The helper component-proteinase of Sweet potato feathery mottle virus facilitates systemic spread of Potato virus X in Ipomoea nil

When ipomoea nil was coinfected with Sweet potato feathery mottle virus (SP FMV), a member of the genus Potyvirus, and Potato virus X (PVX) typical sym ptoms caused by PVX were observed on those by SPFMV on the first upper true leaves at 14 days postinoculation (dpi). On the other hand. no PVX-induced symptoms were observed on the first upper true leaves at 14 dpi when plant s were infected with PVX alone. In the case of coinfection with PVX and SPF MV, PVX RNA was detected not only in the inoculated cotyledonary leaves but also in the first upper true leaves at 14 dpi. In the case of single infec tion with PVX, PVX RNA was detected in the inoculated cotyledonary leaves b ut not in the first upper true leaves at 14 dpi. The accumulation of SPFMV remained unchanged, regardless of whether the inoculum consisted of SPFMV a lone or a mixture of SPFMV and PVX. Although recombinant PVX engineered to express the helper component-proteinase (HC-Pro) of SPFMV (PVX.HC) enhanced symptoms severity in Nicotiana benthamiana, PVX.HC induced the synergism c haracterized by an enhanced viral movement in Ipomoea nil. Immunofluorescen ce microscopic examination revealed that the HC-Pro was present in phloem o f SPFMV-infected I. nil. These results suggest that SPFMV HC-Pro acts as an enhancer of long distance movement for PVX in I. nil.