Nitrogenase activity is required for the activation of iron‐stress response in iron‐inefficient T203 soybean

Abstract

Iron (Fe)‐stress response and symbiotic nitrogen (N₂) fixation are chemical reduction processes within the roots which interact to the extent that nodulated, Fe‐stressed, Fe‐inefficient soybean initiates Fe‐stress response, but similar nonnodulated soybean does not. The objective of this study was to examine the relationship between nitrogenase activity and Fe‐stress response in the Fe‐inefficient T203 soybean by inoculating plants with effective or ineffective strains of Bradyrhizobium japonicum on either side of split‐root systems. Half the roots of each plant were either inoculated with effective or ineffective strains or left uninoculated. Nodules formed on roots inoculated with the ineffective B. japonicum strain, SM5, were similar in number and shape to those on roots inoculated with the effective strain, 61A209, but roots inoculated with SM5 were free of nitrogenase activity. Hydrogen ion and reductant were not released from uninoculated roots or roots inoculated with SM5, but were released from active, effective nodules and the roots below the active nodule clusters of roots inoculated with effective B. japonicum. Plants with effective and ineffective strains inoculated on either half of the roots exhibited Fe‐stress response only on the half with nitrogenase activity. Plants inoculated with both effective and ineffective strains, but provided nitrate to inhibit nitrogenase activity, lacked Fe‐stress response and became severely Fe chlorotic. Iron reduction and hydrogen (H⁺) release around nodules and roots of inoculated Fe‐inefficient T203 soybean is keyed to nitrogenase activity.