INCREASED HYPERSENSITIVITY INDUCED IN TOBACCO BY SYSTEMIC INFECTION BY POTATO VIRUS Y

A resistance, characterized by an enhanced capacity of the host to localize and restrict cell-to-cell movement of local lesion-inducing viruses, was induced in plants of Nicotiana tabacum L. var. Samsun NN by systemic infection by potato virus Y (PVY). Diameters of lesions induced by challenge inoculation with tobacco mosaic virus (TMV) or with tobacco necrosis virus were consistently smaller in leaves systemically infected by PVY than in comparable leaves of PVY-free plants. The resistance appeared similar or identical to that induced by localized infections by TMV. The effects of the two types of resistance-inducing infections were not additive, except possibly under conditions such that neither type of infection induced its maximum effect. Brief immersion of leaves in hot water (50° for 45 seconds) 24 hours after inoculation with TMV retarded lesion development similarly in resistant and nonresistant leaves. This heat treatment applied 3 days after inoculation caused collapse (within 12 hours) of a narrow band of tissue surrounding each lesion in both resistant and nonresistant leaves. Lesions did not subsequently enlarge beyond the collapsed areas. The response to heat treatment of lesions in PVY-infected leaves was the same as that of lesions in leaves with TMV-induced resistance. Inclusion of potato virus X (PVX) in challenge inocula, or application of 2,4-dichlorophenoxyacetic acid to leaves 24 hours after challenge inoculation, increased the size of TMV lesions in both normal leaves and PVY-infected leaves. The results suggest that the mechanisms for localization of TMV in resistant leaves, whether the resistance is induced by PVY infection or by localized TMV infections, are similar to those in normal leaves; thus, the induced resistance appears to constitute an enhancement of natural host hypersensitivity. © 1968.