ISSN X. Flexible filamentous viruses make up a large fraction of the known plant viruses, but by comparison with other viruses, very little is known about their structures. We have used fiber diffraction, cryo-electron microscopy, and scanning transmission electron microscopy to determine the symmetry of a potyvirus, soybean mosaic virus, to confirm the symmetry of a potexvirus, potato virus X, and to determine low resolution structures of both viruses. We conclude that these viruses and, by implication, most or all flexible filamentous plant viruses share a common coat protein fold and helical symmetry, with slightly less than 9 subunits per helical turn.
Repository Staff Only: item control page. A Caltech Library Service. Structure of Flexible Filamentous Plant Viruses. Abstract Flexible filamentous viruses make up a large fraction of the known plant viruses, but by comparison with other viruses, very little is known about their structures.
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Virus nomenclature has had a chequered history. The paucity of nucleic acid sequence data for many of these makes it difficult to assign them confidently to established virus groups. In addition, there are numerous reports of apparently unencapsidated dsRNAs in fungi. These are usually assumed to be viral in nature, but sequence data are needed before they can be confirmed as viruses and assigned to a virus genus.
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- The Molecular Basis for Flexibility in the Flexible Filamentous Plant Viruses.
ICTV convention is that names of species assigned to genera are written in italics but the names of tentative members of genera or unassigned viruses are written in normal text. A notable development since that time has been the discovery and characterization of a significant number of ssRNA mycoviruses. A significant taxonomic change in the 8th report is the creation of the family Chrysoviridae to accommodate Penicillium chrysogenum virus PcV and related viruses with four dsRNA segments in the genus Chrysovirus , which were previously classified in the family Partitiviridae.
This followed the molecular characterization of PcV by Jiang and Ghabrial Plant pathologists have a long been interested in mycoviruses because of their potential as biological control agents. Also of interest is that several of the more recently described viruses from plant pathogenic fungi show some sequence identity with plant viruses.
Although these mycoviruses are sufficiently distinct from each other and their closest plant virus relatives to be considered as new genera, several of them clearly belong within families that predominently contain plant viruses, for example BCVF, BVX and SsDRV in the Flexiviridae. These close phylogenetic relationships raise the possibility that at least some mycoviruses may have originated from plant viruses see later.
They found a high diversity in dsRNA profiles between isolates from different blocks of trees, different trees within a block, and even between fruit from the same tree. The incidence of one virus may be affected by the presence of other viruses. A curious situation arises with the complex of dsRNA viruses putatively infecting an unknown fungus associated with cherry chlorotic rusty spot and Amasya cherry diseases of cherry.
There is currently insufficient information to address this issue adequately for RNA mycoviruses. Two main hypotheses have been proposed to explain the origins and evolutionary history of mycoviruses Ghabrial, The first, the ancient coevolution hypothesis, proposes that infections are ancient from an unknown source and have coevolved along with their hosts.
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The second, the plant virus hypothesis, proposes that, at least for plant pathogenic fungi, the viruses have moved relatively recently from the host plant to the fungus. This ancestral coevolution hypothesis is based, at least in part, on the proposal that as mycovirus transmission is limited to intracellular mechanisms, horizontal movement of the virus to other fungal hosts is likely to be rare, and thus the association between the host and virus is likely to be ancient Buck, The Reoviridae , for example, are a large and biologically diverse family, predominantly animal viruses but including three plant virus genera and the single fungal genus, Mycoreovirus.
The host range of individual mycoviruses would also be limited to single species, or closely related species, where host speciation has occurred following mycovirus infection. Koonin et al. They suggest that the most plausible explanation is that a saprophytic or pathogenic fungus acquired an ssRNA virus from a plant followed by loss of the coat protein gene and a shift towards the dominance of the dsRNA replicative form of the virus due to different evolutionary pressures such as horizontal transmission by hyphal anastamosis. However, they also acknowledge that the opposite is possible and that ssRNA plant viruses could have been derived from a dsRNA fungal virus.
The ancient infection hypothesis, reflecting a long period of coevolution, can also account for the apparently symptomless phenotype of many mycovirus infections. However, not all mycovirus associations are consistent with this hypothesis. Under this hypothesis, the evolution of mycoviruses and their fungal hosts may be incongruent.
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Evidence in support of this hypothesis has come from sequence comparisons between mycoviruses and plant viruses. However, this is not likely to be a common mechanism and it is more probable, especially in the case of fungal plant pathogens, that the fungus becomes infected directly during infection of its plant host. Adams personal communication, concluded that BVX is likely to be a part of a new genus within the family Flexiviridae. This may indicate either a recent recombination event between the two viruses or a relatively recent divergence from a common ancestor.
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Furthermore, in the case of fungi that infect plants, it is also possible that mycoviruses have moved from their fungal host into the plant host. Mycoviruses have no known extracellular mode of transmission and under natural conditions are reliant on their fungal hosts for intracellular transmission Buck, This can occur in two ways, horizontally via protoplasmic fusion and vertically by sporulation. There are no documented examples of intact filamentous fungi being infected with purified mycoviruses, although under experimental conditions protoplasts of several fungi have been infected with purified viruses, or transfer via protoplast fusion.
Asexual spores are produced from modified hyphae and mycoviruses are readily transmitted to these spores through the cytoplasm as the spores develop Buck, Although the c.
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In summary, it appears there are no consistent patterns or differences in rates of virus transmission between sexual and asexual spores, and that transmission of mycoviruses through sexual spores is not a rare event. Anastomosis occurs when hyphae from different fungal individuals fuse together and genetic and cytoplasmic exchange occurs. Mycoviruses exist within the cytoplasm, and are therefore included in this exchange. Successful anastomosis relies on the two fungi being vegetatively compatible, i. In ascomycete fungi, this process is controlled by a series of gene loci, variously known as vic vegetative incompatibility or het heterokaryon incompatibility loci Glass and Dementhon, ; Saupe, Conversely, the existence of VCGs in many fungi provides a natural barrier to horizontal transmission of mycoviruses.
Although incompatibility has long been known to restrict virus transmission, notably in Cryphonectria parasitica Milgroom and Cortesi, , the strength of the incompatibility reaction affects the rate of transmission in a manner somewhat analogous to the hypersensitive reaction between plants and pathogens.
Many plant viruses infect multiple host species, often from different plant genera or families. In principle it would appear much more difficult for mycoviruses to jump hosts due to the apparent absence of mechanical and vector transmission, but there is evidence for the natural horizontal transmission of mycoviruses between different species, adding an extra dimension to the debate over the origins of some mycoviruses.
However, there is no known natural transmission mechanism for this to occur in field populations. Mycoviruses can confer a range of phenotypes on their fungal hosts, both advantageous and deleterious, but many appear symptomless cryptic.
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Apparently symptomless infections are commonly observed between mycoviruses and their fungal hosts Buck, , and the widespread nature of mycoviruses and their lack of obvious impact has led many investigators to propose that mycoviruses have no effect on fungal biology. However, the absence of symptoms under one set of conditions does not exclude the possibility that the virus might induce symptoms under some unexplored environmental conditions and, although many mycoviruses produce no obvious phenotypic changes, it is reasonable to assume that many virus infections will have some, albeit small, effect on growth.
Consequently, the term cryptic is favoured, implying that symptoms can be expressed under some conditions. Considering plant pathogenic fungi, there is increasing evidence for the negative effects of mycoviruses on pathogen growth and pathogenicity on plant hosts, although in many cases the evidence is circumstantial and proving causal relationships are often hampered by the lack of genetic manipulation tools Nuss, The best studied examples of mycoviruses that confer a hypovirulent phenotype on plant pathogenic fungi are the hypoviruses of the Chestnut Blight fungus Chryphonectria parasitica Choi and Nuss, ; Nuss There is also, perhaps not surprisingly, great variability in reactions between a single host and different viruses or dsRNAs.
However, on examining ten additional F. In contrast to hypovirulent interactions, there is good evidence that some mycoviruses are benefical to their hosts. Killer phenotyes in yeasts and Ustilago represent an extreme form of beneficial interactions. Also, Ahn and Lee provide strong evidence that a 6. Previously, it had been demonstrated that endophytic infection confers thermal tolerance to the fungus and the host plant, allowing both to survive high temperatures. However, more subtle beneficial effects occur with other species, and such effects could underlie the widespread occurrence and persistence of mycoviruses in many hosts, given that a parasite limited to vertical transmission is unlikely to persist if it lowers fitness Fine, If the same difference occurs in the field it may be that virus infection provides an advantage for substrate colonization.
Fungal pathogens are a major source of plant disease. Various biological approaches to plant disease management have been proposed but few are, as yet, widely successful in practice. The potential of mycoviruses as biological control agents of plant pathogenic fungi was first demonstrated for Chryphonectria parasitica Nuss, However, even when mycoviruses clearly have the ability to reduce the virulence of fungal plant pathogens, given that hyphal interaction is the only known mechanism for transmission between fungal colonies, the vegetative incompatibility shown by many fungal species is a major barrier to their adoption as biological control agents.
In the case of the closely related Botrytis cinerea , however, field strains are much less clonal Beever and Weeds, and such matching is likely to be challenging. Killer phenotypes characterized to date are from yeasts or genera such as Ustilago with a yeast phase. This may reflect the largely liquid habitat of yeasts in contrast to filamentous fungi. As many mycoviruses appear to have only minimal effect on their host fungi, a complementary approach to biological control might be the use of mycoviruses as gene vectors.
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