Nature 431: 356-363 (16 Sept 2004)
RNA silencing in plants
David Baulcombe
The Sainsbury Laboratory, John Innes Centre, Colney Lane, Norwich NR4 7UH, UK

There are at least three RNA silencing pathways for silencing specific genes in plants. In these pathways, silencing signals can be amplified and transmitted between cells, and may even be self-regulated by feedback mechanisms. Diverse biological roles of these pathways have been established, including defence against viruses, regulation of gene expression and the condensation of chromatin into heterochromatin. We are now in a good position to investigate the full extent of this functional diversity in genetic and epigenetic mechanisms of genome control.

Although RNA silencing has only emerged as a topic of general interest in the past six years, the first RNA silencing paper may have been published as long ago as 1928. In that paper Wingard described tobacco plants in which only the initially infected leaves were necrotic and diseased owing to tobacco ringspot virus1 (Fig. 1). The upper leaves had somehow become immune to the virus and consequently were asymptomatic and resistant to secondary infection. At the time this ‘recovery’ was a mystery: there was no obvious way to explain the specificity of the resistance to secondary infection.

Figure 1 Recovery in tobacco plants infected with tobacco ringspot virus. The original legend1 to the figure reads ‘Turkish tobacco plant 23 days after inoculation with ringspot. Note the gradual decline in the development of ringspot symptoms on the upper leaves until finally the top leaves appear perfectly normal’. We now know that the virus causing the initial symptoms had activated viral RNA silencing that inhibited spread of the infection into the upper leaves, and caused them to be specifically immune to tobacco ringspot virus secondary infection.

1) Wingard, S. A. Hosts and symptoms of ring spot, a virus disease of plants. J. Agric. Res. 37, 127–153 (1928).