Cross-protection reactions of two tobamoviruses, sunn-hemp mosaic virus (SHMV) and common tobacco mosaic virus (TMV-C), were investigated and compared. A mutant of SHMV (SHMV-n), produced by nitrous acid treatment, induced necrotic lesions in bean. SHMV protected completely against this mutant and against SHMV-n RNA. SHMV in bean protected only weakly, however, against TMV-C. To determine whether the coat protein of these viruses affected the ability to superinfect, RNA of each virus was encapsidated in the coat protein of the other. TMV-C RNA encapsidated in SHMV coat protein was five- to 27-fold less infectious on SHMV-infected bean leaves than TMV-C RNA re-encapsidated in TMV-C coat protein. When homologous or heterologous coat protein was added to inocula, infectivity for healthy plants was diminished markedly more by homologous protein, suggesting that extraneous homologous protein diminished infectivity by inhibiting viral uncoating. SHMV-n RNA encapsidated in TMV-C coat protein did not superinfect SHMV-infected bean leaves. Thus, although coat protein was shown to be a factor in cross-protection in some situations, other factors must also be involved.


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