Mechanism of Interference between Two Strains of Tobacco Mosaic Virus

Interference between two strains of tobacco mosaic virus-ordinary strain (TMV OM) and necrotic strain (TMV Nl)-was investigated. The number of local lesions caused by TMV OM or by TMV Nl on the leaves of Phaseolus vulgaris (cv. Hirasaya Shakugosun) decreased by addition of 1) ultraviolet light-irradiated TMV OM (UV TMV OM), 2) TMV OM protein or of 3) UV TMV Nl.The mixture which was composed of TMV OM and UV TMV OM showed the same sedimentation pattern as that of TMV OM or UV TMV OM alone. When the mixture composed of TMV OM and concentrated UV TMV OM was inoculated, the number of local lesions was greatly reduced. However, after dilutions of the mixture, infectivity of TMV OM recovered to some extent. Similar recovery of infectivity after dilutions also appeared in interference between TMV Nl and UV TMV OM.Leaves of Nicotiana tabacum (cv. Bright Yellow), on which TMV Nl induces local lesions but TMV OM does not, produced decreased number of local lesions when TMV OM was mixed with TMV Nl inoculum. A lesser amount of TMV OM was also detected in Bright Yellow inoculated with the mixture of UV TMV OM and TMV OM than in the plant inoculated with TMV OM alone. Ultraviolet light irradiation of TMV OM before mixing lowered the rate of inhibition of lesion production. Low doses of ultraviolet light irradiation resulted in a decrease of the inhibition depending on the time exposed to ultraviolet light. With high doses, the rate of inhibition by irradiated TMV OM was lowered but not completely, being independent of the time exposed to ultraviolet light where the infectivity of TMV OM had been already lost.On the leaves of Bright Yellow, proteinous moiety of TMV OM inhibited local lesion production by TMV Nl, but did not diminish the interfering level even after exposure to ultraviolet light. Similar results were obtained using Hirasaya Shakugosun as the test plant.From these results the mechanism of inhibition of TMV infectivity by UV TMV is discussed.