Lysis Of Zoospores Research Articles

Inhibitory Effects of Macrotetrolides from Streptomyces spp. On Zoosporogenesis and Motility of Peronosporomycete Zoospores Are Likely Linked with Enhanced ATPase Activity in Mitochondria.

The release of zoospores from sporangia and motility of the released zoospores are critical in the disease cycle of the Peronosporomycetes that cause devastating diseases in plants, fishes, animals and humans. Disruption of any of these asexual life stages eliminates the possibility of pathogenesis. In the course of screening novel bioactive secondary metabolites, we found that extracts of some strains of marine Streptomyces spp. rapidly impaired motility and caused subsequent lysis of zoospores of the grapevine downy mildew pathogen Plasmopara viticola at 10 μg/ml. We tested a number of secondary metabolites previously isolated from these strains and found that macrotetrolide antibiotics such as nonactin, monactin, dinactin and trinactin, and nactic acids such as (+)-nonactic acid, (+)-homonactic acid, nonactic acid methyl ester, homonactic acid methyl ester, bonactin and feigrisolide C impaired motility and caused subsequent lysis of P. viticola zoospores in a dose- and time-dependent manners with dinactin being the most active compound (MIC 0.3 μg/ml). A cation channel-forming compound, gramicidin, and a carrier of monovalent cations, nigericin also showed similar biological activities. Among all 12 compounds tested, gramicidin most potently arrested the motility of zoospores at concentrations starting from 0.1 μg/ml. All macrotetrolide antibiotics also displayed similar motility impairing activities against P. viticola, Phytophthora capsici, and Aphanomyces cochlioides zoospores indicating non-specific biological effects of these compounds toward peronosporomyctes. Furthermore, macrotetrolide antibiotics and gramicidin also markedly suppressed the release of zoospores from sporangia of P. viticola in a dose-dependent manner. As macrotetrolide antibiotics and gramicidin are known as enhancers of mitochondrial ATPase activity, inhibition of zoosporogenesis and motility of zoospores by these compounds are likely linked with hydrolysis of ATP through enhanced ATPase activity in mitochondria. This is the first report on motility inhibitory and lytic activities of macrotetrolide antibiotics and nactic acids against the zoospores of peronosporomycete phytopathogens.

Oligomycins and pamamycin homologs impair motility and induce lysis of zoospores of the grapevine downy mildew pathogen, Plasmopara viticola.

Four antibiotics (pamamycin, oligomycin A, oligomycin B and echinosporin) were isolated and characterized from the fermentation broth of the marine Streptomyces strains B8496 and B8739. Bioassays revealed that each of these compounds impaired motility and caused subsequent lysis of P.viticola zoospores in a dose- and time-dependent manner. Pamamycin displayed the strongest motility inhibitory and lytic activities (IC50 0.1 μgmL(-1)) followed by oligomycin B (IC50 0.15 and 0.2 μgmL(-1)) and oligomycin F (IC50 0.3 and 0.5 μgmL(-1)). Oligomycin A and echinosporin also showed motility inhibitory activities against the zoospores with IC50 values of 3.0 and 10.0 μgmL(-1), respectively. This is the first report of motility inhibitory and lytic activities of these antibiotics against zoospores of a phytopathogenic peronosporomycete. Structures of all the isolated compounds were determined based on detailed spectroscopic analysis.

Biosynthesis, Chemical Structure, and Structure-Activity Relationship of Orfamide Lipopeptides Produced by Pseudomonas protegens and Related Species.

Orfamide-type cyclic lipopeptides (CLPs) are biosurfactants produced by Pseudomonas and involved in lysis of oomycete zoospores, biocontrol of Rhizoctonia and insecticidal activity against aphids. In this study, we compared the biosynthesis, structural diversity, in vitro and in planta activities of orfamides produced by rhizosphere-derived Pseudomonas protegens and related Pseudomonas species. Genetic characterization together with chemical identification revealed that the main orfamide compound produced by the P. protegens group is orfamide A, while the related strains Pseudomonas sp. CMR5c and CMR12a produce orfamide B. Comparison of orfamide fingerprints led to the discovery of two new orfamide homologs (orfamide F and orfamide G) in Pseudomonas sp. CMR5c. The structures of these two CLPs were determined by nuclear magnetic resonance (NMR) and mass spectrometry (MS) analysis. Mutagenesis and complementation showed that orfamides determine the swarming motility of parental Pseudomonas sp. strain CMR5c and their production was regulated by luxR type regulators. Orfamide A and orfamide B differ only in the identity of a single amino acid, while orfamide B and orfamide G share the same amino acid sequence but differ in length of the fatty acid part. The biological activities of orfamide A, orfamide B, and orfamide G were compared in further bioassays. The three compounds were equally active against Magnaporthe oryzae on rice, against Rhizoctonia solani AG 4-HGI in in vitro assays, and caused zoospore lysis of Phytophthora and Pythium. Furthermore, we could show that orfamides decrease blast severity in rice plants by blocking appressorium formation in M. oryzae. Taken all together, our study shows that orfamides produced by P. protegens and related species have potential in biological control of a broad spectrum of fungal plant pathogens.

Medicinal plant extracts and protein kinase C inhibitor suppress zoosporogenesis and impair motility of Phytophthora capsici zoospores

Ansary M.W.R., Haque E., West H.M., Rahman M.M., Akanda A.M., Wang Y., Islam M. T. (2016): Medicinal plant extracts and protein kinase C inhibitor suppress zoosporogenesis and impair motility of Phytophthora capsici zoospores. Plant Protect. Sci., 52: 113–122. The effects of water and acetone extracts from 100 medicinal plants growing in Bangladesh, along with a selective inhibitor of protein kinase C (PKC), chelerythrine chloride, were tested on zoosporogenesis (release of zoospores from the sporangia) and motility of Phytophthora capsici zoospores. Among 10 active crude acetone extracts, those from Psidium guajava and Nigella sativa (100 μg/ml) suppressed zoosporogenesis relative to the control (100% zoospore release) to 60 and 40% released, respectively and inhibited motility of 100% of the zoospores within 60 min of treat ment. Chelerythrine chloride also suppressed zoosporogenesis (30% released) at 0.1 μg/ml and inhibited motility of 100% zoospores at 0.2 μg/ml within 60 minutes. Among water extracts of 100 medicinal plants, 56 impaired motility of zoospores in a dose- and time-dependent manner. Diluted (20-fold) water extracts of 10 plants including Ocinum gratissimum, Terminalia bohera , and Duranta plumeri inhibited motility and subsequently caused lysis of zoospores.

Zoospore interspecific signaling promotes plant infection by Phytophthora

BackgroundOomycetes attack a huge variety of economically and ecologically important plants. These pathogens release, detect and respond to signal molecules to coordinate their communal behaviors including the infection process. When signal molecules are present at or above threshold level, single zoospores can infect plants. However, at the beginning of a growing season population densities of individual species are likely below those required to reach a quorum and produce threshold levels of signal molecules to trigger infection. It is unclear whether these molecules are shared among related species and what their chemistries are.ResultsZoospore-free fluids (ZFF) from Phytophthora capsici, P. hydropathica, P. nicotianae (ZFFnic), P. sojae (ZFFsoj) and Pythium aphanidermatum were cross tested for stimulating plant infection in three pathosystems. All ZFFs tested significantly increased infection of Catharanthus roseus by P. nicotianae. Similar cross activities were observed in infection of Lupinus polyphyllus and Glycine max by P. sojae. Only ZFFnic and ZFFsoj cross induced zoospore aggregation at a density of 2 × 103 ml-1. Pure autoinducer-2 (AI-2), a component in ZFF, caused zoospore lysis of P. nicotianae before encystment and did not stimulate plant infection at concentrations from 0.01 to 1000 μM. P. capsici transformants with a transiently silenced AI-2 synthase gene, ribose phosphate isomerase (RPI), infected Capsicum annuum seedlings at the same inoculum concentration as the wild type. Acyl-homoserine lactones (AHLs) were not detected in any ZFFs. After freeze-thaw treatments, ZFF remained active in promoting plant infection but not zoospore aggregation. Heat treatment by boiling for 5 min also did not affect the infection-stimulating property of ZFFnic.ConclusionOomycetes produce and use different molecules to regulate zoospore aggregation and plant infection. We found that some of these signal molecules could act in an inter-specific manner, though signals for zoospore aggregation were somewhat restricted. This self-interested cooperation among related species gives individual pathogens of the same group a competitive advantage over pathogens and microbes from other groups for limited resources. These findings help to understand why these pathogens often are individually undetectable until severe disease epidemics have developed. The signal molecules for both zoospore aggregation and plant infection are distinct from AI-2 and AHL.

Functional, genetic and chemical characterization of biosurfactants produced by plant growth-promotingPseudomonas putida267

Plant growth-promoting Pseudomonas putida strain 267, originally isolated from the rhizosphere of black pepper, produces biosurfactants that cause lysis of zoospores of the oomycete pathogen Phytophthora capsici. The biosurfactants were characterized, the biosynthesis gene(s) partially identified, and their role in control of Phytophthora damping-off of cucumber evaluated. The biosurfactants were shown to lyse zoospores of Phy. capsici and inhibit growth of the fungal pathogens Botrytis cinerea and Rhizoctonia solani. In vitro assays further showed that the biosurfactants of strain 267 are essential in swarming motility and biofilm formation. In spite of the zoosporicidal activity, the biosurfactants did not play a significant role in control of Phytophthora damping-off of cucumber, since both wild type strain 267 and its biosurfactant-deficient mutant were equally effective, and addition of the biosurfactants did not provide control. Genetic characterization revealed that surfactant biosynthesis in strain 267 is governed by homologues of PsoA and PsoB, two nonribosomal peptide synthetases involved in production of the cyclic lipopeptides (CLPs) putisolvin I and II. The structural relatedness of the biosurfactants of strain 267 to putisolvins I and II was supported by LC-MS and MS-MS analyses. The biosurfactants produced by Ps. putida 267 were identified as putisolvin-like CLPs; they are essential in swarming motility and biofilm formation, and have zoosporicidal and antifungal activities. Strain 267 provides excellent biocontrol activity against Phytophthora damping-off of cucumber, but the lipopeptide surfactants are not involved in disease suppression. Pseudomonas putida 267 suppresses Phy. capsici damping-off of cucumber and provides a potential supplementary strategy to control this economically important oomycete pathogen. The putisolvin-like biosurfactants exhibit zoosporicidal and antifungal activities, yet they do not contribute to biocontrol of Phy. capsici and colonization of cucumber roots by Ps. putida 267. These results suggest that Ps. putida 267 employs other, yet uncharacterized, mechanisms to suppress Phy. capsici.

Biological Mode of Action of Dimethomorph on Pseudoperonospora cubensis and Its Systemic Activity in Cucumber

Dimethomorph is a fungicide with high activity against Peronosporomycetes plant pathogens. The present study showed that dimethomorph is effective on controlling the oomycete fungal pathogen Pseudoperonospora cubensis causing downy mildew on cucumber. The fungicide did not affect zoospores discharge from sporangia of P. cubensis, but it strongly inhibited mycelial growth and sporangial production in vitro and increased lysis of zoospores. Dose of 2 mg L −1 of dimethomorph was sufficient to inhibit mycelial growth and sporangial production of P. cubensis on leaf disks, 5 mg L −1 was enough to lyse zoospores of P. cubensis, and 25 mg L −1 was required to inhibit sporangial production on detached leaves. In whole plant tests, dimethomorph exhibited strong protective and curative activity. Dimethomorph when applied at a dose of 300 mg L −1 for 1, 3, 5, 7 days before inoculation exhibited 100% efficacy on disease control. On the other hand, efficacies of 67.1 and 31.5% were obtained when the same dose of dimethomorph was applied for 1 and 3 days after inoculation, respectively. So dimethomorph had persistence effect on leaves for 7 days at least and exhibited strong protective and curative activity. Bioassay analyses showed that dimethomorph could be translocated in the xylem system, redistributed in the leaf, and penetrated from the upper surface to the lower surface of the leaf but could not be translocated in phloem system or transferred from the roots to leaves of cucumber plants in sufficient amounts for disease control. The biocharacteristics of dimethomorph make it well suitable for integration of a control programme against downy mildew disease on cucumber and as a component to delay other peronosporomycetes fungicide-resistance development.

In vitro antimicrobial and in vivo antioomycete activities of the novel antibiotic thiobutacin

A number of synthetic fungicides are not effective when confronted by oomycete pathogens because many fungicide targets are absent from oomycetes. Moreover, resistance to fungicides has already arisen in oomycete species, and thus development of new, effective and safe compounds for use in oomycete disease control is necessary. Zoospore lysis began at 10 microg mL(-1) of thiobutacin, and most of the zoospores were collapsed at 50 microg mL(-1). Thiobutacin also revealed inhibitory activity against the cyst germination and hyphal growth of Phytophthora capsici at 50 microg mL(-1). Treatment with thiobutacin exhibited protective activity against development of Phytophthora disease on pepper plants. The authors verified in vitro antioomycete activity of thiobutacin against P. capsici and its control efficacy against Phytophthora blight in vivo. This is the first report to demonstrate in vivo antioomycete activity of the novel antibiotic thiobutacin against P. capsici infection.

Efficacy of alcohol alkoxylate surfactants differing in the molecular structure of the hydrophilic portion to control Phytophthora nicotianae in tomato substrate culture

Phytophthora nicotianae is a common and destructive pathogen of numerous ornamental, agronomic and horticultural crops such as tomato, tobacco and citrus. Three monobranched C 13 alcohol alkoxylate non-ionic surfactants were evaluated for in vitro inhibitory activity against the different asexual structures of P. nicotianae. The same surfactants, labelled MBA1301, MBA1303 and MBA1306, were tested for their in vivo control capacity against P. nicotianae root rot of tomato ( Lycopersicon esculentum) under glasshouse conditions. MBA1301, MBA1303 and MBA1306 differ in the molecular structure of the hydrophilic portion. The molecular weight of MBA1301 is comparable to that of MBA1303 and is eight times lower than that of MBA1306. The main in vitro activity for MBA1301 and MBA1303 was a direct lytic effect on the zoospores. Zoospore lysis was already observed in the presence of 1 μg ml −1 of these two surfactants and almost no zoospores survived an addition of 5 μg ml −1 surfactant. In addition, MBA1301 and MBA1303 reduced sporangia formation at a concentration of 5 μg ml −1. Both surfactants only affect mycelium growth at concentrations as high as 100 μg ml −1. MBA1306 did not show any effect on sporangia formation, zoospore release and mycelium growth of P. nicotianae at a concentration 10 times that of the other two surfactants. A good in vivo control of P. nicotianae on tomato in substrate culture was obtained for MBA1301 and MBA1303 whereas the control capacity of MBA1306 was significantly lower. The results of this research indicate that non-ionic alcohol alkoxylate surfactants can be used to control tomato root rot caused by P. nicotianae in substrate culture. In addition, the size rather than the arrangement of the polymers in the hydrophilic portion of the surfactant molecule determines the efficacy to control tomato root rot caused by P. nicotianae in substrate culture.

In Vitro Anti-Oomycete Activity and In Vivo Control Efficacy of Phenylacetic Acid Against Phytophthora capsici

Phenylacetic acid (PAA) was evaluated for in vitro anti-oomycete activity and in vivo control efficacy against Phytophthora capsici. Microscopic observation revealed that the high level of anti-oomycete activity of PAA (10 /ml) against P. capsici is mainly due to the lytic effect on zoospores. Zoospore lysis began in the presence of 5 u/ml of PAA and most of the zoospores were collapsed at 10 /ml. PAA showed inhibitory activity against the zoospore germination and hyphal growth of P. capsici at the concentration of 50 /ml. In the glasshouse, the protective effect of PAA against Phytophthora blight was high on pepper plants when treated just before inoculation with P. capsici. In the artificially infested field, protection of pepper plants against the Phyto-phthora epidemic was achieved at a considerable level by PAA treatment.

Biochemical, genetic, and zoosporicidal properties of cyclic lipopeptide surfactants produced by Pseudomonas fluorescens.

Zoospores play an important role in the infection of plant and animal hosts by oomycetes and other zoosporic fungi. In this study, six fluorescent Pseudomonas isolates with zoosporicidal activities were obtained from the wheat rhizosphere. Zoospores of multiple oomycetes, including Pythium species, Albugo candida, and Phytophthora infestans, were rendered immotile within 30 s of exposure to cell suspensions or cell culture supernatants of the six isolates, and subsequent lysis occurred within 60 s. The representative strain SS101, identified as Pseudomonas fluorescens biovar II, reduced the surface tension of water from 73 to 30 mN m-1. The application of cell suspensions of strain SS101 to soil or hyacinth bulbs provided significant protection against root rot caused by Pythium intermedium. Five Tn5 mutants of strain SS101lacked the abilities to reduce the surface tension of water and to cause lysis of zoospores. Genetic characterization of two surfactant-deficient mutants showed that the transposons had integrated into condensation domains of peptide synthetases. A partially purified extract from strain SS101 reduced the surface tension of water to 30 mN m-1 and reached the critical micelle concentration at 25 micrograms ml-1. Reverse-phase high-performance liquid chromatography yielded eight different fractions, five of which had surface activity and caused lysis of zoospores. Mass spectrometry and nuclear magnetic resonance analyses allowed the identification of the main constituent as a cyclic lipopeptide (1,139 Da) containing nine amino acids and a 10-carbon hydroxy fatty acid. The other four zoosporicidal fractions were closely related to the main constituent, with molecular massesranging from 1,111 to 1,169 Da.

Zoosporicidal activities of anacardic acids against Aphanomyces cochlioides.

The EtOAc soluble constituents of the unripe fruits of Ginkgo biloba showed motility inhibition followed by lysis of zoospores of the phytopathogenic Aphanomyces cochlioides. We purified 22:1-omega7-anacardic acid (1), 24:1-omega9-anacardic acid (2) and 22:0-anacardic acid (3), together with other related compounds, 21:1-omega7-cardol (4) and 21:1-omega7-cardanol (5) from the crude extracts of Ginkgo fruits. Amongst them, compound 1 was a major active agent in quality and quantity, and showed potent motility inhibition (98% in 30 min) followed by lysis (55% in 3 h) of the zoospores at 1 x 10(-7) M. The 2-O-methyl derivative (1-c) of 1 displayed antibacterial activity against Bacillus subtilis, but practically inactive to Escherichia coli. A brief study on structure-activity relationships revealed that a carboxyl group on the aromatic ring and an unsaturated side chain in the anacardic acid derivative are important for strong motility inhibitory and lytic activities against the zoospore.

The Effect of Synthetic Surfactants on Disease Severity of White Rust on Spinach.

White rust, caused by Albugo occidentalis, is an economically important disease of spinach (Spinacia oleracea). Although cultural practices and partial host resistance are used for disease management, control strategies often rely on fungicides. An alternative approach, the use of ionic (cationic or anionic) and nonionic surfactants, was evaluated for its effect on white rust in greenhouse and field experiments. Surfactant treatments were compared with a water control and two commercial fungicides, azoxystrobin (Quadris) and 1,2,3-benzothiadiazole-7-carbothioic acid S-methyl ester (Actigard). Greenhouse plants were treated with a single surfactant application followed by inoculation with sporangia. Disease severity was rated on leaves 8 to 12 days after inoculation. Field tests were conducted in Arkansas and Texas and received three to five surfactant applications during the season. Disease severity was determined at the end of the growing season. In greenhouse and field tests, all surfactant treatments showed significant reductions in white rust severity compared with water controls. The surfactants Naiad and sodium dodecyl sulfate (SDS) were highly effective and comparable to fungicides in reducing white rust severity. In a laboratory assay, microscopic examination revealed that most of the surfactants at low concentrations caused rapid (<2 min) zoospore lysis.

Biological mode of action of Famoxadone on Plasmopara viticola and Phytophthora infestans

Famoxadone (DPX-JE874, Famoxate ®) is a highly active, broad-spectrum fungicide from a new class of chemicals, the oxazolidinediones. Famoxadone inhibited sporangial differentiation and zoospore release and caused lysis of zoospores within minutes. Doses in the order of 0.01 mg l −1 were sufficient to lyse zoospores of both Plasmopara viticola and Phytophthora infestans. In the case of P. infestans, mycelial growth and direct germination of sporangia were also inhibited but higher doses were needed. In whole plant tests, famoxadone exhibited strong preventative control of both grape downy mildew and potato late blight with good residual activity of at least 7–10 days. No significant effect on disease control was observed when famoxadone was applied after inoculation. Both the in vitro and in vivo activities of famoxadone appeared complementary to that of cymoxanil which exhibited shorter residual activity but strong curative properties.

Zoospore homing and infection events: effects of the biocontrol bacterium Burkholderia cepacia AMMDR1 on two oomycete pathogens of pea (Pisum sativum L.).

Burkholderia cepacia AMMDR1 is a biocontrol agent that protects pea and sweet corn seeds from Pythium damping-off in field experiments. The goal of this work was to understand the effect of B. cepacia AMMDR1 on Pythium aphanidermatum and Aphanomyces euteiches zoospore homing events and on infection of pea seeds or roots. In vitro, B. cepacia AMMDR1 caused zoospore lysis, prevented cyst germination, and inhibited germ tube growth of both oomycetes. B. cepacia AMMDR1 also reduced the attractiveness of seed exudates to Pythium zoospores to nondetectable levels. However, when present at high levels on seeds, B. cepacia AMMDR1 had little net effect on zoospore attraction, probably because it also enhanced seed exudation. Seed-applied B. cepacia AMMDR1 dramatically reduced the incidence of infection by Pythium zoospores in situ compared with an antibiosis-deficient Tn5 mutant strain. This mutant strain also decreased Pythium infection incidence to some extent, but only when the pathogen inoculum potential was low. B. cepacia AMMDR1 did not affect attraction of Aphanomyces zoospores or Aphanomyces root rot incidence. These results suggest that B. cepacia AMMDR1 controls P. aphanidermatum largely through antibiosis, but competition for zoospore-attracting compounds can contribute to the effect. Differences in suppression of Aphanomyces and Pythium are discussed in relation to differences in the ecology of the two pathogens.

In vivo control andin vitro antifungal activity of rhamnolipid B, a glycolipid antibiotic, againstPhytophthora capsici andColletotrichum orbiculare

The glycolipid antibiotic rhamnolipid B isolated from Pseudomonas aeruginosa strain B5 was evaluated for in vitro antifungal activity and in vivo control against phytophthora blight and anthracnose under glasshouse conditions. Rhamnolipid B showed antifungal activity against Cercospora kikuchii, Cladosporium cucumerinum, Colletotrichum orbiculare, Cylindrocarpon destructans, Magnaporthe grisea and Phytophthora capsici. Microscopic observation revealed that the high level of antifungal activity (10 µg ml −1) against P capsici was mainly due to a lytic effect on zoospores. Zoospore lysis began in the presence of 10 µg ml −1 of rhamnolipid B and most of the zoospores were collapsed at 25 µg ml −1. Rhamnolipid B showed inhibitory activity against the germination of zoospores and hyphal growth of P capsici at concentrations of 50 µg ml −1. Spore germination of the anthracnose plant pathogen C orbiculare was also inhibited in the presence of 50 µg ml −1 of rhamnolipid B, although hyphal growth was not affected at this concentration. In the glasshouse, the efficacy of rhamnolipid B against phytophthora blight was similar to that of metalaxyl on pepper plants when treated just before inoculation with P capsici. Treatment with either at 500 µg ml −1 completely protected pepper plants from phytophthora blight. Rhamnolipid B also suppressed the development of C orbiculare infection on leaves of cucumber plants. © 2000 Society of Chemical Industry

Studies on Antagonistic Effects of Trichoderma Isolates Against Phytophthora cactorum

AbstractBioassays were used to demonstrate the antibiotic effect of Trichoderma isolates on P. cactorum. When both fungi were grown on benomyl‐containing PDA medium, the mycelial growth of Trichoderma was suppressed. However, the production of antibiotics by this fungus remained active, leading to inhibition of the mycelial growth of P. cactorum. The antibiotic effect of Trichoderma on zoospores and cysts was tested on a PDA substrate precultured with Trichoderma on cellophane sheets. On the substrate of some Trichoderma isolates, lysis of zoospores, formation of extracellular vesicles, and hypertrophy of the water expulsion vesicle did occur, both resulting in the death of the zoospores. Conidial suspensions of Trichoderma isolates also induced zoospore lysis. It is presumed that membrane‐active peptide antibiotics (peptaibols) are involved in zoospore lysis. The peptaibol paracelsin caused lysins of zoospores at a concentration of 2.5 × 10−4 M. The effect on cysts depended on the Trichoderma isolate tested and the age of Trichoderma preculture. Old cultures (after beginning of sporulation) affected cysts more severely than young cultures (before sporulation) which usually were not lethal to the cysts but induced preferably microsporangium formation, inhibition of cyst germination, and retardation of germ tube growth.

Role of ammonia and calcium in lysis of zoospores of Phytophthora cactorum by Bacillus cereus strain UW85

Cultures and cell-free culture filtrates of the biological control agent Bacillus cereus strain UW85 lysed zoospores of Phytophthora cactorum in vitro. Changes in the ionic composition of the growth medium caused by growth of UW85 account for the lytic activity. UW85 raised the pH, excreted ammonia, and removed calcium from the medium during growth and sporulation. Zoospores lysed when pCa 2+:pNH 3 was greater than 0.8. The lytic activity was produced in uninoculated growth medium by adding ammonium chloride and base to create a pCa 2+:pNH 3 ratio similar to that of UW85 culture filtrate.

Toxicity of oat roots, oat root extracts, and saponins to zoospores of Pythium spp. and other fungi

In laboratory experiments, roots of oats (Avena sativa) and the grass Arrhenatherum elatius caused attraction and subsequent lysis of zoospores of Allomyces arbuscula, Aphanomyces sp., Phytophthora cinnamomi, Pythium aphanidermatum, P. arrhenomanes, P. graminicola, P. intermedium, P. ultimum var. sporangiferum and Saprolegnia litoralis. The lytic compound, thought to be the saponin avenacin, was released from apparently undamaged oat roots. It prevented cyst wall formation by zoospores, but it was only weakly active or inactive against pre-encysted zoospores and vegetative mycelia of Pythium spp. Its effects on zoospores were paralleled by the saponin β-aescin. In preliminary tests, these compounds retained activity after passage through, or incubation in, soil.

Potato lectin lyses zoospores of Phytophthora infestans and precipitates elicitors of terpenoid accumulation produced by the fungus

Lectins were isolated and partially purified from 4 potato cullivars with different R-genes for resistance to Phytophthora infestans. The 4 lectins lysed zoospores from 5 races of the fungus, and the ability to lyse was not related to the resistance of the cultivar. Solutions containing 25 μg glucose equivalent of lectin ml−1 caused 80 to 100% lysis of zoospores within 1 h. Highly purified lectins prepared from 2 potato cultivars exhibited a similar lytic effect on zoospores. Lysis was prevented when solutions containing lectin were mixed with solutions containing trimers and tetramers of N-acetyl- D -glucosamine prior to mixing with the zoospores. Sucrose, dextran, soluble starch, inulin, bovine serum albumin, laminarin and collagen at 300 μg ml−1 did not lyse zoospores. Agglutination or lysis was not observed when cystospores, sporangia or mycelia were treated with potato lectins, Concanavalin A, and lectins from Ricinus communis and Dolichos biflorns were tested for their effect on zoospores, and only concanavalin A was as active as the potato lectins in causing lysis. Like potato lectin, concanavalin A did not agglutinate, lyse or affect the germination or growth of cystospores or sporangia. Potato lectin (100 μg glucose equivalent ml−1) precipitated elicitors of terpenoid accumulation extracted from the fungus. The lectin-elicitor complexes nonetheless retained most of the elicitor activity when applied to potato discs. In contrast to potato lectin, concanavalin A in the absence of salts did not precipitate the elicitor. In the presence of Mn2+ and Ga2+, concanavalin A at 100 and 200 μg ml−1 precipitated some but not all the elicitor activity. Though evidence is lacking that potato lectin has a direct role in determining specificity in the interaction between potato and P. infestans, the results from studies of lysis, agglutination and the precipitation of elicitor by lectins suggest that the elicitor may not be exposed on the surface of cystospores, sporangia and mycelium.