Mapping helper virus functions for cucumber mosaic virus satellite RNA with pseudorecombinants derived from cucumber mosaic and tomato aspermy viruses.

P-TAV is a strain of tomato aspermy virus (TAV) able to efficiently support the systemic accumulation of some (i.e., B2-satRNA) but not of other (i.e., Ix-satRNA) strains of the satellite RNA (satRNA) of cucumber mosaic virus (CMV) in both tobacco and in tomato. As reported for V-TAV, the failure to support the systemic accumulation of Ix-satRNA seems to be due to an inefficient support of its systemic movement. Pseudorecombinants obtained by the exchange of RNAs 1 + 2 between P-TAV and Trk7-CMV, an efficient helper for the systemic accumulation of Ix-satRNA, were assayed for their ability to support the accumulation of CMV-satRNAs in tobacco plants and protoplasts. Pseudorecombinants having RNAs 1 + 2 from CMV supported the systemic movement and accumulation of CMV-satRNA as efficiently as CMV, whereas pseudorecombinants having RNAs 1 + 2 from TAV supported the CMV-satRNA very poorly. Thus, the ability to support the systemic movement and accumulation of CMV-satRNA is determined primarily by RNAs 1 + 2 and not by RNA 3, which is presumed to encode movement functions in the cucumoviruses and only has a minor, modulating effect on the systemic accumulation of satRNA. This suggests that for systemic movement CMV-satRNA has to interact with (the gene products of) RNAs 1 and/or 2 or that these viral RNAs compete with the satRNA for interaction with the coat or other movement proteins.