Abscisic acid alleviates ammonium toxicity by enhancing NH4+ assimilation through synergistic carbon and nitrogen metabolism

Rapeseed（ Brassica napus L.）is highly sensitive to ammonium (NH 4 + )，and ABA is widely involved in plant stress responses, but its physiological regulatory role in rapeseed under ammonium stress remains unclear. Our study shows that ammonium stress boosts ABA synthesis in rapeseed, which enhances ammonium tolerance in a concentration-dependent manner. Under ammonium stress, inhibiting ABA synthesis downregulates carbon and nitrogen metabolism genes, exacerbating ammonium toxicity symptoms, while ABA addition reverses this effect. Structural equation modeling revealed that ABA primarily promotes ammonium assimilation by upregulating BnaGSs expression and enhancing its enzyme activity, while its effect on carbon metabolism likely serves a synergistic role by providing the carbon skeleton for ammonium assimilation. Further, the response of BnaGSs gene expression to ABA and the phenotype of gs1.1 s mutants confirmed the above results. Additionally, ABA can alleviate ammonium induced oxidative damage, ultimately significantly restoring leaf photosynthetic performance. Collectively, our results suggest that the synthesis of ABA in response to ammonium stress alleviates ammonium toxicity in rapeseed by enhancing NH 4 + assimilation through synergistic carbon and nitrogen metabolism. • ABA alleviates ammonium toxicity, while the effect ABA biosynthesis inhibitor is the opposite. • The regulation of GS genes by ABA mitigated NH 4 + accumulation in rapeseed. • ABA mediates sugar metabolism to promote NH 4 + assimilation. • The ROS damage caused by ammonium stress is relieved by ABA.