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|Title:||Enzymatic defenses against plant phototoxins in phytophagous insects|
|Department / Program:||Entomology|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
Biology, Plant Physiology
|Abstract:||Enzymatic defenses of phytophagous insects against two biosynthetically distinct types of plant phototoxins, furanocoumarins and $\beta$-carboline alkaloid, which are known to produce toxic oxygen species, were studied. Questions raised for the study were (1) Are antioxidant enzyme activity levels higher in insects adapted to feed on plants containing phototoxins? (2) Are antioxidant enzymes inducible by ingestion of phototoxins? (3) How do antioxidant enzymes interact with other metabolic detoxification enzymes?
Antioxidant enzyme activity levels were highest in the parsnip webworm, Depressaria pastinacella, a specialist on plants in only three genera of Apiaceae, rich in furanocoumarins, moderately high in the black swallowtail, Papilio polyxenes, a species adapted to feed on plants in Apiaceae and Rutaceae known to contain phototoxins, and lowest in the cabbage looper, Trichoplusia ni, a generalist. Antioxidant enzyme activites were induced by dietary phototoxins in the generalist T. ni, but not induced in the specialists P. polyxenes and D. pastinacella. T. ni, which only rarely encounter furanocoumarins in its host range, had the lowest cytochrome P-450 monooxygenase activity toward xanthotoxin. Slow detoxification of phototoxin might allow a considerable amount of unmetabolized phototoxin to generate toxic oxyradicals which in turn induce antioxidant enzymes. When piperonyl butoxide, a cytochrome P-450-specific inhibitor, was fed along with xanthotoxin to D. pastinacella, superoxide dismutase activity was increased.
These comparative studies suggest a hypothesis that the action of cytochrome P-450 and antioxidant enzymes is independent but consequential--the induction of antioxidant enzymes by phototoxins occurs when low P-450 activity toward the phototoxin permits the accumulation of oxidative stress from unmetabolized phototoxin.
Antioxidant enzymes and glutathione-$S$-transferase activities were studied in three Papilio species (Lepidoptera: Papilionidae) which encounter different chemical profiles in their food plants. In general, high antioxidant enzyme titers were found in P. polyxenes and the giant swallowtail, P. cresphontes, both of which feed on plants rich in phototoxins, and low titers were in the tiger swallowtail, P. glaucus, which is less likely to encounter phototoxins in its broad host range. GST activity was highest in P. polyxenes, and low in the other congeners. Antioxidant enzyme activites, in general, were localized in the midgut and fat body tissues. GST activity was highly localized in fat body, and lowest in hemolymph.
Although GST activity was induced by dietary xanthotoxin, xanthotoxin was not directly metabolized by GST. Some plant phenols including quercetin and ellagic acid inhibited GST and glutathione peroxidase in vitro. GST inhibition patterns differed among plant phenols.
|Rights Information:||Copyright 1989 Lee, Keywan|
|Date Available in IDEALS:||2011-05-07|
|Identifier in Online Catalog:||AAI9010928|