Reverse-transcribing viruses, which synthesize a copy of genomic DNA from an RNA template, are widespread in animals, plants, algae and fungi (1, 2).….

Rice tungro is a viral disease seriously affecting rice production in South and Southeast Asia. Tungro is caused by the simultaneous infection in rice of Rice tungro bacilliform virus (RTBV), a double-stranded DNA virus and Rice tungro spherical virus (RTSV), a single-stranded RNA virus. To apply the concept of RNA-interference (RNAi) for the control of RTBV infection, transgenic rice plants expressing DNA encoding ORF IV of RTBV, both in sense as well as in anti-sense orientation, resulting in the formation of double-stranded (ds) RNA, were raised. RNA blot analysis of two representative lines indicated specific degradation of the transgene transcripts and the accumulation of small molecular weight RNA, a hallmark for RNA-interference. In the two transgenic lines expressing ds-RNA, different resistance responses were observed against RTBV. In one of the above lines (RTBV-O-Ds1), there was an initial rapid buildup of RTBV levels following inoculation, comparable to that of untransformed controls, followed by a sharp reduction, resulting in approximately 50-fold lower viral titers, whereas the untransformed controls maintained high levels of the virus till 40 days post-inoculation (dpi). In RTBV-O-Ds2, RTBV DNA levels gradually rose from an initial low to almost 60% levels of the control by 40 dpi. Line RTBV-O-Ds1 showed symptoms of tungro similar to the untransformed control lines, whereas line RTBV-O-Ds2 showed extremely mild symptoms.

Of the various proteins encoded by plant viruses, one of the most interesting is the movement protein (MP). MPs are unique to plant viruses and show surprising structural and functional variability while maintaining their core function, which is to facilitate the intercellular transport of viruses or viral nucleoprotein complexes. MPs interact with components of the intercellular channels, the plasmodesmata (PD), modifying their size exclusion limits and thus allowing larger particles, including virions, to pass through. The interaction of MPs with the components of PD, the formation of transport complexes and the recruitment of host cellular components have all revealed different facets of their functions. Multitasking is an inherent property of most viral proteins, and MPs are no exception. Some MPs carry out multitasking, which includes gene silencing suppression, viral replication and modulation of host protein turnover machinery. This review brings together the current knowledge on MPs, focusing on their structural variability, various functions and interactions with host proteins.