Integration of Transcription Factors, Photosynthesis, and Nitrogen Metabolic Genes Modulates Nitrogen Stress with Abscisic Acid in Rapeseed

Nitrogen (N) is essential for plant growth, but excessive fertilizer use decreases nitrogen use efficiency (NUE) and raises environmental concerns. This study investigated the effect of exogenous abscisic acid (ABA; 50 μM) application on rapeseed (Brassica napus L.) plants under hydroponic conditions with high (7.5 mM NO₃ ^-) and low (0.25 mM NO₃ ^-) nitrate. N deficiency significantly reduced growth, chlorophyll, gas exchange, and stomatal size (length and width). Exogenous ABA application under low N conditions increased the N content (23.1%) and the activities of nitrate reductase (NR; 15.7%), glutamine synthetase (GS; 30.3%), and GS/glutamate synthase (GOGAT; 19.3%). It also enhanced activities of antioxidant enzymes: superoxide dismutase (SOD; 18.3%), peroxidase (POD; 30.2%), ascorbate peroxidase (APX; 39%), catalase (CAT; 16.3%), and endogenous hormones: ABA (35.3%), salicylic acid (SA; 20.5%), indole acetic acid (IAA; 49.8%), and jasmonic acid (JA; 90.3%) compared to untreated low N conditions. Histochemical staining (NBT and DAB) confirmed that ABA alleviated oxidative stress under N deficiency. Transcriptomic analysis identified differentially expressed genes related to photosynthesis, antioxidant defense, N metabolism, and ABA signaling (PYL/PYR/RCAR receptors, PP2C phosphatases, SnRK2 kinases). Transcription factors from bZIP, MYB, and AP2/ERF families were significantly regulated. These results highlight the mechanisms of ABA-mediated N stress mitigation and provide a basis for improving NUE in rapeseed through genetic approaches such as overexpression CRISPR/Cas9, supporting sustainable agriculture.