Structure and growth of infection threads in the legume symbiosis with Rhizobium leguminosarum

Specific antibodies and enzyme–gold probes were used to study the structure and development of infection threads in nodules induced by Rhizobium leguminosarum on the roots of Vicia, Pisum and Phaseolus. In Pisum nodules, the tubular infection thread wall contains polysaccharides antigenically similar to those of the cell wall, including cellulose, xyloglucan, methyl-esterified pectin and non-esterified pectin, but none of these wall components is present around the infection droplet structures from which bacteria are internalized by plant plasma membrane. As reported previously for pea nodules, the luminal matrix of infection threads and infection droplets contains a plant glycoprotein; this glycoprotein is also secreted by infected and uninfected cortical cells of a Vicia root at the earliest stages of nodule initiation. Synthesis of a transcellular infection thread apparently involves reorganized deposition of components normally targeted to the cell wall, and infection thread growth is orientated anticlinally through the outer cortex in the same plane observed for the deposition of new cell walls following mitosis. Both the development of infection threads in the outer cortex and the initiation of cell division in the inner cortex are preceded by a similar process of cell reactivation involving centralization of nuclei and the development of anticlinal transvacuolar strands. It is therefore suggested that the two Rhizobium-induced processes of infection thread growth and cortical cell division may both be consequences of a similar plant cell response in the inner and outer root cortex, respectively. Phaseolus nodules contained only short intracellular infection structures which terminated within individual cells and contained no luminal matrix material. The differences in infection thread structure between Pisum and Phaseolus nodules may reflect differences in ontogeny between “indeterminate” and “determinate” nodule meristems.     

The role of reactive oxygen species in the virulence of wheat leaf rust fungus Puccinia triticina

Reactive oxygen species (ROS) play an important role during host–pathogen interactions and are often an indication of induced host defence responses. In this study, we demonstrate for the first time that Puccinia triticina (Pt) generates ROS, including superoxide, H₂O₂ and hydroxyl radicals, during wheat infection. Through pharmacological inhibition, we found that ROS are critical for both Pt urediniospore germination and pathogenic development on wheat. A comparative RNA-Seq analysis of different stages of Pt infection process revealed 291 putative Pt genes associated with the oxidation–reduction process. Thirty-seven of these genes encode known proteins. The expressions of five Pt genes, including PtNoxA, PtNoxB, PtNoxR, PtCat and PtSod, were subsequently verified using RT-qPCR analysis. The results show that the expressions of PtNoxA, PtNoxB, PtNoxR, PtCat and PtSod are up-regulated during urediniospore germination. In comparison, the expressions of PtNoxA, PtNoxB, PtNoxR and PtCat are down-regulated during wheat infection from 12 to 120 h after inoculation (HAI), whereas the expression of PtSod is up-regulated with a peak of expression at 120 HAI. We conclude that ROS are critical for the full virulence of Pt and a coordinate down-regulation of PtNox genes may be important for successful infection in wheat.     

MHC‐I provides both quantitative resistance and susceptibility to blood parasites in blue tits in the wild

Major histocompatibility complex (MHC) genes are central for the adaptive immune response against parasites. Here, we investigated potential associations among MHC‐I alleles and blood parasite infections in a natural breeding population of a passerine bird, the blue tit Cyanistes caeruleus, in central Spain. We screened both infection status (presence/absence of infection) and infection intensity to the pathogenic blood parasites Haemoproteus and Leucocytozoon. Three MHC‐I alleles (UA104, UA108 and UA117) were associated with higher or lower infection intensities by Leucocytozoon. Interestingly, these associations were dependent on age and were found both among young and adult birds. No MHC alleles were associated with infection intensity by Haemoproteus parasites. In addition, no significant relationships were detected between infection status by Haemoproteus and Leucocytozoon infections and MHC alleles. The very high prevalence of these two parasites in our study population (79–100%) poses challenges to identify associations with infection status and also suggests that clearance of infections may be rare. In conclusion, associations between specific MHC‐I alleles and Leucocytozoon parasites were related to either high or low infection intensities, and hence increased susceptibility or resistance to infection.     

The role of Rhizobium conserved and host specific nodulation genes in the infection of the non-legume Parasponia andersonii

Summary Rhizobium and Bradyrhizobium bacteria gain intercellular entry into roots of the non-legume Parasponia andersonii by stimulating localized sites of cell division which disrupt the epidermis. Infection threads are then initiated from intercellular colonies within the cortex. Infection via the information of infection threads within curled root hairs, which commonly occurs in legumes, was not observed in Parasponia. The conserved nodulation genes nodABC, necded for the curling of legume root hairs, were not essential for the initiation of infection, however, these genes were required for Parasponia prenodule development. In contrast, the nodD gene of Rhizobium strain NGR234 was essential for the initiation of infection. In addition, successful infection required not only nodD but a region of the NGR234 symbiotic plasmid which is not needed for the nodulation of legumes. Agrobacterium tumefaciens carrying this Parasponia specific region, as well as legume nod genes, was able to form nodules on Parasponia which reached an advanced stage of development.     

Dissemination of Chlamydia pneumoniae to the vessel wall in atherosclerosis

Heart disease and stroke are the result of atherosclerotic vascular lesions. It is becoming increasingly clear that an infection may be an important initiating component within the atherogenic process. However, in order for the infection to contribute to atherosclerosis, it must first be capable of disseminating to the vessel wall. Chlamydia pneumoniae is an example of an infectious atherogenic stimulus. The present treatise reviews our knowledge concerning dissemination of infectious agents like C. pneumoniae. Three factors can be identified that modulate the severity of the infection in the vascular wall. First, although all vascular cell types appear to be infected with agents like C. pneumoniae, there are differences in the sensitivity to infection amongst these cell types. Second, the lipid environment is important in defining the effects of C. pneumoniae on atherosclerotic disease. Third, the inflammatory/atherosclerotic interaction is influenced by the specific infectious stimuli employed. The in situ atherogenic effects of C. pneumoniae may be specific to this organism and may not occur with related infectious agents like C. trachomatis. Despite the identification of these three factors, controversy exists surrounding specific characteristics of these effects. This may be the result of a plethora of differing experimental conditions (different labs, different lipids, different cell types or lines, and different C. pneumoniae characteristics (infection, dosage, duration, etc.)). Further study of these important phenomena is clearly warranted in view of the potential importance of infection to the atherosclerotic disease.     

Rhizobial infection does not require cortical expression of upstream common symbiosis genes responsible for the induction of Ca2+ spiking

For the establishment of an effective root nodule symbiosis, a coordinated regulation of the infection processes between the epidermis and cortex is required. However, it remains unclear whether the symbiotic genes identified so far are involved in epidermal and/or cortical infection, e.g. epidermal and cortical infection thread formation or cortical cell division. To analyze the symbiotic gene requirements of the infection process, we have developed an epidermis‐specific expression system (pEpi expression system) and examined the symbiotic genes NFR1, NFR5, NUP85, NUP133, CASTOR, POLLUX, CCaMK, CYCLOPS, NSP1 and NSP2 for involvement in the infection process in the epidermis and cortex. Our study shows that expression of the upstream common symbiosis genes CASTOR, POLLUX, NUP85 and NUP133 in the epidermis is sufficient to induce formation of infection threads and cortical cell division, leading to the development of fully effective nodules. Our system also shows a requirement of CCaMK, CYCLOPS, NSP1 and NSP2 for the entire nodulation process, and the different contributions of NFR1 and NFR5 to cortical infection thread formation. Based on these analyses using the pEpi expression system, we propose a functional model of symbiotic genes for epidermal and cortical infection.     

Allee Effect in the Infection Dynamics of the Entomopathogenic Fungus Beauveria bassiana (Bals) Vuill. on the Beetle, Mylabris pustulata

Successful infection by Beauveria bassiana as with all other entomopathogenic fungi, is accomplished only at a high conidial dose while, theoretically, a single conidium should be sufficient. Indeed, this is a major deterrent in its use as a biocontrol agent. High pathogen load for infection is required by organisms which display ‘Allee’ effect. In such organisms, a threshold exists for pathogen dose, below which no infection can be caused. B. bassiana has a semelparous life cycle and, therefore, its infection dynamics are expected to conform to the mass action principle with a linear relationship between dose and successful infection observable as mortality of the insect. Whether the need for a high conidial dose to induce insect mortality by B. bassiana is due to the operation of Allee effect was examined. A sample of 34 isolates was bioassayed on Mylabris pustulata (Coleoptera: Meloidae) at four conidial concentrations. With more than half of the isolates in the sample, the lowest dose tested (10⁴ conidia/insect) did not cause insect mortality. Thus, a threshold pathogen load is required to cause successful infection. In these isolates, the dose–mortality relationship was sigmoid. Allee effect is thus identified in the infection dynamics of B. bassiana–M. pustulata system. The isolates that induced mortality at the lowest dose tested are concluded to be highly virulent with a lower threshold dose required for successful infection. With some isolates, at high conidial dose, the infection rate decreased either due to a decrease in the proportion of insects showing mycosis, to the speed of death, or both. Such a response could result from intra scramble competition arising from overload of pathogen at very high dose.     

Effect of the air pollution component ammonium sulphate on the VAM infection rate of three heathland species

Three heathand species, Antennaria dioica, Arnica montana and Hieracium pilosella, were artificially rained with ammonium sulphate solutions at increasing concentrations in a greenhouse experiment. The same species were also artificially rained with increasing ammonium sulphate solutions under field conditions. Dry weights of the plants in the field experiments did not change with increasing ammonium sulphate applications. Nor did the dry weights of plants in the greenhouse experiments change with increasing ammonium sulphate concentrations, except for Arnica montana, which showed an increase in dry weight. VAM infection percentage of Antennaria dioica increased in both the greenhouse and the field experiment. The results of the field experiment show that VAM infection rates are reduced after two years of artificial rain in the plant species Arnica montana, which grows naturally under nutrient poor conditions and is presently declining in its natural habitat in the Netherlands. In the greenhouse experiment, VAM infection of Arnica montana did not change with increasing ammonium sulphate concentrations. VAM infection rates of Hieracium pilosella, which presently is not declining, did not change with increasing ammonium sulphate concentrations.     

Wolbachia affects survival to different oxidative stressors dependent upon the genetic background in Drosophila melanogaster

It is well known that the rate of ageing varies among individuals dependent on the genetic background. In the present study, we explore how Wolbachia infection (a common insect endosymbiont bacterium) and oxidative stress interact in ageing with respect to two different genetic backgrounds of Drosophila melanogaster. Naturally infected and cured lines of Drosophila are challenged with either paraquat or l ‐arginine to generate two different types of oxidative stress. We first observe that removing Wolbachia infection shortens the lifespan in one genetic background but not in the other. Wolbachia infection only makes one of the genetic lines more sensitive to paraquat. However, only the line unaffected by Wolbachia in the paraquat treatment is protected by Wolbachia from l ‐arginine induced stress. Hence, Wolbachia is modifying free radical defence via two different mechanisms dependent on the genetic background. This supports the idea that factors that can govern ageing (infection and oxidative stress) are not universal, but are specific to the genetic make‐up of an individual.     

Effect of <Emphasis Type="Italic">Tetrahymena</Emphasis> on the occurrence of achlyosis in the guppy

Tetrahymena infection has become the most problematic parasitic disease of the guppy Poecilia reticulata in Southeast Asia. Tetrahymena corlissi was isolated from guppies with a fungal infection in Thailand, and the fungus was identified as Achlya bisexualis. Male and female guppies were artificially infected with both organisms. The results showed that guppies could easily be artificially infected with a culture of Tetrahymena corlissi and that female guppies were more sensitive than male guppies. Achlya bisexualis infection was shown to be a secondary infection after the Tetrahymena infection. Received: April 29, 2001 / Accepted: September 6, 2001     

An in vitro method for the analysis of infection‐related morphogenesis in Fusarium graminearum

Fusarium graminearum is a significant pathogen of many cereal crops. With its genetic tractability, ease of culture, genome sequence availability and economic significance, F. graminearum has become the subject of intensive molecular research. Although molecular tools have been developed to enhance research into virulence determinants of F. graminearum, simple assays for infection‐related development are lacking. As such, the objective of this study was to develop an in vitro protocol for the analysis of infection‐related morphogenesis in F. graminearum. We demonstrate that two morphologically distinct hyphal structures are produced by F. graminearum during the invasion of detached wheat glumes: subcuticular hyphae and bulbous infection hyphae. Specialized wheat epidermal cells (papillae) appear to act as sites of invasion by F. graminearum on the adaxial side of detached wheat glumes. In addition, the development of bulbous infection hyphae is dependent on the pathogenicity mitogen‐activated protein kinase Gpmk1, further supporting the infection‐related nature of these structures. This relatively simple assay will contribute to the tractability of the F. graminearum system and help to uncover molecular requirements for infection‐related development.     

Genetic variation of Batrachochytrium dendrobatidis is linked to skin bacterial diversity in the Pacific treefrog Hyliola regilla (hypochondriaca)

Symbiotic bacterial communities are crucial to combating infections and contribute to host health. The amphibian skin microbiome plays an important role in protecting their hosts against pathogens such as Batrachochytrium dendrobatidis (Bd), one of the causative agents of chytridiomycosis, which is responsible for dramatic amphibian population declines worldwide. Although symbiotic skin bacteria are known to inhibit Bd growth, an understanding of the relationship between Bd genetic variability, environmental conditions, and skin bacterial communities is limited. Therefore, we examined the associations between Bd infection load, Bd genetic diversity and skin bacterial communities in five populations of Hyliola regilla (hypochondriaca) from environmentally contrasting sites in Baja California, Mexico. We observed differences in Bd genetics and infection load among sites and environments. Genetic analysis of Bd isolates revealed patterns of spatial structure corresponding to the five sites sampled. Amphibian skin bacterial diversity and community structure differed among environments and sites. Bacterial community composition was correlated with Bd genetic differences and infection load, with specific bacterial taxa enriched on infected and un-infected frogs. Our results indicate that skin-associated bacteria and Bd strains likely interact on the host skin, with consequences for microbial community structure and Bd infection intensity.     

Exploring the effect of the Cardinium endosymbiont on spiders

Spiders have recently emerged as important diversity hot spots for endosymbiotic bacteria, but the consequences of these symbiotic interactions are largely unknown. In this article, we examined the evolutionary history and effect of the intracellular bacterium Cardinium hertigii in the marbled cellar spider Holocnemus pluchei. We showed that Cardinium infection is primarily transmitted in spider populations maternally via egg cytoplasm, with 100% of the progeny from infected mothers being also infected. Examination of a co‐inherited marker, mitochondrial DNA (mtDNA), revealed that Cardinium infection is associated with a wide diversity of mtDNA haplotypes, showing that the interaction between Cardinium and H. pluchei has a long‐term evolutionary dimension and that horizontal transmission among individuals could also occur. Although Cardinium is well known to exert sex ratio distortion or cytoplasmic incompatibility in various arthropod hosts, we show, however, that Cardinium does not interact with the reproductive biology of H. pluchei. A field survey shows a clear geographical structuring of Cardinium infection, with a marked gradual variation of infection frequencies from ca. 0.80 to 0. We discuss different mechanistic and evolutionary explanations for these results as well as their consequences for spider phenotypes. Notably, we suggest that Cardinium can either behave as a neutral cytoplasmic element within H. pluchei or exhibit a context‐dependent effect, depending on the environmental conditions.     

Antigenic Similarity Between the Coccidian Parasites Toxoplasma gondii and Hammondia hammondi1

The antigens that are present in the coccidian parasites Toxoplasma gondii and Hammondia hammondi were demonstrated and defined by using SDS-PAGE and immunoenzymatic techniques with ¹²⁵I-labeled and unlabeled antigens of T. gondii and sera of mice infected orally or intraperitoneally with H. hammondi. All cell surface antigens of T. gondii that were labeled with ¹²⁵I were recognized by antibodies in the sera of the mice infected with H. hammondi except the antigen of approximate molecular weight of 21.5 Kd. This suggests that this antigen is specific for T. gondii. Various antigens in the T. gondii-lysed antigen preparations were recognized by antibodies to H. hammondi. The number of recognized antigens increased as the infection of the mice with H. hammondi progressed. Oral infection with H. hammondi appeared to induce the formation of antibodies that recognized more T. gondii antigens than infection by intraperitoneal inoculation.     

Male rescue maintains low frequency parthenogenesis-inducing <Emphasis Type="Italic">Wolbachia</Emphasis> infection in <Emphasis Type="Italic">Trichogramma</Emphasis> populations

Parthenogenesis-inducing (PI) Wolbachia bacteria are reproductive parasites that cause infected (W ⁺) female haplodiploid parasitoids to produce daughters without fertilization by males. Theoretically, PI Wolbachia infection should spread to fixation within Trichogramma populations as males are no longer required to produce female offspring. Infections in some naturally occurring Trichogramma populations are, however, maintained at frequencies ranging from 4 to 26%. Here we describe discrete equation models to examine if the PI Wolbachia infection in Trichogramma populations can be maintained at relatively low frequencies by mating regularity. Model outcomes suggest the probability of W ⁺ females mating could stabilize Wolbachia infection frequency at low levels in Trichogramma populations. The primary mechanism maintaining low-level PI Wolbachia infection in Trichogramma populations is reducing the survivorship from egg to adult in infected relative to uninfected females. The model successfully demonstrates that the relatively low PI Wolbachia infection frequency in host populations can be maintained by fertilization, or male rescue, of infected eggs, which avoids potentially hazardous gamete duplication that occurs during Wolbachia-induced parthenogenesis.     

Influences of two coexisting endosymbionts,CI‐inducing Wolbachia and male‐killing Spiroplasma,on the performance of their host Laodelphax striatellus (Hemiptera: Delphacidae)

The small brown planthopper Laodelphax striatellus (Hemiptera: Delphacidae) is reported to have the endosymbiont Wolbachia, which shows a strong cytoplasmic incompatibility (CI) between infected males and uninfected females. In the 2000s, female‐biased L. striatellus populations were found in Taiwan, and this sex ratio distortion was the result of male‐killing induced by the infection of another endosymbiont, Spiroplasma. Spiroplasma infection is considered to negatively affect both L. striatellus and Wolbachia because the male‐killing halves the offspring of L. striatellus and hinders the spread of Wolbachia infection via CI. Spiroplasma could have traits that increase the fitness of infected L. striatellus and/or coexisting organisms because the coinfection rates of Wolbachia and Spiroplasma were rather high in some areas. In this study, we investigated the influences of the infection of these two endosymbionts on the development, reproduction, and insecticide resistance of L. striatellus in the laboratory. Our results show that the single‐infection state of Spiroplasma had a negative influence on the fertility of L. striatellus, while the double‐infection state had no significant influence. At late nymphal and adult stages, the abundance of Spiroplasma was lower in the double‐infection state than in the single‐infection state. In the double‐infection state, the reduction of Spiroplasma density may be caused by competition between the two endosymbionts, and the negative influence of Spiroplasma on the fertility of host may be relieved. The resistance of L. striatellus to four insecticides was compared among different infection states of endosymbionts, but Spiroplasma infection did not contribute to increase insecticide resistance. Because positive influences of Spiroplasma infection were not found in terms of the development, reproduction, and insecticide resistance of L. striatellus, other factors improving the fitness of Spiroplasma‐infected L. striatellus may be related to the high frequency of double infection in some L. striatellus populations.     

Mycorrhizal fungi and the nutrient ecology of three oldfield annual plant species

Summary Three oldfield annual species (Abutilon theophrasti Medic., Ambrosia artemisiifolia L. and Setaria lutescens (Weigel) Hubb.) were investigated. All three developed substantial mycorrhizal infections when inoculated with Glomus etunicatum Becker & Gerd. Mycorrhizal infection dramatically increased phosphorus content and dry weight of both Abutilon and Ambrosia, but did not significantly affect dry weight and only modestly increased phosphorus content of Setaria. These results were consistent with a lower level of infection and much greater root density in Setaria than in the other species. When Abutilon was grown in the presence of Setaria, mycorrhizal infection had no effect on Abutilon phosphorus content or dry weight. The depressive effect of Setaria on the response to inoculation in Abutilon was probably not caused by water soluble allelopathic chemicals from Setaria roots, but soil leachate from Abutilon plants did inhibit infection in other Abutilon plants. The data were consistent with the hypothesis that the very high root density and effective soil exploitation of Setaria reduced the benefit from mycorrhizal infection in Abutilon via phosphorus depletion in a large proportion of the available soil volume. Furthermore, even if mycorrhizal infection were capable of increasing phosphorus content of Abutilon in the presence of Setaria, the very high competitive ability of Setaria for nitrogen in the soil could have reduced the benefit of an enhanced phosphorus content. Carbon isotope ratios were reduced in Abutilon by mycorrhizal infection, indicating a possible reduction in water use efficiency.