Mitochondrial plasmid-like elements in some hypovirulent strains of Cryphonectria parasitica

Hypovirus infection induces proliferation and perturbs functions of mitochondria in the chestnut blight fungus

Introduction

The chestnut blight fungus, Cryphonectria parasitica, and hypovirus have been used as a model to probe the mechanism of virulence and regulation of traits important to the host fungus. Previous studies have indicated that mitochondria could be the primary target of the hypovirus.

Methods

In this study, we report a comprehensive and comparative study comprising mitochondrion quantification, reactive oxygen species (ROS) and respiratory efficiency, and quantitative mitochondrial proteomics of the wild-type and virus-infected strains of the chestnut blight fungus.

Results and discussion

Our data show that hypovirus infection increases the total number of mitochondria, lowers the general ROS level, and increases mitochondrial respiratory efficiency. Quantification of mitochondrial proteomes revealed that a set of proteins functioning in energy metabolism and mitochondrial morphogenesis, as well as virulence, were regulated by the virus. In addition, two viral proteins, p29 and p48, were found to co-fractionate with the mitochondrial membrane and matrix. These results suggest that hypovirus perturbs the host mitochondrial functions to result in hypovirulence.

In vivo conformation and replication intermediates of circular mitochondrial plasmids in Neurospora and Cryphonectria parasitica

The in vivo conformation and replication intermediates of fungal circular mitochondrial plasmids and plasmid-like mitochondrial element (plMEs) were analyzed by two-dimensional gel electrophoresis and electron microscopy. Plasmids with circular restriction maps exist predominantly as circular molecules and were found to replicate by rolling circle mechanisms. However, the reverse transcriptase-encoding Mauriceville plasmid of Neurospora crassa was observed to replicate by two possible mechanisms: one that is consistent with a reverse transcriptase-mediated process and a second one might involve rolling circle DNA replication. Like the mtDNA-derived plasmid-like elements of N. crassa (Hausner et al. 2006a, b), a plasmid-like element of Cryphonectria parasitica (plME-C9), which consists predominantly of a 1.4 kb nucleotide sequence different from mitochondrial DNA, also was found to replicate by a rolling circle mechanism. Although the techniques used in this study were not suited for the establishment of the in vivo conformation and mode of replication of the mtDNAs of Neurospora or Cryphonectria, we surmise that the rolling circle mechanism might be the predominant mode of DNA replication in fungal mitochondria.