Double stranded RNA (dsRNA) viruses replicate in the cytoplasm. After initial infection, a new, positive sense strand is transcribed within the virus particle itself, is extruded into the cytoplasm, and serves as an mRNA template for the synthesis of viral proteins. After the coat and RNA-dependent RNA polymerase proteins have been generated, the positive sense strand and the RNA-dependent RNA polymerase are encapsulated into a new viral particle. The positive sense strand then acts as a template to form a new double stranded RNA molecule generating a mature viral particle.
Family Reoviridae is a family of dsRNA unenveloped RNA viruses with cubic symmetry. The twelve genera (Orthoreovirus, Orbivirus, Coltivirus, Rotavirus, Aquareovirus, Cypovirus, Phytoreovirus, Fijivirus, Seadornavirus, Idnoreovirus, Mycoreovirus, and Oryzavirus) include pathogens of animals and plants.
Plant reoviruses have segmented genomes consisting of 10 to 12 dsRNAs. They have been classified into three different groups, Phytoreovirus, Fijivirus and the Rice Ragged Stunt Virus (RRSV) group (possible genus), based on particle morphology, number of dsRNA genome segments and vector species.
Viruses – dsRNA viruses – Reoviridae –
|Spinareovirinae – Fijivirus – Fijivirus group 2 – Maize rough dwarf virus
Synonym: virus del nanismo ruvido del mais
|MRDV is a large spherical virus 66 nm in diameter. It has a double capsid shell. The outer shell is probably made up of 92 morphological units, with 12 projections or spikes (A-spikes). Virions contain no lipids. Genome made of 10 parts (named S1-10) of double-stranded linear RNA; its size is 26.169 kb. This virus has been known for several years in countries in Europe and Asia, as has its variant, “Mal de Rio Cuarto”, in central Argentina and Uruguay.
||Infected plants show stunting; secondary veins become chlorotic and thick. Younger leaves roll upwards with characteristic overgrowths (enations) on the veins on the underside. Symptoms can be detected in seedlings at approximately one month of age. In later stages, infected plants develop a reddish color. The tassels, ears and upper leaves are malformed and underdeveloped. MRDV can reduce yields by 20-30%.
||Transmitted by several delphacid planthoppers: Laodelphax striatellus, Delphacodes propinqua, Dicranotropis bamata, Javasella pellucida, and Sogatella vibix. Mechanical transmission by needle inoculation is also possible. Transmission is for most of the life of the vector after feeding on an infected plant, and females can pass the virus to the next generation through the eggs.
||Resistant maize plants are the best option for farmers, but in the short term delayed sowing and efforts to kill the virus-carrying planthoppers about 20-30 days before maize is planted will offer adequate control of the disease.
|Spinareovirinae – Fijivirus – Fijivirus group 2 – Rice black streaked dwarf virus
Synonym: rice black-streaked dwarf virus RBSDV; Rice black-streaked dwarf virus
|The RBSDV virion is an icosahedral, double-layered particle with a diameter of 75-80 nm and consists of ten genomic dsRNA segments. Protein sequence analysis suggested that S1 encodes a putative 168.8-kDa RNA-dependent RNA polymerase. S2 and S4 encode a core protein and an outer-shell B-spike protein, respectively. The protein encoded by S3 is assumed to have some guanylyltransferase activity. Proteins translated from S8 and S10 are the components of the major capsid and outer capsid, respectively. Both S7 and S9 encode nonstructural proteins. S7 ORF1 P7-1 and S9 ORF1 P9-1 are components of the tubular structures and viroplasm produced in infected cells, respectively.
||Affects 25 species of Graminae including rice, maize, wheat, barley, oat. The typical symptom is marked white, waxy swelling (enation) on cums, leaf sheath, and undersurface of the leaves, which at a later stage turns black. Pronounced dwarfing and dark green leaves. Sometimes the major veins and whole young leaves become twisted. Plants exhibit severe stunting and the dark green leaves with clearly marked pale streaks between veins, twisted leaves, and breaking of the leaf margins.
||Mechanically transmissible and transmitted in a persistent manner by planthoppers. Three planthopper species are known to transmit the virus in a persistent manner: Laodelphax striatellus, Unkanodes sapporona, and U. albifascia . No transmission occurs through the eggs. Insects acquire the virus after feeding on infected rice plants for 30 min. Minimum inoculation feeding time is about 5 min. Incubation period in feeding insects is 7-35 days, after which the insects are viruliferous until death.
||Control meadures include roguing, elimination off-season volunteer rice plants and wild hosts around the fields, vector control, and cultivation of resistant or tolerant varieties.
|Spinareovirinae – Oryzavirus – Rice ragged stunt virus
Synonym: rice ragged stunt oryzavirus
|Viral particles are 63-65 nm in diameter and consist of five proteins. They are mostly found in phloem and gall cells. The genome consists of ten double-stranded RNA segments.
||Aside from the rice plant, the virus also infects Oryza latifolia and O. nivara. Plants are severely stunted during early growth stages; leaves are short and dark green with ragged yellow to yellow-brown edges; leaf blades twisted which results in the spiral shape; vein swellings develop on the leaf blades and sheaths. Flowering is delayed. Partially exserted panicles contain unfilled grains. Can causes up to 90% crop losses.
||The brown planthopper transmits the disease. The early instar nymphs of the insect are more efficient in transmitting the disease than older ones. Five-day-old nymphs are the most efficient transmitters. The virus is acquired within a feeding period of 24 hours. The brown planthoppers can pick up the virus from infected plants in a 1 day acquisition access period. The latent period is 3-35 days (with an average of 8-6 days). The vectors transmit the disease in a 6 hour-inoculation access period (minimum of 1 hour). The vector retains the virus after each molt and remains infective for life.
||There are no specific control measures for the ragged virus disease except for the use of resistant varieties. Some rice varieties are resistant to the brown planthopper, to the virus, or both. Cultivars resistant to the vector have low disease incidence. The application of insecticides to migratory planthoppers is being used in temperate countries to reduce disease incidence.
|Sedoreovirinae – Phytoreovirus – Rice gall dwarf virus
Synonym: rice gall dwarf virus RGDV
|Viral particles are polyhedral about 65 nm in diameter. Genome consists of double-stranded RNA. Total genome size 25.191 kb. Genome consists of 12 parts (named S1-12).
||Spreads in China. Found, but with no evidence of spread, in Malaysia and Thailand. The characteristic symptoms of the disease are stunting, formation of galls on the leaf blades and sheaths, and slight twisting of diseased plants.
||Transmitted in a persistent manner by five species of leafhoppers Nephotettix nigropictus, N. cincticeps, N. malayanus, N. virescens, and Recilia dorsalis. Virus retained between molts; multiplies in the vector; transmitted vertically to the progeny of the vector; not transmitted by mechanical inoculation; not transmitted by contact between plants; not transmitted by seed; not transmitted by pollen.
||The epidemiology and control methods are yet to be clearly worked out.