Reverse transcribing viruses replicate in two stages. Upon entering the cell the circular, discontinuous dsDNA genomic molecule uncoats and is transported into the nucleus where the molecule associates with host cell histones to generate a minichromosome. Host RNA polymerase then transcribes a full length transcript of the genome that is transported to the cytoplasm. In the next stage, the full length RNA transcript is primed via a cytosolic initiator methionyl tRNA for reverse transcription to DNA via the virally encoded reverse transcriptase. The newly formed, discontinuous dsDNA molecule then is encapsulated by coat proteins.
Rice tungro bacilliform virus (RTBV)
|Caulimoviridae – Tungrovirus – Rice tungro bacilliform virus
Synonym: rice tungro bacilliform virus, RTBV; rice yellow orange leaf virus
|Virions are bacilliform, not enveloped, 110-400 nm in length, 30-35 nm wide. The virion contains circular double-stranded DNA in a single molecule and is a single coat protein species.
||One of the most important viral disease of rice, widespread in South and Southeast Asia, annual losses are estimated in billions of US dollars worldwide. Symptoms include stunting, red to yellow-orange discoloration, and the reduction of the amounts of grain harvested. Early infections will cause the death of the plant. May produce mottling, striping or interveinal necrosis of the leaves. Panicle exertion is delayed and often incomplete, short and partially filled.
||Transmitted by leafhoppers especially Nephotettix virescens, but also by other species in a semi-persistent manner. Virus lost by the vector when it molts; does not multiply in the vector; not transmitted vertically; for transmission requires a helper virus (rice tungro spherical waikavirus). The viruses are retained for 3-5 days by N. virescens, but for 1 day only by other species.
||Among the many possible methods of controlling tungro, the most practical at present are (1) growing resistant varieties and (2) cultural practices (cultural control). The latter include timing and synchrony of planting, plowing under infected stubbles, roguing, direct seeding instead of transplanting. Use of insecticides is often not effective.