Chestnut Blight: Biological Control by Transmissible Hypovirulence in Endothia parasitica

Randomly amplified polymorphic DNA fingerprinting identifies subgroups of Trichoderma viride and other Trichoderma sp. capable of chestnut blight biocontrol

Eleven strains of Trichoderma viride, 2 strains of the putative teleomorph Hypocrea rufa and 9 of several other Trichoderma sp. were characterized by random polymorphic DNA amplification (RAPD) fingerprinting and screened for their ability to antagonize growth of European strains of the chestnut blight causing fungus Cryphonectria parasitica, using a dual-culture assay. The best strains were found in the species T. harzianum, T. parceramosum, a distinguishable subgroup of T. viride and a not named Trichoderma sp. The successful application of these strains against chestnut blight in vivo is demonstrated.

Virus-mediated or transgenic suppression of a G-protein alpha subunit and attenuation of fungal virulence

Strains of the chestnut blight fungus Cryphonectria parasitica harboring RNA viruses of the genus Hypovirus exhibit significantly reduced levels of virulence (called hypovirulence). The accumulation of a heterotrimeric GTP-binding protein (G protein) alpha subunit of the Gi class was found to be reduced in hypovirus-containing C. parasitica strains. Transgenic cosuppression, a phenomenon frequently observed in transgenic plants, reduced the accumulation of this alpha subunit in virus-free fungal strains. Significantly, the resulting transgenic fungal strains were also hypovirulent. These results indicate a crucial role for G-protein-linked signal transduction in fungal pathogenesis and suggest a molecular basis for virus-mediated attenuation of fungal virulence.

Isolation and characterization of a virus-resistant mutant of Cryphonectria parasitica

Hypovirulent strain NB58 of Cryphonectria parasitica contains a dsRNA virus with a genome size of approximately 12.5 kb. Although NB58 is very stable in culture, a phenotypically-distinct sector arose which was found to be dsRNA-free. Attempts to infect the mutant strain, termed NB58F, by pairing with the parent strain (NB58) or other conversion-compatible, virus-containing strains have been unsuccessful. DNA fingerprint analysis showed that NB58, NB58F, and a representative dsRNA-free single-conidial isolate of NB58 termed NB58-19, were isogenic. The mutant culture was phenotypically stable, and all single-conidial progeny had the NB58F morphology. NB58F was intermediate between NB58 and NB58-19 in laccase production and virulence. Pigmentation and sporulation of NB58F, however, were reduced to near the level of NB58. In mating studies, NB58F functioned only as the male in sexual crosses. The mutant phenotype (F) predominated by a ratio of 5:2 among the ascospore progeny of F-type x wild-type crosses. These data suggest the lesion is nuclear and may be associated with a chromosomal abnormality. Attempts to infect the NB58F-type ascospore progeny failed, whereas the wild-type progeny were successfully infected with strains compatible with one or the other parent at a frequency of about 34%. Hyphal anastomosis and movement of cytoplasmic material occurred when NB58F was paired with a compatible strain, suggesting that the lesion is involved in viral maintenance as opposed to initial virus infection. NB58F represents the first virus-resistant isolate of C. parasitica to be described.

A small mitochondrial double-stranded (ds) RNA element associated with a hypovirulent strain of the chestnut blight fungus and ancestrally related to yeast cytoplasmic T and W dsRNAs

A small double-stranded (ds) RNA element was isolated from a moderately hypovirulent strain of the chestnut blight fungus Cryphonectria parasitica (Murr.) Barr. from eastern New Jersey. Virulence was somewhat lower in the dsRNA-containing strain than in a virulent dsRNA-free control strain, but colony morphology and sporulation levels were comparable. A library of cDNA clones was constructed, and overlapping clones representing the entire genome were sequenced. The 2728-bp dsRNA was considerably smaller than previously characterized C. parasitica dsRNAs, which are 12-13 kb and ancestrally related to the Potyviridae family of plant viruses. Sequence analysis revealed one large open reading frame, but only if mitochondrial codon usage (UGA = Trp) was invoked. Nuclease assays of purified mitochondria confirmed that the dsRNA was localized within mitochondria. Assuming mitochondrial translation, the deduced amino acid sequence had landmarks typical of RNA-dependent RNA polymerases. Alignments of the conserved regions indicate that this dsRNA is more closely related to yeast T and W dsRNAs and single-stranded RNA bacteriophages such as Q beta than to other hypovirulence-associated dsRNAs.

Attenuation of fungal virulence by synthetic infectious hypovirus transcripts

Noninfectious, cytoplasmically transmissible viral double-stranded RNAs of the genus Hypovirus cause reduced virulence (hypovirulence) in the chestnut blight fungus Cryphonectria parasitica, providing the basis for virus-mediated biological control of a fungal disease. Synthetic transcripts corresponding to a full-length hypovirus RNA coding strand are infectious when introduced into fungal spheroplasts by electroporation. Hypovirus infections were readily established in Cryphonectria parasitica and in related fungal species not previously reported to harbor viruses. These results demonstrate the use of a synthetic mycovirus transcript to expand fungal host range, thereby broadening the potential application of virus-mediated hypovirulence to control fungal pathogenesis.

New developments in fungal virology

Although viruses are widely distributed in fungi, their biological significance to their hosts is still poorly understood. A large number of fungal viruses are associated with latent infections of their hosts. With the exception of the killer-immune character in the yeasts, smuts, and hypovirulence in the chestnut blight fungus, fungal properties that can specifically be related to virus infection are not well defined. Mycoviruses are not known to have natural vectors; they are transmitted in nature intracellularly by hyphal anastomosis and heterokaryosis, and are disseminated via spores. Because fungi have a potential for plasmogamy and cytoplasmic exchange during extended periods of their life cycles and because they produce many types of propagules (sexual and asexual spores), often in great profusion, mycoviruses have them accessible to highly efficient means for transmission and spread. It is no surprise, therefore, that fungal viruses are not known to have an extracellular phase to their life cycles. Although extracellular transmission of a few fungal viruses have been demonstrated, using fungal protoplasts, the lack of conventional methods for experimental transmission of these viruses have been, and remains, an obstacle to understanding their biology. The recent application of molecular biological approaches to the study of mycoviral dsRNAs and the improvements in DNA-mediated fungal transformation systems, have allowed a clearer understanding of the molecular biology of mycoviruses to emerge. Considerable progress has been made in elucidating the genome organization and expression strategies of the yeast L-A virus and the unencapsidated RNA virus associated with hypovirulence in the chestnut blight fungus. These recent advances in the biochemical and molecular characterization of the genomes of fungal viruses and associated satellite dsRNAs, as they relate to the biological properties of these viruses and to their interactions with their hosts are the focus of this chapter.

Papain-like protease p29 as a symptom determinant encoded by a hypovirulence-associated virus of the chestnut blight fungus

Viral double-stranded RNAs (dsRNAs) responsible for virulence attenuation (hypovirulence) of the chestnut blight fungus, Cryphonectria parasitica, profoundly influence a range of host functions in addition to virulence. The 5'-proximal open reading frame, A, of the prototypical hypovirulence-associated viral dsRNA, L-dsRNA, present in hypovirulent strain EP713, was recently shown by DNA-mediated transformation analysis to suppress fungal sporulation, pigmentation, and accumulation of the enzyme laccase (G. H. Choi and D. L. Nuss, EMBO J. 11:473-477, 1992). We mapped this suppressive activity to the autocatalytic papain-like protease, p29, present within the amino-terminal portion of open reading frame A-encoded polyprotein p69. Mutational analysis revealed that the ability of p29 to alter fungal phenotype is dependent upon release from the polyprotein precursor but is independent of intrinsic proteolytic activity. Deletion of the p29-coding domain within the context of an infectious L-dsRNA cDNA clone resulted in a replication-competent viral dsRNA that exhibited intermediate suppressive activity while retaining the ability to confer hypovirulence. Thus, p29 is necessary but not sufficient for the level of virus-mediated suppression of fungal pigmentation, sporulation, and laccase accumulation observed for wild-type hypovirulent strain EP713 and is nonessential for viral RNA replication and virulence attenuation. These results also illustrate the feasibility of engineering infectious viral cDNA for construction of hypovirulent fungal strains with specific phenotypic traits.

Biological control of chestnut blight: an example of virus-mediated attenuation of fungal pathogenesis

Environmental concerns have focused attention on natural forms of disease control as potentially safe and effective alternatives to chemical pesticides. This has led to increased efforts to develop control strategies that rely on natural predators and parasites or that involve genetically engineered microbial pest control agents. This review deals with a natural form of biological control in which the virulence of a fungal pathogen is attenuated by an endogenous viral RNA genetic element: the phenomenon of transmissible hypovirulence in the chestnut blight fungus, Cryphonectria parasitica. Recent progress in the molecular characterization of a hypovirulence-associated viral RNA has provided an emerging view of the genetic organization and basic expression strategy of this class of genetic elements. Several lines of evidence now suggest that specific hypovirulence-associated virus-encoded gene products selectively modulate the expression of subsets of fungal genes and the activity of specific regulatory pathways. The construction of an infectious cDNA clone of a hypovirulence-associated viral RNA represents a major advancement that provides exciting new opportunities for examining the molecular basis of transmissible hypovirulence and for engineering hypovirulent strains for improved biocontrol. These developments have significantly improved the prospects of using this system to identify molecular determinants of virulence and elucidate signal transduction pathways involved in pathogenic responses. In addition, novel approaches are now available for extending the application of transmissible hypovirulence for management of chestnut blight and possibly other fungal diseases.

DNA fingerprinting and analysis of population structure in the chestnut blight fungus, Cryphonectria parasitica

We analyzed DNA fingerprints in the chestnut blight fungus, Cryphonectria parasitica, for stability, inheritance, linkage and variability in a natural population. DNA fingerprints resulting from hybridization with a dispersed moderately repetitive DNA sequence of C. parasitica in plasmid pMS5.1 hybridized to 6-17 restriction fragments per individual isolate. In a laboratory cross and from progeny from a single perithecium collected from a field population, the presence/absence of 11 fragments in the laboratory cross and 12 fragments in the field progeny set segregated in 1:1 ratios. Two fragments in each progeny set cosegregated; no other linkage was detected among the segregating fragments. Mutations, identified by missing bands, were detected for only one fragment in which 4 of 43 progeny lacked a band present in both parents; no novel fragments were detected in any progeny. All other fragments appeared to be stably inherited. Hybridization patterns did not change during vegetative growth or sporulation. However, fingerprint patterns of single conidial isolates of strains EP155 and EP67 were found to be heterogenous due to mutations that occurred during culturing in the laboratory since these strains were first isolated in 1976-1977. In a population sample of 39 C. parasitica isolates, we found 33 different fingerprint patterns with pMS5.1. Most isolates differed from all other isolates by the presence or absence of several fragments. Six fingerprint patterns each occurred twice. Isolates with identical fingerprints occurred in cankers on the same chestnut stems three times; isolates within the other three pairs were isolated from cankers more than 5 m apart. The null hypothesis of random mating in this population could not be rejected if the six putative clones were removed from the analysis. Thus, a rough estimate of the clonal fraction of this population is 6 in 39 isolates (15.4%).

Hypovirulence of chestnut blight fungus conferred by an infectious viral cDNA

Strains of the chestnut blight fungus Cryphonectria parasitica that contain viral double-stranded RNAs often exhibit reduced virulence. Such hypovirulent strains act as biocontrol agents by virtue of their ability to convert virulent strains to hypovirulence after anastomosis. Transformation of virulent C. parasitica strains with a full-length complementary DNA copy of a hypovirulence-associated viral RNA conferred the complete hypovirulence phenotype. Cytoplasmic double-stranded RNA was resurrected from the chromosomally integrated complementary DNA copy and was able to convert compatible virulent strains to hypovirulence. These results establish viral double-stranded RNA as the casual agent of hypovirulence and demonstrate the feasibility of engineering hypovirulent fungal strains.

Two nonhomologus viruses of Cryphonectria (Endothia) parasitica reduce accumulation of specific virulence-associated polypeptides

Double-stranded RNA responsible for transmissible hypovirulence of Cryphonectria (Endothia) parasitica affected the accumulation of specific polypeptides. Nonhomologous hypovirulence-causing double-stranded RNAs, originating in Europe or North America, affected accumulation of the same polypeptides. Fewer than 5% of detectable proteins were affected, indicating that hypovirulence is probably not the result of general debilitation of the fungus.

The adaptive significance of sexuality

A theory of sexuality and polymorphism is proposed in which diversity at the molecular level is the adaptive response of multicellular organisms to the challenge of microparasites that have smaller genomes, shorter generation times and which can evolve more quickly than their hosts. The theory has implications for genetically homogenized crops and other cultivated plants as well as for immunology. A different function of sexuality is proposed for microorganisms that reproduce both asexually and sexually. Several possible experimental tests are discussed. Mathematical modelling techniques are outlined qualitatively and compared with game-theoretical methods which may be interpreted as simplifications of population dynamics of polymorphic host-parasite populations are referenced.