Temporal Transcriptomic and Metabolomic Reprogramming Unveils a Two-Phase Salt Tolerance Mechanism in Apocynum venetum

Soil salinization poses a major constraint to global agriculture. Apocynum venetum, a salt-tolerant halophyte, provides an effective model for investigating salt-adaptive strategies; however, the temporal dynamics of its tolerance-associated genes and metabolites remain unclear. In this study, integrated transcriptomics, metabolomics (UHPLC-MS), physiological assays, and weighted gene co-expression network analysis (WGCNA) were conducted to characterize early (7-day) and late (18-day) responses to 200 mM NaCl stress. NaCl stress significantly reduced chlorophyll content while increasing Na+ accumulation, MDA levels, antioxidant enzyme activities (SOD and CAT), and total flavonoid content. Early responses (NaCl7) were marked by accumulation of ferulic acid, rhamnetin, and 3,4-dihydrocoumarin, with activation of plant hormone (ABA, auxin, zeatin) and MAPK signaling pathways. Late responses (NaCl18) exhibited increased accumulation of scopoletin, formononetin, and caffeyl-alcohol, with enrichment of phenylpropanoid biosynthesis, glutathione metabolism, and photosynthesis-related pathways. WGCNA identified early-response hub genes, including AOC, MAPKKK17/18, CYP98A, and CCoAOMT, coordinating stress signaling and antioxidant metabolism. Late stress responses involved genes like CPK, GST, CYCD3, and ARF, modulating calcium signaling and ROS detoxification. Genes shared across phases included CYP90C1, HD-ZIP, HSP20, and PP2C, regulating protein stabilization and stress signaling. These findings reveal a two-phase salt tolerance strategy in A. venetum, integrating early signaling and late metabolic adaptation.