Near immunity to rice tungro spherical virus achieved in rice by a replicase-mediated resistance strategy

Rice tungro disease is caused by rice tungro bacilliform virus (RTBV), whic h is responsible for the symptoms, and rice tungro spherical virus (RTSV), which assists transmission of both viruses by leafhoppers. Transgenic japon ica rice plants (Oryza sativa) were produced containing the RTSV replicase (Rep) gene in the sense or antisense orientation. Over 70% of the plants co ntained one to five copies of the Rep gene, with integration occurring at a single locus in most cases. Plants producing anti-sense sequences exhibite d significant but moderate resistance to RTSV (60%); accumulation of antise nse RNA was substantial, indicating that the protection was not of the homo logy-dependent type. Plants expressing the full-length Rep gene, as well as a truncated Rep gene, in the (+)-sense orientation were 100% resistant to RTSV even when challenged with a high level of inoculum. Accumulation of vi ral RNA was low, leading us to conclude that RTSV Rep-mediated resistance i s not protein-mediated but is of the cosuppression type. Resistance was eff ective against geographically distinct RTSV isolates. In addition, RTSV-res istant transgenic rice plants were unable to assist transmission of RTBV. S uch transgenic plants could be used in an epidemiological approach to comba t the spread of the tungro disease.