Inoculation with Efficient Nitrogen Fixing and Indoleacetic Acid Producing Bacterial Microsymbiont Enhance Tolerance of the Model Legume Medicago truncatula to Iron Deficiency

The aim of this study was to assess the effect of symbiotic bacteria inoculation on the response of Medicago truncatula genotypes to iron deficiency. The present work was conducted on three Medicago truncatula genotypes: A17, TN8.20, and TN1.11. Three treatments were performed: control (C), direct Fe deficiency (DD), and induced Fe deficiency by bicarbonate (ID). Plants were nitrogen-fertilized (T) or inoculated with two bacterial strains: Sinorhizobium meliloti TII7 and Sinorhizobium medicae SII4. Biometric, physiological, and biochemical parameters were analyzed. Iron deficiency had a significant lowering effect on plant biomass and chlorophyll content in all Medicago truncatula genotypes. TN1.11 showed the highest lipid peroxidation and leakage of electrolyte under iron deficiency conditions, which suggest that TN1.11 was more affected than A17 and TN8.20 by Fe starvation. Iron deficiency affected symbiotic performance indices of all Medicago truncatula genotypes inoculated with both Sinorhizobium strains, mainly nodules number and biomass as well as nitrogen-fixing capacity. Nevertheless, inoculation with Sinorhizobium strains mitigates the negative effect of Fe deficiency on plant growth and oxidative stress compared to nitrogen-fertilized plants. The highest auxin producing strain, TII7, preserves relatively high growth and root biomass and length when inoculated to TN8.20 and A17. On the other hand, both TII7 and SII4 strains improve the performance of sensitive genotype TN1.11 through reduction of the negative effect of iron deficiency on chlorophyll and plant Fe content. The bacterial inoculation improved Fe-deficient plant response to oxidative stress via the induction of the activities of antioxidant enzymes.