Nitrogen uptake and carbon–nitrogen synergistic translocation improve yield and nitrogen use efficiency in the dep1 rice line

Improving rice yield and nitrogen use efficiency (NUE) are crucial challenges for coordinating food production and environmental health. However, little is known about the physiological mechanisms underlying the synergistic effects of high yield and NUE in rice. Using two near-isogenic rice lines (named DEP1 and dep1), a two-year field experiment was conducted to assess agronomic characteristics and the physiological characteristics of carbon and nitrogen translocation under three nitrogen levels. Compared with DEP1, dep1 had higher grain yield, grain filling percentage, nitrogen (N) uptake, and NUE. More non-structural carbohydrates (NSCs) and N in the stems were translocated to grains during grain filling in dep1 than in DEP1. Furthermore, stem NSCs translocation was significantly positively correlated with grain yield, while stem N translocation was significantly positively correlated with NUE. Key carbon metabolism enzyme activities (α-amylase, β-amylase and sucrose-phosphate synthase in stems, and sucrose synthase, ADP-glucose pyrophosphorylase and starch synthase in grains) and stem sucrose transporter gene (OsSUT1 and OsSWEET13) expression were higher in dep1 than in DEP1. This contributed to high stem NSCs translocation. Higher N translocation in the stems occurred due to the higher expression of OsNPF2.4. Moreover, the higher values of root morphological traits (root dry weight, root surface area, root length and root volume) and structural characteristics (stele diameter, cortical thickness and vessel section area) in dep1 explained its high nitrogen uptake. In addition, higher expression of OsNADH-GOGAT1 and OsGS1.3 promoted the assimilation of ammonium and contributed to higher nitrogen uptake in dep1. The application of N reduced carbon translocation but enhanced N translocation by regulating the corresponding metabolic enzyme activities and gene expression. Overall, these findings highlighted the roles of nitrogen uptake, and carbon and nitrogen translocation from stems as crucial characteristics for synergistically improving yield and NUE in the dep1 rice line.