Correspondence Yuichiro Watanabe

The sequence profiles of small interfering RNAs (siRNAs) in Arabidopsis infected with the crucifer tobamovirus tobacco mosaic virus (TMV)-Cg were determined by using a small RNA cloning technique. The majority of TMV-derived siRNAs were 21 nt in length. The size of the most abundant endogenous small RNAs in TMV-infected plants was 21 nt, whilst in mock-inoculated plants, it was 24 nt. Northern blot analysis revealed that some microRNAs (miRNAs) accumulated more in TMV-infected plants than in mock-inoculated plants. The question of whether the TMV-Cg-encoded 126K replication protein, an RNA-silencing suppressor, caused small RNA enrichment was examined. Transient expression of the replication protein did not change the pattern of miRNA processing. However, miRNA, miRNA * (the opposite strand of the miRNA duplex) and hairpin-derived siRNA all co-immunoprecipitated with the replication protein. Gel mobility-shift assays indicated that the replication protein binds small RNA duplexes. These results suggest that the tobamovirus replication protein functions as a silencing suppressor by binding small RNA duplexes, changing the small RNA profile in infected plants. RNA silencing in plants acts as an immune system, defend-ing against invading nucleic acids such as viruses, trans