Biomedical subjects

G W Felton

Publications and source records attributed to G W Felton.

8 recordsLinked to original sources

PubMed2002

Influence of the host plant on occluded virus production and lethal infectivity of a baculovirus.

Intra- and inter-specific effects of cotton, soybean, and clover on the time until death of Helicoverpa zea (Boddie) and Heliothis virescens (F.) larvae lethally infected with H. zea nucleopolyhedrovirus (HzSNPV) were evaluated in the laboratory. In the first test, on second instar only, the time until death of lethally infected larvae of both species differed with the plant tissues (vegetative or reproductive) and plant species. The total viral activity produced per larva in LC(50) units (occluded viral bodies (OBs) per larva/LC(50) in OBs/mm(2) of diet surface) was greater from H. virescens larvae fed vegetative than reproductive tissues of all host plants, but from H. zea virus production was greater only when fed vegetative tissue of soybean. In a second test that compared second and fourth instar H. virescens on cotton, total viral activity from larvae treated in both instars was greater when fed vegetative than reproductive tissues. Results of these tests suggest that the ability of host plants to influence baculovirus disease is more complex than previously believed. When examining the epizootic potential of a baculovirus, more attention must be given to the effects of the host plant on the insect-virus interactions.

Animals↗

PubMed2001

Plant phenolics as dietary antioxidants for herbivorous insects: a test with genetically modified tobacco.

High foliar phenolics are generally assumed to increase resistance to insect herbivores, but recent studies show that tobacco lines modified to over- and underexpress phenolics do not exhibit higher constitutive resistance to caterpillars. This is contrary to the expectation that ingestion of tobacco phenolics, particularly chlorogenic acid, should cause oxidative stress in herbivores. We investigated free radical production and antioxidant capacity of fresh crushed leaves of tobacco lines exhibiting over a sixfold difference in chlorogenic acid content to test whether high phenolic concentrations are associated with increased production of reactive oxygen species (ROS). The effects of in planta phenolic levels on feeding behavior, growth, biochemical markers of oxidative stress, and the antioxidant capacity of midgut fluid and hemolymph were assessed in tobacco budworm, Heliothis virescens. The experiments showed that high phenolic foliage was more prooxidant than low phenolic foliage, but the net balance in crushed tissue was antioxidant in comparison to buffer and the commercial antioxidant standard, Trolox. In H. virescens, the antioxidant capacity of midgut fluid was also powerful, and caterpillars fed high phenolic foliage did not exhibit the expected markers of oxidative stress in midgut tissues (altered ascorbate ratios, disulfides, or total hydroperoxides). Instead, hemolymph of larvae fed high phenolic foliage exhibited improved total Trolox equivalent antioxidant capacity (TEAC). These results suggest that the elevated foliar phenolics in some plants may have beneficial antioxidant properties for herbivorous insects, much as dietary phenolics do in mammals.

Adaptation, Physiological↗

PubMed2000

Digestive proteinase activity in corn earworm (Helicoverpa zea) after molting and in response to lowered redox potential.

Insect digestive proteinases are often strongly influenced by ambient physicochemical conditions, such as pH, ionic strength, and oxidation-reduction potential. Although the effects of the former two parameters are well documented, the influence of redox potential on catalytic rates of digestive enzymes is not well understood. In this study, we manipulated the midgut redox potential of a generalist caterpillar (the corn earworm, Helicoverpa zea) by augmenting artificial diet with dithiothreitol, a powerful thiol reducing agent that lowers the redox potential in the lumen by 40-45 mV. Effects on total proteolytic activity, as well as on elastase, chymotrypsin, trypsin, leucine aminopeptidase, and carboxypeptidase A and B activities were measured using azocasein and nitroanilide model substrates. The profiles of proteinase activities in the epithelium and lumen were also monitored on days 1, 2, and 3 after the molt in penultimate instar larvae. Although the reducing agent strongly inhibited the activity of some proteinases in vitro, ingestion of the reducing diet failed to affect in vivo proteinase activities. There was also no effect on larval relative growth, consumption, or digestive efficiencies. We conclude that dietary reducing agents must lower midgut redox potential to below -40 mV to significantly impact digestive efficiency. Arch.

Animals↗

PubMed2000

Trade-offs between pathogen and herbivore resistance.

During the past year genetic and pharmacological experiments have revealed a molecular basis for the cross-talk between signaling pathways mediating pathogen and herbivore resistance. These findings provide considerable insight into the apparently contradictory results reported for trade-offs between pathogen and herbivore resistance.

Animals↗

PubMed1999

Inverse relationship between systemic resistance of plants to microorganisms and to insect herbivory.

Pre-inoculation of plants with a pathogen that induces necrosis leads to the development of systemic acquired resistance (SAR) to subsequent pathogen attack [1]. The phenylpropanoid-derived compound salicylic acid (SA) is necessary for the full expression of both local resistance and SAR [2] [3]. A separate signaling pathway involving jasmonic acid (JA) is involved in systemic responses to wounding and insect herbivory [4] [5]. There is evidence both supporting and opposing the idea of cross-protection against microbial pathogens and insect herbivores [6] [7]. This is a controversial area because pharmacological experiments point to negative cross-talk between responses to systemic pathogens and responses to wounding [8] [9] [10], although this has not been demonstrated functionally in vivo. Here, we report that reducing phenylpropanoid biosynthesis by silencing the expression of phenylalanine ammonialyase (PAL) reduces SAR to tobacco mosaic virus (TMV), whereas overexpression of PAL enhances SAR. Tobacco plants with reduced SAR exhibited more effective grazing-induced systemic resistance to larvae of Heliothis virescens, but larval resistance was reduced in plants with elevated phenylpropanoid levels. Furthermore, genetic modification of components involved in phenylpropanoid synthesis revealed an inverse relationship between SA and JA levels. These results demonstrate phenylpropanoid-mediated cross-talk in vivo between microbially induced and herbivore-induced pathways of systemic resistance.

Adaptation, Physiological↗

PubMed1999

Cross-talk between the signal pathways for pathogen-induced systemic acquired resistance and grazing-induced insect resistance.

Reducing phenylpropanoid biosynthesis in transgenic tobacco compromises systemic acquired resistance (SAR) to tobacco mosaic virus, while increasing phenylpropanoid biosynthesis enhances SAR. Surprisingly, transgenic tobacco plants compromised in SAR exhibit more effective grazing-induced systemic resistance to larvae of Heliothis virescens, whereas induced insect resistance is compromised in transgenic plants with elevated phenylpropanoid levels. Levels of the phenylpropanoid-derived signal salicylic acid are directly correlated with overall phenylpropanoid biosynthesis in this series of transgenic plants. Moreover, while pathogen-induced SAR is almost completely compromised in salicylic acid-deficient plants expressing the bacterial nahG salicylate hydroxylase gene, these plants show enhanced grazing-induced insect resistance compared to wild-type. Hence, suppression of grazing-induced insect resistance is mediated at least in part by salicylic acid and likely reflects salicylic acid inhibition of the synthesis and action of the wound signal jasmonic acid. We propose that the dual functions of salicylic acid contribute to a signal poise which constrains constitutive expression of disease and insect resistance mechanisms, and reciprocally switches their selective activation.

Animals↗

PubMed1995

Antioxidant systems in insects.

Insects possess a suite of antioxidant enzymes and small molecular weight antioxidants that may form a concatenated response to an onslaught of dietary and endogenously produced oxidants. Antioxidant enzymes such as superoxide dismutase, catalase, glutathione transferase, and glutathione reductase have been characterized in insects. Water-soluble and lipid-soluble antioxidants such as ascorbate, glutathione, tocopherols, and carotenoids have not been well studied in insects but may play very important antioxidant roles. Additionally, the peritrophic matrix and trehalose may possess important antioxidant functions in insects. The enzymatic recycling of ascorbate, first noted in green plants, may also exist in insects. A greater understanding of these antioxidant systems may provide greater understanding about the ecological relationships of insects with their hosts.

Animals↗

PubMed1993

Antioxidant role of dehydroascorbic acid reductase in insects.

Dehydroascorbic acid reductase, which catalyses the regeneration of ascorbic acid from dehydroascorbic acid, is reported here to occur widely among insects. Due to the reported absence of glutathione peroxidase in insects and the generally low affinity of catalase for hydrogen peroxide, dehydroascorbic acid reductase may play a pivotal role in the elimination of hydrogen peroxide in insects.

Animals↗