Lipid remodeling and phosphatidic acid signaling maintain the vigor of recalcitrant Panax notoginseng seeds under low temperature

BACKGROUND: Low-temperature exposure during the early stage of seed imbibition markedly constrains seedling emergence in direct-seeded crops. Recalcitrant seeds are commonly accepted to be highly sensitive to low temperatures. Although Panax notoginseng seeds are characterized by recalcitrance, physiological and molecular responses to low temperature remain poorly defined. RESULTS: Herein, P. notoginseng seeds from three-year-old plants were treated at 0, 4, 10, and 25 °C for 24 or 48 h and analyzed by the method of integrated physiological biochemistry, transcriptomics, and lipidomics. Relative to other temperatures at 48 h, the 0 °C treatment significantly delayed germination, whereas neither 0-4 °C reduced the final germination rate. At 4 °C, reactive oxygen species (ROS) levels exceeded those at 0 °C at 24 h and 48 h, accompanied by increased catalase (CAT) activity. Compared with the 25 °C treatment at 24 h and 48 h, both the 0 °C and 4 °C treatment suppressed phospholipase D (PLD) activity, with a significant inhibition at 0 °C treatment. PLD is a membrane-associated phospholipase that contributes to stress-induced phospholipid hydrolysis. Suppressed PLD activity at 0 °C and 4 °C suggests tighter restriction of membrane lipid catabolism, consistent with enhanced membrane integrity under low temperature. Transcriptome profiling showed that relative to the 25 °C treatment for 48 h, both the 0 °C and 4 °C treatment consistently enriched pathways for unsaturated fatty-acid biosynthesis and elongation, mitogen-activated protein kinase (MAPK) signaling, abscisic acid (ABA) pathway, and phosphatidylinositol signaling. Lipidomics further revealed temperature-specific remodeling: lysophosphatidylcholine, LPC (16:0) accumulated specifically at the 0 °C treatment, whereas phosphatidylcholine, PC (34:1), phosphatidic acid, PA (34:1), and the glycerolipids diacylglycerol (DG) and triacylglycerol (TG) accumulated at 4 °C treatment. Integrative analysis revealed that both PA and LPC are associated with transport and signaling genes. This study also identified the ABA core module centered on pyrabactin resistance 1-like proteins (PYLs) and protein phosphatase 2Cs (PP2Cs) whose module eigengene was negatively correlated with electrolyte conductivity and negatively correlated with phospholipase D activity. This inverse relationship suggests that ABA signaling helps maintain membrane integrity. It may reduce electrolyte leakage and limit PLD-driven lipid hydrolysis. CONCLUSIONS: In conclusion, recalcitrant P. notoginseng seeds appear insensitive to low temperature at 0 °C and 4 °C. Changes in the membrane-lipid microenvironment initiate an integrated signaling mechanism that transduces signals via the core ABA pathway. This integrated mechanism ultimately converges on NAM, ATAF1/2, and CUC2 (NAC) transcription factors and dehydration-responsive element-binding protein (DREB) transcription factors to coordinate stress responses.