Location: Chemistry Research2012 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:
This year’s progress report focuses on signals generated from insect herbivory rather than pathogen infection. One anticipated effect of increasing atmospheric carbon dioxide (CO2) is increased frequency and duration of drought. Plants respond to abiotic and biotic stresses by activating signal transduction cascades to coordinate physiological responses necessary for adaptation and survival. Abiotic stress affects some of the same hormonal signals stimulated by herbivory and can interfere with defense responses. Therefore, the combined effects of elevated CO2 and drought on biotic stress induced volatile production was measured on Golden Queen sweet corn at ambient (400 ppm) and elevated (800 ppm) CO2. Three-week-old plants were subjected to drought for seven days. Leaf volatile production was not significantly altered by abiotic stress alone. Next the production of volatiles induced by Fall Armyworm feeding on the fifth leaf for 24 hours was measured. Less volatiles were produced by drought stressed plants and by plants grown at elevated CO2. An Herbivory Response Peptide was used to stimulate volatile production. Leaves from different environmental conditions were excised and placed in this peptide solution for 16 hours before collecting volatiles. Drought stressed leaves treated with this peptide emitted less volatiles than well-watered leaves. Similarly, peptide treated leaves grown at elevated CO2 emitted less volatiles than peptide treated leaves grown at ambient CO2. In contrast to volatiles emitted, the internal leaf volatile concentrations were not lower in drought stressed or elevated CO2 leaves, nor were the transcript levels of caryophyllene (TPS23) and bergamotene (TPS10) terpene synthases significantly attenuated by the environmental stresses. Results indicate that induced corn volatile emission may be hindered by physiological changes caused by climate change. In a second study, Golden Queen sweet corn was exposed to three levels of drought stress (none, moderate, and severe) with and without Fall Armyworm. Third instar larvae fed on leaves for 24 hours. Volatile organic compounds from the leaves were collected and analyzed. After 24 hours of feeding, leaves of non-stressed plants emitted more volatiles, including monoterpenes and sesquiterpenes, and volatile production decreased as drought stress increased. Without herbivory, there was less volatile production. Under severe drought stress, Fall Armyworm feeding induced more volatiles than with no Fall Armyworm. The reduction of emissions with increased drought stress is possibly related to stomatal closure. Under drought stress, plant roots also produced less volatiles. Further research is needed to determine if this reduction was caused directly from drought or was due to escape of volatiles through larger air-space pores of the dryer soil. Reductions of volatile emissions imply that plants under drought stress have less defenses against herbivory and may be more vulnerable in future climates. Corn photosynthetic capacity was affected by severe drought but no significant interaction was found between Fall Armyworm feeding and photosynthetic capacity of the corn plant.
1. Plant produced attractants for entomophagous nematodes. Ultraviolet (UV) radiation exclusion by polycarbonate greenhouses causes abnormal soybean internode elongation. Climate change studies focusing on soybean seed set and yield responses to elevated temperatures were conducted in controlled environments of an 8-room polycarbonate greenhouse. ARS researchers and University of Florida collaborators at Gainesville, Florida discovered that two soybean cultivars produced abnormally long internodes that increased mainstem lengths by about 3-fold, but seed set and yield were affected little. Spectral radiometer measurements showed essentially no transmission through polycarbonate materials below 400 nm wavelengths; however greenhouse glass, special acrylics, and certain thin film claddings transmitted UV-A (320-400 nm) and sometimes UV-B (280-320 nm) as well. Other studies are few but outdoor UV exclusion experiments in India showed that soybean, guar bean, cowpea, and some types of mung bean exhibited internode elongation, but not corn and other types of mung bean. In the U.S., lettuce internodes were found to be longer with combined exclusion of blue light and UV-A. These findings should provide essential information for (A) selection of appropriate greenhouse or growth chamber materials for research or for commercial production of certain plants, or (B) lead to use of supplemental UV lighting. Furthermore, these findings should stimulate additional fundamental research into UV photoreceptors that govern genetic control of plant photomorphogenic responses.
2. Identification of dispersal pheromones by entomopathogenic and plant parasytic nematodes. Biological control is an attractive alternative to soil fumigation with methyl bromide. Or potential replacements, that not only will kill unwanted plant parasitic organisms but also beneficial organisms that in a natural situations often can control a pest to acceptable levels. Larvae of the root weevil Diaprepes abbreviates are serious pests on numerous agricultural plants and infestations are therefore routinely treated with pesticides or soil fumigation. We found that beneficial entomopathogenic nematodes were attracted by damage induced root volatiles and thus selection of suitable root stock, or cultivars, could reduce the need for chemical control of root feeding pest insects. We have now demonstrated in field trials that the citrus produced attractant, pregeijerene, strongly attracted several species of beneficial native Entomopathogenic nematodes (EPN) species and thus appear to be a potential key signal for below ground chemical control.
3. An alternative to host plant signal for below ground biological control is the use of pheromones, like routinely utilized above ground. We have earlier discovered that many nematode species utilize pheromones to regulate numerous behaviors, such as mating and entry in to resting stages. We have now discovered that dispersal by nematode infectious stages is also regulated by pheromones, and furthermore that multiple entomopathogenic as well as plant pathogenic nematode species can distinguish and respond to other nematode signals. These signals can be used to improve the host searching behavior of commercially reared entomopathogenic nematodes when applied to insect infested fields. Furthermore, dispersal pheromones can also be used for temporary nematode control of vegetable seedlings when transplanted from a green house to the field thus eliminating the need for insecticide treatment.Jiang, Y., Wu, C., Zhang, L., Hu, P., Hou, W., Zu, W., Han, T. 2011. Long-day effects on the terminal inflorescence development of a photoperiod-sensitive soybean [Glycine max (L.) Merr.] variety. Plant Science. 180:504-510.