Behavioural changes in Schistocerca gregaria following infection with a fungal pathogen: implications for susceptibility to predation

1. Field observations have indicated that infection of locusts and grasshoppers by the fungal entomopathogen Metarhizium anisopliae var. acridum may result in a substantial increase in the host's susceptibility to predation, before death is caused directly by the disease. 2. Laboratory experiments were conducted to examine how the behaviour of the desert locust Schistocerca gregaria Forskål changes following infection by M. anisopliae var. acridum to explore some potential mechanisms underlying this phenomenon. 3. In the first experiment, which involved monitoring general locust activity in small cages throughout the disease incubation period, infected locusts were observed to increase locomotion and bodily movement from 3 days after infection until death (average survival time of 11 days). There was some evidence of reduced feeding and mating behaviour following infection. 4. In a second experiment, locusts were exposed individually to a simulated predator attack and the initiation and strength of any escape responses were measured. Infected locusts were observed to have a reduced escape capability (both the propensity to escape and the strength of the response). In contrast to the relatively early changes in general activity observed in the first experiment, this was only apparent at the late stages of infection shortly before death. 5. Both an increase in movement and general apparency early in the infection process, and reduced escape capability late on, suggest mechanisms whereby the susceptibility of locusts and grasshoppers to predation might be enhanced following infection with M. anisopliae var. acridum.     

Stage-specific DNA methylation in a fungal plant pathogen.

Significantly more 5-methylcytosine residues were found in the DNA from the dormant sclerotia of Phymatotrichum omnivorum than in the DNA from the metabolically active mycelia of the fungus, as shown by high-pressure liquid chromatography of acid-hydrolyzed DNA digests and by restriction of the DNA with the isoschizomers MspI and HpaII. N6-Methyladenine was not detected in GATC sequences in the DNA isolated from either stage.     

Auxin signaling and transport promote susceptibility to the root-infecting fungal pathogen Fusarium oxysporum in Arabidopsis

Fusarium oxysporum is a root-infecting fungal pathogen that causes wilt disease on a broad range of plant species, including the model plant Arabidopsis thaliana. Currently, very little is known about the molecular or physiological processes that are activated in the host during infection and the roles these processes play in resistance and susceptibility to F. oxysporum. In this study, we analyzed global gene expression profiles of F. oxysporum-infected Arabidopsis plants. Genes involved in jasmonate biosynthesis as well as jasmonate-dependent defense were coordinately induced by F. oxysporum. Similarly, tryptophan pathway genes, including those involved in both indole-glucosinolate and auxin biosynthesis, were upregulated in both the leaves and the roots of inoculated plants. Analysis of plants expressing the DR5:GUS construct suggested that root auxin homeostasis was altered during F. oxysporum infection. However, Arabidopsis mutants with altered auxin and tryptophan-derived metabolites such as indole-glucosinolates and camalexin did not show an altered resistance to this pathogen. In contrast, several auxin-signaling mutants were more resistant to F. oxysporum. Chemical or genetic alteration of polar auxin transport also conferred increased pathogen resistance. Our results suggest that, similarly to many other pathogenic and nonpathogenic or beneficial soil organisms, F. oxysporum requires components of auxin signaling and transport to colonize the plant more effectively. Potential mechanisms of auxin signaling and transport-mediated F. oxysporum susceptibility are discussed.     

Arabidopsis nonhost resistance gene PSS1 confers immunity against an oomycete and a fungal pathogen but not a bacterial pathogen that cause diseases in soybean

ABSTRACT: BACKGROUND: Nonhost resistance (NHR) provides immunity to all members of a plant species against all isolates of a microorganism that is pathogenic to other plant species. Three Arabidopsis thaliana PEN (penetration deficient) genes, PEN1, 2 and 3 have been shown to provide NHR against the barley pathogen Blumeria graminis f. sp. hordei at the prehaustorial level. Arabidopsis pen1-1 mutant lacking the PEN1 gene is penetrated by the hemibiotrophic oomycete pathogen Phytophthora sojae, the causal organism of the root and stem rot disease in soybean. We investigated if there is any novel nonhost resistance mechanism in Arabidopsis against the soybean pathogen, P. sojae. RESULTS: The P. sojae susceptible (pss) 1 mutant was identified by screening a mutant population created in the Arabidopsis pen1-1 mutant that lacks penetration resistance against the non adapted barley biotrophic fungal pathogen, Blumeria graminis f. sp. hordei. Segregation data suggested that PEN1 is not epistatic to PSS1. Responses of pss1 and pen1-1 to P. sojae invasion were distinct and suggest that PSS1 may act at both pre- and post-haustorial levels, while PEN1 acts at the pre-haustorial level against this soybean pathogen. Therefore, PSS1 encodes a new form of nonhost resistance. The pss1 mutant is also infected by the necrotrophic fungal pathogen, Fusarium virguliforme, which causes sudden death syndrome in soybean. Thus, a common NHR mechanism is operative in Arabidopsis against both hemibiotrophic oomycetes and necrotrophic fungal pathogens that are pathogenic to soybean. However, PSS1 does not play any role in immunity against the bacterial pathogen, Pseudomonas syringae pv. glycinea, that causes bacterial blight in soybean. We mapped PSS1 to a region very close to the southern telomere of chromosome 3 that carries no known disease resistance genes. CONCLUSIONS: The study revealed that Arabidopsis PSS1 is a novel nonhost resistance gene that confers a new form of nonhost resistance against both a hemibiotrophic oomycete pathogen, P. sojae and a necrotrophic fungal pathogen, F. virguliforme that cause diseases in soybean. However, this gene does not play any role in the immunity of Arabidopsis to the bacterial pathogen, P. syringae pv. glycinea, which causes bacterial blight in soybean. Identification and further characterization of the PSS1 gene would provide further insights into a new form of nonhost resistance in Arabidopsis, which could be utilized in improving resistance of soybean to two serious pathogens.     

Hirsutella thompsoniU a fungal pathogen of mites. II. Host-pathogen interactions

The moniliaceous fungus, Hirsutella thompsoniU grown on potato-dextrose agar (PDA) or on sterile wheat bran, was highly pathogenic to the carmine spider mite, Tetranychus cinnabarinus, and, grown on PDA, to the oriental spider mite, Eutetranychus orientalis. The fungus penetrated the mites' integument mainly through the legs and formed hyphal bodies in chains in the haemolymph. Hyphae, on which the spores were produced, began to emerge through the genital and anal apertures and then all over the body. Various mycophagous mites, a predaceous species and the tick Argas persicus were immune. H. thompsonii grew well on cadavers of diverse insects and mites. The fungus killed most mites and quickest, usually by the 2nd day, at 25^o, 27^o and 30 ^oC; least at 13^o and 35 ^oC. It sporulated best on mites at 24^o, 27^o and 30 ^oC (full sporulation at 27 ^oC took place within 12 h after death); it was good, but slower, at 13^o and 35 ^oC. The fungus germinated, penetrated mites and sporulated there very poorly below 100% r.h. When the daily number of hours at 100% r.h. was reduced from 24 to 18 or 6 h, fungus-associated mortality dropped greatly. In the field, use of the fungus for mite control would be suitable particularly in tropical and humid subtropical areas, and its success in Israel would not be assured. In glasshouses, H. thompsonii could induce epizootics in Israel and elsewhere only if humidities were raised to saturation and the daytime temperature kept below 37 ^oC.     

Cryptococcus gattii: a resurgent fungal pathogen

Cryptococcus gattii and Cryptococcus neoformans are causal agents of cryptococcosis, which manifests as pneumonia and meningitis. C. gattii has recently received widespread attention owing to outbreaks in British Columbia, Canada and the US Pacific Northwest. The biology of this tree-dwelling yeast is relatively unexplored, and there are few clues about how it causes infections in humans and animals. In this review, we summarize recent discoveries about C. gattii genetics and its ecological niche and highlight areas ripe for future exploration. Increased focus on epidemiology, ecological modeling and host-pathogen interactions is expected to yield a better understanding of this enigmatic yeast, and ultimately lead to better measures for its control.     

No evidence for immune priming in ants exposed to a fungal pathogen

There is accumulating evidence that invertebrates can acquire long-term protection against pathogens through immune priming. However, the range of pathogens eliciting immune priming and the specificity of the response remain unclear. Here, we tested if the exposure to a natural fungal pathogen elicited immune priming in ants. We found no evidence for immune priming in Formica selysi workers exposed to Beauveria bassiana. The initial exposure of ants to the fungus did not alter their resistance in a subsequent challenge with the same fungus. There was no sign of priming when using homologous and heterologous combinations of fungal strains for exposure and subsequent challenges at two time intervals. Hence, within the range of conditions tested, the immune response of this social insect to the fungal pathogen appears to lack memory and strain-specificity. These results show that immune priming is not ubiquitous across pathogens, hosts and conditions, possibly because of immune evasion by the pathogen or efficient social defences by the host.     

Isolation of ethylene-insensitive soybean mutants that are altered in pathogen susceptibility and gene-for-gene disease resistance

Plants commonly respond to pathogen infection by increasing ethylene production, but it is not clear if this ethylene does more to promote disease susceptibility or disease resistance. Ethylene production and/or responsiveness can be altered by genetic manipulation. The present study used mutagenesis to identify soybean (Glycine max L. Merr.) lines with reduced sensitivity to ethylene. Two new genetic loci were identified, Etr1 and Etr2. Mutants were compared with isogenic wild-type parents for their response to different soybean pathogens. Plant lines with reduced ethylene sensitivity developed similar or less-severe disease symptoms in response to virulent Pseudomonas syringae pv glycinea and Phytophthora sojae, but some of the mutants developed similar or more-severe symptoms in response to Septoria glycines and Rhizoctonia solani. Gene-for-gene resistance against P. syringae expressing avrRpt2 remained effective, but Rps1-k-mediated resistance against P. sojae races 4 and 7 was disrupted in the strong ethylene-insensitive etr1-1 mutant. Rps1-k-mediated resistance against P. sojae race 1 remained effective, suggesting that the Rps1-k locus may encode more than one gene for disease resistance. Overall, our results suggest that reduced ethylene sensitivity can be beneficial against some pathogens but deleterious to resistance against other pathogens.     

Antibiotic growth promoter olaquindox increases pathogen susceptibility in fish by inducing gut microbiota dysbiosis

Low dose antibiotics have been used as growth promoters in livestock and fish. The use of antibiotics has been associated with reduced pathogen infections in livestock. In contrast, antibiotic growth promoter has been suspected of leading to disease outbreaks in aquaculture. However, this phenomenon is circumstantial and has not been confirmed in experimental conditions. In this study,we showed that antibiotic olaquindox increased the susceptibility of zebrafish to A. hydrophila infection. Olaquindox led to profound alterations in the intestinal microbiota of zebrafish, with a drastic bloom of Enterobacter and diminishing of Cetobacterium. Moreover, the innate immune responses of zebrafish were compromised by olaquindox(P0.05). Transfer of microbiota to GF zebrafish indicated that while the immuo-suppression effect of olaquindox is a combined effect mediated by both OLA-altered micro biota and direct action of the antibiotic(P0.05), the increased pathogen susceptibility was driven by the OLA-altered microbiota and was not dependent on direct antibiotic effect. Taken together, these data indicate that low level of OLA induced gut microbiota dysbiosis in zebrafish, which led to increased pathogen susceptibility.     

Effects of temperature during soybean seed development on defense-related gene expression and fungal pathogen accumulation

Soybean [Glycine max (L.) Merr] plants were exposed to three temperature regimens during seed development to investigate the effect of temperature on the expression of eight defense-related genes and the accumulation of two fungal pathogens in inoculated seeds. In seeds prior to inoculation, either a day/night warm (34/26°C) or a cool temperature (22/18°C) relative to normal (26/22°C) resulted in altered patterns of gene expression including substantially lower expression of PR1, PR3 and PR10. After seed inoculation with Cercospora kikuchii, pathogen accumulation was lowest in seeds produced at 22/18°C in which of all defense genes, MMP2 was uniquely most highly induced. For seeds inoculated with Diaporthe phaseolorum, pathogen accumulation was lowest in seeds produced at 34/26°C in which of all defense genes, PR10 was uniquely most highly induced. Our detached seed assays clearly demonstrated that the temperature regimens we applied during seed development produced significant changes in seed defense-related gene expression both pre- and post inoculation and our findings support the hypothesis that global climate change may alter plant–pathogen interactions and thereby potentially crop productivity.     

Wolbachia increases susceptibility to Plasmodium infection in a natural system

Current views about the impact of Wolbachia on Plasmodium infections are almost entirely based on data regarding artificially transfected mosquitoes. This work has shown that Wolbachia reduces the intensity of Plasmodium infections in mosquitoes, raising the exciting possibility of using Wolbachia to control or limit the spread of malaria. Whether natural Wolbachia infections have the same parasite-inhibiting properties is not yet clear. Wolbachia–mosquito combinations with a long evolutionary history are, however, key for understanding what may happen with Wolbachia-transfected mosquitoes after several generations of coevolution. We investigate this issue using an entirely natural mosquito–Wolbachia–Plasmodium combination. In contrast to most previous studies, which have been centred on the quantification of the midgut stages of Plasmodium, we obtain a measurement of parasitaemia that relates directly to transmission by following infections to the salivary gland stages. We show that Wolbachia increases the susceptibility of Culex pipiens mosquitoes to Plasmodium relictum, significantly increasing the prevalence of salivary gland stage infections. This effect is independent of the density of Wolbachia in the mosquito. These results suggest that naturally Wolbachia-infected mosquitoes may, in fact, be better vectors of malaria than Wolbachia-free ones.     

Growth of the fungal pathogen Candida in parotid saliva of patients with burning mouth syndrome

Subclinical Candida infection has been suggested as one of the aetiological factors in patients with burning mouth syndrome (BMS). In order to investigate the possible factors which contribute to the relatively high isolation rate of Candida in BMS, parotid saliva samples (20 in toto) from patients with this condition were collected and the growth of Candida in each sample dynamically observed using a computerized turbidometric assay system. A total of thirteen parotid saliva samples obtained from healthy individuals served as normal controls. The results showed no significant growth differential within the test and control saliva samples, when a single isolate each of Candida albicans and Candida tropicalis were cultured for 24 h, at 37 degrees C. A single isolate of Candida glabrata tended to grow better in the saliva from BMS patients than the controls. These results indicate that the composition of saliva may be a contributory factor for the high isolation rate of Candida in saliva of BMS patients.     

Host inbreeding increases susceptibility to ectoparasitism

Inbreeding, which increases homozygosity throughout the genome by increasing the proportion of alleles that are identical by descent, is expected to compromise resistance against parasitism. Here, we demonstrate that host inbreeding increases susceptibility to ectoparasitism in a natural fruit fly (Drosophila nigrospiracula) - mite (Macrocheles subbadius) association, and that this effect depends on host genetic background. Moreover, flies generated from reciprocal crosses between susceptible inbred lines exhibited elevated levels of resistance similar to that in the mass-bred base population, confirming in reverse direction the causative link between expected heterozygosity and resistance. We also show that inbreeding reduces the host's ability to sustain energetically expensive behaviours, and that host exhaustion dramatically increases susceptibility. These findings suggest that inbreeding depression for resistance results from an inability to sustain defensive behaviours because of compromised physiological competence.     

Peroxidase activity and inducibility in the sea fan coral exposed to a fungal pathogen

The enzymatic defense mechanisms of Gorgonia ventalina to the fungal pathogen Aspergillus sydowii may play important roles in colony resistance to infection. In this study, we examined the role of the superfamily of peroxidase enzymes in the coral response to a naturally occurring pathogen. We examined the inducibility of peroxidases by experimentally exposing corals to A. sydowii and found that peroxidase activity was induced after an 8 day incubation period. In contrast, naturally infected corals collected from the reef had lower peroxidase activity when compared to healthy corals. Infected sea fans from the field also had less measurable protein in their tissues and increased purple sclerites near infection sites and it is likely that these infections are months old. Using native-PAGE activity gels, we detected 5 peroxidase isozymes in healthy corals, indicating that multiple isoforms of peroxidase with a plurality of possible functions are present in this coral. The role of the peroxidase enzymes in disease resistance was examined by testing anti-fungal activity of commercially available and partially purified sea fan peroxidases. In both cases there was significant, dose-dependent anti-fungal activity. While peroxidases are ubiquitous enzymes involved in many cellular pathways, we also hypothesize that G. ventalina utilizes these enzymes as an integral component in disease resistance pathways. As such, they may also contribute to the initiation of physiochemical defenses such as melanization and lipid soluble anti-fungal metabolites.     

Fine-root system development and susceptibility to pathogen colonization

Root development may exert control on plant–pathogen interactions with soil-borne pathogens by shaping the spatial and temporal availability of susceptible tissues and in turn the impact of pathogen colonization on root function. To evaluate the relationship between root development and resistance to apple replant disease (ARD) pathogens, pathogen abundance was compared across root branching orders in a bioassay with two rootstock genotypes, M.26 (highly susceptible) and CG.210 (less susceptible). Root growth, anatomical development and secondary metabolite production were evaluated as tissue resistance mechanisms. ARD pathogens primarily colonized first and second order roots, which corresponded with cortical tissue senescence and loss in second and third order roots. Defense compounds were differentially allocated across root branching orders, while defense induction or stress response was only detected in first order and pioneer roots. Our results suggest disease development is based largely on fine-root tip attrition. In accordance, the less susceptible rootstock supported lower ARD pathogen abundance and altered defense compound production in first order and pioneer roots and maintained higher rates of root growth in both the ARD soil and pasteurized control compared to the more susceptible. Thus, this rootstock’s ability to maintain shoot growth in replant soil may be attributable to relative replant pathogen resistance in distal root branches as well as tolerance of infection based on rates of root growth.     

Characterization of a proteinase inhibitor isolated from the fungal pathogen Coccidioides immitis.

1. Human erythrocytes were incubated in autologous plasma containing [32P]Pi, and sampled by a method which avoids washing the cells. 2. In experiments of up to 3 h duration, the specific radioactivity of cellular Pi stabilized at a value below that of extracellular Pi. This can be explained on the basis of a single cellular Pi pool exchanging with a large unlabelled pool of cellular organic phosphates. 3. However, a rapid initial phase of labelling, occurring within 30 s, was inconsistent with the situation described in point 2. A possible explanation is that about 1/4 of cellular Pi occurs in a separate, fast-labelling pool. 4. When the extracellular Pi concentration was doubled, most of the corresponding increase in the steady-state cellular Pi concentration was accounted for by the apparent fast-labelling Pi pool, which also doubled. 5. The observed initial rate of labelling of cellular organic phosphates [which probably occurs through the reaction catalysed by glyceraldehyde-3-phosphate dehydrogenase (E.C. 1.2.1.12)] was considerably lower than that predicted from the flux through the Embden-Meyerhof pathway. This implies that the enzyme is exposed to Pi whose specific radioactivity is lower than the mean specific radioactivity of cellular Pi, and fails to support earlier suggestions that this enzyme uses extracellular Pi. 6. In 3 h incubations, the rate of organic phosphate labelling was roughly constant throughout, even though the specific radioactivity of cellular Pi had risen slowly to a plateau. Viewed in conjunction with point 5, this again suggests some inhomogeneity in cellular Pi. 7. Cellular Pi and extracellular Pi only reached isotopic steady state after 2 days. At this stage some organic phosphates were probably still incompletely labelled. 8. We conclude that, whatever their physical or technical reasons, such labelling inhomogeneities and slow attainment of isotopic steady state may cause serious misinterpretation of results if ignored during 32P-labelling of intact cells.     

Viruses accumulate in aging infection centers of a fungal forest pathogen

Fungal viruses (mycoviruses) with RNA genomes are believed to lack extracellular infective particles. These viruses are transmitted laterally among fungal strains through mycelial anastomoses or vertically via their infected spores, but little is known regarding their prevalence and patterns of dispersal under natural conditions. Here, we examined, in detail, the spatial and temporal changes in a mycovirus community and its host fungus Heterobasidion parviporum, the most devastating fungal pathogen of conifers in the Boreal forest region. During the 7-year sampling period, viruses accumulated in clonal host individuals as a result of indigenous viruses spreading within and between clones as well as novel strains arriving via airborne spores. Viral community changes produced pockets of heterogeneity within large H. parviporum clones. The appearance of novel viral infections in aging clones indicated that transient cell-to-cell contacts between Heterobasidion strains are likely to occur more frequently than what was inferred from genotypic analyses. Intraspecific variation was low among the three partitivirus species at the study site, whereas the unassigned viral species HetRV6 was highly polymorphic. The accumulation of point mutations during persistent infections resulted in viral diversification, that is, the presence of nearly identical viral sequence variants within single clones. Our results also suggest that co-infections by distantly related viral species are more stable than those between conspecific strains, and mutual exclusion may play a role in determining mycoviral communities.