Cryphonectria hypovirus 1-Induced Changes of Stress Enzyme Activity in Transfected Phytopathogenic Fungus Cryphonectria parasitica

Molecular genetic variability of Cryphonectria hypovirus 1 associated with Cryphonectria parasitica in South Tyrol (northern Italy)

Cryphonectria hypovirus 1 (CHV-1) has been widely studied and used as a biocontrol agent because of its ability to infect the chestnut blight fungus, Cryphonectria parasitica, and to reduce its virulence. Knowledge about the hypovirus, its presence, and diversity is completely lacking in South Tyrol (northern Italy), which may obstruct biocontrol measures for chestnut blight based on CHV-1. This work aimed to study the occurrence of CHV-1 infecting C. parasitica in South Tyrol and to perform a genetic characterization of the hypovirus. In South Tyrol, CHV-1 was found to occur in 29.2% of the fungal isolates investigated, varying in frequency between different regions and chestnut stands. Twenty-three haplotypes based on partial cDNA (complementary DNA) sequences of open reading frame (ORF)-A and 30 haplotypes based on partial cDNA sequences of ORF-B were identified among 47 and 56 hypovirulent fungal isolates, respectively. Phylogenetic analysis showed that all the haplotypes belonged to the Italian subtype of CHV-1 and that they were closely related to the populations of Italy, Switzerland, Croatia and Slovenia. Evidence of recombination was not found in the sequences and point mutations were the main source of diversity. Overall, this study indicated that the prevalence of CHV-1 in South Tyrol is low compared to many other central and western European populations and determined a need to actively impose biocontrol measures. Using sequence analysis, we identified some variants of interest of CHV-1 that should be studied in detail for their potential use in biocontrol.

AfuPmV-1-Infected Aspergillus fumigatus Is More Susceptible to Stress Than Virus-Free Fungus

Infection with Aspergillus fumigatus polymycovirus 1 (AfuPmV-1) affects Aspergillus fumigatus Af293's growth in vitro, iron metabolism, resistance in intermicrobial competition with Pseudomonas aeruginosa, resistance to osmotic stress, and resistance to the chitin synthase inhibitor nikkomycin Z. Here, we show that response to high temperature, Congo Red-induced stress, and hydrogen peroxide are also dependent on the viral infection status of A. fumigatus. AfuPmV-1- infected Af293 was more susceptible than virus-free Af293 to growth inhibition by high temperature, hydrogen peroxide, Congo Red exposure, and nutrient restriction. Increased resistance of virus-free fungus was observed when cultures were started from conidia but, in the case of high temperature and hydrogen peroxide, not when cultures were started from hyphae. This indicates that the virus impairs the stress response during the growth phase of germination of conidia and development into hyphae. In conclusion, our work indicates that AfuPmV-1 infection in A. fumigatus impairs host responses to stress, as shown by exposure to high temperature, oxidative stress such as hydrogen peroxide, and some cell wall stresses, as shown by exposure to Congo Red (in agreement with our previous observations using nikkomycin Z) and nutrient restriction.

Physiological variations in hypovirus-infected wild and model long-term laboratory strains of Cryphonectria parasitica

Introduction

Forest ecosystems are highly threatened by the simultaneous effects of climate change and invasive pathogens. Chestnut blight, caused by the invasive phytopathogenic fungus Cryphonectria parasitica, has caused severe damage to European chestnut groves and catastrophic dieback of American chestnut in North America. Within Europe, the impacts of the fungus are widely mitigated through biological control that utilizes the RNA mycovirus: Cryphonectria hypovirus 1 (CHV1). Viral infections, similarly to abiotic factors, can cause oxidative stress in their hosts leading to physiological attrition through stimulating ROS (reactive oxygen species) and NOx production.

Methods

To fully understand the interactions leading to the biocontrol of chestnut blight, it is vital to determine oxidative stress damage arising during CHV1 infection, especially considering that other abiotic factors, like long-term cultivation of model fungal strains, can also impact oxidative stress. Our study compared CHV1-infected C. parasitica isolates from two Croatian wild populations with CHV1-infected model strains (EP713, Euro7 and CR23) that have experienced long-term laboratory cultivation.

Results and Discussion

We determined the level of oxidative stress in the samples by measuring stress enzymes' activity and oxidative stress biomarkers. Furthermore, for the wild populations, we studied the activity of fungal laccases, expression of the laccase gene lac1, and a possible effect of CHV1 intra-host diversity on the observed biochemical responses. Relative to the wild isolates, the long-term model strains had lower enzymatic activities of superoxide dismutase (SOD) and glutathione S-transferase (GST), and higher content of malondialdehyde (MDA) and total non-protein thiols. This indicated generally higher oxidative stress, likely arising from their decades-long history of subculturing and freeze-thaw cycles. When comparing the two wild populations, differences between them in stress resilience and levels of oxidative stress were also observed, as evident from the different MDA content. The intra-host genetic diversity of the CHV1 had no discernible effect on the stress levels of the virus-infected fungal cultures. Our research indicated that an important determinant modulating both lac1 expression and laccase enzyme activity is intrinsic to the fungus itself, possibly related to the vc type of the fungus, i.e., vegetative incompatibility genotype.

Legacy of Plant Virology in Croatia-From Virus Identification to Molecular Epidemiology, Evolution, Genomics and Beyond

This paper showcases the development of plant virology in Croatia at the University of Zagreb, Faculty of Science, from its beginning in the 1950s until today, more than 70 years later. The main achievements of the previous and current group members are highlighted according to various research topics and fields. Expectedly, some of those accomplishments remained within the field of plant virology, but others make part of a much-extended research spectrum exploring subviral pathogens, prokaryotic plant pathogens, fungi and their viruses, as well as their interactions within ecosystems. Thus, the legacy of plant virology in Croatia continues to contribute to the state of the art of microbiology far beyond virology. Research problems pertinent for directing the future research endeavors are also proposed in this review.

Laccase Activity in Fungus Cryphonectria parasitica Is Affected by Growth Conditions and Fungal-Viral Genotypic Interactions

Laccase activity reduction in the chestnut blight fungus Cryphonectria parasitica usually accompanies the hypovirulence caused by the infection of fungus with Cryphonectria hypovirus 1 (CHV1). However, the different methods utilized for assessing this phenomenon has produced varied and often conflicting results. Furthermore, the majority of experimental setups included only one prototypic system, further confounding the results. Considering the diversity of fungal isolates, viral strains, and variability of their effects on the phytopathogenic process observed in nature, our goal was to ascertain if laccase activity variability is affected by (1) different C. parasitica isolates infected with several CHV1 strains, and (2) growth conditions. We have demonstrated that some CHV1 strains, contrary to previous assumptions, increase the activity of C. parasitica laccases. The specific fungal isolates used in the experiments and culture conditions also affected the results. Furthermore, we showed that two commonly used laccase substrates, 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) and 2,4-dimethoxyphenol, cannot be used interchangeably in C. parasitica laccase activity measurements. Our results illustrate the importance of conducting this type of study in experimental systems and culture conditions that resemble natural conditions as much as possible to be able to infer the most relevant conclusions applicable to natural populations.

The Physiological and Biochemical Responses of European Chestnut (Castanea sativa L.) to Blight Fungus (Cryphonectria parasitica (Murill) Barr)

The most important disease of European chestnut (Castanea sativa Mill.) is chestnut blight caused by the fungus Cryphonectria parasitica (Murrill) Barr which induces yield reduction in Europe and North America. This study aimed to investigate the impacts of C. parasitica infection on the physiological and biochemical characteristics of European chestnut at two different growth stages, 3 and 6 weeks after the infection. The amount of photosynthetic pigments (chlorophyll-a, chlorophyll-b, and carotenoids), the relative chlorophyll content, and the photochemical efficiency of the photosystem II (PSII) were measured in the leaves above and below the virulent and hypovirulent C. parasitica infections. The highest values were measured in the control leaves, the lowest values were in the leaves of the upper part of virulent necrosis. Antioxidant enzyme activities such as ascorbate peroxidase (APX), guaiacol peroxidase (POD), and superoxide dismutase (SOD), proline, and malondialdehyde concentrations were also investigated. In each of these measured values, the lowest level was measured in the control leaves, while the highest was in leaves infected with the virulent fungal strain. By measuring all of these stress indicator parameters the responses of chestnut to C. parasitica infection can be monitored and determined. The results of this study showed that the virulent strain caused more pronounced defense responses of chestnut’s defense system. The measured parameter above the infection was more exposed to the blight fungus disease relative to the leaves below the infection.

Genetic Diversity of Cryphonectria hypovirus 1, a Biocontrol Agent of Chestnut Blight, in Croatia and Slovenia

Transmissible hypovirulence associated with Cryphonectria hypovirus 1 (CHV1) has been used for biological control of chestnut blight, devastating disease of chestnut caused by the fungus Cryphonectria parasitica. The main aims of this study were to provide molecular characterization of CHV1 from Croatia and Slovenia and to reveal its genetic variability, phylogeny, and diversification of populations. Fifty-one CHV1 haplotypes were detected among 54 partially sequenced CHV1 isolates, all belonging to Italian subtype (I). Diversity was mainly generated by point mutations while evidence of recombination was not found. The level of conservation over analyzed parts of ORF-A proteins p29 and p40 varied, but functional sites were highly conserved. Phylogenetic analysis revealed close relatedness and intermixing of Croatian and Slovenian CHV1 populations. Our CHV1 isolates were also related to Swiss and Bosnian hypoviruses supporting previously suggested course of CHV1 invasion in Europe. Overall, this study indicates that phylogeny of CHV1 subtype I in Europe is complex and characterized with frequent point mutations resulting in many closely related variants of the virus. Possible association between variations within CHV1 ORF-A and growth of the hypovirulent fungal isolates is tested and presented.

Link between epigenetic diversity and invasive status of south-eastern European populations of phytopathogenic fungus Cryphonectria parasitica

Epigenetic modifications may play an important role in invasion and adaptation of clonal and invasive populations to different environments. The aim of this study was to analyse epigenetic diversity and structure within and among populations of invasive pathogenic fungus Cryphonectria parasitica from south-eastern Europe, where one haplotype S12 dominates. The highest level of epigenetic diversity was found in haplotype S1, followed by S2, while the lowest level of epigenetic diversity was found in haplotype S12. Similar pattern of epigenetic diversity was detected in the control, genetically diverse Croatian population where S1 haplotype dominates. In four south-eastern European populations, the highest level of epigenetic diversity was observed in the Italian population, the oldest population in the studied area, while the lowest diversity was found in most recently established Bulgarian population. This relationship between epigenetic diversity and population age implies the important role of epigenetic modifications on the process of invasion. Our data suggest that epigenetic differences might affect the success of expansion of certain haplotype into new regions. Understanding the role of epigenetic processes in expansion and (pre)adaptation of fungal plant pathogens, besides fundamental knowledge, can contribute to development of strategies for control of fungal spread and pathogenesis.

Cryphonectria hypovirus 1-Induced Epigenetic Changes in Infected Phytopathogenic Fungus Cryphonectria parasitica

Biotic stress caused by virus infections induces epigenetic changes in infected plants and animals, but this is the first report on methylation pattern changes in a fungus after mycovirus infection. As a model pathosystem for mycovirus-host interactions, we used Cryphonectria hypovirus 1 (CHV1) and its host fungus Cryphonectria parasitica, in which deregulation of methylation cycle enzymes upon virus infection was observed previously. Six CHV1 strains of different subtypes were transferred into three different C. parasitica isolates in order to assess the effect of different CHV1 strains and/or subtypes on global cytosine methylation level in infected fungus, using methylation-sensitive amplification polymorphism (MSAP). Infection with CHV1 affected the methylation pattern of the C. parasitica genome; it increased the number and diversity of methylated, hemi-methylated, and total MSAP markers found in infected fungal isolates compared to virus-free controls. The increase in methylation levels correlated well with the CHV1-induced reduction of fungal growth in vitro, indicating that C. parasitica genome methylation upon CHV1 infection, rather than being the defensive mechanism of the fungus, is more likely to be the virulence determinant of the virus. Furthermore, the severity of CHV1 effect on methylation levels of infected C. parasitica isolates depended mostly on individual CHV1 strains and on the combination of host and virus genomes, rather than on the virus subtype. These novel findings broaden our knowledge about CHV1 strains which could potentially be used in human-aided biocontrol of chestnut blight, a disease caused by C. parasitica in chestnut forest ecosystems and orchards.

Molecular evolution and invasion pattern of Cryphonectria hypovirus 1 in Europe: Mutation rate, and selection pressure differ between genome domains

Understanding virus evolution is a fundamental goal of virology, evolutionary biology, and disease epidemiology. We provide a detailed analysis of evolution and origin of Cryphonectria hypovirus 1 (CHV1) populations in Europe, based on the complete genome sequence of all European subtypes. Phylogenetic analyses divided European strains into two closely related clades. Strains of the subtype I belong to the first, while strains of the subtypes F1, D and E belong to the second clade suggesting that the subtypes F1, D and E are more closely related than previously thought. Strains of the subtype F2 appeared to be recombinant; subtypes F1/D/E contributed a larger fraction of sequence while subtype I contributed a smaller fraction. The p29 was the most variable domain, while the replication-associated large ORF B protein was the most conserved domain within the CHV1. Low sequence similarity, predominant negative selection and frequent recombination characterise the evolution of CHV1.