PiRV-2 stimulates sporulation in Phytophthora infestans.

Phytophthora infestans is the causal agent of potato and tomato late blight. This pathogen, which caused the Irish potato famine, is of profound historical significance and still poses a major threat in today's agroecosystems. Research on late blight epidemics usually focuses on pathogen virulence, host resistance, environmental factors and fungicide resistance. In this study, we examined the effect of PiRV-2, an RNA virus harbored by some P. infestans isolates, on its host. Comparing isogenic isolates with or without the virus demonstrated that the virus stimulated sporangia production in P. infestans. Transcriptome analysis suggested that it achieved sporulation stimulation likely through down-regulation of ammonium and amino acid intake in P. infestans. Survey of a limited P. infestans collection found PiRV-2 presence in most strains in the US-8 lineage, a very successful clonal lineage of P. infestans in North America. We suggest that PiRV-2 may affect the ecological fitness of P. infestans and thus could contribute to late blight epidemiology.

Tobacco mosaic virus infection triggers an RNAi-based response in Phytophthora infestans.

RNA interference (RNAi) is a sequence identity-dependent RNA degradation mechanism conserved in eukaryotic organisms. One of the roles of RNAi is as a defense system against viral infections, which has been demonstrated in filamentous fungi but not in oomycetes. We investigated the virus-RNAi interplay in the oomycete Phytophthora infestans using a crucifer-infecting strain of the plant virus tobacco mosaic virus (TMVcr) and its derivative TMVcr-Δ122 that is mutated in the sequence of the p122 replicase subunit and thus inhibited in RNA suppression activity. In this study we provide evidence that replication of TMVcr-Δ122 but not of TMVcr was impaired in P. infestans as well as in tobacco plants used as positive control. The interference was associated with induction of high transcription of dicer-like genes Pidcl2 and NtDCL2 and of RNA-dependent-RNA-polymerase Pirdr1 and NtRDR1 in P. infestans and tobacco, respectively. These high transcription levels suggest an RNAi-based response that TMVcr-Δ122 mutant was not able to suppress. Taken altogether, results of this study demonstrated that an antiviral silencing activity operates also in P. infestans and that a plant virus could be a simple and feasible tool for functional studies also in oomycetes.

Phytophthora infestans RNA virus 2, a novel RNA virus from Phytophthora infestans, does not belong to any known virus group.

Phytophthora infestans is the causal agent of potato and tomato late blight. In this study, we obtained the complete genome sequence of a novel RNA virus from this plant pathogen, tentatively named "Phytophthora infestans RNA virus 2" (PiRV-2). The PiRV-2 genome is 11,170 nt in length and lacks a polyA tail. It contains a single large open reading frame (ORF) with short 5' and 3' untranslated regions. The ORF is predicted to encode a polyprotein of 3710 aa (calculated molecular weight, 410.94 kDa). This virus lacks significant similarity to any other known viruses, even in the conserved RNA-dependent RNA polymerase region. Phylogenetic analysis demonstrated that it did not cluster with any known virus group. We conclude that PiRV-2 belongs to a new virus family yet to be described. This virus was found to be faithfully transmitted through asexual reproduction.

The family narnaviridae: simplest of RNA viruses.

Members of the virus family Narnaviridae contain the simplest genomes of any RNA virus, ranging from 2.3 to 3.6 kb and encoding only a single polypeptide that has an RNA-dependent RNA polymerase domain. The family is subdivided into two genera based on subcellular location: members of the genus Narnavirus have been found in the yeast Saccharomyces cerevisiae and in the oomycete Phytophthora infestans and are confined to the cytosol, while members of the genus Mitovirus have been found only in filamentous fungi and are found in mitochondria. None identified thus far encodes a capsid protein; like several other RNA viruses of lower eukaryotes, their genomes are confined within lipid vesicles. As more family members are discovered, their importance as genetic elements is becoming evident. The unique association of the genus Mitovirus with mitochondria renders them potentially valuable tools to study biology of lower eukaryotes.

A new virus from the plant pathogenic oomycete Phytophthora infestans with an 8 kb dsRNA genome: the sixth member of a proposed new virus genus.

A virus designated Phytophthora infestans RNA virus 3 (PiRV-3) was characterized from an isolate of P. infestans that was co-infected with a second previously described virus, PiRV-4, a member of the virus family Narnaviridae (Cai et al., 2012). The genome of PiRV-3 is 8112 nt and one strand, designated the positive strand, has two deduced overlapping open reading frames linked by a potential frameshift sequence. The first open reading frame (ORF1) is predicted to encode a protein of unknown function, and ORF2 is predicted to encode an RNA-dependent RNA polymerase (RdRp) most closely related to six unclassified dsRNA viruses of filamentous fungi. The genome organizations of five of the related viruses are similar to PiRV-3, indicating taxonomic linkage among those viruses. We suggest that PiRV-3 and related viruses should be collected into a new virus taxon.

A member of the virus family Narnaviridae from the plant pathogenic oomycete Phytophthora infestans.

A virus that has properties consistent with inclusion in the virus family Narnaviridae was described in Phytophthora infestans, the oomycete that caused the Irish potato famine. The genome of phytophthora infestans RNA virus 4 (PiRV-4) is 2,984 nt with short complementary terminal sequences and a single open reading frame predicted to encode an RNA-dependent RNA polymerase (RdRp) most closely related to saccharomyces cerevisiae narnavirus 20S (ScNV-20S) and ScNV-23S, the members of the genus Narnavirus, family Narnaviridae. This report constitutes the first description of a member of the family Narnaviridae from a host taxon outside of the kingdom Fungi.

A novel virus of the late blight pathogen, Phytophthora infestans, with two RNA segments and a supergroup 1 RNA-dependent RNA polymerase.

Double-stranded RNA representing four distinct electrophoretic patterns was found in a screen of Phytophthora infestans isolates. Two dsRNAs that always appeared together were sequenced. RNA 1, which was 3160 nt plus a poly (A) tail, contained a single deduced ORF with the potential to encode a polyprotein of 977 aa with motifs characteristic of supergroup I viral RdRps. The 2776 nt, polyadenylated RNA2 contained an ORF with a potential to encode a polyprotein of 847 aa including a possible trypsin-like serine protease, and a second putative ORF of unknown function. An alternative form of RNA2, in which a 19-nt stretch was replaced by a 9-nt sequence, was detected in 4 of 17 clones sequenced. Based on genome structure and phylogenetic analysis, this virus did not fit into any known virus family and we tentatively named it Phytophthora infestans RNA virus 1 (PiRV-1).