Molecular Analysis of the Genome of an Attenuated Strain of Cucumber Green Mottle Mosaic Virus.

Cucumber green mottle mosaic virus (CGMMV), a member of tobamovirus, is a serious disease agent of cucurbit crops. For instance, in a district of Japan where greenhouse-grown muskmelon is an important crop, the disease caused by CGMMV has been prevalent, causing significant economical losses. Once a greenhouse is contaminated, the virus is extremely difficult to eliminate, since it propagates to very high levels in infected plants. It also remains stable in dead plant tissues in the soil, which later become sources of the virus to infect newly transplanted young seedlings. In addition, the virus is readily transmitted from infected plants to healthy plants by contact with contaminated growers' hands. The infected plants show growth retardation, mottle mosaic symptoms on the leaves and water-soaked spots on the surface of fruit. Since preinoculation of tomatoes and some fruit trees with an attenuated strain of a virus is effective for protecting plants from later infection with the wild-type virus, it was attempted to attenuate a wild-type CGMMV strain SH isolated from an infected musk melon plant15). A stable attenuated strain that causes no or only very mild symptoms on muskmelon was obtained from an ultraviolet light-treated RNA sample of a virus isolate which had been partially attenuated by mutagenization with nitrous acid15). The attenuated strain was designated SH33b, and the virus was propagated in muskmelon for practical use in growers' greenhouses. SH33b showed remarkable effect in protecting muskmelon plants from outbreaks of severe symptoms, and in eliminating the wild-type CGMMV from the greenhouse. The complete nucleotide sequence of the genomic RNA of the parental wild-type strain SH was determined22), and shown to be very closely related to W strain of CGMMV, an isolate from watermelon13,21). However, the entire nucleotide sequence was not fully compared, because the 186 kDa protein gene of W had only been partially sequenced. The sequence of the SH genome also had high homologies to that of tobacco mosaic virus (TMV) strain vulgare6), tobacco mild green mosaic virus (TMGCV)20) and tomato mosaic virus strain L (ToMV-L)17). The abbreviation ToMV-L is used in this paper instead of TMV-L17), since the virus is taxonomically a strain of tomato mosaic virus2). Because of the practical usefulness of SH33b for cross-protecting muskmelon against wild-type CGMMV, we need to understand how SH33b differs from the