Polyacrylamide disc electrophoresis of the soluble leaf proteins from Nicotiana tabacum var. 'Samsun' and 'Samsun NN'. IV. Similarity of qualitative changes of specific proteins after infection with different viruses and their relationship to acquired resistance.

The development of systemic mosaic symptoms on Nicotiana tabacum var. 'Samsun' or 'Samsun NN' after infection with potato virus X (PVX), potato virus Yo (PVYo) or cucumber mosaic virus (CMV) was accompanied by differing alterations in protein constitution upon electrophoresis in polyacrylamide gels. Small amounts of four new protein components I-IV, identical to those found earlier in tobacco mosaic virus (TMV) infected Samsun NN, were observed in both varieties after infection with CMV. Only bands I and II were present after infection with PVYo, whereas no new components were detected in plants infected with PVX. Symptom expression differed in CMV-infected Samsun and Samsun NN, but changes in protein constitution were essentially similar for the two varieties. The induction of localized or systemic necrosis with tobacco necrosis virus, tobacco rattle virus or potato virus Yo always led to similar alterations and the appearance of the four new protein components in both varieties. Bands I-IV also occurred in symptomless leaves that developed after inoculation. However, the relative proportions of the bands varied with the variety used and with the characteristic of the infecting virus to remain localized or become systemic. In the variety Samsun NN, the presence of the new protein components closely correlated with the expression of acquired resistance after challenge inoculation with TMV. Systemic acquired resistance was also expressed in plants sprayed with mercuric chloride, although no new components had been induced. Inoculation with yeast-RNA induced only localized resistance by a mechanism that seems to differ from that of systemic acquired resistance and does not involve new protein components. In detached leaves of TMV-infected Samsun NN, lesions enlarged to a greater extent, and new protein components I-IV were induced to lower levels than in attached leaves. These observations are consistent with a role of the new protein components in limiting virus multiplication or spread.