Antimicrobial Cyclic Peptides for Plant Disease Control

Cyclic peptides lysing cell membrane

Cyclic lipopeptides (CLPs) are composed of a fatty acid tail linked to a cyclized oligopeptide, and possess antifungal, antibacterial, cytotoxic or surfactant properties. One of the modes of action of the CLPs is through membrane integration and the formation of pores, causing leakage and an imbalance of the ionic potential across the membrane, followed by cell lysis. This is typically seen in the syringomycins secreted by Pseudomonas syringae pv. syringae.

Syringomycins (Fig. 1) and syringopeptins act as virulence factors in Pseudomonas syringae, and also possess strong inhibitory activities against the growth of Gram-positive bacteria at a concentration of 18.75 μM (Grgurina et al., 2005) and mycelial growth of Botrytis cinerea at 1.56–12.5 μM (Lavermicocca et al., 1997). The syringopeptin-producing strain P. syringae 508 has antagonistic property against Venturia inaequalis, the causal agent of apple scab (Burr et al., 1996). Tolaasin D inhibits the growth of Rhizoctonia solani and the Gram-positive bacterium Rhodococcus fascians at a minimal inhibitory quantity (MIQ) of 0.16 μg (Bassarello et al., 2004). Pseudophomins A and B produced by Pseudomonas fluorescens BRG100 significantly inhibit the growth of Phoma lingam, Alternaria brassicae and Sclerotinia sclerotiorum (Pedras et al., 2003). Massetolide A is produced by P. fluorescens SS101, and possesses a biosurfactant property that affects plant pathogenic oomycetes. Massetolide A is also zoosporicidal towards Pythium intermedium (de Souza et al., 2003), where zoospores initially lose their motility before rupturing into fragments. Culture extracts of P. fluorescens SS101 can reduce the surface tension of water to 30 mN/m and attain a critical micelle concentration at 25 μg/ml. Amphisin, lokisin, tensin, and viscosinamide are produced by P. fluorescens spp., and possess in vitro antagonistic activities against the oomycete Pythium ultimum and the basidiomycete R. solani (Nielsen et al., 2002; Nielsen and Sorensen, 2003).

Fig. 1

Molecular structures of cyclic lipopeptides (CLPs) that lyse the microbial cell membrane.

In addition to the pseudomonads, Bacillus spp. are major producers of CLPs. Fusaricidins, iturins, fengycins, polymixins, and agrastatins have been identified in culture extracts of several species of Bacillus (Stein, 2005). These CLPs are composed of seven (surfactins, iturins) or ten α-amino acids (fengycins) linked to one unique β-amino (iturins) or β-hydroxy (surfactins, fengycins) fatty acids. The carbon numbers of β-amino fatty acid chain of surfactins, iturins, and fengycins are C₁₃–C₁₆, C₁₄–C₁₇, and C₁₄–C₁₈, respectively. Multiple derivatives of each lipopeptide family are usually coproduced by a strain (Akpa et al., 2001; Leenders et al., 1999). Iturins have a potent antifungal activity against plant pathogens, affecting the surface tension of membranes, resulting in the formation of pores that allow leakage of potassium and other vital ions, and ultimately cell death (Thimon et al., 1992). Iturin production of antagonistic bacteria was related to their control efficacy as biocontrol agents. The iturin-producing strain Bacillus subtilis B-3 was able to control peach brown rot caused by Monilinia fructicola (Gueldner et al., 1988). Fengycin-type cyclic peptides produced by B. subtilis M4 inhibit the mycelial growths of Fusarium oxysporum, R. solani and B. cinerea at an MIQ of 1.85 μg, and also reduce the incidence of grey mold disease of apple fruits caused by B. cinerea during post-harvest storage (Ongena et al., 2005). B. subtilis CMB32 produces iturin A, fengycin, and surfactin A, that are each able to suppress the growth of Colletotrichum gloeosporioides, the causative agent of anthracnose in a variety of crops (Kim et al., 2010). Surfactin (Fig. 1) produced by B. subtilis RB14 suppresses damping-off by R. solani in tomato seedlings (Asaka and Shoda, 1996). Surfactin is the most powerful biosurfactant known to date, and it has strong antimicrobial activities on Candida albicans, Lactococcus lacti subsp. Lactis, Mycobacterium smegmatis, Staphylococcus aureus, and Streptococcus mutant (Peypoux et al., 1999; Singh and Cameotra, 2004). The derivatives of iturin such as bacillomycins have seven α-amino acids and a β-amino fatty acids with 14–16 C atoms, which can control a broad range of plant diseases. Podosphaera fusca, the principal cause of powdery mildew in melons is controlled by different strains of B. subtilis producing bacillomycin, fengycin or iturin (Romero et al., 2007). Derivatives of bacillomycin D produced by Bacillus amyloliquefaciens SD-32 inhibits infections of B. cinerea in cucumber leaves at concentrations of 30–80 μM (Tanaka et al., 2014), and bacillomycin F prevents the growth of phytopathogenic fungi such as B. cinerea (MIC 20 μg/ml), Mycosphaerella pinode (MIC 10 μg/ml), and Pleospora herbarum (MIC 10 μg/ml) (Mhammedi et al., 1982). Other antibiotics of the iturin group, mycosubtilin, consists of seven α-amino acids in an L-Asn-D-Tyr-D-Asn-L-Gln-L-Pro-D-Ser-L-Asn sequence closed by a β-amino acid linkage (Peypoux et al., 1986). Overproducing mycosubtilin mutants of B. subtilis ATCC6633 are more effective at controlling Pythium damping-off on tomato plants than the parent strain (Leclère et al., 2005). Paenibacillus sp. strain B2 isolated from the sorghum rhizosphere and was found to produce polymixin B and had antagonistic characteristics towards Erwinia carotovora. Polymixin B is also effective against the root pathogenic fungus Fusarium solani (2.6 μg/ml; isolated from Lascaux caves, Lot, France) and Fusarium acuminatum (8.0 μg/ml) (Selim et al., 2005). The fusaricidins are a group of cyclic depsipeptides with an unusual 15-guanidino-3-hydroxypentadecanoic acid moiety bound to a free amino group (Kajimura and Kaneda, 1997) (Fig. 1). The compound is produced by Paenibacillus polymyxa E681 and controls Phytophthora blight infections in red pepper caused by Phytophthora capsici (Lee et al., 2013). The antibiotic agrastatin A is produced by B. subtilis AQ713 and has a broad fungicidal spectrum in vitro. Agrastatin A is a potent control agent on grey mold disease of bean and geranium leaves and, early blight of tomato seedlings caused by Alternaria solani. Grape downy mildew caused by Plasmopara viticolar is also effectively controlled by agrastatin A and the efficacy is comparable with the synthetic fungicide metalaxyl (Heins et al., 2000). Gramicidin S (Fig. 1) is considered unusual because it has both antifungal activity against Fusarium nivale and sporicidal qualities towards the spores of Bacillus sp. and Dictyostelium discoideum (Murray et al., 1986). Brevibacillus brevis (formerly Bacillus brevis) synthesizes gramicidin S, and acts as a biocontrol agent against B. cinerea, the causative agent of grey mold of Chinese cabbage (Edwards and Seddon, 2001).

Cyclic peptides are also widely produced by Streptomyces spp. Valinomycin (Fig. 1) was first identified from culture extract of Streptomyces griseus. Valinomycin has insecticidal and acaricidal activities (Heisey et al., 1988) and acts as an ionophore that specifically modulates the transport of potassium ions across biological membranes. Based on these properties, valinomycin has been used as an insecticide and nematocide, and also as a key component in the instrumental measurement of potassium ions in biomedicine (Perkins et al., 1990). Antifungal property of valinomycin was subsequently identified from Streptomyces sp. M10 against B. cinerea, and its disease control efficacy was confirmed against Botrytis blight (Park et al., 2008). Antioomycete activity has also been shown against P. capsici at an IC₅₀ value of 15.9 μg/ml (Lim et al., 2007).

Chitin synthase inhibitors

Chitin is a linear homopolymer of β-(1,4)-linked N-acetylglucosamine (GlcNAc) residues, and is a major component of fungal cell walls. It is synthesized on the cytoplasmic surface of the plasma membrane, extruded perpendicularly to the cell surface as microfibrils, and then crystallized outside the cell through extensive hydrogen bonding (the poly-GlcNAc chains run antiparallel) (Bulawa, 1993; Georgopapadakou, 1992). Polyoxins and nikkomycins are nucleoside-peptide antibiotics that are competitive inhibitors of chitin synthase, acting as analogs of the substrate UDP-GlcNAc (Isono and Suzuki, 1979). Their potency and excellent selectivity ensure they have much commercial appeal as fungicides.

Arthrichitin (Fig. 2) is produced by Arthrinium phaeospermum and is non-nucleoside type inhibitor of chitin biosynthesis. Arthrichitin has a cyclic depsipeptide structure and causes morphological abnormalities in B. cinerea in vitro. It also showed 75 and 85% disease control efficacies against Pyricularia oryzae infection in rice and B. cineria infection in cucumber at 5 mg/ml, respectively (Vijayakumar et al., 1996).

Fig. 2

Molecular structures of antimicrobial cyclic peptides that modulate the biosynthesis of chitin, glucan, and inositol phosphorylceramide (IPC).

Thiopeptide antibiotics are an important class of natural products resulting from post-translational modifications of ribosomally synthesized peptides. Cyclothiazomycin is a typical thiopeptide antibiotic that has a unique bridged macrocyclic structure derived from an 18 amino acid structural peptide (Wang et al., 2010). Cyclothiazomycin B1 (Fig. 2) is an antifungal cyclic thiopeptide isolated from the culture broth of Streptomyces sp. HA 125-40 (Mizuhara et al., 2011). Cyclothiazomycin B1 binds to chitin in the fungal cell wall and disrupts the crystallization of the chitin chain. This mechanism inhibits the growth of filamentous fungi such as plant pathogens, and causes swelling of the hyphae and spores. Growths of Mucor and Fusarium spp. can be inhibited at low concentrations (0.020–0.33 μM) of cyclothiazomycin B1.

Cyclic peptides affecting glucan synthesis

Glucans are homopolymers of glucose arranged as long (≥60 units) coiling strings of β-(1,3)-linked residues with occasional side chains involving β-(1,6)-linkages. The enzyme β-(1,3)-glucan synthase catalyzes polymerization and consists of two functional components: a catalytic part that acts on the UDP-glucose substrate and a regulatory component that binds GTP (Arellano et al., 1996; Hartland et al., 1991). The β-1,3-glucan polymers form a network of fibrils that surround the fungal cell, helping to maintain mechanical strength and osmotic resistance of the fungal cell wall (Smits et al., 1999). Glucan synthases hold much appeal as targets for antifungal drug development.

Neopeptins (Fig. 2) are produced by Streptomyces spp. and are cyclic lipopeptides composed of nine unusual amino acids and one acyl chain. Neopeptins have potent antifungal properties, inhibiting mannoprotein, proteoheteroglycan, and β-1,3-glucan synthetases found in Saccharomyces cerevisiae (Satomi et al., 1982; Ubukata et al., 1984; Ubukata et al., 1986), and causing mycelial swelling in several plant pathogens. The growth of Glomerella cingulata, P. oryzae, Cochliobolus miyabeanus, and Colletotrichum lagenaium can be completely inhibited by neopeptins at a concentration of 4 μg/ml. In one study, the development of powdery mildew on cucumber plants was effectively controlled by a mixture of neopeptins isolated from a culture of Streptomyces neopeptinius (Han et al., 2008; Kim et al., 2007). The tyrocidines (Fig. 2) are a group of antimicrobial cyclic decapeptides identified from the soil bacterium Bacillus aneurinolyticus. They have a potent antifungal activity against a range of phytopathogens including F. solani and B. cinerea, where they induce morphological abnormalities of the mycelia. The potency of tyrocidines against agronomically important phytopathogens (significantly higher activity than that of the commercial fungicide bifonazole), combined with their relative salt stability, highlights the potential for their development as antifungal agents in the agricultural sector (Troskie, 2014).

Cyclic peptides affecting inositol phosphorylceramide synthase

Aureobasidins (Fig. 2) are cyclic depsipeptide antifungal antibiotics produced by Aureobasidium pullulans (Takesako et al., 1991). Their molecular structures consist of eight lipophilic amino acid residues and one α-hydroxy acid (Ikai et al., 1991a; Ikai et al., 1991b). Most of the aureobasidin derivatives are effective against a wide range of fungi and protozoa. Aureobasidin A inhibits inositol phosphorylceramide (IPC) synthase, a key enzyme catalyzing sphingolipid synthesis in fungi (Nagiec et al., 1997). The inhibition of IPC synthase affects spore germination, germ tube elongation and hyphal growth of the pathogenic fungi Penicillium digitatum, Penicillium italicum, Penicillium expansum, B. cinerea and M. fructicola, which are major pathogens causing postharvest diseases of a variety of fruits (Liu et al., 2007). Aureobasidin A is effective (50 μg/ml) at controlling the citrus green mold (P. digitatum) and also reducing the incidence and severity of strawberry gray mold (B. cinerea).

Other cyclic peptides with unknown antimicrobial mechanism

The modes of action of several antifungal cyclic peptides isolated from bacteria, fungi, blue-green alga and cyanobacterium remain unclear, but their potent activities against plant pathogens are well established.

Cepacidines A1 and A2 (Fig. 3) are glycopeptides produced by Pseudomonas cepacia (Lee et al., 1994; Lim et al., 1994). The MICs for cepacidine A range from 0.098 to 0.391 mg/ml against Aspergillus niger, F. oxysporum, and Rhizopus stolonifer. Fungicin M4 is a hydrophilic antifungal peptide produced by Bacillus licheniformis M-4 (Lebbadi et al., 1994). Fungicin M4 is resistant to proteolytic enzymes and lipase, and inhibits the growth of Microsporum canis, Mucor species, Bacillus megaterium, Corynebacterium glutamicum, and Sporothrix schenckii. Peptide antibiotic PKB1 (Fig. 3) is produced by P. polymyxa PKB1 and has antifungal activity against the causative agent of blackleg disease in canola (Leptosphaeria maculans) as well as other economically destructive pathogens including S. sclerotiorum, Marasmius oreades, Pythium pythioides, R. solani, Fusarium avenaceum, and Alternaria brassicae (Kharbanda et al., 2003). P. polymyxa PKB1 can also act as a biocontrol agent against blackleg and other fungal diseases of canola. Success in using P. polymyxa PKB1 as a biocontrol agent was achieved by coating canola seeds with freeze-dried or living cells of the bacterium to protect germinating shoots from L. maculans infection in stubble.

Fig. 3

Molecular structures of cyclic peptides with unknown antimicrobial mechanisms.

Cryptocandin (Fig. 3) is a potent lipopeptide antimycotic produced by the endophytic fungus Cryptosporiopsis cf. quercina (Strobel et al., 1999), and is active against several plant pathogenic fungi including S. sclerotiorum (MIC 0.78 μg/ml) and B. cinerea (MIC 0.62 μg/ml). Verlamelin (Fig. 3) is a peptide antibiotic produced by Acremonium strictum (Kim et al., 2002) and the entomopathogenic fungus Lecanicillum sp. (Ishidoh et al., 2014). It has in vitro antifungal activity against phytopathogenic fungi such as Margnaporthe grisea, Bipolaris maydis, and B. cinerea. In vivo analysis has shown strong protective and curative activities, particularly against barley powdery mildew caused by Blumeria graminis f. sp. hordei. The development of barley powdery mildew can be controlled by an application of 100 μg/ml verlamelin in 7-day protective treatments and 2-day curative applications.

Calophycin is a broad-spectrum antifungal cyclic decapeptide that has been isolated from Calothrix fusca EU-10-1, a terrestrial blue-green alga belonging to the Nostocaceae family. In experiments using disc-diffusion assays in soft agar, calophycin showed zones of inhibition of 13 mm (1.2 μg/disc) against Aspergillus oryza (Moon et al., 1992).

The cyanobacterium Schizotrix (TAU strain IL-89-2) produces schizotrin A (Fig. 3), a cyclic undecapeptide (Pergament and Carmeli, 1994), that inhibited the radial growth of Sclerotium rolfsii (13.4 nM/disc), R. solani (13.4 nM/disc), F. oxysporum (33.5 nM/disc) and C. gloeosporioides (13.4 nM/disc) in zone-of-inhibition assays.

Xanthostatin (Fig. 3) produced by Streptomyces spiroverticillatus is a depsipeptide antibiotic with an N-acetylglycine side chain and selective antimicrobial activity against Xanthomonas spp. (Kim et al., 2003). Bacterial canker affecting citrus plants by causing necrotic spots on citrus fruits, leaves, and stems, is induced by Xanthomonas campestris pv. citri. Control measures include windbreaks of trees or netting, pruning of diseased shoots, and chemical sprays, but the canker on susceptible or highly susceptible trees has not yet been properly controlled (Osada and Isono, 1986; Stall and Seymour, 1983).