In vivo generated Citrus exocortis viroid progeny variants display a range of phenotypes with altered levels of replication, systemic accumulation and pathogenicity

Citrus exocortis viroid (CEVd) exists as populations of heterogeneous variants in infected hosts. In vivo generated CEVd progeny variants (CEVd-PVs) populations from citrus protoplasts, seedlings and mature plants, following inoculation with transcripts of a single CEVd cDNA-clone (wild-type, WT), were studied. The CEVd-PVs population in protoplasts was heterogeneous and became progressively more homogeneous in seedlings and mature plants. The infectivity and pathogenicity of selected CEVd-PVs was evaluated in citrus and herbaceous experimental hosts. The CEVd-PVs U30C, G128A and U182C were not infectious; G50A and 108U+ were infectious but reverted back to WT and 62A+, U129A and U278A were infectious, genetically stable and more severe than WT. The 62A+ and U278A and U129A accumulated at higher levels than WT in protoplasts and seedlings respectively. The effect of specific mutations on the predicted secondary structure of the CEVd-PVs' RNA coupled with the infectivity and replication studies suggested complex structure-to-function relationships for CEVd.