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.