ON THE RELATIONSHIP BETWEEN X-BODIES AND SYMPTOM DEVELOPMENT IN PLANTS INFECTED WITH DIFFERENT TOBAMOVIRUSES

The relationship between systemic mosaic symptoms and the occurrence o f viral 126-kDa protein in X-bodies was studied in tobacco infected wi th the tobacco mild green mosaic virus (TMGMV) strains U2, U5, and rib grass mosaic virus (RMV) strain HR, and in other plant species infecte d with tobacco mosaic virus (TMV) strain W U 1. Strains U 2, U 5, and HR coded for proteins of 126, 126, and 130 kDa, respectively, but thes e were not recognized by antisera against the corresponding protein fr om W U 1. Only the HR 130-kDa protein reacted with an antiserum raised against a peptide of amino acids 849-863 from the sequence of W U 1. Electron microscopic analysis established the presence of virus cluste rs in the cytoplasm, as well as in chloroplasts, in leaf tissue infect ed with U 2 or U 5, and adjacent to nuclei and chloroplasts in scatter ed cells infected with HR. X-bodies were not detected after infection with any of these strains, but were large and adjacent to nuclei in W U 1-infected tomato displaying severe mosaic symptoms. Large X-bodies were detected near nuclei in W U 1-infected tomato displaying severe m osaic symptoms, but none were detected after infection of tobacco with any of the other tobamoviruses. The induction of X-bodies appears to be characteristic of some tobamovirus only and, at best, can only be a ssociated with, rather than causative of, the severity of symptoms ind uced by those viruses.