Macrophage migration inhibitory factor MtMIF3 prevents the premature aging of Medicago truncatula nodules

Macrophage migration inhibitory factor (MIF) is a proinflammatory cytokine involved in immune response in animals. However, the role of MIFs in plants such as Medicago truncatula, particularly in symbiotic nitrogen fixation, remains unclear. An investigation of M. truncatula-Sinorhizobium meliloti symbiosis revealed that MtMIF3 was mainly expressed in the nitrogen-fixing zone of the nodules. Silencing MtMIF3 using RNA interference (Ri) technology resulted in increased nodule numbers but higher levels of bacteroid degradation in the infected cells of the nitrogen-fixing zone, suggesting that premature aging was induced in MtMIF3-Ri nodules. In agreement with this conclusion, the activities of nitrogenase, superoxide dismutase and catalase were lower than those in controls, but cysteine proteinase activity was increased in nodulated roots at 28 days postinoculation. In contrast, the overexpression of MtMIF3 inhibited nodule senescence. MtMIF3 is localized in the plasma membrane, nucleus, and cytoplasm, where it interacts with methionine sulfoxide reductase B (MsrB), which is also localized in the chloroplasts of tobacco leaf cells. Taken together, these results suggest that MtMIF3 prevents premature nodule aging and protects against oxidation by interacting with MtMsrB.