Dynamic changes in the activities of glucose-6-phosphate dehydrogenase, ribulose bisphosphate carboxylase and ribonuclease in tobacco leaves, leaf discs and mesophyll protoplasts in relation to TMV multiplication

Three metabolic pathways of virus-RNA biosynthesis (reductive and/or oxidative pentose phosphate cycles, rRNA degradation pathway) were investigated using three tobacco systems: leaves, leaf discs and mesophyll protoplasts infected with tobacco mosaic virus (TMV). TMV multiplication differed in each system. The maximum concentration of TMV (7.2 mg TMV g-1 fresh wt) was reached at 18 days post inoculation (p.i.) in leaves, but in leaf discs the TMV multiplication curve culminated at 9 days p.i. and virus content was lower (204 mg TMV g-1 fresh wt). In protoplasts, most virus was synthesized between 8 and 30 h p.i. and TMV concentration reached 45.6 micrograms TMV per 10(6) living protoplasts at 72 h p.i. Virus infection significantly increased ribonuclease (RNase) activity in all three systems. Maximum enhancement to 140% was observed at 16 days p.i. in leaves, to 241.8% at 8 days p.i. in discs and to 146.1% at 30 h p.i. in protoplasts in comparison with mock-inoculated healthy controls (100%). Significant linear correlation between enhanced RNase activity and TMV content was found in leaves and discs (r = 0.872+++ and r = 0.979+++, respectively) but not in protoplasts. Activity of glucose-6-phosphate dehydrogenase increased to 128% 12 days p.i. in infected leaves. The enhancement of enzyme activity linearily correlated with the TMV accumulation (r = 0.781+++) over the whole monitored period. In leaf discs the activity of this enzyme was initially enhanced to as much as 148.7% at 6 days p.i., and then a slow decrease was observed. In protoplasts this enzyme was not detectable. Activity of RUBISCO did not differ significantly during the first 10 days between healthy and infected plants, but then a rapid fall to 60-65% of healthy control values was found in infected leaves. In infected leaf discs the enzyme activity gradually decreased after inoculation. At the time of the culmination of the TMV multiplication curve the activity was 16% of control mock-inoculated disc tissues. Similarly, in protoplasts this enzyme was not detectable.