Queensland Department of Primary Industries and Fisheries, Emerging Technologies, Queensland Bioscience Precinct, The University of Queensland, St. Lucia, QLD, 4072, Australia.
Abstracts of the papers presented at the International Conference of Indian Virological Society on “Emerging and Re-emerging viral Diseases of the Tropics and Subtropics” at Indian Agricultural Research Institute, New Delhi, India, December 11–14, 2007.
RNA silencing, a cytoplasmic RNA degradation mechanism, which is highly conserved in all eukaryotes, processes double- stranded (ds) RNAs into small interfering (si) RNAs that direct ribonucleases to target cognate RNAs. Also, it has been shown that in at least plants and worms, RNAi signals can be transmitted from one tissue to another, where they switch off (silence) or regulate the amount of expression of genes. RNA silencing can provide high-level virus resistance by specific targeting of viral sequences in transgenic plants. Transgene-induced RNA silencing is triggered by sequence-specific double-stranded RNA which can be derived from sense, antisense or hairpin RNA transgenes. Transgene design features such as inverted repeat sequences separated by splicable introns have considerably increased the efficiency of generating virus-immune plants. Viruses in turn encode potent suppressors of RNA silencing leading to breakdown of resistance.
We have investigated the mechanism underlying the long-distance transmission of mRNA silencing. We have shown that RNA silencing based immunity against Potato virus Y in transgenic tobacco can be overcome by infection with Cucumber mosaic virus, making the plant susceptible to the virus it was engineered to resist. This may jeopardise the use of silencing for virus resistance or even silencing of any other trait in transgenic crops. Transcript levels of both the inverted repeat sequence targeting the gene of interest and the sense stuffer sequence are uniquely affected by RNA silencing and its suppression in transgenic tobacco. We have also demonstrated graft transmission of RNA silencing based CMV resistance. We are now working on strategies to overcome the unwanted suppression of RNA silencing.
