Fe-Based Nanomaterial-Induced Root Nodulation Is Modulated by Flavonoids to Improve Soybean ( Glycine max ) Growth and Quality.

Innovative technology to increase efficient nitrogen (N) use while avoiding environmental damages is needed because of the increasing food demand of the rapidly growing global population. Soybean ( Glycine max ) has evolved a complex symbiosis with N-fixing bacteria that forms nodules to fix N. Herein, foliar application of 10 mg L -1 Fe 7 (PO 4 ) 6 and Fe 3 O 4 nanomaterials (NMs) (Fe-based NMs) promoted soybean growth and root nodulation, thus improving the yield and quality over that of the unexposed control, EDTA-control, and 1 and 5 mg L -1 NMs. Mechanistically, flavonoids, key signaling molecules at the initial signaling steps in nodulation, were increased by more than 20% upon exposure to 10 mg L -1 Fe-based NMs, due to enhanced key enzyme (phenylalanine ammonia-lyase, PAL) activity and up-regulation of flavonoid biosynthetic genes ( GmPAL , GmC4H , Gm4CL , and GmCHS ). Accumulated flavonoids were secreted to the rhizosphere, recruiting rhizobia for colonization. Fe 7 (PO 4 ) 6 NMs increased Allorhizobium by 87.3%, and Fe 3 O 4 NMs increased Allorhizobium and Mesorhizobium by 142.2% and 34.9%, leading to increased root nodules by 50.0% and 35.4% over the unexposed control, respectively. Leghemoglobin content was also noticeably improved by 8.2-46.5% upon Fe-based NMs. The higher levels of nodule number and leghemoglobin content resulted in enhanced N content by 15.5-181.2% during the whole growth period. Finally, the yield (pod number and grain biomass) and quality (flavonoids, soluble protein, and elemental nutrients) were significantly increased more than 14% by Fe-based NMs. Our study provides an effective nanoenabled strategy for inducing root nodules to increase N use efficiency, and then both yield and quality of soybean.