Impaired antioxidant defense associated with flavonoid biosynthesis reprogramming in the mangrove Acanthus ilicifolius under high salinity.

INTRODUCTION: This study investigates how varying salinity levels influence the physiological and metabolic responses of the mangrove plant Acanthus ilicifolius, aiming to clarify its salt adaptation mechanisms. METHODS: Field experiments were conducted at three sites with stable salinity conditions (approximately 0.73, 2.26, and 3.09 g/L), combined with physiological assays, metabolomics, and transcriptomics. RESULTS: Increasing salinity elevated intracellular reactive oxygen species, reduced antioxidant enzyme activity, and aggravated membrane lipid peroxidation. Metabolomics revealed that medium salinity primarily affected terpenoid (18.7%) and lipid (14.5%) metabolism, while high salinity regulated flavonoid (11%) and terpenoid (16.4%) biosynthesis. Transcriptomic data indicated that the antioxidant defense system plays a central role in mitigating oxidative stress, with WRKY and AP2/ERF transcription factors significantly upregulated under high salinity, enhancing kaempferol-flavanone isomerase activity and flavonoid accumulation (e.g., hesperidin). DISCUSSION: These findings show that A. ilicifolius compensates for impaired antioxidant defenses by redirecting metabolism toward flavonoid synthesis, providing novel insights into the molecular basis of mangrove salt tolerance and its potential medicinal applications.