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United States Department of Agriculture

Agricultural Research Service

Research Project: Impact of Climate Change on Plant Defense Responses Induced by Insect Herbivores and Plant Pathogens

Location: Chemistry Research Unit

2013 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:
Studies were conducted to determine the effects of drought on the terpene metabolite profile in Zea mays var. Golden Queen. In response to drought stress maize overall foliar terpene volatile production was reduced. Linalool and DMNT were reduced by more than 50%, while caryophyllene and bergamotene had no significant difference. Herbivore and pathogen induced terpenes were also affected in a similar manner. Fall army worm feeding induced foliar volatile terpene production in both well watered and drought stressed plants, however, well watered plants produced significantly more terpene volatiles than drought stressed plants (Linalool reduced by 50%, caryophyllene reduced by 80% and bergamotene reduced by 55%). Well watered and drought stressed leaves inoculated with equal amounts of Herbivory Response Peptide (HRP) showed a similar weakened induction of volatile terpenes. Correspondingly, maize stem tissue of well watered plants produced significantly more terpene phytoalexins in response to fungal pathogen infection by Fusarium verticillioides. Drought stress had the complete opposite effect on maize root terpene metabolites. The amount of linalool and caryophyllene in maize roots more than doubled with drought stress. Furthermore, uninfected drought stressed root tissue produced copious amounts of Zealexin and Kauralexin phytoalexins. The addition of Diabrotica balteata larvae feeding on drought stressed roots enhanced both the production of the volatile caryophyllene and the phytoalexins. It is clear that the allocation of resources for terpene metabolites strongly favor the belowground tissues of the plant during drought stress. Studies on semiochemical communication by nematodes led to the documentation that dispersal by nematode infectious stages is regulated by pheromones. Studies on entomopathogenic nematodes that attack the citrus root weevil resulted in identification of a plant produced chemical, pregeijerene is attractive to the nematodes and released when the tree is attacked by the weevil larvae. This attraction was in field trials shown to be caused by a terpene named pregeijerene which is released by some root stock cultivars only. We have now demonstrated in extended field trials that this compound also attracts several species of beneficial native EPN species in blueberry fields and thus appear to be a potential key signal for below ground chemical control. These results have significant implications for improving biological control of root feeding insect pests using nematode predators.


4.Accomplishments
1. Elevated carbon dioxide and drought alter plant defense against Furasium pathogen attack. With the rising atmospheric carbon dioxide concentration ([CO2]), climate changes are predict to intensify drought. We studied the individual and combined effects of elevated CO2 and drought stress on susceptibility of Zea mays (maize) to Fusarium verticillioides stalk rot pathogen. Drought at ambient [CO2] reduces F. verticillioides growth, but the combination of drought and elevated [CO2] enhances pathogen proliferation and increases production of the fungal toxin fumonisin. Furthermore, the concentrations of plant-produced defense metabolites are reduced at elevated [CO2]. Examination of levels of the plant hormone, jasmonic acid (JA) to F. verticillioides inculcation revealed that the production of JA is dampened in plants at elevated [CO2]. However, wound induced priming, which increases JA levels prior to pathogen infection, is sufficient to negate the increased susceptibility at elevated [CO2]. The research provides an important basis upon which disease management strategies to cope with biotic stresses und conditions of rising [CO2] and increased drought stress con be developed.


Last Modified: 9/20/2014
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