Viroid-like RNAs from cherry: the first satellite RNAs of micoviruses?

Several years ago, two cherry small circular RNAs (cscRNA1 and cscRNA2) and 10-12 double stranded RNAs (dsRNAs) of presumable viral origin were identified in sweet and sour cherry plants grown in Italy. These RNAs were found to be closely associated to a disease termed cherry chlorotic rusty spot (CCRS). The Italian CCRS disease is symptomatologically very similar to the Amasya cherry disease (ACD) described previously in Turkey, and both diseases have been closely associated with similar mycelium-like structures and with double-stranded (ds) RNAs of mycoviruses from the genera Chrysovirus, Partitivirus and Totivirus. More recently it has been realized that cherry trees from Spain affected by cherry leaf scorch (CLS), a fungal disease reported to be caused by Apiognomonia erythrostoma, family Gnomoniaceae (order Diaporthales), show a very similar symptomatology (translucent-chlorotic leaf spots evolving into rusty areas). However, although CLSand CCRS-affected trees have been associated with similar mycelia, fungal fructifications, fungal genes and mycoviral dsRNAs sharing high sequence similarity ? further supporting a close relationship between these two disorders the involvement of A. erythrostoma in CCRS has not been conclusively shown (Carrieri et al., J. Plant Pathol. 2012). Here we report that a small viroid-like RNA similar to cscRNA1 associated with CCRS is also present in CLS-affected trees, thus providing an additional link between them. Comparisons between cscRNAs from CLS and CCRS isolates have shown several common features, including sequence identity (83%), a quasi rod-like conformation with short bifurcations at both termini, and the presence of hammerhead ribozymes in both polarity strands. However, the CLS isolate lacks the recombinant molecules of smaller size (cscRNA2). Although the biological nature of cscRNAs remain to be conclusively shown, the identification of at least cscRNA1 in different cherry cultivars and geographic areas (Spain and Italy) in close association with the same mycoviral dsRNAs, strongly support the satellite nature of the cscRNAs, which could thus be the first mycovirus satellite RNAs.