Rice transcription factors OsLBD37/38/39 regulate nitrate uptake by repressing OsNRT2.1/2.2/2.3 under high-nitrogen conditions

Nitrate (NO₃⁻) uptake involves a finely regulated and complex multilevel response system. Elucidating the molecular mechanism of nitrate uptake may lead to improving the growth and productivity of plants in the presence of dynamic variation in nitrate concentration. In this study, we identified three lateral organ boundaries domain (LBD) transcription factors, OsLBD37, OsLBD38, and OsLBD39, as regulators of nitrate uptake in response to nitrogen (N) availability. OsLBD37, OsLBD38, and OsLBD39 were induced by ammonium and glutamine in rice roots. Individual or collective knockout of OsLBD37, OsLBD38, and OsLBD39 led to increased concentrations of nitrate and increased expression of OsNRT2.1, OsNRT2.2, and OsNRT2.3 respectively under high-N conditions, whereas overexpression of each of these three LBD genes produced opposite effects where N accumulation was reduced. Dual-luciferase reporter assay further confirmed that OsLBD37, OsLBD38, and OsLBD39 possessed transcription inhibitory activities in rice protoplast cells, downregulating the expression of OsNRT2.1/OsNRT2.2/OsNRT2.3. Yeast two-hybrid and bimolecular fluorescence complementation assays showed that OsLBD37 interacted with OsLBD37, OsLBD38, and OsLBD39 in the nucleus. Together, these results show that OsLBD37, OsLBD38, and OsLBD39 collaborate to inhibit the expression of OsNRT2.1/OsNRT2.2/OsNRT2.3 transporters under N-sufficient conditions, thereby helping rice plants avoid excessive nitrate accumulation that may affect their growth.