Virus-like factors

Viruses and virus-like RNAs are of common occurrence in fungi {[642]}. In 1983, Brasier {[521]} described an extranuclear virus-like factor (the “d-factor”) that causes degenerative disease in O. novo-ulmi. Since then, a number of different d-factors (subsequently named d¹ to dⁿ) have been identified within the population of O. novo-ulmi {[531]}. Similar virus-like elements appear to be present in O. himal-ulmi {[855]}. Most d-factors are associated with multiple virus-like RNA segments {[521],[533]}. Depending on the particular d-factor involved, infection of the DED fungus with these virus-like RNAs results in a more or less severe reduction in growth rate and reproductive fitness. Infected isolates show an unstable amoeboid colony morphology in vitro {[105],[521],[531]}. In Table 9, thirteen d-factors reported for O. novo-ulmi race EAN and NAN in Europe are listed. The factors d¹² and d¹³ differ only in one dsRNA band and are considered to be variants of the same d-factor {[105]}. Generally, infection of healthy O. novo-ulmi isolates with one of these factors results in vitro in reduced cerato-ulmin production, decreased conidial viability, and formation of shorter germ tubes {[8],[105]}. Different d-factors vary in their effect on in vivo pathogenicity of O. novo-ulmi. By determination of the threshold number of spores required to persist in an artificial feeding groove and subsequently invade the elm xylem, the latter effect was analyzed (Table 9,{[8],[643]}).

Of all d-factors, the d² factor has been studied in most detail. The large reduction in the levels of cytochrome oxidase found for d²-infected O. novo-ulmi suggests a mitochondrial malfunction {[196]}. Sutherland et al. {[8]} showed that xylem infection of the moderately DED-resistant “Commelin” elm with d²-(or d⁹-)infected O. novo-ulmi requires as many as 50,000-500,000 spores. When a healthy isolate of this fungal species is used, 5,000 spores are sufficient to establish infection. A d²-diseased O. novo-ulmi isolate contains mitochondria infected with 12 unencapsidateddsRNA (and additional ssRNA) virus-like elements (termed RNA 1 to 12), varying in size from 0.33 to 3.49 kbp {[249],[516],[ 533],[ 400]}. The 12 dsRNAs are comprised of several independent replicons {[516]}. Loss of dsRNA 4, 7, and 10 results in a healthy revertant O. novo-ulmi isolate {[533]}. RNA 7 and 10 can be regarded as defective RNAs (in)directly derived from RNA 4. These defective RNAs depend on RNA 4 for their replication. Recently, Hong et al. {[666]} showed that the virus-like elements RNA 3a, 4, 5, and 6 present in d²-diseased O. novo-ulmi correspond to the genomes of four different viruses which replicate independently in the same fungal cell. Since these four viruses appear to be related to (mitochondrial RNA) viruses described for Cryphonectria parasitica and Rhizoctonia solani, it is proposed to assign them to the Mitovirus genus of the Narnaviridae family.

Transmission of d-factors occurs through anastomose {[532],[533]}. After cytoplasmic transmission of the disease, mitochondrial plasmids derived from different regions of the recipient's mitochondrial DNA are generated de novo. {[196]}. Recombination events appear to be involved in this process. Experiments by Abu-Amero et al. {[68]} indicate that the 2.2 kb DNA plasmid present in d²-infected O. novo-ulmi isolate is obtained by recombination between two long repeat sequences in the mitochondrial large subunit ribosomal RNA gene.

Table 9:  Effect of virus-like elements (d-factors) on the level of xylem infection on U. minor var. vulgaris by O. novo-ulmi {[8],[643]}

1. O. novo-ulmi race NAN (North American) and EAN (Eurasian)

The spread of d-factors through a population of O. novo-ulmi appears to be rather limited. Transmission is regulated by the vegetative incompatibility system. Strongly restricted d-factor transmission occurs between fully incompatible mycelia (“w” reaction). Unrestricted spread of the virus-like elements occurs between compatible mycelia (“c” reaction). Between partially compatible mycelia (“l”, “lg”, “n” reactions), transmission will be more or less hampered {[531]}.

Further limitation of the spread of d-factors results from the sexual reproduction of the DED fungus. D-factors are not sexually inherited via the ascospores even if the diseased isolate acts as the female parent {[531],[516]}. Additional loss of d-factors may occur during conidiogenesis and multiplication of O. novo-ulmi by yeast-like budding {[532],[533]}.