Characterisation of a novel emaravirus identified in mosaic-diseased Eurasian aspen (Populus tremula). novel emaravirus in Eurasian aspen

Five genome segments of a novel emaravirus from mosaic-diseased Eurasian aspen (Populus tremula) have been completely determined. The monocistronic, segmented ssRNA genome of the virus shows a genome organisation typical for emaraviruses encoding the viral RNA-dependent RNA polymerase (RdRP, 268.2 kDa) on RNA1 (7.1 kb), a glycoprotein precursor protein (GPP, 73.5 kDa) on RNA2 (2.3 kb), the viral nucleocapsid protein (N, 35.6 kDa) on RNA3 (1.6 kb), and a putative movement protein (MP, 41.0 kDa) on RNA4 (1.6 kb). The fifth identified genome segment (RNA5, 1.3 kb) encodes a protein of unknown function (P28, 28.1 kDa). We discovered that it is distantly related to proteins encoded by emaraviruses, such as P4 of European mountain ash ringspot-associated emaravirus. All proteins contain a central region with a conserved secondary structure and a hydrophobic amino acid stretch, bordered by two highly conserved positions, thus clearly representing a new group of homologs of emaraviruses. The virus identified in aspen could be closely associated with observed leaf symptoms, such as mottle, yellow blotching, variegation and chloroses along veins. All five viral RNAs were regularly detectable by RT-PCR in mosaic-diseased P. tremula in Norway, Finland and Sweden. Observed symptoms and testing of mosaic-diseased aspen by virus-specific RT-PCR targeting RNA3 and RNA4 confirmed a wide geographic distribution of the virus in Fennoscandia.We could demonstrate that the mosaic-disease is graft-transmissible and confirmed that the virus is the causal agent by detection in symptomatic, graft-inoculated seedlings used as rootstocks as well as in the virus-infected scions used for graft-inoculation. Owing to these characteristics, the virus represents a novel species within the genus emaravirus and was tentatively denominated aspen mosaic-associated virus (AsMaV).