Changes in a Nucleic Acid and a Protein Component of Rice Dwarf Virus Particles Associated with an Increase in Symptom Severity

Repeated selection of plants with unusually severe symptoms after their inoculation by insect vectors which had been injected with dilute inoculum from crude extracts of a stock culture (O strain) of rice dwarf virus (RDV) resulted in the emergence of a severe isolate (S strain). Of the 12 segments of RDV RNA, the fourth largest RNA of the S strain had an apparent Mr about 20000 larger than that of the corresponding segment of the O strain. The Mr of the protein corresponding to the M r 43000 protein of the O strain, which is located on the outside of the outer capsid, was 44000 in the S strain. The implication of the differences in the RNA and protein components between the S and O strains is discussed. Mutants and variants have provided some of the most promising materials for comparative studies of the mechanisms of biological functions of viruses, such as the pathological disorders they induce and their transmissibilities (Nishiguchi et al., 1985; Ohno et al., 1983). Since none of the viruses belonging to the plant reovirus group can be cloned by single local lesion selection, the isolation of mutants of these viruses has generally been considered to be difficult. However, mutants that lost transmissibility by the insect vector were obtained with clover wound tumou