|
Type Species |
(RSV) |
Virions have a thin filamentous shape; they consist of nucleocapsids, 3-10 nm in diameter, with lengths proportional to the size of their RNA. The filamentous particles may appear to be spiral-shaped, branched or circular (Fig. 1). No envelope has been observed.
Physicochemical and Physical Properties
Virus preparations are separated into four or five components by sucrose density gradient centrifugation, but form one component with a buoyant density 1.282-1.288 g/cm3 when centrifuged to equilibrium in CsCl.
The ssRNA genome consists of four or more segments. The sizes are ~ 9 kb (RNA-1, negative polarity), 3.3 to 3.6 kb (two species in Rice grassy stunt virus: RGSV) (RNA-2, ambisense), 2.2 to 2.5 kb (RNA-3, ambisense) and 1.9 to 2.2 kb (RNA-4, ambisense). Maize stripe virus (MSpV) and Echinochloa hoja blanca virus (EHBV) preparations contain a 5th RNA of negative polarity and with a size of 1.3 kb. A fifth RNA segment has also been reported for some isolates of Rice stripe virus (RSV). RGSV preparations contain six segments.
The nucleocapsid proteins are of Mr 34-35 103. Small amounts of a minor protein (Mr > 230 103), have been found to co-purify with particles RGSV, which may have been an RNA-dependent RNA polymerase, as this activity is associated with filamentous nucleoprotein particles.
None reported.
None reported.
Genome Organization and Replication
The 3- and 5-terminal sequences of each ssRNA are almost complementary for about 20 bases. Several RNA segments encode two proteins in an ambisense arrangement (Fig. 2). The nucleocapsid protein (pc3; N) is encoded by the 5-proximal region of the virus complementary strand of RNA-3. Virus sense RNA-4 encodes in its 5-proximal region a major non-structural protein (pc4; NSP) that accumulates in infected plants. Some of the intergenic non-coding regions between the ORFs could adopt hairpin structures. Some segments (e.g., RNA-1 of RSV and RNA-5 of MSpV) are of negative polarity. RNA-1 encodes the RNA-dependent RNA polymerase (pc1; RdRp). Some proteins are translated from sgRNAs (Fig. 3). For MSpV, RHBV and RSV mRNA, the production of mRNAs involves a cap-snatching mechanism. An RNA polymerase has been found associated with purified preparations of RSV, RHBV and RGSV. The RNA polymerase activity of RHBV is capable of replicating and transcribing the RNA segments in vitro.
The N proteins of RSV and MSpV are serologically related, and both the N and NSP proteins of RSV and RGSV are related. RSV capsid protein (CP) reacts weakly with antibodies to RGSV or RHBV.
Plant hosts of tenuiviruses are all in the family Graminae.
Each species is transmitted by a particular species of planthopper in a circulative, propagative manner. The major vectors are Laodelphax striatellus (RSV), Peregrinus maidis (MStV), Tagosodes orizicolus (RHBV), T. cubanus (EHBV), Nilaparvata lugens (RGSV), Caenodelphax teapae (Urochloa hoja blanca virus; UHBV), Ukanodes tanasijevici (Iranian wheat stripe virus; IWSV), Javesella pellucida (European wheat striate mosaic virus; EWSMV) and Sogatella kolophon (Brazilian wheat spike virus; BWSV).
Offspring can be infected by transovarial transmission by viruliferous females or through sperm from viruliferous males. Mechanical transmission using sap extracts is difficult.
Characteristic inclusion bodies that consist almost entirely of pc4 are formed in cells of infected plants.
List of Species Demarcation Criteria in the Genus
The criteria demarcating species in the genus are:
1. |
Vector specificity, i.e., transmission by different species of vector, |
2. |
Host range, i.e., different abilities to infect key plant species, |
3. |
Different sizes and/or numbers of RNA components, |
4. |
<85% amino acid sequence identity between any corresponding gene products, |
5. |
<60% sequence identity between corresponding non-coding intergenic regions. |
An example of species discrimination is that between RSV and MSpV. RSV is transmitted by Laodelphax striatellus and infects 37 species in the Graminae including wheat and rice. MSpV is transmitted by Peregrinus maidis and infects maize, occasionally sorghum and a few other graminaceous plants but not wheat or rice. Most RSV isolates have genomes of four RNA segments of 9090, 3514, 2475 to 2504 and 2137 to 2157 nts; the MSpV genome has five segments (9121, 3575, 2357, 2227 and 1317 nts) and RGSV has six RNA segments. Also, the differences in sequence among the components of these viruses all fall outside the limits set by the species parameters.
Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names, CMI/AAB description numbers ( ), genome sequence accession numbers [ ], and assigned abbreviations ( ) are:
|
Echinochloa hoja blanca virus |
(EHBV) | |
|
Maize stripe virus (300) |
(MSpV) | |
|
Rice grassy stunt virus (320) |
[AB 000403; AB 000404] |
(RGSV) |
|
Rice hoja blanca virus (299) |
(RHBV) | |
|
Rice stripe virus (269) |
[D01164, D31879, D13176, X53563] |
(RSV) |
|
Urochloa hoja blanca virus |
(UHBV) |
Tentative Species in the Genus
|
Brazilian wheat spike virus |
(BWSpV) |
|
|
European wheat striate mosaic virus |
(EWSMV) |
|
|
Iranian wheat stripe virus |
(IWSV) |
|
|
Rice wilted stunt virus |
(RWSV) |
|
|
Winter wheat mosaic virus |
(WWMV) |
Phylogenetic Relationships within the Genus
See Fig. 4.
Tenuiviruses have some similarities with viruses classified in the family Bunyaviridae, particularly those in the genus Phlebovirus. The multipartite genomes contain negative sense and ambisense components. Ribonucleoprotein (RNPs) containing the genomic RNAs can be purified from infected plants. The genomic RNA 5 and 3-ends can base pair, and probably give rise to circular RNPs. Generation of mRNA involves a cap-snatching mechanism. Like viruses in most genera in the Bunyaviridae, tenuiviruses infect their insect vectors as well as their primary hosts, plants. The number of genome components (four or five) and the apparent lack of a membrane-bound virus particle distinguish tenuiviruses from viruses in the family Bunyaviridae. Recent data raise the possibility that RGSV be classified in a separate genus. It has six RNA segments that in total encode four or five proteins in addition to those characteristic of the expression of a tenuivirus genome. Moreover, the sequence relatedness of RGSV gene products with those of other tenuiviruses are all unusually low.
Tenui: from Latin tenuis, “thin, fine, weak”.
|
|