PSTVd and ASBVd are selected to represent members of Pospiviroidae and Avsunviroidae, respectively. Red and black lines refer to (+)- and (−)-strands of PSTVd, respectively. Purple and yellow lines refer to (+)- and (−)-strands of ASBVd, respectively. Pol II, DNA-dependent RNA polymerase II. NEP, nuclear-encoded plastid RNA polymerase. P, phosphate terminus. OH, hydroxyl terminus.