Abstract
Legumes have the ability to form root nodules that fix atmospheric nitrogen through a symbiotic interaction with nitrogen-fixing bacteria. As a first step in dissecting the molecular process of nodulation, proteome reference maps of soybean roots and nodules were constructed. Time course analysis revealed that the transition from root to nodule was accompanied with downregulation of defense-response related proteins, including Mn-superoxide dismutase, peroxidase (Prx), PR10, and stress-induced protein, leading to the initiation of a symbiotic interaction between the two partners. Following nitrogenase biosynthesis, the host plant cooperated with the rhizobia to fix atmospheric nitrogen under microaerobic conditions via expression of leghemoglobins and antioxidant proteins. Comparative proteome analysis indicated lower expression of malate dehydrogenase (MDH), leghemoglobins and nitrogenase in the nodule development of the supernodulation mutant, SS2-2, as compared to the wild type, indicating that SS2-2 forms functionally immature nodules in higher numbers with the lower activity of nitrogen fixation.
