Coleus blumei viroid 8: identification and characterization of a novel coleviroid harboring a unique terminal conserved hairpin

Abstract Six viroids belonging to the genus Coleviroid (family Pospiviroidae ) are currently reported all of which naturally infect coleus ( Coleus scutellarioides ). In 2025, a novel viroid-like circular RNA was identified in a vegetatively propagated coleus plant along with Coleus blumei viroid 1 (CbVd-1). Sequence analysis showed that the RNA is 268 nucleotides (nt) long and folds into a rod-like secondary structure. Although the sequence of its central region is identical to that of CbVd-5, its right- and left-hand regions differ significantly from those of all previously described coleviroids. The RNA also contains the central conserved region characteristic of the genus Coleviroid and a unique terminal conserved hairpin in its left-terminal region, comprising a 3-nt terminal loop shared with small-genome coleviroids and a 6-bp stem. Maximum likelihood phylogenetic analysis further showed that this RNA is genetically distant from known coleviroids, forming a separate clade. Bioassays with dimeric transcripts confirmed that it is infectious and replicates autonomously in coleus. Collectively, these findings demonstrate that this 268-nt infectious circular RNA represents a novel coleviroid, for which we propose the name Coleus blumei viroid 8 (CbVd-8).