NITROGENASE ACTIVITY IS REQUIRED FOR THE ACTIVATION OF IRON-STRESS RESPONSE IN IRON-INEFFICIENT T203 SOYBEAN

Iron (Fe)-stress response and symbiotic nitrogen (N2) fixation are che mical reduction processes within the roots which interact to the exten t that nodulated, Fe-stressed, Fe-inefficient soybean initiates Fe-str ess response, but similar nonnodulated soybean does not. The objective of this study was to examine the relationship between nitrogenase act ivity and Fe-stress response in the Fe-inefficient T203 soybean by ino culating plants with effective or ineffective strains of Bradyrhizobiu m japonicum on either side of split-root systems. Half the roots of ea ch plant were either inoculated with effective or ineffective strains or left uninoculated. Nodules formed on roots inoculated with the inef fective 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 a nd reductant were not released from uninoculated roots or roots inocul ated with SM5, but were released from active, effective nodules and th e roots below the active nodule clusters of roots inoculated with effe ctive B. japonicum. Plants with effective and ineffective strains inoc ulated on either half of the roots exhibited Fe-stress response only o n the half with nitrogenase activity. Plants inoculated with both effe ctive and ineffective strains, but provided nitrate to inhibit nitroge nase activity, lacked Fe-stress response and became severely Fe chloro tic. Iron reduction and hydrogen (Hf) release around nodules and roots of inoculated Fe-inefficient T203 soybean is keyed to nitrogenase act ivity.