ECOLOGICAL BIOCHEMISTRY - Study Guide - V. M. Isaenko 2005

Chapter I. VIRAL INFECTIONS AND ORGANISMAL RESPONSES TO THEM (General Concepts)

1.2. Fungal Viruses

Among Basidiomycetes, cap fungi are susceptible to viral infections that cause diseases known by various names, such as watery FOOT, browning, fundulus, etc.

Sporophores of virus-infected fungi are stunted, deformed, and appear wrinkled or watery depending on ambient humidity. The mycelium of a healthy fungus grows vigorously; it is white, fluffy, and features short, radially oriented strands. In contrast, the mycelium of an infected fungus exhibits slow growth and a brown, velvety surface lacking radial strands.

Fungal infection can occur via several routes. For instance, during the artificial cultivation of the button mushroom Agaricus bisporus, viruses are disseminated through infected mycelium and spores. Transmission of these fungal viruses by insects (such as the scuttle fly Megaselia halterata, etc.) is also possible.

Extracts from infected cap fungi contain multiple types of viruses: rod-shaped particles 19 nm in diameter and 50 nm in length, as well as spherical particles 25 nm and 34 nm in diameter, which are found either free in the Cytoplasm or associated with the vacuolar membranes of the mycelium. Only the 34 nm viruses have been detected in the cytoplasm of spores.

Isometric viruses with a diameter of 28 nm were discovered in extracts of sporophores of Laccaria tocqueta, where they cause a transmissible disease. Isometric viruses measuring 40 nm in diameter were identified in cultures of the rust fungus Puccinia graminis f. sp. tritici that succumbed to infection. Similar viruses were found in spores of Uromyces phaseoli and Cronartium ribicola. Furthermore, isometric viruses with a diameter of 130 nm were isolated from a culture of Schizophyllum commune.

Other fungi are likewise susceptible to viral infections. For example, viruses responsible for take-all disease in wheat and barley have been found in the ascomycete Gaeumannomyces graminis. When these crops are grown continuously on the same fields for consecutive years, the incidence of take-all disease increases over a 3–5 year period. It has been established that the decline in disease severity correlates with a decrease in the intracellular concentration of 18 nm isometric fungal viruses.

Viruses with a diameter of 30 nm have been detected in Cells of the toxin-producing Yeast Saccharomyces cerevisiae. These viruses contain two double-stranded RNA fragments with molecular weights of 2.5 • 10⁶ and 1.4 • 10⁶ Da. Toxin synthesis is regulated by the RNA component with a Molecular Weight of 1.4 • 10⁶ Da.

Viruses causing powdery mildew have been isolated from conidia of the fungus Sphaerotheca lanestris (on oak) and Erysiphe graminis (on barley).

Relatively large isometric viruses resembling Herpesviruses have been discovered in saprophytic fungi of the genus Thraustochytrium. Smaller isometric and iridovirus-like particles have been identified in the fungus Labyrinthomyxa marina.

The deuteromycete Penicillium stoloniferum harbors Two Types of viruses, measuring 34 nm and 40–45 nm in diameter. Viruses of the first type comprise multiple components and may lack RNA entirely or contain single- and double-stranded RNA, either separately or as mixtures. Infected fungal cultures are characterized by stunted, uneven growth. These viruses are transmitted via hyphal anastomoses.

Cells of infected Penicillium chrysogenum contain isometric viruses 40 nm in diameter, located freely in the cytoplasm or within vacuoles. These viruses contain a double-stranded RNA genome consisting of three fragments. Isometric viruses have also been detected in Mycogone perniciosa, a fungus causing bubble disease in cultivated mushrooms, as well as in Pyricularia oryzae, a major rice pathogen.

Viral infections in lower and higher fungi are by no means limited to the Examples discussed above.