title: Viral small interfering RNAs target host genes to mediate disease symptoms in plants.
abstract: The Cucumber mosaic virus (CMV) Y-satellite RNA (Y-Sat) has a small non-protein-coding RNA genome that induces yellowing symptoms in infected Nicotiana tabacum (tobacco). How this RNA pathogen induces such symptoms has been a longstanding question. We show that the yellowing symptoms are a result of small interfering RNA (siRNA)-directed RNA silencing of the chlorophyll biosynthetic gene, CHLI. The CHLI mRNA contains a 22-nucleotide (nt) complementary sequence to the Y-Sat genome, and in Y-Sat-infected plants, CHLI expression is dramatically down-regulated. Small RNA sequencing and 5' RACE analyses confirmed that this 22-nt sequence was targeted for mRNA cleavage by Y-Sat-derived siRNAs. Transformation of tobacco with a RNA interference (RNAi) vector targeting CHLI induced Y-Sat-like symptoms. In addition, the symptoms of Y-Sat infection can be completely prevented by transforming tobacco with a silencing-resistant variant of the CHLI gene. These results suggest that siRNA-directed silencing of CHLI is solely responsible for the Y-Sat-induced symptoms. Furthermore, we demonstrate that two Nicotiana species, which do not develop yellowing symptoms upon Y-Sat infection, contain a single nucleotide polymorphism within the siRNA-targeted CHLI sequence. This suggests that the previously observed species specificity of Y-Sat-induced symptoms is due to natural sequence variation in the CHLI gene, preventing CHLI silencing in species with a mismatch to the Y-Sat siRNA. Taken together, these findings provide the first demonstration of small RNA-mediated viral disease symptom production and offer an explanation of the species specificity of the viral disease.
authors.affiliation: CSIRO Plant Industry, Canberra, Australia.
authors.forename: Neil A
authors.initials: NA
authors.lastname: Smith
authors.forename: Andrew L
authors.initials: AL
authors.lastname: Eamens
authors.forename: Ming-Bo
authors.initials: MB
authors.lastname: Wang
mesh_terms: D016960:Agrobacterium tumefaciens / Q000235:genetics; D000595:Amino Acid Sequence; D015536:Down-Regulation; D020868:Gene Silencing; D059014:High-Throughput Nucleotide Sequencing; D054884:Host-Pathogen Interactions; D008190:Lyases / Q000235:genetics*; D008969:Molecular Sequence Data; D010935:Plant Diseases / Q000235:genetics* / Q000821:virology; D010942:Plant Viruses / Q000502:physiology; D030821:Plants, Genetically Modified / Q000201:enzymology / Q000235:genetics / Q000821:virology; D010957:Plasmids; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D019244:RNA, Satellite / Q000235:genetics / Q000378:metabolism; D034741:RNA, Small Interfering / Q000235:genetics / Q000378:metabolism*; D012367:RNA, Viral / Q000235:genetics / Q000378:metabolism*; D014026:Nicotiana / Q000201:enzymology / Q000235:genetics* / Q000821:virology; D014779:Virus Replication
chemical_list: D012333:RNA, Messenger; D019244:RNA, Satellite; D034741:RNA, Small Interfering; D012367:RNA, Viral; D008190:Lyases; C086788:magnesium chelatase