Microbial Iron Reduction: A Secondary Source of Fe2+ in the Southern Highbush Blueberry Rhizosphere

Iron reduction from Fe³⁺ to Fe²⁺ by plasma membrane ferric chelate reductase (FCR) is the rate-limiting step in the iron uptake mechanism of the southern highbush blueberry (SHB, Vaccinium corymbosum hybrids) and other strategy I plants. Although FCR activity is typically measured from root tissue, root and rhizosphere-associated microorganisms are also able to reduce iron. We hypothesized that microbial iron reduction could increase Fe²⁺ availability in the SHB rhizosphere. The ex situ FCR activity of fungi and oomycota isolated from roots of hydroponically grown SHB was measured, and the taxonomic identity of the isolates was determined by sequencing of ITS genomic regions. Three of the 47 isolates were then selected based on their ex situ FCR activities for an in vitro compatibility study based on their relative growth rate. Basidiomycete Phanerochaete sordida, ascomycete Penicillium polonicum, and oomycete Pythium irregulare were successfully co-cultured to create a microbial consortium which was used to inoculate roots of sterile, micropropagated SHB cv. Snowchaser plants. The effect of inoculation on the iron nutrition of SHB under iron limiting (–Fe) and iron sufficient (+Fe) conditions was tested in two separate experiments carried out in an in vitro growth system. Inoculation with the microbial consortium did not affect root FCR activity or localization, regardless of iron concentration. Iron uptake by inoculated plants was lower than uptake by non-inoculated controls under +Fe conditions, but not under –Fe conditions. Under +Fe conditions, inoculation with the consortium led to increased Fe²⁺ concentration in the medium. The microbial consortium increased Fe²⁺ concentration in the rhizosphere without directly affecting plant FCR activity, suggesting that microbial iron reduction could be a neglected source of Fe²⁺ in the blueberry rhizosphere.