OsCIPK18 mediates nitrate-regulated rice root growth through the nitrogen metabolism pathway: Insights from quantitative proteomic and phosphoproteomic analyses

Nitrate acts as both a nutrient and a signaling molecule to regulate root growth, and this process is closely associated with protein abundance and protein phosphorylation within the nitrogen metabolism pathway. However, the relationship between nitrate-regulated root growth, protein expression and protein phosphorylation remain incompletely understood. Here, we investigated the function and underlying molecular mechanisms of the calcineurin B-like (CBL)-interacting protein kinase OsCIPK18 in nitrate-modulated rice root growth using phenotypic analyses together with quantitative proteomic and phosphoproteomic profiling of wild-type (WT) plants and cipk18 mutants. Knockdown of OsCIPK18 significantly inhibited rice root growth compared with WT plants. In contrast, 2 mM nitrate significantly promoted root growth in the cipk18 mutant, increasing lateral root length by 65% and radicle length by 24%, whereas these effects were not observed in WT plants. Consistently, knockdown of OsCIPK18 altered the accumulation of nitrogen-related proteins (including GS1;2, OsGS2, OsNADH-GOGAT2 and OsbetaCA2) and the phosphorylation status of the high-affinity nitrate transporter OsNRT2.2 in response to nitrate. Together, these findings reveal a central regulatory role of OsCIPK18 in nitrogen signaling and root development and provide a potential molecular target and theoretical basis for breeding rice varieties with improved nitrogen use efficiency.