Molecular characterization of a partitivirus from the plant pathogenic ascomycete Rosellinia necatrix

Jiang

et al. 2014

102

Botryosphaeria dothidea is an important pathogenic fungus causing fruit rot, leaf and stem ring spots and dieback, stem canker, stem death or stool mortality, and decline of pear trees. Seven double-stranded RNAs (dsRNAs; dsRNAs 1 to 7 with sizes of 3,654, 2,773, 2,597, 2,574, 1,823, 1,623, and 511 bp, respectively) were identified in an isolate of B. dothidea exhibiting attenuated growth and virulence and a sectoring phenotype. Characterization of the dsRNAs revealed that they belong to two dsRNA mycoviruses. The four largest dsRNAs (dsRNAs 1 to 4) are the genomic components of a novel member of the family Chrysoviridae (tentatively designated Botryosphaeria dothidea chrysovirus 1 [BdCV1]), a view supported by the morphology of the virions and phylogenetic analysis of the putative RNA-dependent RNA polymerases (RdRps). Two other dsRNAs (dsRNAs 5 and 6) are the genomic components of a novel member of the family Partitiviridae (tentatively designated Botryosphaeria dothidea partitivirus 1 [BdPV1]), which is placed in a clade distinct from other established partitivirus genera on the basis of the phylogenetic analysis of its RdRp. The smallest dsRNA, dsRNA7, seems to be a noncoding satellite RNA of BdPV1 on the basis of the conservation of its terminal sequences in BdPV1 genomic segments and its cosegregation with BdPV1 after horizontal transmission. This is the first report of a chrysovirus and a partitivirus infecting B. dothidea and of a chrysovirus associated with the hypovirulence of a phytopathogenic fungus. IMPORTANCEOur studies identified and characterized two novel mycoviruses, Botryosphaeria dothidea chrysovirus 1 (BdCV1) and Botryosphaeria dothidea partitivirus 1 (BdPV1), associated with the hypovirulence of an important fungus pathogenic to fruit trees. This is the first report of a chrysovirus and a partitivirus infecting B. dothidea and of a chrysovirus associated with the hypovirulence of a phytopathogenic fungus. BdCV1 appears to be a good candidate for the biological control of the serious disease induced by B. dothidea. Additionally, BdPV1 is placed in a clade distinct from the established genera. The BdCV1 capsid has two major structural proteins, and the capsid is distinct from that made up by a single polypeptide of the typical chrysoviruses. BdPV1 is the second partitivirus in which the putative capsid protein shares no significant identity with any mycovirus protein.A small accompanying dsRNA that is presumed to be a noncoding satellite RNA of BdPV1 is the first of its kind reported for a partitivirus.

Section: Discussionmentioning

confidence: 99%

“…Biocontrol measures may represent an important way to combat fungal diseases. Apart from R. necatrix and Helicobasidium mompa, no other fungi attacking fruit trees have been reported to be infected by mycoviruses (6,(11)(12)(13)(14)(15)(16)(17). Here we report on a strain of B. dothidea (LW-1) isolated from a pear tree from Wuhan, China.…”

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confidence: 95%

Hypovirulence of the Phytopathogenic Fungus Botryosphaeria dothidea: Association with a Coinfecting Chrysovirus and a Partitivirus

Jiang

et al. 2014

102

“…As in some other phytopathogenic fungi such as the chestnut blight fungus, Cryphonectria parasitica (16) and the white mold fungus, Sclerotinia scleotiorum (17), R. necatrix hosts a variety of viruses, including partitiviruses (18)(19)(20). For example, a field hypovirulent fungal strain, W8, was shown to be infected with a partitivirus Rosellinia necatrix partitivirus 1 (RnPV1) (21) and a novel megabirnavirus, Rosellinia necatrix megabirnavirus 2 (RnMBV2) (A. Sasaki, personal communication). Transfection with purified RnPV1 particles showed that RnPV1 alone conferred neither hypovirulence nor any symptoms (22).…”

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confidence: 99%

Effects of Defective Interfering RNA on Symptom Induction by, and Replication of, a Novel Partitivirus from a Phytopathogenic Fungus, Rosellinia necatrix

Chiba

Lin

Kondō

et al. 2013

A novel mycovirus termed Rosellinia necatrix partitivirus 2 (RnPV2), isolated from a phytopathogenic fungus, Rosellinina necatrix strain W57, was molecularly and biologically characterized in both natural and experimental host fungi. Three doublestranded RNA (dsRNA) segments, dsRNA1, dsRNA2, and defective interfering dsRNA1 (DI-dsRNA1), whose sizes were approximately 2.0, 1.8, and 1.7 kbp, respectively, were detected in W57. While the dsRNA2 sequence, encoding the coat protein, was reported previously, dsRNA1 and DI-dsRNA1 were shown to encode competent and defective (truncated) RNA-dependent RNA polymerase, respectively. Artificial introduction of RnPV2 into an RNA silencing-defective, Dicer-like 2 knockout mutant (⌬dcl-2) of a nonnatural host, Cryphonectria parasitica (chestnut blight fungus), resulted in successful infection by the DI-dsR-NA1-carrying and -free RnPV2. The DI-dsRNA1-free RnPV2 strain was characterized by a higher ratio of accumulation of the intact dsRNA1 to dsRNA2, enhanced replication and severer symptom expression, compared with the DI-carrying strain. These findings confirmed the nature of DI-dsRNA1 as a DI-RNA. Both viral strains replicated to higher levels in a ⌬dcl-2 mutant than in a wild-type C. parasitica fungal strain (EP155) and induced severe symptoms in the ⌬dcl-2 mutant but subtle symptoms in EP155, indicating that the host RNA silencing targets the partitivirus. No obvious phenotypic effects of infection by either virus strain were detected in the natural host fungus. These combined results represent the first example of a partitivirus with DI-RNA that alters viral symptom induction in a host-dependent manner.

“…Due to the lack of effective control measures, virocontrol (biological control) of the disease using mycoviruses that attenuate the fungal virulence is an attractive option (13,29,30). To this end, an extensive search of a large number of field isolates was conducted allowing the detection of diverse dsRNA viruses such as a bipartite dsRNA virus, Rosellinia necatrix megabirnavirus 1 (RnMBV1), a quadrivirus, Rosellinia necatrix quadrivirus 1 (RnQV1), the reovirus mycoreovirus 3 (MyRV3), and the Rosellinia necatrix partitiviruses 1 and 2 (RnPV1 and RnPV2) (13,17,(31)(32)(33)(34)(35). Some of these viruses were shown to be able to confer hypovirulence under laboratory conditions.…”

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confidence: 99%

A Novel Victorivirus from a Phytopathogenic Fungus, Rosellinia necatrix, Is Infectious as Particles and Targeted by RNA Silencing

Chiba

Lin

Kondō

et al. 2013

Japan bA novel victorivirus, termed Rosellinia necatrix victorivirus 1 (RnVV1), was isolated from a plant pathogenic ascomycete, white root rot fungus Rosellinia necatrix, coinfected with a partitivirus. The virus was molecularly and biologically characterized using the natural and experimental hosts (chestnut blight fungus, Cryphonectria parasitica). RnVV1 was shown to have typical molecular victorivirus attributes, including a monopartite double-stranded RNA genome with two open reading frames (ORFs) encoding capsid protein (CP) and RNA-dependent RNA polymerase (RdRp), a UAAUG pentamer presumed to facilitate the coupled termination/reinitiation for translation of the two ORFs, a spherical particle structure ϳ40 nm in diameter, and moderate levels of CP and RdRp sequence identity (34 to 58%) to those of members of the genus Victorivirus within the family Totiviridae. A reproducible transfection system with purified RnVV1 virions was developed for the two distinct fungal hosts. Transfection assay with purified RnVV1 virions combined with virus elimination by hyphal tipping showed that the effects of RnVV1 on the phenotype of the natural host were negligible. Interestingly, comparison of the RNA silencing-competent (standard strain EP155) and -defective (⌬dcl-2) strains of C. parasitica infected with RnVV1 showed that RNA silencing acted against the virus to repress its replication, which was restored by coinfection with hypovirus or transgenic expression of an RNA silencing suppressor, hypovirus p29. Phenotypic changes were observed in the ⌬dcl-2 strain but not in EP155. This is the first reported study on the host range expansion of a Totiviridae member that is targeted by RNA silencing.