Plant–Microbe Partnerships in the Southern Highbush Blueberry Rhizosphere: The Case of Iron Nutrition

Iron uptake in Strategy I plants is the product of the coordinated action of three processes in the plasma membrane (PM) of root epidermal cells: rhizosphere acidification, iron reduction, and iron transmembrane transport. Iron reduction by a PM-bound ferric chelate reductase (FCR) has been shown to be the rate-limiting step in this process. However, hydroponically grown Vaccinium spp. have yielded inconsistent FRC activity results. We hypothesized that microbiota in the plants’ rhizosphere might be partners or competitors for iron acquisition and that they may be a source of variability. To investigate the microbial role in root-level iron reduction, we isolated and cultured bacteria, fungi and oomycota from the roots and rhizosphere of hydroponically grown clonal southern highbush blueberry (SHB; Vaccinium corymbosum L. hybrids) cv. Emerald plants. The plants were grown in a complete nutrient solution for five weeks before representative root and nutrient solution samples were used to inoculate nutrient agar, PARP and potato dextrose agar plates. A total of 67 unique colonies were sub-cultured from all media types and their FCR activity was measured. Bacteria exhibited extremely low FCR activity that was significantly greater than zero in only seven of the isolates. However, most fungi and oomycota exhibited significant FCR activity. We used amplicon sequencing of the ITS and COX genomic regions of these cultures to assign them to taxa. Oomycete Pythium irregulare, as well as fungi Penicillium polonicum and Bjerkandera adusta emerged as relevant taxa for iron uptake based on the ex-situ measurements of FCR activity. To measure the in-situ effect of microorganisms on rhizosphere FCR activity, we used micro-cuttings of SHB cv. Emerald rooted in peat under sterile or non-sterile conditions, with the assumption that non-sterile peat contained native microbiota. Plants were then transferred to a complete nutrient solution solidified with phytagel. Under these conditions, microbial colonization of the rhizosphere led to increased iron reduction over sterile controls. These results indicate that plant-microbe interactions might be beneficial to V. corymbosum in terms of iron reduction. Moreover, they suggest that management of the soil microbiota might have a direct impact on the iron nutrition of SHB.