Suppression of virus induced local lesions in plasmolysed leaf tissue

It has been demonstrated previously that protoplasts derived from leaves which normally respond in vivo to virus inoculation, with the formation of necrotic local lesions show no such response when inoculated in vitro [1]. In this study mesophyll cells, isolated from one another by plasmolysis of whole leaves, after infection were investigated. When primary leaves of cowpea (Vigna sinensis Endl. var. Blackeye) were floated on the non-permeating, non-metabolizable carbohydrate mannitol, at plasmolysing concentration (0.5 M), immediately, or at any time up to 24 h after inoculation with tobacco necrosis virus (TNV R/1 : 1.5/19 : S/S : S/Fu, [2]), complete suppression of normal local lesion symptoms was observed. Concurrent with this suppression was a decrease in virus content, as determined by local lesion estimates of viral RNA, and nucleoprotein. Infected leaves only produced lesions if deplasmolysed immediately after initial plasmolysis. Floating TNV infected leaves on a series of mannitol solutions from 0 to 0.5 M, showed a progressive decrease in lesion diameter, and these leaves contained the expected infectious levels of viral RNA and nucleoprotein, with significantly more RNA and whole virus in 0.2 M mannitol treated leaves and significantly less in 0.4 and 0.5 M samples. The implications of these results with regard to necrogenesis in virus-infected, hypersensitively reacting plants are discussed