A systematic review of plant secondary metabolites and associated genes underlying salinity tolerance in mangroves

Mangroves survive in the tidal zones where salinity, periodic inundation, flooding, UV radiation, and heat cripple most plants, yet the biochemical basis of this resilience remains incompletely understood. Plant secondary metabolites (PSMs) are strongly implicated in this tolerance, but the evidence is fragmented across species, assays, and salinity protocols. To systematically evaluate the role of PSMs in mangrove salinity resilience, identify the key biosynthetic genes and pathways involved, and highlight candidate markers relevant to engineering salt-tolerant crops. Original experimental studies on true mangrove species subjected to defined salinity challenges and reporting quantified secondary metabolites were included; reviews, commentaries, and studies on mangrove associates, halophytes, or non-salinity stresses were excluded. A PRISMA-informed systematic review of Scopus, Web of Science, and PubMed (2005–2025) was conducted, retrieving four experimental studies that met the strict inclusion criteria. Across Kandelia candel (L.) Druce, Bruguiera gymnorhiza (L.) Lam., Avicennia marina (Forssk.) Vierh., and Laguncularia racemosa (L.) C.F. Gaertn., phenolics and flavonoids were the most frequently assayed classes, followed by lignin, triterpenoids, and compatible sugars. Salinity ≥ 500 mM NaCl increased total phenolics by 17% and proanthocyanidins by > 75% in K. candel, while a natural salinity gradient (0.5–30 PPT) produced zone-specific two-fold differences in phenolic content in A. officinalis. Root lignin deposition rose from 40 µg mg⁻¹ (freshwater) t 145 µg mg⁻¹ at 20 PPT, and drught stress triggered hexose accumulation with concurrent sucrose depletion, indicating osmotic adjustment. Transcriptomic evidence implicates a core gene set - including KcMS, BgLUS, BgbAS, KcCAS, lupeol/β-amyrin synthases, and flavonoid-biosynthesis enzymes - in coordinating salinity-induced metabolite shifts. The small evidence base (n = 4), a taxonomic bias toward K. candel, and methodological heterogeneity in salinity units and metabolite assays precluded quantitative meta-analysis and limited the generalizability of these findings. Collectively, the available evidence demonstrates the species-dependent and environment/concentration-dependent changes in PSMs under salinity stress, but does not yet support metabolite responses across all mangroves universally. Bridging these review outcomes with previous molecular investigations identifies candidate genes and metabolic markers for engineering or selecting salt-tolerant crops suited to increasingly saline agroecosystems. To our knowledge, this is the first PRISMA-guided systematic review of PSMs in mangroves under salinity stress. • The mangrove metabolites pool protects them from adverse environmental effects. • Antioxidants (Flavonoids, Phenolics, and Phytosterols) are the dominant mangrove metabolites • Multiple pathways are engaged in the mangrove resilience • Mangrove metabolites can be introduced into engineering the salt-tolerant crops • Mangrove metabolites can be the research topic for ecological studies related to salinity intrusion, global climate change, seawater evaporation, and sea level rise