Recently, genome sequencing in plants has revealed what appear to be retrotransposons containing genes with similarity to coat protein genes, fueling the speculation that plants do have retroviruses after all. But to date nobody has actually seen a full-fledged plant retrovirus, much less found a disease caused by a retrovirus. Viroids are infectious agents with a single-stranded, circular, RNA genome of just a few hundred base pairs in length. Viroids do not code for proteinaceous gene products, and are replicated by host factors. Viroids are transmitted by insects.
Viroids are a particular problem with fruit trees.
The common region is a cis acting sequence element, which serves both as a replication origin and as a bidirectional promoter of transcription. Within the common region there is a short inverted repeat sequence that is thought to form a hairpin structure when single stranded. BL1 plasmodesmatal interaction A and B stand for the DNA component, R and L stand for 'right' and 'left', indicating the polarity of transcription. Interestingly, the viral AL1 protein interacts with p48, which is one of the Rb associated proteins. BL1 - increase the size exclusion limit of the plasmodesmata.