Synergistic interactions of a Potyvirus and a phloem-limited crinivirus inSweet potato plants

When infecting alone, Sweet potato feathery mottle virus (SPFMV, genus Poty virus) and Sweet potato chlorotic stunt virus (SPCSV, genus Crinivirus) cau se no or only mild symptoms (slight stunting end purpling), respectively, i n the sweet potato (Ipomoea batatas L.). in the SPFMV-resistant cv. Tanzani a, SPFMV is also present at extremely low titers, though plants are systemi cally infected. However, infection with both viruses results in the develop ment of sweet potato virus disease (SPVD) characterized by severe symptoms in leaves and stunting of the plants. Data from this study showed that SPCS V remains confined to phloem and at a similar or slightly Lower titer in th e SPVD-affected plants, whereas the amounts of SPFMV RNA and CP antigen inc rease 600-fold. SPFMV was not confined to phloem, and the movement from the inoculated leaf to the upper leaves occurred at a similar rate, regardless of whether or not the plants were infected with SPCSV. Hence, resistance t o SPFMV in cv. Tanzania was not based on restricted virus movement, neither did SPCSV significantly enhance the phloem loading or unloading of SPFMV. It is also noteworthy that SPVD is an unusual synergistic interaction in th at the potyvirus component is not the cause of synergism but is the benefic iary. It is hypothesized that SPCSV is able to enhance the multiplication o f SPFMV in tissues other than where it occurs itself, perhaps by interferin g with systemic phloem-dependent signaling required in a resistance mechani sm directed against SPFMV. (C) 2000 Academic Press.