Genetic analysis of resistance to PVY in androgenetic doubled haploid lines, Fl, F2 and backcross progenies of the Mexican pepper line, CM 334 (Capsicum annuum L.), was performed . Three reaction types were observed when seedlings were inoculated with several PVY strains of different pathotypes and with an American PeMV strain . Resistant genotypes never showed systemic symptoms although some individuals sporadically developed necrotic local lesions on inoculated cotyledons . Susceptible genotypes exhibited either a typical systemic mosaic or a systemic necrosis that caused the death of the inoculated seedlings . Segregation analyses indicated that resistance to pepper potyviruses in CM 334 is conferred by two genes . The first one, tentatively named Pr4, is dominant and confers the resistance to all now known pathotypes of PVY and to PeMV The second one, tentatively named pry, is recessive ; it confers only the resistance to common strains of PVY The systemic necrotic response is conferred by an independent dominant gene, tentatively named Pnl . 231

Immunofluorescence microscopy, azur A staining of viral inclusion bodies, and ELISA tests revealed differences between resistant and susceptible decapitated plants of pepper for virus distribution through the plant but not for virus multiplication and spread in the artificially inoculated leaves. Within 7 to 10 days after inoculation, viral antigens were detected with ELISA tests in all organs of the susceptible plants; only in inoculated leaves of highly resistant plants; and in inoculated leaves, primary stems, and primary roots of partially resistant plants. Immunofluorescence microscopy revealed that infection in petioles, stems, and roots of resistant varieties was restricted to one or two phloem bundles and did not spread to neighbouring tissues or to other plant organs. However, in some partially resistant plants the virus spread lately to other tissues and organs, resulting in a delayed systemic infection and mosaic symptoms on one or two axillary shoots.

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In single inoculations, both PVY and PVMV replicated in inoculated leaves of Nicotiana tabacum cv. ‘Xanthi nc’ plants, but only PVY infected the tobacco plants systemically, whereas PVMV caused localized infection. A mixed infection by the PVY‐To72 and PVMV‐type strains was experimentally realized in ‘Xanthi nc’ plants. In the presence of PVY, PVMV migrated systemically into the upper leaves of the tobacco plant, as was proved by back inoculation. It would appear that in tobacco, PVY acts as a “helper” virus, providing PVMV with the necessary component factor for migration. In extracts from the co–infected leaves. Immune Electron Microscopy (IEM) revealed phenotypic mixed particles which contained a mixture of coat proteins of PVY and PVMV. The role of the structural and functional interactions between the two viruses, which enable PVMV to migrate systemically in tobacco plants, is discussed.

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