Impact of Phenazine-Producing Pseudomonads upon Fe and Mn Bioavailability in the Rhizosphere of Dryland Wheat.

Large populations of plant-growth-promoting Pseudomonas fluorescens strains producing up to 1 μg/g root of the redox-active antibiotic phenazine-1-carboxylic acid (PCA) have been discovered in dryland but not irrigated wheat fields throughout the low precipitation zone of the Columbia Plateau of the Pacific Northwest (1).  In culture systems, PCA has been shown to reductively dissolve iron and manganese oxides (2) and to promote biofilm development via solubilization of Fe (3), but these effects have not been established in agroecosystems.  Using a wild-type PCA-producing strain and an isogenic mutant impaired in PCA synthesis, we have developed growth chamber experiments to test the hypothesis that PCA-producing pseudomonads enhance the bioavailability of Fe and Mn in the rhizosphere of dryland wheat.  Contrary to the initial hypothesis, comparisons of poorly-crystalline and total free Fe and Mn in mutant- and wild-type-inoculated rhizospheres suggest that mutant-inoculated roots take up more Fe than wild-type-inoculated roots.  Additional soil and plant analyses to provide a mechanistic explanation for this unexpected result are underway.

(1) Mavrodi et al.(2012) Appl Environ Microb 78, 804-812.

(2) Hernandez et al.(2004) Appl Environ Microb 70, 921-928.

(3) Wang et al. (2011) J Bacteriol 192, 365-369.