Uclacyanin <scp>MtUC1</scp> Is Involved in the Regulation of Nodule Senescence in <i>Medicago truncatula</i>

Phytocyanins (PCs) are ancient plant-specific blue copper proteins that play an important role in plant growth and development, and stress tolerance. In this study, the role of MtUC1, a member of the uclacyanin subfamily of the PC family, was analysed in the nodule symbiosis of Medicago truncatula. MtUC1 was mainly expressed in the nodule interzone and strongly induced in the later nodule developmental stage. RNA interference (RNAi) and mutation of MtUC1 led to reduced root nodule formation and degeneration of bacteroids within nodules. Cysteine protease activity in the MtUC1-RNAi inoculated roots and uc1 mutant nodules was significantly increased, the leghaemoglobin content and the expression of nitrogen-fixing enzyme genes in the uc1 mutant nodules were significantly reduced, and the nodule cells showed signs of senescence, suggesting that MtUC1 expression is required to avert nodule senescence. Transcriptomic analysis indicated that many symbiotic genes were significantly downregulated, and the senescence/defence-related genes were significantly upregulated in roots 7 days post-inoculation (dpi) and in the nodules of the uc1 mutant at 28 dpi. Yeast two-hybrid and bimolecular fluorescence complementation experiments showed that MtUC1 interacted with MtBI-1 (Bax-Inhibitor 1). Both MtUC1 and MtBI-1 were localised and co-localised to the endoplasmic reticulum and plasma membrane. In addition, MtBI-1 also showed a significantly high expression level in the mature nodules. In summary, MtUC1 may prevent the premature aging of root nodules by interacting with MtBI-1.