Induced Resistance to Viral Infection and Soluble Protein Alterations in Cucumber and Cowpea Plants

Abstract

The formation of tobacco necrosis virus‐elicited local lesions led to a localized “resistance” in cowpea leaves and cucumber cotyledons with an “acquired systemic resistance” induced in cucumber first leaves, following homogolous challenge. The “resistance” was against symptom expression rather than against virus multiplication. A similar “resistance” in both species could be elicited by spraying healthy tissue with polyacrylic acid 4 days before inoculation or osmotically‐stressing cowpea leaves 24 h before inoculation. Virus‐elicited necrosis resulted in reproducible alterations to the soluble protein profile of both species. Apparently novel host proteins, termed the α and β fractions in cowpea leaves and the γ fraction in cucumber tissue were induced during necrosis, and quantitatively reduced by concomitant actinomycin D treatment. Tissue of both species showing “resistance” contained either small amounts or more often none of the apparently novel fractions and any direct involvement in “resistance” is thought unlikely. It is proposed that the tissue exhibiting a degree of “resistance” to virus inoculation has an altered susceptibility to infection. This altered susceptibility appears to be engendered by treatments (e.g. PA spraying, osmotic stressing or previous inoculations on the same plants) that reduce the turgidity of the cells to be challenged.

Pathogenesis-related proteins

“…Although these observations strengthen the hypothesis that PRs are involved in acquired resistance, several findings are difficult to reconcile with this view (19). Similar discrepancies have also been noted in N. glutinosa (18), cucumber (15,20) and cowpea (15). This finding shows that the connection between the presence of PRs and the reduction in virus multiplication and spread may not be direct, and suggests that PRs are not responsible for acquired resistance.…”