PHENAZINE-MEDIATED FE AND MN MOBILIZATION AND BIOFILM FORMATION IN THE RHIZOSPHERE OF DRYLAND WHEAT

Melissa Kay LeTourneau

Phenazine-1-carboxylic acid (PCA) is a redox-active antibiotic produced by root-associated fluorescent pseudomonads in dryland wheat fields throughout the low-precipitation zone of the Columbia Plateau in Oregon and Washington state. In culture systems, PCA promotes development of robust bacterial biofilms by transferring respiratory electrons to iron and manganese hydroxide minerals in lieu of oxygen, which is often depleted within dense biofilm matrices. PCA-mediated biofilm formation and redox transformations of Fe and Mn could dramatically influence soil health and crop nutrition because rhizobacterial biofilms contribute to soil organic matter (SOM) pools, and because redox cycling of soil Fe and Mn minerals alters the bioavailability and recalcitrance of mineral-associated nutrients and SOM. However, the impact of PCA upon rhizobacterial biofilm development and bioavailability of soil nutrients has not been reported, and the factors controlling production of PCA in dryland agro-ecosystems with low annual precipitation remain unknown. In this study, the impact of soil moisture upon expression of PCA biosynthesis genes and PCA degradation kinetics, and the combined impact of PCA-producing (PCA+) Pseudomonas rhizobacteria and soil moisture regimes upon the density and morphology of extracellular polymeric substances (EPS), turnover of rhizobacterial nitrogen, and dynamics of Fe and Mn were investigated in wheat rhizospheres. Population-level expression of PCA biosynthesis genes and persistence of PCA were decreased under dryland relative to irrigated conditions, indicating that these factors do not entirely account for accumulation of PCA in the Columbia Plateau. The densities of EPS were enhanced in PCA+-inoculated rhizospheres maintained under dryland conditions relative to rhizospheres inoculated with bacteria unable to produce PCA (PCA-) or maintained under irrigated conditions, indicating that PCA enhanced development of rhizobacterial biofilms under dryland conditions, as observed in culture systems. Turnover of rhizobacterial N was heterogeneous in dryland PCA+ rhizospheres relative to irrigated and PCA- rhizospheres, indicating that PCA influences the dynamics of organic nutrients derived from microbial biomass under dryland conditions. PCA+ and irrigated rhizospheres were enriched with extractable Fe relative to corresponding PCA- and dryland rhizospheres. PCA is therefore likely to impact soil health and crop nutrition in the Columbia Plateau by impacting the retention and bioavailability of soil nutrients.

PHENAZINE-MEDIATED FE AND MN MOBILIZATION AND BIOFILM FORMATION IN THE RHIZOSPHERE OF DRYLAND WHEAT

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