Effect of Figaren, an Antiviral Protein from Cucumis figarei, on Cucumber mosaic virus Infection

Local lesion formation on cowpea leaves was more than 50% inhibited by treatment with a 23 kDa RNase-like glycoprotein from Cucumis figarei, figaren, from 24 hr before to 1 hr after inoculation with Cucumber mosaic virus (CMV). CMV accumulation detected by ELISA in tobacco leaves treated with figaren 6 or 0 hr before inoculation with CMV was suppressed. When upper leaves of tobacco plants were treated with figaren and inoculated 10 min later with CMV, mosaic symptoms were delayed for 5–7 days on most of the tobacco plants, and some plants remained asymptomatic. From fluorescence in situ hybridization, infection sites were present in figaren-treated cowpea or melon leaves after inoculation with CMV, though the sites were reduced in number and size compared with those in water-treated control leaves. The amount of CMV RNAs and CMV antigen in melon protoplasts inoculated with CMV and subsequently incubated with figaren similarly increased with time as did that in the control. ELISA and local lesion assays indicated that CMV infection on the upper surfaces of the leaves of tobacco, melon, cowpea and C. amaranticolor whose lower surfaces had been treated with figaren 5–10 min before CMV inoculation was almost completely inhibited. Figaren did not inhibit CMV infection on the opposite untreated leaf halves of melon, cowpea and C. amaranticolor, whereas it almost completely inhibited CMV infection on the untreated halves of leaves of tobacco. CMV infection was not inhibited in the untreated upper or lower leaves of the four plants. These data suggest that figaren does not completely prevent CMV invasion but does inhibit the initial infection processes. It may also induce localized acquired resistance in host plants.