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Keywords:

  • defensins;
  • peptaibols;
  • cyclopeptides;
  • synthetic peptides;
  • biocontrol agents;
  • transgenic plants

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Antimicrobial peptides from microorganisms
  5. Exploiting antimicrobial peptides in plant disease control
  6. Concluding remarks and future prospects
  7. Acknowledgements
  8. References

Several diseases caused by viruses, bacteria and fungi affect plant crops, resulting in losses and decreasing the quality and safety of agricultural products. Plant disease control relies mainly on chemical pesticides that are currently subject to strong restrictions and regulatory requirements. Antimicrobial peptides are interesting compounds in plant health because there is a need for new products in plant protection that fit into the new regulations. Living organisms secrete a wide range of antimicrobial peptides produced through ribosomal (defensins and small bacteriocins) or non-ribosomal synthesis (peptaibols, cyclopeptides and pseudopeptides). Several antimicrobial peptides are the basis for the design of new synthetic analogues, have been expressed in transgenic plants to confer disease protection or are secreted by microorganisms that are active ingredients of commercial biopesticides.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Antimicrobial peptides from microorganisms
  5. Exploiting antimicrobial peptides in plant disease control
  6. Concluding remarks and future prospects
  7. Acknowledgements
  8. References

Plant diseases caused by viruses, bacteria and fungi affect crops, and are responsible for significant losses or decrease the quality and safety of agricultural products. Their control relies mainly on chemical pesticides (Agrios, 2005). The European Union, USA and other countries have undertaken regulatory changes in pesticide registration requirements expecting a half reduction of the existing active ingredients in the early 1990s, thus retaining compounds being more selective, with lower intrinsic toxicity and reduced negative environmental impact. Upon the implementation of the regulations, several pesticides have been banned and some plant diseases of economic importance are managed with difficulty due to the lack of effective compounds.

Antimicrobial peptides (AMPs) have been the object of attention in past years as candidates for plant protection products. They are short sequence peptides, with generally fewer than 50 amino acid residues reported in living systems, which are a first line of defence in plants and animals. Reviews of AMPs have been carried out in bacteria (Jack & Jung, 2000; Cooter et al., 2005; Raaijmakers et al., 2006), fungi (Degenkolb et al., 2003; Ng, 2004), insects (Hancock, 2001; Bulet et al., 2004), marine invertebrates (Tincu & Taylor, 2004), amphibians and mammals (Andreu & Rivas, 1998; Zasloff, 2002; Toke, 2005), and plants (García-Olmedo et al., 1998; Lay & Anderson, 2005). Around 900 AMPs have been reported that can be classified based on structural characteristics into linear peptides often adopting helical structures, cysteine-rich open-ended peptides containing disulphide bridges, and cyclopeptides forming a peptide ring. Linear and cyclic peptides may have linked fatty acid chains (lipopeptides) or other chemical substitutions, resulting in complex molecules (pseudopeptides) (Fig. 1).

image

Figure 1.  Simplified structure of linear and cyclic antimicrobial peptides. The peptidic moiety is represented in black adopting helical or extended conformation, or β-sheet structure with disulphide bonds (S). Fatty acyl substitutions in lipopeptides are shown in blue. Complex substitutions in pseudopeptides are represented in red.

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In this review, only AMPs produced by microorganisms will be described in more detail as well as compounds that have been synthesized, produced by microbial biocontrol agents or expressed in transgenic plants. The mechanism of action, genes involved and regulation of its biosynthesis, although known in some AMPs, will not be covered. The reader is referred to the reports available on AMPs from animals (Mourgues et al., 1998; López-García et al., 2000; Alan & Earle, 2002; Banzet et al., 2002; Moreno et al., 2003; Cuthbertson et al., 2004; Ferréet al., 2006; Kuzina et al., 2006) and plants (Bohlman, 1994; Terras et al., 1995; Lai et al., 2002; Vila-Perellóet al., 2003; Lay & Anderson, 2005) that are active against a wide range of fungal and bacterial plant pathogens.

Antimicrobial peptides from microorganisms

  1. Top of page
  2. Abstract
  3. Introduction
  4. Antimicrobial peptides from microorganisms
  5. Exploiting antimicrobial peptides in plant disease control
  6. Concluding remarks and future prospects
  7. Acknowledgements
  8. References

Microorganisms produce a wide range of antimicrobial peptides that include small bacteriocins and fungal defensins synthesized through ribosomal synthesis, and peptaibols, cyclopeptides and pseudopeptides that are secondary metabolites produced by non-ribosomal synthesis.

Bacteriocins and fungal defensins

Bacteriocins are a type of protein and peptide secreted by major groups of bacteria that kill closely related species. Examples of large bacteriocins that inhibit plant pathogenic bacteria have been reported from bacteria associated with plants (Ishimaru et al., 1988; Jabrane et al., 2002; Lavermicocca et al., 2002; Pham et al., 2004; Parret et al., 2005), but small bacteriocins have not been tested.

Several filamentous fungi secrete AMPs of 51–58 aminoacid residues similar to defensins from animals and plants, with a compact structure of antiparallel strands stabilized by disulphide bridges. The peptides AFP from Aspergillus giganteus (Lacadena et al., 1995), PAF from Penicillium chrysogenum and Penicillium nalgiovense (Kaiserer et al., 2003) and Anafp from Aspergillus niger (Lee et al., 1999) have antifungal activity. AFP is active against Botrytis cinerea, Fusarium spp. and Pyricularia oryzae, but is inactive against bacteria (Vila et al., 2001; Moreno et al., 2003, 2005).

Peptaibols

Peptaibols are linear peptides, usually composed of a C-terminal amino alcohol and an acyl N-terminus, which are rich in dialkylated aminoacids such as α-diaminobutyric acid (Degenkolb et al., 2003). The lipopeptaibols have an acylated N-terminus composed of a short fatty acid chain (Toniolo et al., 2001). Their synthesis is performed by large multifunctional non-ribosomal peptide synthetases that have been cloned (Wiest et al., 2002). Peptaibols have been reported in several fungi (Table 1). Their antimicrobial activity affects mainly fungi and plant pathogenic Gram-positive bacteria by a mechanism of membrane disruption. Trichokonins are active against the plant pathogenic bacterium Clavibacter michiganensis and the fungi Fusarium oxysporum, Botrytis cinerea, Rhizoctonia solani, Bipolaris sorokiniana and Colletotrichum ssp. (Xiao-Yan et al., 2006). Trichorzins and harzianins show antifungal activity against Sclerotium cepivorum (Goulard et al., 1995). Some strains of fungi that are biological control agents of plant diseases and pertain to species of Trichoderma and Gliocladium, produce such compounds, which have been implicated in its mechanism of action (Schirmbock et al., 1994).

Table 1.   A survey of peptaibols produced by fungi
TypeCompoundComposition*Producer microorganism
  • *

    Px, number of aminoacid residues; Ac, acetyl; Dec, decanyl; Hex, hexanyl; Oc, octanyl; FA, fatty acyl; MPD, N1-methyl-propane-1,2-diamine; AAE, 2-(2-aminopropyl)-aminoethanol; AMAE, 2-(2-aminopropyl)-N-methylamino-ethanol.

Non-lipidicPeptaibolinAc-P4-PheOHSepedonium sp.
HypomurocinAc-P10-LeuOHHypocrea murociana
HarzianinsAc-P10-LeuOHTrichoderma harzianum
AmpullosporinAc-P14-LeuOHSepedonium ampullosporium
EmericinsAc-P15-PheOHEmericellopsis microspora
ClonostachinAc-P15-C(6)OHClonostachys sp.
TrichovirinsAc-P17-LeuOHTrichoderma virens
TrichorzianinsAc-P18-TrpOHTrichoderma harzianum
ChrysosperminsAc-P18-TrpOHApiocrea chrysosperma
TrichokoninAc-P19-PheOHTrichoderma koningii
PolysporinsAc-P19-PheOHTrichoderma polysporum
ParacelsinAc-P19-PheOHTrichoderma reesei
AlamethicinAc-P19-PheOHTrichoderma viride
StilboflavinsAc-P19-ValOHStilbella flaviceps
LipidicTrichodeceninDec-P5-LeuOHTrichoderma viride
LeucinostatinsHex-P8-MPDPaecilomyces/Acremonium spp.
HelioferinsOc-P8-AAEMycogone rosea
TrichopolynsDec-P9-AMAETrichoderma polysporum
LP237Oc-P10-LeuOHTolypocladium geodes
TrichoginOc-P10-LeuOHTrichoderma longibrachiatum
TexenomycinFA-P20-ArgOHScleroderma texenense

Cyclopeptides

Antimicrobial cyclopeptides are secondary metabolites reported from bacteria, fungi and cyanobacteria (Table 2) that are composed of amino acid residues including D- and L-forms as well as allo- and diamino derivatives, arranged in a cyclic ring, usually without disulphide bridges. Depending on the nature of the cyclization, the cyclopeptides have only the peptide ring or may have an additional peptide tail and may be lipidic due to the presence of an acyl group (DeLucca & Walsh, 1999; Bonmatin et al., 2003; Raaijmakers et al., 2006). Lipidic cyclopeptides (LCPs) are produced by several plant-associated and soil-inhabiting bacteria and have antifungal and antibacterial, cytotoxic or surfactant properties.

Table 2.   Antimicrobial cyclic-peptides produced by microorganisms
TypeCompoundComposition*Producer microorganism
  • *

    C, peptide ring size; T, peptide tail size; R, linked fatty acid.

SimpleGramicidinsC10Bacillus brevis
CalophycinC10Calothrix fusca
LaxaphycinsC11Anabaena laxa
TailedBacitracinsT5-C7Bacillus licheniformis
Simple lipidicXanthostatinR-C6Streptomyces spiroverticillatus
EchinocandinsR-C6Aspergillus spp.
CryptocandinsR-C6Cryptosporiopsis quercina
FusaricidinsR-C6Paenibacillus polymixa
IturinsR-C7Bacillus spp./Bacillus amyloliquefaciens
AureobasidinsR-C8Aureobasidium pullulans
SyringomycinsR-C9Pseudomonas syringae/Pseudomonas viridiflava
FengycinsR-C10Bacillus subtilis
Tailed lipidicViscosinsR-T2-C7Pseudomonas fluorescens
PolymixinsR-T3-C7Paenibacillus polymixa
AgrastatinsR-T2-C8Bacillus subtilis
AmphisinsR-T2-C9Pseudomonas fluorescens
PutisolvinsR-T8-C4Pseudomonas putida
TolaasinsR-T11-C4Pseudomonas tolaasi
CorpeptinsR-T17-C5Pseudomonas corrugata
SyringopeptinsR-T14-C8Pseudomonas syringae
Schizotrin AR-T1-C12Schizotrix sp.

LCPs from Pseudomonas ssp. are mainly of the depsipeptide type and are classified into seven major groups (i.e. amphisins, corpeptins, putisolvins, syringomycins, syringopeptins, tolaasins and viscosins), based on the length and composition of the fatty acid, and the peptide ring and tail (Raaijmakers et al., 2006). Syringomicins and syringopeptins play roles as virulence factors in Pseudomonas syringae, but also inhibit Gram-positive bacteria (Grgurina et al., 2005) and Botrytis cinerea (Lavermicocca et al., 1997). The syringopeptin-producing strain Pseudomonas syringae 508 was antagonistic to Venturia inaequalis, the causal agent of apple scab (Burr et al., 1996). Tolaasins are inhibitory to Rhizoctonia solani and Gram-positive bacteria like Rhodococcus fascians, but not to Gram-negative bacteria like Erwinia carotovora (Bassarello et al., 2004). Pseudophomins from Pseudomonas fluorescens BRG100 have antifungal activity against Leptosphaeria maculans and Sclerotinia sclerotiorum (Pedras et al., 2003). Cormycin A and corpeptins from Pseudomonas corrugata, as well as syringomycins and syringopeptins are potent toxic compounds against plants, animals and microorganisms (Scaloni et al., 2004). Several LCPs have biosurfactant properties affecting plant pathogenic oomycetes. The massetolide from strain Pseudomonas fluorescens SS101 is zoosporicidal in Phytium intermedium (de Souza et al., 2003). Amphisin, lokisin, tensin and viscosinamide produced by strains of Pseudomonas fluorescens are inhibitory to Pythium ultimum and Rhizoctonia solani (Nielsen et al., 2002; Nielsen & Sorensen, 2003).

Several species of Bacillus produce fusaricidins, iturins, fengycins, polymixins or agrastatins (Stein, 2005). Iturin derivatives include mycosubtilins, iturins, bacillomycins and surfactin, differing in fatty acid length. Iturins, fengycins and surfactins have synergistic effects and display antifungal activity against a wide range of plant pathogens. Production of fengycin-type cyclopeptides in Bacillus subtilis strains S499 and M4 was inhibitory to F. oxysporum, Rhizoctonia solani and Botrytis cinerea, and was associated with the control of grey mould rot in apple fruits caused by Botrytis cinerea (Ongena et al., 2005). Iturin synthesis by Bacillus subtilis B-3 was related to the control of peach brown rot caused by Monilinia fructicola (Gueldner et al., 1988). The production of surfactin was also involved in the control of Pseudomonas syringae infections in Arabidopsis by Bacillus subtilis 6051 (Bais et al., 2004) and of Rhizoctonia solani by Bacillus subtilis RB14 (Asaka & Shoda, 1996). Secretion of bacillomycin, fengycin or iturin by different strains of Bacillus subtilis has been implicated in the biocontrol of powdery mildew of melon caused by Podosphaera fusca (Romero et al., 2007). Paenibacillus B2, a strain producing polymixin B isolated from the rhizosphere of sorghum, was antagonistic to E. carotovora and the root pathogenic fungus F. solani and Fusarium acuminatum (Selim et al., 2005).

Pseudopeptides

Pseudopeptides of interest in plant disease control have few peptide bonds and complex aminoacid modifications (e.g. nucleosides) and are produced by bacteria (Table 3). Pantocines are derivatives from alanine that inhibit transaminase-catalyzed aminoacid biosynthesis in the bacterium Erwinia amylovora, the causal agent of fire blight of rosaceous plants, and are produced by strains of Pantoea agglomerans (Brady et al., 1999; Jin et al., 2003). The nucleopeptide derivatives polyoxins, nikkomycins, blasticidin and mildiomycin are also pseudopeptides with antifungal activity (Copping & Menn, 2000). Polyoxins are commercially used for fungal plant disease control and are pyrimidinyl dipeptides that inhibit chitin synthesis in fungi like Alternaria spp., Botrytis cinerea, and Rhizoctonia solani. Nikkomycins are pyridinyl derivatives similar to polyoxins. Blasticidin inhibits protein biosynthesis in prokaryotes and also have activity against Pyricularia oryzae. Mildiomycin is a serine derivative active against powdery mildews Podosphera, Sphaerotheca, Erysiphe and Uncinula necator. The antifungal pseudopeptides bacilysin, an alanine-epoxycyclohexane substituted dipeptide, and rhizocticin, a phosphonodipeptide, are produced by Bacillus subtilis strains (Stein, 2005).

Table 3.   Antimicrobial pseudopeptides produced by microorganisms active against plant pathogens
CompoundCompositionProducer microorganism
Pantocines A and BAlanine derivativesPantoea agglomerans
PolyoxinsPyrimidinyl–dipeptideStreptomyces cacaoi
NikkomycinsPyridinyl–dipeptideStreptomyces tendae
RhizocticinPhosphono–oligopeptideBacillus subtilis
BacilysinEpoxycyclohexane–dipeptideBacillus subtilis
BlasticidinNucleopeptideStreptomyces griseochromogenes
MildiomycinNucleopeptideStreptoverticillium rimofaciens

Exploiting antimicrobial peptides in plant disease control

  1. Top of page
  2. Abstract
  3. Introduction
  4. Antimicrobial peptides from microorganisms
  5. Exploiting antimicrobial peptides in plant disease control
  6. Concluding remarks and future prospects
  7. Acknowledgements
  8. References

One of the things that make AMPs attractive as antimicrobial compounds for plant disease control is the mechanism of action against the target microorganism. Most AMPs are cationic and bind to the surface of microorganisms through receptor-mediated interaction and insert into the cytoplasmic membrane. Several AMPs are membrane disruptive, but others are non-membrane disruptive and cross the cell membrane to interact with intracellular targets and inhibit nucleic acid or protein synthesis, or enzymatic activity (Powers & Hancock, 2003; Brogden, 2005).

Unfortunately, AMPs from microbial origin like LCPs and peptaibols have significant phytotoxicity that limits its direct use as plant protection products. However, several AMPs have been used as the basis for the development of shorter and less toxic analogues by synthetic procedures. Some AMPs play a role in biocontrol agents of plant diseases. The fact that animal and plant AMPs are produced by ribosomal synthesis, has provided tools for developing transgenic plants expressing genes coding for the synthesis of these compounds conferring partial or total resistance to plant pathogens.

Synthetic AMPs

Analogues from animal and plant AMPs or newly designed compounds of six to 47 aminoacid residues have been synthesized (Table 4). Synthetic AMPs have been obtained by solid-phase methods (Andreu et al., 1983) and procedures that facilitate the performance of combinatorial libraries (Monroc et al., 2006b). Combinatorial chemistry is a powerful tool for the design of new molecules departing from leader compounds that can be improved, focusing its activity towards selected target pathogens, but minimizing toxicity for plants and animals and susceptibility to protease digestion (Powell et al., 1995; Reed et al., 1997; Oh et al., 1999; Monroc et al., 2006a).

Table 4.   Synthetic antimicrobial peptides active against plant pathogens
CompoundSizeSequenceSource
PEP66FRLKFHSynthetic
PAF266Acetyl-RKKWFW-NH2Synthetic
BPC19410c(KKLKKFKKLQ)Synthetic
PEP311WKLFKKILKVL-NH2Cecropin–melittin hybrid
PEP1111WKLFKKILKVLCecropin–melittin hybrid
BP7611KKLFKKILKFL-NH2Cecropin–melittin hybrid
CAMEL15KWKLFKKIGAVLKVL-NH2Cecropin–melittin hybrid
Iseganan17RGGLCYCRGRFCVCVGR-NH2Protegrin
D4E117FKLRAKIKVRLRAKIKLCecropin
TPY17KWVFRVNYRGIKYRRQRTachyplesin
ESF1218MASRAAGLAARLARLALRMagainin
ESF120MASRAAGLAARLARLALRALMagainin
Pexiganan22GIGKFLKKAKKFGKAFVKILKK-NH2Magainin
MSI-9923GIGKFLKSAKKFGKAFVKILNSMagainin
MB-3939HQPKWKVFKKIEVVGRNIRNGI VKAGPAIAVLGEAKALGCecropin
Pen4-146HSSGYTRPLRKPSRPIFIRPIGCDVCYGI PSSTARLCCFRYGDCHL-NH2Penaedin
D32R47KSCCRNTWARNCYNVCRLPGTISREI CAKKCRCKIISGTTCPSDYPKThionin

Several synthetic AMPs based on compounds of animal origin have been produced. Pep3, a cecropin-melittin hybrid, is active against two Fusarium species, Phytophthora infestans and Thielaviopsis basicola (Andreu et al., 1992; Cavallarin et al., 1998). The analogue of cecropin B, D4E1 inhibits T. basicola, Verticillium dahliae, Fusarium moniliforme, two species of Phytophthora, and the bacterial pathogens Pseudomonas syringae pv. tabaci and Xanthomonas campestris pv. malvacearum (DeLucca & Walsh, 1999). MB39, another cecropin B analogue, is active against the phytopathogenic bacteria E. carotovora ssp. betavasculorum, C. michiganensis, three pathovars of Pseudomonas syringae and two pathovars of X. campestris, and also inhibits the fungi Phytophthora infestans and Rhizoctonia solani (Owens and Heutte, 1997). Several derivatives of tachyplesin are active against two Fusarium species (Rao, 1999). MS1-99, a synthetic derivative of magainin is effective against the fungi Phytophthora infestans and Alternaria solani, and plant pathogenic bacteria (Alan & Earle, 2002). CAMEL, a cecropin-melittin hybrid peptide (Kamysz et al. 2005), Iseganan, a synthetic variant of the porcine protegrin I, and Pexiganan, an analogue from the clawed frog magainin (Chen et al., 2000), are active against the plant pathogenic bacteria of the Pectobacterium species (Fuchs et al., 1998). Pe4-1, an isomorph of penaedin from white shrimp inhibits several plant pathogenic bacteria and Fusarium oxysporum (Cuthbertson et al., 2004).

Plant AMP derivatives have been also prepared. Rs-AFP2, a peptide derived from radish defensin, inhibits growth of several fungi (De Samblanx et al., 1996). D32R, an analogue of the Pyrularia pubera thionin, is active against the phytopathogenic fungi F. oxysporum, Plectosphaerella cucumerina and Botrytis cinerea, and the plant pathogenic bacteria X. campestris pv. translucens and C. michiganensis (Vila-Perellóet al., 2003).

Completely synthetic peptides have been designed by combinatorial chemistry. PEP6 is a hexapeptide with activity against F. oxysporum f. sp. lycopersici, Rhizoctonia solani, Ceratocystis fagacearum and Pythium ultimum (Reed et al., 1997). ESF1 is active against several fungi including Cryphonectria parasitica, F. oxysporum f.sp. lycopersici, Septoria musiva, Phytophthora infestans and Alternaria solani, and to the rot-causing bacteria E. carotovora (Powell et al., 1995; Ali & Reddy, 2000). ESF12 has antibacterial activity against E. amylovora, Agrobacterium tumefaciens and Pseudomonas syringae (Powell et al., 1995). PAF26, a synthetic antifungal hexapeptide, is active against Penicillium italicum, Penicillium digitatum and Botrytis cinerea (López-García et al., 2000). BP76 is an amidated cecropin-melittin hybrid undecapeptide with antibacterial activity against E. amylovora, Pseudomonas syringae and Xanthomonas vesicatoria that was obtained by combinatorial chemistry targeted to plant pathogenic bacteria (Ferréet al., 2006).

Synthetic cyclic peptides active against plant pathogens are less abundant. The cyclodecapeptide BPC194 is active against the plant pathogenic bacteria E. amylovora, Pseudomonas syringae and X. vesicatoria and was obtained in an extended survey from tetra- to decapeptides using combinatorial chemistry (Monroc et al. 2006a, b).

Role of AMPs in biocontrol agents

The production of AMPs has been attributed to the inhibitory activity of several microorganisms that act against fungal and bacterial plant pathogens. However, only in a few cases do the reports provide strong evidence implicating them in the biocontrol mechanism using genetic tools.

Cyclic lipopeptides have been implicated to give Bacillus amyloliquefaciens FZB42 the ability to control F. oxysporum by using structural and functional characterizations of gene clusters involved in their synthesis and analysis of defective mutants unable to produce bacillomycin D and fengycin (Koumoutsi et al., 2004). Deficient mutants of Bacillus subtilis 6051 unable to produce surfactin lost the capacity for biocontrol of root infections caused by Pseudomonas syringae in Arabidopsis (Bais et al., 2004). Using a similar approach, deficient mutants of Pseudomonas fluorescens SS101 lacking the capacity to produce a cyclic lipopeptide similar to massetolide lost their capacity to inhibit root rot of hyacinth caused by Phytium intermedium and the ability to lyse zoospores of several oomycetes (de Souza et al., 2003). Mycosubtilin overproducing mutants of Bacillus subtilis ATCC6633 were more effective than the parent strain in controlling Phytium damping-off on tomato (Leclère et al., 2005). Upon combination of in situ detection and deficient mutant analysis, the production of bacillomycin, fengycin or iturin by several strains of Bacillus subtilis was implicated in the mechanism of control of powdery mildew on melon caused by Podosphaera fusca (Romero et al., 2007). Less robust evidence for the implication of cyclopeptides in biocontrol have also been provided by in situ detection of the compounds on host plant tissues (Touréet al., 2004). Of great interest is that the commercial Bacillus subtilis biocontrol strains FZB42 (Abitep, Berlin, Germany), GB03 (Kodiak, Gustafson Biologicals, Plano, TX), QST713 (Serenade, Agraquest Inc., Davis, CA), and MBI600 (Subtilex, Becker Underwood, Ames, IA) have the genes for fengycin, surfactin, bacillomycin or iturin synthesis (Joshi & McSpadden-Gardener, 2006).

The role of pseudopeptides in the inhibition of E. amylovora by Erwinia herbicola E318 (syn. Pantoea agglomerans) has been determined using defective mutants unable to produce pantocines A and B, which lost their inhibitory capacity against E. amylovora and other species of enterobacteria (Wright et al., 2001).

Transgenic plants expressing antimicrobial peptides

Gene constructions including sequences coding for AMPs have been expressed on model or crop plants providing different degrees of protection against plant pathogens (Table 5).

Table 5.   Antimicrobial peptides expressed in transgenic plants that confer partial resistance to pathogens
OriginAMPSourcePlant transformed
AnimalCecropin A, BMoth haemolynphRice
TachyplesinCrab haemolynphPotato
Heliomicin/drosomycinInsect defensinTobacco
Sarcotoxin IAFruit fly haemolynphTobacco
Mussel defensinMusselTobacco
MagaininFrog skinTobacco
Esculentin-1Frog skinTobacco
PlantRs-AFP2Radish defensinTobacco/tomato
Alf-AFPAlfalfa defensinPotato
Spi1Spruce defensinTobacco
DRR230-aPea defensinCanola/tobacco
BSD1Cabbage defensinTobacco
WT1Wasabi defensinRice
Dm-AMP1Dahlia defensinEggplant
Mj-AMP1Jalapa defensinTomato
Pn-AMPHeveinTobacco
HordothioninBarleyTobacco
Alpha thioninBarleyTobacco
FungalAFPFungal defensinRice
SyntheticSB-37Cecropin analoguePotato, apple
Shiva-1Cecropin analogueAnthurium, Paulownia
SB37, Shiva-1Cecropin analoguesTobacco
MB-39Cecropin analogueApple
MsrA1Cecropin–melittin hybridPotato
MSI-99Magainin analogueGrapevine/banana
Myp30Magainin analogueTobacco
Rev4Indolicidin analogueTobacco/arabidopsis
D4E1SyntheticTobacco/cotton/poplar

Animal defensin genes have been expressed on several plants. Cecropins A and B expressed in rice confer protection against Magneporthe grisea (Coca et al., 2004) and Xanthomonas oryzae (Sharma et al., 2000), magainin expressed on tobacco confers protection against several fungi and bacteria (De Gray et al., 2001), and tachyplesin from crab expressed in potato was effective against infections by E. carotovora (Allefs et al., 1996). The insect defensins heliomicin and drosomycin expressed in tobacco confer protection against B. cinerea (Banzet et al., 2002), and the sarcotoxin from fruit fly expressed in tobacco protected against Pseudomonas syringae pv. tabaci and E. carotovora ssp. carotovora (Ohshima et al., 1999).

Plant defensins have also been expressed in plants. The Rs-AFP2 radish defensin was expressed in tobacco and tomato and confers protection against Alternaria longipes (Terras et al., 1995), Alf-AFP alfalfa defensin expressed in potato protects against V. dahliae (Gao et al., 2000), SPI1 spruce defensin expressed in tobacco protects against Heterobasidium annosum (Elfstrand et al., 2001), DRR206 pea defensin expressed in canola and tobacco protects against Leptosphaeria maculans (Wang et al. 1999), BSD1 cabbage defensin expressed in tobacco protects against Phytophthora parasitica (Park et al., 2002), WT1 wasabi defensin expressed in rice protects against M. grisea (Kanzaki et al., 2002), Dm-AMP1 dahlia defensin expressed in eggplant protect against Botrytis cinerea and Verticillium alboatrum (Turrini et al., 2004), and Mj-AMP1 jalapa defensin expressed in tomato protects against Alternaria solani (Schaefer et al., 2005). The hevein Pn-AMP expressed in tobacco protects against Phytophthora parasitica (Koo et al., 2002), and barley hordothionin expression in tobacco confers protection against C. michiganensis and Pseudomonas syringae pv. tabaci (Carmona et al., 1993).

Synthetic analogues, mostly based on animal defensins, have also been expressed in a wide range of plants. Magainin analogues like Myp30 expressed in tobacco provided protection against Peronospora tabacina (Qingshun et al., 2001), and MSI-99 expressed in grapevine protects against Agrobacterium tumefaciens (Vidal et al., 2006). Synthetic cecropin analogues confer protection against several plant pathogenic bacteria. SB-37 protects against E. carotovora ssp. carotovora on potato (Arce et al., 1999) and MB39 was effective against the plant pathogenic bacterium E. amylovora on Royal Gala apple (Liu et al., 2001). Shiva-1 expressed in Anthurium was effective against Xanthomonas axonopodis pv. dieffenbachia (Kuehnle et al., 2004), in Paulownia protects against phytoplasms (Du et al., 2005), and in tobacco gave protection against Ralstonia solanacearum (Jaynes et al., 1993) and Pseudomonas syringae pv. tabaci (Huang et al., 1997). Synthetic cecropin-melittin hybrids like MsrA2 have been expressed in tobacco and potato and confer protection against several phytopathogens (Osusky et al., 2005; Yevtushenko et al., 2005). The completely synthetic antimicrobial peptide D4E1 has been expressed in tobacco, potato and poplar, protecting against several pathogens (Cary et al., 2000; Mentag et al., 2003). The analogue of indolicidin Rev4 has been expressed in tobacco and arabidopsis and was effective against Peronospora tabacina, Pseudomonas syringae pv. tabaci and E. carotovora infections (Xing et al., 2006).

Examples of antimicrobial peptides from microbial origin expressed in plants are scarce. The AFP fungal defensin from Aspergillus giganteus was expressed in rice and conferred protection against M. grisea (Coca et al., 2006).

Concluding remarks and future prospects

  1. Top of page
  2. Abstract
  3. Introduction
  4. Antimicrobial peptides from microorganisms
  5. Exploiting antimicrobial peptides in plant disease control
  6. Concluding remarks and future prospects
  7. Acknowledgements
  8. References

As in pharmaceutical areas, AMPs could play strong roles in agriculture as plant protection products. Successful use of AMPs has been achieved through the commercial development as biopesticides of several microorganisms secreting these compounds. Although numerous transgenic plants expressing AMPs that confer different degrees of protection against diseases have been developed, commercial cultivars have not been marketed because of regulatory limitations and social concerns. Synthetic approaches to obtain AMPs guided by combinatorial chemical methods provide powerful tools to optimize molecules derived from natural compounds with improved activity against selected target pathogens, including decreased cytotoxicity and increased protease stability. However, exploitation of the great number of AMPs as active ingredients of pesticides has not been accomplished yet. The majority of AMPs with potential uses have been studied at the in vitro level, fewer compounds have been tested on plant pathosystems, and only a few are on the market. Development of compounds suitable for agricultural use as pesticide ingredients have several constraints mainly due to the intrinsic toxicity and low stability of some of the compounds, the requirement to develop suitable formulations, and the need for inexpensive products in plant protection. Therefore, future areas of interest consist of developing less toxic and more stable compounds as well as decreasing production costs by improving preparative synthesis and biotechnological procedures using microbial systems or transgenic crops as plant factories.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Antimicrobial peptides from microorganisms
  5. Exploiting antimicrobial peptides in plant disease control
  6. Concluding remarks and future prospects
  7. Acknowledgements
  8. References

To the ‘Comisión Interministerial de Ciencia y Tecnología (CICYT)’ (AGL2006-13564), Instituto Nacional de Investigaciones Agrarias INIA (CAL03-084) of Spain, and CIRIT-Generalitat de Catalunya (2005SGR00835).

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