal hyphae. In addition, the in vitro synthesized peptide also inhibited the growth of the fungi. These results demonstrate that generating transgenic crops expressing PmAMP1 may be an effective and versatile method to protect susceptible crops against multiple phytopathogens.

Defense gene expression is potentiated in transgenic barley expressing antifungal peptide Metchnikowin throughout powdery mildew challenge

Transgenesis of antimicrobial peptides (AMPs) from different origins has emerged as an option for improvement of crop disease resistance since proof-of-concept for their activities against microbial phytopathogens is provided, persistently. Nevertheless, a more systematic approach based on knowledge of AMPs modes of action including elucidation of their cellular targets and possible impact on immune system considerably improves and diversifies the armory against harmful plant diseases. In present study, the impact of Metchnikowin (Mtk) expression in barley in terms of modulating different immune pathways was investigated. Monitoring of transcript abundance of different genes involved in key immune pathways of SAR, ISR, and redox milieu during interaction of Mtk barley with biotrophic Blumeria graminis f. sp. hordei (Bgh) demonstrated that several immune responses are modulated in Mtk transgenic plants. Present findings substantiate the higher activation of SAR pathway as well as ISR pathway in transgenic plants. Regarding susceptibility factors, nonetheless MLO gene is expressed more in Mtk plants and should lead to an increased cellular accessibility to Bgh, its impact is presumably overwhelmed by other mechanism(s) so that the plants show more resistance when challenging with Bgh. On the other hand, no obvious difference was observed between expression level of Bax inhibitor-1 (BI-1) in transgenic and wild type plants, which could be an indicative of its neutrality in resistance/susceptibility of transgenic plants to Bgh. The provided evidence on the involved pathways in Mtk-induced resistance improves our knowledge concerning impacts of AMPs expressed in diverse plant species on immune system of relevant transgenic plants.

Overexpression of the pepper antimicrobial protein CaAMP1 gene regulates the oxidative stress- and disease-related proteome in Arabidopsis

Proteomics facilitates our understanding of cellular processes and network functions in the plant defense response during abiotic and biotic stresses. Here, we demonstrate that the ectopic expression of the Capsicum annuum antimicrobial protein CaAMP1 gene in Arabidopsis thaliana confers enhanced tolerance to methyl viologen (MV)-induced oxidative stress, which is accompanied by lower levels of lipid peroxidation. Quantitative comparative proteome analyses using two-dimensional gel electrophoresis coupled with mass spectrometry identified some of the oxidative stress- and disease-related proteins that are differentially regulated by CaAMP1 overexpression in Arabidopsis leaves. Antioxidant- and defense-related proteins, such as 2-cys peroxiredoxin, L-ascorbate peroxidase, peroxiredoxin, glutathione S-transferase and copper homeostasis factor, were up-regulated in the CaAMP1 transgenic leaf tissues. In contrast, GSH-dependent dehydroascorbate reductase and WD-40 repeat family protein were down-regulated by CaAMP1 overexpression. In addition, CaAMP1 overexpression enhanced resistance to Pseudomonas syringae pv. tomato (Pst) DC3000 infection and also H(2)O(2) accumulation in Arabidopsis. The identified antioxidant- and defense-related genes were differentially expressed during MV-induced oxidative stress and Pst DC3000 infection. Taken together, we conclude that CaAMP1 overexpression can regulate the differential expression of defense-related proteins in response to environmental stresses to maintain reactive oxygen species (ROS) homeostasis.

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.

Enhanced resistance to fungal pathogens in transgenic Populus tomentosa Carr. by overexpression of an nsLTP-like antimicrobial protein gene from motherwort (Leonurus japonicus)

The antimicrobial protein gene LJAMP2 is a plant non-specific lipid transfer protein from motherwort (Leonurus japonicus). In this study, it was introduced into Chinese white poplar (Populus tomentosa Carr.) via Agrobacterium-mediated transformation with neomycin phosphotransferase II gene conferring kanamycin resistance as selectable marker. A total of 16 poplar lines were obtained, and polymerase chain reaction (PCR) analysis established the stable integration of transgenes in the plant genome. Reverse transcription-PCR detected LJAMP2 expression in transgenic plants. Resistance to fungal pathogens Alternaria alternata (Fr.) Keissler and Colletotrichum gloeosporioides (Penz.) of transgenic poplar lines was tested. In vitro inhibitory activity against the fungal pathogens was evident from the crude leaf extracts from the transformants. In vivo assays showed that, after infection with both A. alternata (Fr.) Keissler and C. gloeosporioides (Penz.), there was a significant reduction in disease symptoms in transgenic poplar plants compared with the control. These results suggest that constitutive expression of the LJAMP2 gene from motherwort can be exploited to improve resistance to fungal pathogens in poplar.

The potential of genetically enhanced plants to address food insecurity

Food insecurity is one of the most important social issues faced today, with 840 million individuals enduring chronic hunger and three billion individuals suffering from nutrient deficiencies. Most of these individuals are poverty stricken and live in developing countries. Strategies to address food insecurity must aim to increase agricultural productivity in the developing world in order to tackle poverty, and must provide long-term improvements in crop yields to keep up with demand as the world's population grows. Genetically enhanced plants provide one route to sustainable higher yields, either by increasing the intrinsic yield capability of crop plants or by protecting them from biotic and abiotic constraints. The present paper discusses a range of transgenic approaches that could increase agricultural productivity if applied on a large scale, including the introduction of genes that confer resistance to pests and diseases, or tolerance of harsh environments, and genes that help to lift the intrinsic yield capacity by increasing metabolic flux towards storage carbohydrates, proteins and oils. The paper also explores how the nutritional value of plants can be improved by genetic engineering. Transgenic plants, as a component of integrated strategies to relieve poverty and deliver sustainable agriculture to subsistence farmers in developing countries, could have a significant impact on food security now and in the future.

A defensin from tomato with dual function in defense and development

Defensins are antimicrobial peptides that are part of the innate immune system, contributing to the first line of defense against invading pathogens. Defensins and defensin-like peptides are functionally diverse, disrupting microbial membranes and acting as ligands for cellular recognition and signaling. Here we show that the tomato defensin DEF2 is expressed during early flower development. Defensin mRNA abundance, peptide expression and processing are differentially regulated in developing flowers. Antisense suppression or constitutive overexpression of DEF2 reduces pollen viability and seed production. Furthermore, overexpression of DEF2 pleiotropically alters the growth of various organs and enhances foliar resistance to the fungal pathogen Botrytis cinerea. Partially purified extracts from leaves of a DEF2-overexpressing line inhibited tip growth of B. cinerea. Besides providing insights into regulation of defensin expression, these data demonstrate that plant defensins, like their animal counterparts, can assume multiple functions related to defense and development.

Transgenic tobacco and peanut plants expressing a mustard defensin show resistance to fungal pathogens

Defensins are small positively charged, antimicrobial peptides (approximately 5 kDa in size) and some of them exhibit potent antifungal activity. We have cloned the complete cDNA containing an ORF of 243 bp of a defensin of mustard. The deduced amino acid sequence of the peptide showed more than 90% identity to the amino acid sequence of the well-characterized defensins, RsAFP-1 and RsAFP-2 of Raphanus sativus. We have generated and characterized transgenic tobacco and peanut plants constitutively expressing the mustard defensin. Transgenic tobacco plants were resistant to the fungal pathogens, Fusarium moniliforme and Phytophthora parasitica pv. nicotianae. Transgenic peanut plants showed enhanced resistance against the pathogens, Pheaoisariopsis personata and Cercospora arachidicola, which jointly cause serious late leaf spot disease. These observations indicate that the mustard defensin gene can be deployed for deriving fungal disease resistance in transgenic crops.

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.

In vitro toxicity towards kiwifruit pollen of the antimicrobial peptides magainins 1 and 2

In vitro toxicity of the antimicrobial peptides (AMPs) magainin 1 and 2 to a higher plant organism, i.e., the bicellular male gametophyte of Actinidia Deliciosa (kiwifruit), is investigated. Heavy damage to the plasma membrane, the primary cellular target of the peptides, was rapidly induced: in as few as 15 min, from 70 to nearly 100 % of pollen grains were rendered unviable by 20 microM magainin 1 or 2, respectively. Therefore, kiwifruit pollen sensitivity to natural magainins seemed to be higher if compared to the sensitivity of other pollen species towards magainin 2 amide or synthetic magainin analogues. Strong dose-dependent inhibitory effects on kiwifruit pollen performance were registered: as for magainin 1, the EC (50) at 120 min varied from 14.0 (germination) to 15.8 microM (tube elongation). The inhibitory effect was much greater when administering magainin 1 to elongating tubes rather than to ungerminated pollen grains. The two peptides differentially affected kiwifruit pollen, in line with the previously documented greater activity of magainin 2 in other cell systems. Furthermore, 20 microM magainin 1-treated pollen grains took on a shrivelled shape within 30 min of incubation, an increasingly widespread effect with higher peptide concentration. At the ultrastructural level, both protoplast shrinkage and striking organelle alterations were evident, including chromatin condensation, swelling and loss of mitochondrial cristae, dilation of rough endoplasmic reticulum cisternae, and vacuolization of cytoplasm. To our knowledge, similar alterations in animal or plant cells treated with AMPs have not been described yet.

Gene stacking in Phalaenopsis orchid enhances dual tolerance to pathogen attack

Cymbidium Mosaic Virus (CymMV) and Erwinia carotovora have been reported to cause severe damage to orchid plants. To enhance the resistance of orchids to both viral and bacterial phytopathogens, gene stacking was applied on Phalaenopsis orchid by double transformation. PLBs originally transformed with CymMV coat protein cDNA (CP) were then re-transformed with sweet pepper ferredoxin-like protein cDNA (Pflp) by Agrobacterium tumefaciens, to enable expression of dual (viral and bacterial) disease resistant traits. A non-antibiotic selection procedure in the second transformation minimized the potential rate of 'stacking' antibiotic genes in the orchid gene pool. Transgene integration in transgenic Phalaenopsis lines was confirmed by Southern blot analysis for both CP and pflp genes. Expression of transgenes was detected by northern blot analysis, and disease resistant assays revealed that transgenic lines exhibited enhanced resistance to CymMV and E. carotovora. This is the first report describing a transgenic Phalaenopsis orchid with dual resistance to phytopathogens.

Resistance enhancement of transgenic tomato to bacterial pathogens by the heterologous expression of sweet pepper ferredoxin-I protein

ABSTRACT Expression of a foreign gene to enhance plant disease resistance to bacterial pathogens is a favorable strategy. It has been demonstrated that expressing sweet pepper ferredoxin-I protein (PFLP) in transgenic plants can enhance disease resistance to bacterial pathogens that infect leaf tissue. In this study, PFLP was applied to protect tomato (Lycopersicon esculentum cv. cherry Cln1558a) from the root-infecting pathogen, Ralstonia solanacearum. Independent R. solanacearum resistant T(1) lines were selected and bred to produce homozygous T(2) generations. Selected T(2) transgenic lines 24-18-7 and 26-2-1a, which showed high expression levels of PFLP in root tissue, were resistant to disease caused by R. solanacearum. In contrast, the transgenic line 23-17-1b and nontransgenic tomato, which showed low expression levels of PFLP in root tissue, were not resistant to R. solanacearum infection. The expansion of R. solanacearum populations in stem tissue of transgenic tomato line 24-18-7 was limited compared with the nontransgenic tomato Cln1558a. Using a detached leaf assay, transgenic line 24-18-7 was also resistant to maceration caused by E. carotovora subsp. carotovora; however, resistance to E. carotovora subsp. carotovora was less apparent in transgenic lines 26-2-1a and 23-17-1b. These results demonstrate that PFLP is able to enhance disease resistance at different levels to bacterial pathogens in individual tissue of transgenic tomato.

Biolistic transformation of grapevine using minimal gene cassette technology

The use of minimal gene cassettes (MCs), which are linear DNA fragments (promoter+open reading frame+terminator) lacking the vector backbone sequence, was compared to the traditional use of whole circular plasmids (CPs) for transformation of grapevine. Embryogenic cell suspensions of 'Chardonnay' (Vitis vinifera L.) were transformed via particle co-bombardment using two nonlinked genes in either MCs or CPs. One construct contained the npt-II selectable marker and the second construct contained the MSI99 antimicrobial peptide gene. A total of five lines each from MC and CP treatments that showed positive signals by PCR for both the npt-II and MSI99 genes were selected. Southern blot analyses revealed up to five integration events in the DNA treatments. Transcription levels determined by semi-quantitative RT-PCR varied among transgenic lines. No significant differences were found in transgene transcription between lines from MC and CP transformation. The correlation between npt-II and MSI99 transcription levels was positive (P<0.05), however, no correlation between the transcription level and the number of integration events was observed. Transgenic lines presented a similar phenotype in leaf morphology and plant vigor compared to non-transgenic lines. Moreover, transgenic lines from both MC and CP DNA treatments produced fruit as did the non-transgenic lines in the third year of growth in the greenhouse. Our data confirm the effectiveness of the minimal cassette technology for genetic transformation of grapevine cultivars.

Disease resistance in plants that carry a feedback-regulated yeast poly(A) binding protein gene

It has been reported that the expression of the yeast poly(A) binding protein gene (PAB1) in plants leads to an induction of disease resistance responses, accompanied by alterations in the growth habit of the plant (Li et al. Plant Mol. Biol. (2000) 42 335). To capitalize on this observation, a feedback-regulated PAB1 gene was assembled and introduced into tobacco and Arabidopsis. The regulation entailed the linking of the expression of the PAB1 gene to control by the lac repressor, and by linking lac repressor expression to the disease resistance state of the plant, such that the induction of systemic defense responses by accumulation of the yeast poly(A) binding protein would turn off the expression of the PAB1 gene. Plants containing this system showed elevated and/or constitutive expression of disease-associated genes and significant resistance to otherwise pathogenic organisms. As well, they displayed a nearly normal growth habit under laboratory and greenhouse settings. These studies indicate that the expression of cytotoxic genes (such as the PAB1 gene) in plants can be controlled so that enhanced disease resistance can be achieved without significantly affecting plant growth and development.

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.

Evaluation of Transgenic ‘Chardonnay’ (Vitis vinifera) Containing Magainin Genes for Resistance to Crown Gall and Powdery Mildew

Magainins, short peptides with broad-spectrum antimicrobial activity in vitro, were assayed for their ability to confer resistance to pathogens in transgenic grapevines. Embryogenic cell suspensions of 'Chardonnay' (Vitis vinifera L.) were co-transformed by microprojectile bombardment with a plasmid carrying the npt-II gene and a second plasmid harboring either a natural magainin-2 (mag2) or a synthetic derivative (MSI99) gene. Magainin genes and the marker gene were driven by Arabidopsis ubiquitin-3 and ubiquitin-11 promoters, respectively. A total of 10 mag2 and 9 MSI99 regenerated lines were studied by Southern blot hybridization, which showed 1-6 transgene integration events into the plant genome. Semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) revealed a variable range in transcription levels among mag2 and MSI99 lines. A positive correlation between number of integration events and transcription level was observed (p<0.05). Plants were acclimated and challenged in the greenhouse with either Agrobacterium vitis strains (bacterial crown gall pathogen) at 10(8) cfu/ml or Uncinula necator (fungal powdery mildew pathogen) at 10(5) conidia/ml for evaluation of disease resistance. A total of 6 mag2 and 5 MSI99 lines expressing the antimicrobial genes exhibited significant reductions of crown gall symptoms as compared to non-transformed controls. However, only two mag2 lines showed measurable symptom reductions in response to U. necator, but not strong resistance. Our results suggest that the expression of magainin-type genes in grapevines may be more effective against bacteria than fungi. Additional strategies to enhance transgene expression and the spectrum of resistance to grape diseases are suggested.

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.

Ribosomally synthesized peptides with antimicrobial properties: biosynthesis, structure, function, and applications

Ribosomally synthesized peptides with antimicrobial properties (antimicrobial peptides-AMPs) are produced by eukaryotes and prokaryotes and represent crucial components of their defense systems against microorganisms. Although they differ in structure, they are nearly all cationic and very often amphiphilic, which reflects the fact that many of them attack their target cells by permeabilizing the cell membrane. They can be roughly categorized into those that have a high content of a certain amino acid, most often proline, those that contain intramolecular disulfide bridges, and those with an amphiphilic region in their molecule if they assume an alpha-helical structure. Most of the known ribosomally synthesized peptides with antimicrobial functions have been identified and studied during the last 20 years. As a result of these studies, new knowledge has been acquired into biology and biochemistry. It has become evident that these peptides may be developed into useful antimicrobial additives and drugs. The use of two-peptide antimicrobial peptides as replacement for clinical antibiotics is promising, though their applications in preservation of foods (safe and effective for use in meat, vegetables, and dairy products), in veterinary medicine, and in dentistry are more immediate. This review focuses on the current status of some of the main types of ribosomally synthesized AMPs produced by eucaryotes and procaryotes and discusses the novel antimicrobial functions, new developments, e.g. heterologous production of bacteriocins by lactic acid bacteria, or construction of multibacteriocinogenic strains, novel applications related to these peptides, and future research paradigms.

The potyviral suppressor of RNA silencing confers enhanced resistance to multiple pathogens

Helper component-protease (HC-Pro) is a plant viral suppressor of RNA silencing, and transgenic tobacco expressing HC-Pro has increased susceptibility to a broad range of viral pathogens. Here we report that these plants also exhibit enhanced resistance to unrelated heterologous pathogens. Tobacco mosaic virus (TMV) infection of HC-Pro-expressing plants carrying the N resistance gene results in fewer and smaller lesions compared to controls without HC-Pro. The resistance to TMV is compromised but not eliminated by expression of nahG, which prevents accumulation of salicylic acid (SA), an important defense signaling molecule. HC-Pro-expressing plants are also more resistant to tomato black ring nepovirus (TBRV) and to the oomycete Peronospora tabacina. Enhanced TBRV resistance is SA-independent, whereas the response to P. tabacina is associated with early induction of markers characteristic of SA-dependent defense. Thus, a plant viral suppressor of RNA silencing enhances resistance to multiple pathogens via both SA-dependent and SA-independent mechanisms.

Expressing an antibacterial protein in bacteria for raising antibodies

Magainins are small peptides with broad-spectrum activity against a range of plant and animal microbial pathogens. To detect magainin peptides in applications such as Western blot analysis and enzyme-linked immunosorbent assays, specific antibodies that recognize magainin peptides are required. The production of antibodies against small peptides injected into host animals poses problems with respect to eliciting an adequate immunogenic response due to the small size of the molecules. To increase the immunogenicity of a target peptide, it may be expressed as part of a larger fusion protein. However, expression of an antimicrobial peptide in bacteria may be cytotoxic to the host or subjected to degradation by host-derived peptidases. To overcome these potential problems, we fused the DNA coding sequence of a magainin gene analogue within the sequence of a bacterial thioredoxin gene. The subsequent gene fusion comprising a bacterial thioredoxin gene with a magainin coding sequence ligated at the active site of thioredoxin was successfully translated in a bacterial expression system. The fusion protein was non-toxic to the host bacteria. This represents a novel strategy to express antimicrobial peptides in a bacterial expression system. The fusion protein, purified by molecular size separation, was recovered in a soluble form following electroelution from polyacrylamide gels. Sufficient fusion protein was obtained for injection into rabbits and antibodies were obtained from rabbit sera that selectively recognized magainin peptides in Western blot analysis.

Activity of the Antifungal Protein from Aspergillus giganteus Against Botrytis cinerea

ABSTRACT Botrytis blight (gray mold), caused by Botrytis cinerea, is one of the most widely distributed diseases of ornamental plants. In geranium plants, gray mold is responsible for important losses in production. The mold Aspergillus giganteus is known to produce and secrete a basic low-molecular-weight protein, the antifungal protein (AFP). Here, the antifungal properties of the Aspergillus AFP against various B. cinerea isolates obtained from naturally infected geranium plants were investigated. AFP strongly inhibited mycelial growth as well as conidial germination of B. cinerea. Microscopic observations of fungal cultures treated with AFP revealed reduced hyphal elongation and swollen hyphal tips. Washout experiments in which B. cinerea was incubated with AFP for different periods of time and then washed away revealed a fungicidal activity of AFP. Application of AFP on geranium plants protected leaves against Botrytis infection. Cecropin A also was active against this pathogen. An additive effect against the fungus was observed when AFP was combined with cecropin A. These results are discussed in relation to the potential of the afp gene to enhance crop protection against B. cinerea diseases.

Ethylene-insensitive tobacco shows differentially altered susceptibility to different pathogens

ABSTRACT Transgenic tobacco plants (Tetr) expressing the mutant etr1-1 gene from Arabidopsis thaliana are insensitive to ethylene and develop symptoms of wilting and stem rot when grown in nonautoclaved soil. Several isolates of Fusarium, Thielaviopsis, and Pythium were recovered from stems of diseased Tetr plants. Inoculation with each of these isolates of 6-week-old plants growing in autoclaved soil caused disease in Tetr plants but not in nontransformed plants. Also, when 2-week-old seedlings were used, nontransformed tobacco appeared nonsusceptible to the Fusarium isolates, whereas Tetr seedlings did develop disease. Tetr seedlings were not susceptible to several nonhost Fusarium isolates. In contrast to results with Fusarium isolates, inoculation of 2-week-old seedlings with a Thielaviopsis isolate resulted in equal symptom development of nontransformed and Tetr tobacco. In order to explore the potential range of pathogens to which Tetr tobacco plants display enhanced susceptibility, the pathogenicity of several root and leaf pathogens was tested. Tetr plants were more susceptible to the necrotrophic fungi Botrytis cinerea and Cercospora nicotianae and the bacterium Erwinia carotovora, but only marginally more to the bacterium Ralstonia solanacearum. In contrast, the biotrophic fungus Oidium neolycopersici, the oomycete Peronospora tabacina, and Tobacco mosaic virus caused similar or less severe symptoms on Tetr plants than on nontransformed plants. Total peroxidase activity of Tetr plants was lower than that of nontransformed plants, suggesting a role for peroxidases in resistance against necrotrophic microorganisms. A comparable range of pathogens was examined on Arabidopsis and its ethylene-insensitive mutants etr1-1 and ein2-1. With the exception of one Fusarium isolate, ethylene insensitivity increased susceptibility of Arabidopsis plants to a similar spectrum of necrotizing pathogens as in tobacco. Thus, both ethylene-insensitive tobacco and Arabidopsis plants appear to be impaired in their resistance to necrotrophic pathogens.

The sweet pepper ferredoxin-like protein (pflp) conferred resistance against soft rot disease in Oncidium orchid

Genetic engineering to date has not been used to introduce disease resistance genes into the orchid gene pool. The ferredoxin-like protein gene originally isolated from sweet pepper is thought to function as a natural defense against infection due to its antimicrobial properties. Hence it was reasoned that introduction of this gene might produce Oncidium plants resistant to Erwinia carotovora, the causal agent for the soft rot disease. An expression vector containing sweet pepper ferredoxin-like protein (pflp) cDNA, hph and gusA coding sequence was successfully transformed into protocorm-like bodies (PLBs) of Oncidium orchid, using Agrobacterium tumefaciens strain EHA105. A total of 17 independent transgenic orchid lines was obtained, out of which six transgenic lines (beta-glucuronidase (GUS) positive) were randomly selected and confirmed by Southern, northern and western blot analyses. A bioassay was conducted on the transgenic lines. Transgenic plants showed enhanced resistance to E. carotovora, even when the entire plant was challenged with the pathogen. Our results suggest that pflp may be an extremely useful gene for genetic engineering strategies in orchids to confer resistance against soft rot disease.

MSI-99, a magainin analogue, imparts enhanced disease resistance in transgenic tobacco and banana

Magainin is one of the earliest reported antimicrobial peptides isolated from skin secretions of the African clawed frog Xenopus laevis. A synthetic substitution analogue of magainin, MSI-99, is employed in this study to impart disease resistance in transgenic tobacco ( Nicotiana tabacumL.) and banana [( Musaspp. cv. Rasthali (AAB)]. This peptide inhibited the growth and spore germination of Fusarium oxysporumf.sp. cubenseat 16 micro g/ml. MSI-99 has been subcloned into plant expression vectors pMSI164 and pMSI168, targeting the peptide into the cytoplasm and extracellular spaces, respectively. Tobacco plants transformed with pMSI168 showed enhanced resistance against Sclerotinia sclerotiorum, Alternaria alternataand Botrytis cinerea. Transgenic banana pants were obtained for both pMSI164 and pMSI168 transformations and showed resistance to F. oxysporumf.sp. cubenseand Mycosphaerella musicola. The transgenic nature of the transformants and expression of this peptide was confirmed through polymerase chain reaction (PCR) and reverse transcription (RT)-PCR. The results suggest that MSI-99 can be useful in imparting enhanced disease resistance in transgenic plants.

Antimicrobial peptides from amphibian skin: an expanding scenario

Many organisms employ antimicrobial peptides to fend off microbial pathogens. Amphibian skin is one of the most generous sources of these peptides. In the past couple of years, intriguing additional insights on various aspects of frog skin peptides have been reported. Several novel molecules, often with unprecedented structural features, have been discovered. Studies focusing on the factors that regulate the in vivo synthesis of skin peptides in response to infection have gained in prominence. Moreover, recent results indicate new possibilities for the development of effective human therapeutics based on antimicrobial peptides and partially disclosed the biotechnological potential of these molecules.

Expression and affinity purification of recombinant proteins from plants

With recent advances in plant biotechnology, transgenic plants have been targeted as an inexpensive means for the mass production of proteins for biopharmaceutical and industrial uses. However, the current plant purification techniques lack a generally applicable, economic, large-scale strategy. In this study, we demonstrate the purification of a model protein, beta-glucuronidase (GUS), by employing the protein calmodulin (CaM) as an affinity tag. In the proposed system, CaM is fused to GUS. In the presence of calcium, the calmodulin fusion protein binds specifically to a phenothiazine-modified surface of an affinity column. When calcium is removed with a complexing agent, e.g., EDTA, calmodulin undergoes a conformational change allowing the dissociation of the calmodulin-phenothiazine complex and, therefore, permitting the elution of the GUS-CaM fusion protein. The advantages of this approach are the fast, efficient, and economical isolation of the target protein under mild elution conditions, thus preserving the activity of the target protein. Two types of transformation methods were used in this study, namely, the Agrobacterium-mediated system and the viral-vector-mediated transformation system.

A tridecapeptide possesses both antimicrobial and protease-inhibitory activities

A 13-residue synthetic peptide (Rev4) was designed based on indolicidin, an antimicrobial peptide from bovine neutrophils. The synthetic peptide retains high antimicrobial activity. When tested for its stability in tobacco leaf extracts, Rev4 was highly stable compared to another antimicrobial peptide, magainin. When mixed with Rev4, magainin was protected from degradation by the leaf extract. Our results show that Rev4 is a potent protease inhibitor which selectively inhibits three out of four different types of proteases. Four other synthetic peptides were tested and the results were suggestive of no correlation between their antimicrobial and protease inhibitory activities.

Expression of an antimicrobial peptide via the chloroplast genome to control phytopathogenic bacteria and fungi

The antimicrobial peptide MSI-99, an analog of magainin 2, was expressed via the chloroplast genome to obtain high levels of expression in transgenic tobacco (Nicotiana tabacum var. Petit Havana) plants. Polymerase chain reaction products and Southern blots confirmed integration of MSI-99 into the chloroplast genome and achievement of homoplasmy, whereas northern blots confirmed transcription. Contrary to previous predictions, accumulation of MSI-99 in transgenic chloroplasts did not affect normal growth and development of the transgenic plants. This may be due to differences in the lipid composition of plastid membranes compared with the membranes of susceptible target microbes. In vitro assays with protein extracts from T(1) and T(2) plants confirmed that MSI-99 was expressed at high levels to provide 88% (T(1)) and 96% (T(2)) inhibition of growth against Pseudomonas syringae pv tabaci, a major plant pathogen. When germinated in the absence of spectinomycin selection, leaf extracts from T(2) generation plants showed 96% inhibition of growth against P. syringae pv tabaci. In addition, leaf extracts from transgenic plants (T(1)) inhibited the growth of pregerminated spores of three fungal species, Aspergillus flavus, Fusarium moniliforme, and Verticillium dahliae, by more than 95% compared with non-transformed control plant extracts. In planta assays with the bacterial pathogen P. syringae pv tabaci resulted in areas of necrosis around the point of inoculation in control leaves, whereas transformed leaves showed no signs of necrosis, demonstrating high-dose release of the peptide at the site of infection by chloroplast lysis. In planta assays with the fungal pathogen, Colletotrichum destructivum, showed necrotic anthracnose lesions in non-transformed control leaves, whereas transformed leaves showed no lesions. Genetically engineering crop plants for disease resistance via the chloroplast genome instead of the nuclear genome is desirable to achieve high levels of expression and to prevent pollen-mediated escape of transgenes.