METABOLIC ALTERATIONS IN COTYLEDONS OF CUCURBITA-PEPO INFECTED BY CUCUMBER MOSAIC-VIRUS

Changes in the capacities of enzymes in various metabolic pathways hav e been measured during infection of cotyledons of Cucurbita pepo L. wi th cucumber mosaic virus (CMV). Starch accumulation and low sucrose co ntent, which are characteristic of the early stages of infection, are reversed in the later stages of infection. The decline in starch corre lated with a reduced capacity for starch synthesis (ADP glucose pyroph osphorylase) and a rise in the capacity for starch degradation (total starch hydrolase, starch phosphorylase). (CO2)-C-14 feeding experiment s, conducted at saturating CO2 concentration, show that the newly-assi milated carbon was lost at a lower rate from infected cotyledons and l ess was incorporated into structural carbohydrates, phosphorylated int ermediates plus organic acids, more into soluble sugars, amino acids a nd proteins. At a later stage of infection there were dramatic increas es in respiratory capacity and a substantial alteration of carbohydrat e metabolism. The infection had a large stimulatory effect on the capa city for oxidative pentose-phosphate pathway (glucose-6-phosphate dehy drogenase, 6-phospho-gluconate dehydrogenase), glycolysis (ATP- and py rophosphate-dependent phosphofructokinases), triacid cycle (isocitrate dehydrogenase, hydratase), anaplerotic reactions (NAD-dependent malic enzyme, phosphoenolpyruvate carboxylase) and oxidative electron trans port (cytochrome c oxidase). While there were no overall changes in ph otosynthetic rate (measured in saturating CO2), infection either reduc ed (Rubisco and glycerate kinase) or did not affect (chloroplastic fru ctose bisphosphatase and hydroxypyruvate kinase) the capacities of the photosynthetic carbon reduction pathway or the photosynthetic carbon oxidation pathway.