A Viral Satellite RNA Induces Yellow Symptoms on Tobacco By Targeting a Gene Involved in Chlorophyll Biosynthesis Using the RNA Silencing Machinery

Symptoms on virus-infected plants are often very specific to the given virus. the molecular mechanisms involved in viral symptom induction have been extensively studied,but are still poorly understood. cucumber mosaic virus (cmv) y satellite rna (y-sat) is a non-coding subviral rna and modifies the typical symptom induced by cmv in specific hosts; y-sat causes a bright yellow mosaic on its natural host nicotiana tabacum. the y-sat-induced yellow mosaic failed to develop in the infected arabidopsis and tomato plants suggesting a very specific interaction between y-sat and its host. in this study,we revealed that y-sat produces specific short interfering rnas (sirnas),which interfere with a host gene,thus inducing the specific symptom. we found that the mrna of tobacco magnesium protoporphyrin chelatase subunit i (chli,the key gene involved in chlorophyll synthesis) had a 22-nt sequence that was complementary to the y-sat sequence,including four g-u pairs,and that the y-sat-derived sirnas in the virus-infected plant downregulate the mrna of chli by targeting the complementary sequence. chli mrna was also downregulated in the transgenic lines that express y-sat inverted repeats. strikingly,modifying the y-sat sequence in order to restore the 22-nt complementarity to arabidopsis and tomato chli mrna resulted in yellowing symptoms in y-sat-infected arabidopsis and tomato,respectively. in 5′-race experiments,the chli transcript was cleaved at the expected middle position of the 22-nt complementary sequence. in gfp sensor experiments using agroinfiltration,we further demonstrated that y-sat specifically targeted the sensor mrna containing the 22-nt complementary sequence of chli. our findings provide direct evidence that the identified sirnas derived from viral satellite rna directly modulate the viral disease symptom by rna silencing-based regulation of a host gene. © 2011 shimura et al.