Location: Chemistry Research2011 Annual Report
1a. Objectives (from AD-416)
1. Determine the impacts of high temperatures, limited water availability, and elevated carbon dioxide (CO2) concentrations on: A) plant-grazing insect pest interactions and signaling; and B) plant pathogen relationships with host plants. C) Elucidate chemical mechanisms responsible for regulation of nematode repellence and attraction to hosts and development of nematodes. 2. Determine plant physiological mechanisms that mediate the effects of elevated atmospheric CO2, temperature, and limited water availability on plant-grazing insect pest interactions and plant pathogen interactions.
1b. Approach (from AD-416)
Experiments will be carried out in controlled environment chambers. Studies for objectives 1a, b and 2 will focus on maize (cultivar golden Queen) as the host crop plant, on European corn borer as the insect herbivore, and on Fusarium graminearum as the fungal pathogen. The main focus will be on determining the impact of treatments on generation of plant volatiles that may contribute to changes in plant defense mechanisms and signaling, or generation of plant toxins, but measurements will also be made to assess the impacts on plant productive capacity as well. Experiments will determine the effects of elevated carbon dioxide concentrations and limited water availability, individually and jointly, on the induction of plant defense chemicals in response to planned infestation of maize plants with European corn borer or with infections of F. graminearum. The second experiments will determine the effects on interactions of elevated carbon dioxide and high temperatures (associated with predicted climate change). Studies on sub-objective 1c will focus on plant and nematode produced compounds that repel, attract or inhibit development of Meloidogyne (root knot) nematodes. We will collect volatiles and water soluble exudates from plants and nematodes and conduct bioassays to determine repellence, attractiveness and developmental regulators. Chemicals will be purified by chromatographic methods coupled with bioassays. Compounds will be identified by mass spectrometry, FTIR and NMR and synthesized. Synthesized compounds will be tested in laboratory and field assays to determine efficacy.
3. Progress Report
ARS scientists at Gainesville, Florida embarked on new research on the impact of abiotic stresses related to Global Climate Change on plant defense responses induced by biotic stresses of insect herbivory and plant pathogens. Increase in atmospheric carbon dioxide (CO2) is anticipated to alter rainfall, which could decrease the yield and quality food crops. Drought stress alone can impair plant productivity, but in nature, plants are exposed to both abiotic stresses and biotic attacks. Fusarium verticillioides is an important fungal pathogen of corn that causes seedling decay, stalk and ear rot, and produces carcinogenic mycotoxins. Consumption of mycotoxins through contaminated food can cause serious illness that affect both humans and animals. Water stress and herbivore damage promotes the production of these mycotoxins in infected plants. Stems of the sweet corn cultivar “Golden Queen” were inoculated with the fungus F. verticilliodes and the production of phytoalexins (antimicrobial defense chemicals synthesized by plants) under well-watered and drought stress conditions were measured. At the site of F. verticilliodes inoculation, a significant decrease in phytoalexin production was detected in drought stressed plants compared to well-watered plants. Phytoalexin production is stimulated through the jasmomic acid-ethylene signaling pathway. Jasmonic acid production was suppressed in drought stressed plants infected with F. verticilliodes. This suppression might have been caused by an increase in abscisic acid, a plant hormone produced in response to water-deficit conditions, which is thought to inhibit Jasmonic acid-ethylene dependent resistance. Unlike the infected stem tissue, drought stressed roots of uninfected and infected plants had increased phytoalexin concentrations. The leaf area and root mass of plants exposed to double ambient CO2 levels were significantly higher than ambient CO2 grown plants. Phytoalexin production of infected stem tissue was unaltered by elevated CO2 concentrations and there was no induction the plant roots. Leaf photosynthesis decreased with drought stress, and drought stressed plants that had pathogen infection did not recover as well after rehydration. We are continuing to examine the effect of both elevated CO2 and drought stress on plant phytoalexin production. Regarding infection of corn with European Corn Borer, total quantities of benzoxazinoids and phytoalexins (kauralexins) were increased as early as 24 hrs after feeding started. With the history of the baseline of sweet corn defenses under normal (current climate) conditions at this location, we can further investigate modifications of plant defenses that could occur with climate change. Our results indicate that the perception and response to abiotic stress may dictate or alter subsequent biotic stress responses.