| Abstract: | Five experimental approaches were utilized to support or negate the hypothesis that infection of the systemic host of TMV, N. tabacum var. Samsun, induces an increase in the hexose monophosphate (HMP) shunt pathway of glucose catabolism. All experiments were conducted on plant tissues grown under controlled cultural and environmental conditions. Experiments were conducted on tissues 5 days post inoculation and tissues expressing visual symptoms of infection (13 to 21 days post inoculation). The experiments conducted were as follows: (1) Identical samples of healthy and TMV infected leaf tissues were allowed to metabolize glucose-1- and glucose-6-C¹⁴. The activity of the C¹⁴O₂ trapped from samples metabolizing each intermediate was used to compute C₆/C₁ ratios. (2) The ability of infected and healthy leaf tissues to metabolize the HMP shunt intermediates ribose-1-, xylose-1- and gluconic acid-6-C¹⁴ to C¹⁴O₂ was investigated. (3) The relative activities of three HMP shunt enzymes (glucose-6-phosphate, 6-phosphogluconic acid dehydrogenases and phosphoriboisomerase) were investigated in root tissues from plants grown in nutrient culture. (4) The inhibitory effect of malonic acid on respiration rate was investigated in infected and healthy leaf tissues. (5) The level of endogenous sterols was investigated in infected and healthy leaf, stem and root tissues. The ability to synthesize the steroid nucleus was investigated by infusing mevalonic acid-2-C¹⁴ (MVA-C¹⁴) into leaf tissues at the time of inoculation with TMV and subsequently assaying the lipid soluble fraction for C¹⁴-labeled sterols. Data from the C₆/C₁ ratio investigations indicates that the HMP shunt is responsible for about 20% of the total respiration in both TMV infected and healthy tissues. Both types of tissues were equally effective in degrading the HMP shunt intermediates to C¹⁴O₂. There was no difference in the activities of the three HMP shunt enzymes in infected and healthy root tissues. At no time was there a measurable differential in malonate-induced respiratory inhibition in infected and healthy leaf tissue. There was no variation in the concentrations of the four sterols comprising the total sterol fraction in either infected or healthy leaf tissues. Similarly, the total sterol fractions from infected and healthy tissues infused with MVA-C¹⁴ were identical. The sterols present in the extracts are tentatively identified as cholesterol, campesterol, stigmasterol and β-sitosterol. All were present in leaf and stem extracts; all but cholesterol were detected in root extracts. The results of these experiments lead to the conclusion that (a) there is no increase in the activity of the HMP shunt pathway upon infection of N. tabacum var. Samsun with TMV, and (b) expression of the visual symptoms of infection have no effect on the pathways of glucose catabolism in TMV infected tissues. |
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