The overall objective of the proposed research is to understand how certain plant growth-promoting rhizobacteria (PGPR) promote plant growth of crops. The specific objective is to determine why certain root-colonizing phytopathogenic microorganisms are susceptible to PGPR and their siderophires (high-affinity microbial iron transport agents) and why others are not. The mode of antibiotic action of siderophores from PGPR against deleterious rhizosphere pseudomonads will be determined. Experiments are designed to differentiate between classical antibiosis and iron deprivation. Siderophores will be isolated from PGPR and phytopathogenic microorganisms, and their chemical structures and equilibrium stability constants for iron(III) will be compared. The efficiency of the iron transport systems in PGPR and phytopathogenic microorganisms will also be compared. The research plan will provide information needed to evaluate the feasibility of the widespread application of PGPR to enhance field crop yields and the use of these siderophores for the treatment of iron overload diseases and accidental plutonium poisoning.