Endoplasmic reticulum activation via tunicamycin seed priming enhances salt acclimation in rice seedlings

Seed priming is a promising approach to increasing salt acclimation, but the role of endoplasmic reticulum (ER) remains unclear. This study investigated if ER activation by tunicamycin (TM) as a seed priming agent promotes salt acclimation in rice (Oryza sativa L.) seedlings. The results showed that salinity (150 mM NaCl) decreased the seedling growth. However, priming seeds with TM increased dry mass, length, photosynthetic pigments and K⁺ contents, osmotic potential, and antioxidant enzyme activities under salinity. Conversely, it decreased intracellular Na⁺ accumulation, electrolyte leakage, lipid peroxidation, and ROS levels. Additionally, TM priming enhanced the expression of ER response gene markers OsIRE1, OsbZIP50, and OsbZIP60 in seedlings under salinity. Metabolomic profiling revealed that TM priming and salinity positively modulated salt-defensive metabolites in shoots, including the osmoprotectants β-alanine and maltose. In roots, it led to a higher accumulation of phosphoric acid, the amino acids O-acetylserine and N-acetylserine (involved in Fe and S metabolism), and the sugars maltose, raffinose, and sorbitol, which also function as osmoprotectants and energy sources. In conclusion, TM seed priming followed by salt stress activated ER unfolded protein response (UPR). This may enhance antioxidant enzyme activity, reducing ROS levels and intracellular Na⁺ content, thereby mitigating salt stress through the positive modulation of defense-related and energy-related metabolites. These findings suggest an efficient strategy to improve salt acclimation during the early growth stages of rice seedlings.