A viroid from Nematanthus wettsteinii plants closely related to the Columnea latent viroid

1 Agriculture Canada Research Branch, Research Station, P.O. Box 20280, Fredericton, New Brunswick, Canada E3B 4Z7 and 2 Department of Plant Biology and Pathology, University of Maine, Orono, Maine 04469, U.S.A. A viroid was isolated from symptomless Nematanthus wettsteinii plants using the return-PAGE method for analysis of low M r nucleic acids. The RNA was transmitted to tomato, three cultivars of potato, and Scopolia sinensis plants by mechanical inoculation or by grafting. Infected solanaceous plants developed symptoms similar to those caused by potato spindle tuber viroid (PSTVd). The Nematanthus viroid consists of 372 nucleotides, 214 G+C, 158 A+U, with a G+C/A+U ratio of 1.35. One of seven cDNA clones showed a sequence heterogeneity (G to A) at position 73. The most stable secondary structure of this viroid has 78 G:C, 37 A:U and 11 G:U base pairs with a minimum free energy of -456.9 kJ. The viroid is closely related to the 370 nucleotide Columnea latent viroid. The Nematanthus viroid possesses regions of 100% sequence identity with six viroids belonging to the PSTVd and apple scar skin viroid groups. The viroid also replicated in tomato plants when mixed with PSTVd. Tomato plants were cross-protected against PSTVd when preinfected with the viroid from N. wettsteinii . Received 14 April 1992; accepted 21 July 1992.