Interference in Infections of Tobacco Protoplasts with Two Bromoviruses

Summary Freshly prepared tobacco mesophyll protoplasts behave as if they contain two classes, one of which is resistant to infection but the other is susceptible and can readily be doubly infected with brome mosaic virus (BMV) and cowpea chlorotic mottle virus (CCMV). BMV dominates in all double infections and although both types of virus particles are produced, only those of BMV are infectious. The defective CCMV particles contain normal amounts of RNA but only RNA 3 could be detected in them. No CCMV RNA 2 could be detected in doubly infected protoplasts, showing that there was only partial replication of the genome of CCMV. The proteins encoded by CCMV RNA 3 were produced in mixed infections. It is possible that BMV may exploit the gene products of CCMV while suppressing synthesis of infectious CCMV particles. There was no evidence that BMV RNA was encapsidated in CCMV coat protein.