Genetic dissection of a putative nucleolar localization signal reveals the role of Ourmiavirus coat protein in host specific systemic spread, specific tissue tropism and silencing suppression

Ourmia melon virus is the type member of a recently characterized plant virus group with a tri-segmented (+)ssRNA genome: RNA1 encodes for the RNA-dependent RNA polymerase (RdRP), RNA2 for the movement protein (MP) and RNA3 for the coat protein (CP). GFP-CP fusion locates into the nucleus and preferentially into the nucleolus of N. benthamiana epidermal cells. By alanine scanning and deletion mutagenesis, we identified an arginine rich nucleolar localization signal (NoLS) at the N-terminal of CP. In the context of virus infection, we showed that either CP nucleolar trafficking and/or CP specific arginine residues seem(s) to be essential for efficient virus accumulation and N. benthamiana systemic infection without affecting virion formation. In cucurbits, the integrity of the NoLS seemed critical for the plant systemic infection suggesting a possible role of this protein region in determining the host range. Standard tests for identifying a silencing suppressor in OuMV genome failed; nevertheless, a specific defect in virus accumulation of our NoLS mutant could be fully complemented in Arabidopsis Dicer mutants. Such result is a preliminary indication that the integrity of NoLS in the OuMV CP is necessary for its silencing suppression activity during virus replication.