RNA AND GENE SILENCING: CONTINUED
Both cross protection of papaya(see page 13) and coat-protein-mediated resistance are reliant on this RNA-dependent silencing mechanism.33 In cross protection the initial infection with the mild virus activates the silencing mechanism. When a subsequentinfection occurs the plant is then able to break down viral RNA that is similar to the first virus and in doing so stops the infection. In GMO papayasthe silencing process is given some help. The mRNA for the viral coat protein is made by the plant in large quantities. When a certain RNA molecule is present in large quantities plants react in the same way as when they encounter double stranded RNA, recognizing it as foreign material and breaking it down. This reaction was observed for the first time during research into Petunia flower color. In order to make a purple color more intense American and Dutch researchers activated the expression of extra pigment genes.34,35 To their surprise the resulting plants had less intensely colored flowers and in some cases even had white petals. The plants with white flowers had so much pigment mRNA that the RNA was broken down before pigment could be produced. The GMO papaya’svirus resistance is based on the same principle. The plant makes the mRNA that codes for the viral coat protein in large quantities causing the silencing mechanism in the plant to be activated which results in the RNA for the viral coat protein being broken down. 33 Ratcliff FG et al. (1999). Gene silencing without DNA: RNA-mediated cross-protection between viruses. The Plant Cell 11, 1207-1215. 34 Van der Krol AR et al. (1990). Flavonoid genes in Petunia: addition of a limited number of gene copies may lead to a suppression of gene expression. The Plant Cell 2, 291-299.35 Napoli C et al. (1990). Introduction of a chimeric chalcone synthase gene into Petunia results in reversible co-suppression of homologous genes in trans. The Plant Cell 2, 279-289. RNA AND GENE SILENCING: CONTINUED figure