Cross protection among four viroids

Abstract

Cross-protection tests were performed on tomato and/or chrysanthemum plants with a severe strain (PS) of potato spindle tuber viroid (PSTV), a mild strain of PSTV (PM), citrus exocortis viroid (CEV), chrysanthemum stunt viroid (CSV), and chrysanthemum chlorotic mottle viroid (ChCMV). The identities of all viroids used, except ChCMV, were confirmed by fingerprinting the purified 125I-labeled RNAs. On tomato plants, PS and CEV caused identical, severe symptoms; PM caused mild symptoms; CSV replicated without causing symptoms; and replication of ChCMV could not be detected. Each viroid produced distinctive symptoms on chrysanthemum plants. On tomato, PM protected against the expression of symptoms by PS and CEV. On chrysanthemum, CSV, PM, and PS protected against CEV, but ChCMV did not protect against PS, CEV, or CSV; and PS failed to protect against ChCMV. In two cases of cross-protection, bioassays indicated that both the protecting and the challenge viroids had replicated. Thus, despite their different fingerprints (and hence RNA sequences), CEV, CSV, and two strains of PSTV affect a common biological process not affected by ChCMV.