Serine palmitoyltransferase (SPT),a pyridoxyl-5-phosphate-dependent enzyme,catalyzes the first and rate-limiting step in sphingolipid biosynthesis.In humans and yeast,the orosomucoid proteins (ORMs) negatively regulate SPT and thus play an important role in maintaining the levels of sphingolipids.Despite the importance of sphingoid intermediates as bioactive molecules,the regulation of sphingolipid biosynthesis through SPT is not well understood in plants.In the present study we identified and characterized the Arabidopsis thaliana ORMs,AtORM1 and AtORM2.Loss-of-function of both AtORM1 and AtORM2 (orm1 amiR-ORM2) stimulated de novo sphingolipid biosynthesis,leading to strong sphingolipid accumulation,especially of tong-chain bases and ceramides.Yeast two-hybrid,bimolecular fluorescence complementation,and coimmunoprecipitation assays confirmed that ORM1 and ORM2 physically interact with the small subunit of SPT (ssSPT),indicating that ORMs inhibit ssSPT function.We found that orm1 amiR-ORM2 plants exhibited an early-senescence phenotype accompanied by H2O2 production at the cell wall and in mitochondria,active vesicular trafficking,and formation of cell wall appositions.Strikingly,the orm1 amiR-ORM2 plants not only showed increased the expression of genes related to ER stress and defenses,but also enhanced resistance to oxidative stress and pathogen infection.Our results indicate that AtORMs function to regulate sphingolipid biosynthesis in maintaining sphingolipid homeostasis and play a role in response to abiotic and biotic stresses.