The 2b protein and C-terminal region of the 2a protein indispensably facilitate systemic movement of cucumber mosaic virus in radish with supplementary function by either the 3a or the coat protein

Background In Raphanus sativus (Japanese radish), strain D8 of cucumber mosaic virus (CMV-D8) establishes a systemic infection and induces mild mosaic on upper, non-inoculated leaves, whereas strain Y of CMV (CMV-Y) causes only a local infection in the inoculated leaves. Here, we further analyzed the specific viral factor(s) of CMV-D8 that is (are) indispensable for systemic infection in Japanese radish. Methods To identify which genomic RNA(s) is (are) involved in systemic infection in radish, we carried out a pseudorecombination analysis between CMV-D8 and CMV-Y. With recombination analyses between CMV-D8 and CMV-Y using mutant/recombinant RNA2s, chimeric and point-mutated RNA3s, we identified viral factors that are indispensable for systemic infection. Results Viral RNA2 and RNA3 of CMV-D8 facilitated efficient virus spread into the upper, non-inoculated plant tissues of radish (cv. Tokinashi), but not those of CMV-Y. Recombinant RNA2s demonstrated that the 2b protein (2b) and the C-terminus of the 2a protein (2a) of CMV-D8 have a crucial role in systemic infection. In addition, we used chimeric and point-mutated RNA3s to that Pro(17) and Pro(129) in the coat protein (CP) of CMV-D8 are involved in efficient systemic infection and that Ser(51) in the 3a protein (3a) of CMV-D8 has positive effects on systemic spread. The results suggested that these viral factors facilitate systemic infection of CMV-D8 in Japanese radish. Conclusion The C-terminal region of 2a, the entire region of 2b, and supplementary function of either Ser(51) in 3a or Pro(17)/Pro (129) in CP confer systemic infectivity on CMV-D8 in radish. These results further elucidate the complex interaction of viral proteins of CMV to complete systemic infection as a host-specific manner.