1a. Objectives (from AD-416):
The objectives of this cooperative research are: 1) to test the hypothesis that phenazine-1-carboxylic acid (PCA) is involved in the mobilization and plant uptake of iron (Fe) and manganese (Mn) from whole soils or synthetic (hydr)oxides, and 2) to determine the role of water content in mineral mobility and uptake by wheat.
1b. Approach (from AD-416):
Experiments will be conducted in plastic microcosms containing sterile soil amended with grains of sand coated with radioactively labeled iron or manganese. The sand will be prepared by established procedures and characterized for surface area, porosity, microscopic morphology (SEM), structure (x-ray diffraction) and iron (Fe) and manganese (Mn) concentration. Plastic columns filled with soil and containing each labeled sand will be sown with wheat seeds inoculated with Pseudomonas fluorescens strain 2-79 or a mutant deficient in the production of phenazine-1-carboxylic acid (PCA). Soils will be maintained at different water potentials. Controls will be sterilized soil plus sand (coated or noncoated) with and without plants. After several weeks of growth, shoots and roots will be harvested, washed free of adhering soil or sand and extracted to remove and quantify extracellular Fe and Mn. Roots and shoots will be prepared for liquid scintillation counting of labeled and total Fe and Mn. The ratio of the radioisotope to the total will allow calculation of the fraction of metal supplied by the coated sand. The growth medium will be extracted with water for determination of total soluble Fe and Mn. Sand grains will be separated from the soil by dry sieving and analyzed to determine final Fe and Mn concentrations and any physical, structural or morphological changes of the oxide coatings. Individual grains differing in distance from the root and root apex also will be selected in order to determine if dissolution of the oxide is dependent on location. Populations of PCA-producing strain 2-79 or the non-producing mutant strains will be determined and PCA produced in situ will be extracted and quantified.
3. Progress Report:
This research relates directly to Objective 2b because it identifies how the phenazine antibiotic contributes to the sustainability of dryland wheat. Root diseases, including take-all, Pythium, Rhizoctonia and common root rots, and Fusarium crown rot, cause $3.5 billion in losses annually to U.S. wheat and barley growers. For most of these diseases, there are no resistant varieties and chemical treatments are not available or perform inconsistently. Modern production practices of direct seeding, used to control soil erosion, exacerbate the incidence and severity of root diseases. This research focuses on fundamental microbe-plant interactions on the roots of wheat in order to develop effective biocontrol strategies that perform consistently against root diseases in sustainable cereal-based cropping systems. Specifically the research focuses on indigenous Pseudomonas bacteria that produce the antibiotic phenazine-1-carboxylic acid (PCA) on the roots of cereals grown in dryland agroecosystems, and on Pseudomonas strain 2-79, a well-studied biocontrol agent that produces PCA and was isolated from the roots of dryland wheat. The goal is to understand the factors leading to the enrichment of natural PCA-producing bacteria and their role in biocontrol and plant growth promotion in dryland wheat. To identify bacterial factors that contribute to the ability of PCA-producing bacteria to survive in dryland agroecosystems, researchers at Washington State University, in collaboration with ARS scientists at Pullman, Washingotn, are testing the hypothesis that PCA production and activity are strongly influenced by soil moisture. Soil microcosms have been established to evaluate PCA biosynthesis at reduced water potentials and to monitor its role in the mobilization of iron and manganese, which are biologically unavailable in dryland soils of the Columbia Plateau. This three-year study will provide wheat growers with new approaches to make use of the beneficial activities provided by PCA-producing bacteria.