Cross-protection in Arabidopsis against crucifer tobamovirus Cg by an attenuated strain of the virus

Cross-protection is a procedure that has been utilized to protect crops against virulent strains of viruses by pre-treatment with closely related attenuated strains of the virus. We constructed a mutant of crucifer tobamovirus Cg, which is analogous to L,,A, an attenuated strain of Tomato mosaic virus-L (ToMV-L). This mutant, named CgYD, caused few disease symptoms and could spread throughout Arabidopsis thaliana Col-0 plants. Initial infection with CgYD was shown to efficiently cross-protect against a challenge with wild-type Cg. Thus, we have established in Arabidopsis a powerful system for investigating mechanisms of cross-protection. Using this system, we showed that cross-protection was not overcome, even if a higher concentration of the virion, or purified virion RNA, were used in the challenge. We also demonstrated that cross-protection requires that the second virus be very similar in sequence to Cg, which is a characteristic of RNA silencing. However the RNA dependent RNA polymerase SDE1/SGS2 associated with post-transcriptional gene silencing was not required for cross-protection.