Nuclear Accumulation of OsMADS27 Was Promoted by OsNAR2.1 in Response to Nitrate to Control Root Growth by Directly Targeting OsMADS57 in Rice.

Nitrate serves not only as an essential nitrogen source but also as a signalling molecule to regulate plant growth and development. However, the mechanisms by which plants respond to nitrate to regulate root growth in rice remain largely unclear. In this study, we demonstrated that the overexpression of OsMADS27 significantly promoted root growth and nitrate absorption in response to nitrate supply, whereas RNA interference (RNAi) lines exhibited the opposite phenotype. OsMADS27 is predominantly expressed in the root, and the mutation of OsNAR2.1 impaired both the response rate and intensity of OsMADS27 under nitrate treatment. Notably, OsMADS27 is co-localised with OsNAR2.1, and the mutation of OsNAR2.1 affected the nuclear localisation of OsMADS27. Yeast two-hybrid (Y2H), bimolecular fluorescence complementation (BiFC) and co-immunoprecipitation (Co-IP) assays confirmed a direct protein interaction between OsMADS27 and OsNAR2.1. More importantly, chromatin immunoprecipitation followed by qPCR (ChIP-qPCR), dual-luciferase (Dual-LUC) and yeast one-hybrid (Y1H) assays demonstrated that OsMADS27 could directly regulate the expression of OsMADS57 to control root growth. Furthermore, OsMADS27 could also promote the expression of several nitrate transporter genes, including OsNRT1.2, OsNRT2.1, OsNRT2.3a and OsNRT2.4, to regulate nitrate absorption in rice. Collectively, this study uncovers a novel pathway in which OsMADS27 promotes root growth in response to nitrate by directly regulating OsMADS57 expression in an OsNAR2.1-dependent manner. These findings will broaden our understanding and provide a novel regulatory network of OsMADS genes mediating root development through nitrate signalling in rice, which also has an important application value for molecular breeding to improve nitrogen use efficiency.