Exogenous abscisic acid application enhances nitrogen use efficiency and root development in rapeseed: Transcriptomic and morphological evidence.

Nitrogen (N) is an essential macronutrient governing plant growth and development. However, its excessive application in agricultural systems has precipitated environmental degradation while simultaneously reducing nitrogen use efficiency in crops. This study examined the effects of abscisic acid (ABA; 50 μM) under both high nitrogen (7.5 mM NO) and low nitrogen (0.25 mM NO) conditions on the roots system of rapeseed (Brassica napus) using a hydroponic system. Exogenous ABA application significantly enhanced root morphological parameters (root fresh weight by 16 %, total root length by 7 %, root surface area by 5 %, root dry weight by 16 %, and root volume by 24 %), substantially increased nitrogen concentration (30 %), and upregulated the activities of key nitrogen assimilation enzymes including nitrate reductase (18 %), nitrite reductase (17.8 %), glutamine synthetase (49 %), and glutamate synthase (10 %). Additionally, ABA application enhanced the activities of antioxidant enzymes including peroxidase (30 %), catalase (11 %), ascorbate peroxidase (29 %), and superoxide dismutase (25 %), while simultaneously increasing endogenous phytohormone concentrations of abscisic acid (30.4 %), indole-3-acetic acid (64.9 %), salicylic acid (72.9 %), and jasmonic acid (90.3 %) under low nitrogen (LN) stress conditions. Additionally, transcriptomic analysis explored the differentially expressed genes (DEG) significantly associated with antioxidant enzymes, nitrogen metabolism, transcription factors from the bZIP, AP2/ERF, and MYB families, as well as endogenous hormones including ABA signaling components (PYL/PYR/RCAR, PP2C, SnRK2). Overall, our findings establish a mechanistic foundation for enhancing nitrogen use efficiency (NUE) in Brassica napus through targeted molecular approaches such as CRISPR-Cas9 gene editing or transgenic overexpression, thereby contributing to the advancement of sustainable agricultural practices.