CEVd-induced symptom modification as a response to a host-specific temperature-sensitive reaction

Natural selection of two new variants of citrus exocortis viroid (CEVd) was detected by observing tissues displaying both severe and mild symptoms from a single Gynura aurantiaca. The variants CEVd-S (severe) and CEVd-M (mild), differing by only five nucleotides confined to the pathogenic (P) domain, remained stable when propagated by rooted cuttings or from successive plants inoculated with tissue extracts or transcripts from cDNA clones. CEVd-S induces a very severe reaction in Gynura that is consistent throughout a range of environmental conditions. However, symptoms resulting from CEVd-M infection can vary from a nonsymptomatic condition to a severe reaction when grown at 40 degrees C. This differential response was confined to a single host, Gynura aurantiaca, and expressed under standard growing conditions. The distinct host responses induced by these variants could not be correlated with any changes in sequence or conformation of the dominant viroid variant, as predicted by molecular modeling. Therefore, the variable symptom expression appears to be associated with a specific temperature-sensitive response of Gynura aurantiaca.