Root-level inoculation with iron-reducing microorganisms does not enhance iron uptake by southern highbush blueberry plants

The rate-limiting step in the iron uptake mechanism of strategy-I plants is ferric chelate reductase (FCR)-mediated iron reduction. Southern highbush blueberry (SHB, Vaccinium corymbosum L. interspecific hybrids) is a specialty crop that commonly exhibits iron deficiency in agricultural soils due to its low FCR activity. Extracellular iron reduction has been reported in some fungi and peronosporomycetes (subsequently referred to as oomycetes), and this has the potential to increase iron reduction capacity in the rhizosphere and impact plant iron uptake. Here, we isolated and identified fungi and oomycetes from the roots and nutrient solution of hydroponically grown SHB. Then, we built a consortium of iron-reducing microorganisms and used this to study the iron nutrition of inoculated and non-inoculated SHB plants in vitro and ex vitro. We found isolates of nine fungal species and the oomycete Pythium irregulare complex, all of which exhibited detectable levels of extracellular iron reduction. Isolates of Alternaria alternata, Bjerkandera adusta, and Pythium irregulare exhibited extracellular iron reduction that was equal to or higher than that of SHB roots, and they were used to create a microbial consortium for the inoculation experiments. In vitro, inoculation with a consortium of iron-reducing microorganisms did not affect iron reduction capacity in the SHB rhizosphere under iron deficiency conditions, but it increased iron reduction capacity in the SHB rhizosphere under iron abundance conditions. Nevertheless, inoculation treatments generally decreased SHB iron uptake from the medium. Ex vitro, inoculation with a consortium of iron-reducing microorganisms did not affect iron reduction capacity in the SHB rhizosphere. In all experiments, the inoculation treatment did not increase leaf active iron concentration. Altogether, our results suggest that inoculation with iron-reducing microorganisms is not a viable alternative for enhancing SHB iron nutrition.