Strong host resistance targeted against a viral suppressor of the plant gene silencing defence mechanism

The 2b protein encoded by cucumber mosaic cucumovirus (Cmv2b) acts as an im portant virulence determinant by suppressing post-transcriptional gene sile ncing (PTGS), a natural plant defence mechanism against viruses. We report here that the tomato aspermy cucumovirus 2b protein (Tav2b), when expressed from the unrelated tobacco mosaic tobamovirus (TMV) RNA genome, activates strong host resistance responses to TMV in tobacco which are typical of the gene-for-gene disease resistance mechanism. Domain swapping between Cmv2b, which does not elicit these responses, and Tav2b, revealed functional doma ins in Tav2b critical for triggering virus resistance and hypersensitive ce ll death. Furthermore, substitution of two amino acids from Tav2b by those found at the same positions in Cmv2b, Lys21-->Val and Arg28-->Ser, abolishe d the ability to induce hypersensitive cell death and virus resistance. How ever, in Nicotiana benthamiana, a species related to tobacco, Tav2b functio ns as a virulence determinant and suppresses PTGS, Thus, a viral suppressor of the host gene silencing defence mechanism is the target of another inde pendent host resistance mechanism, Our results provide new insights into th e complex molecular strategies employed by viruses and their hosts for defe nce, counter-defence and counter counter-defence.