Arabidopsis thaliana plants that lack ceramide kinase, encoded by ACCELERATED CELL DEATH5 (ACD5), display spontaneous programmed cell death late in development and accumulate substrates of ACD5. Here, we compared ceramide accumulation kinetics, defense responses, ultrastructural features, and sites of reactive oxygen species (ROS) production in wild-type and acd5 plants during development and/or Botrytis cinerea infection. Quantitative sphingolipid profiling indicated that ceramide accumulation in acd5 paralleled the appearance of spontaneous cell death, and it was accompanied by autophagy and mitochondrial ROS accumulation. Plants lacking ACD5 differed significantly from the wild type in their responses to B. cinerea, showing earlier and higher increases in ceramides, greater disease, smaller cell wall appositions (papillae), reduced callose deposition and apoplastic ROS, and increased mitochondrial ROS. Together, these data show that ceramide kinase greatly affects sphingolipid metabolism and the site of ROS accumulation during development and infection, which likely explains the developmental and infection-related cell death phenotypes. The acd5 plants also showed an early defect in restricting B. cinerea germination and growth, which occurred prior to the onset of cell death. This early defect in B. cinerea restriction in acd5 points to a role for ceramide phosphate and/or the balance of ceramides in mediating early antifungal responses that are independent of cell death.

SUMMARYCeramidases hydrolyze ceramide into sphingosine and fatty acids. In mammals, ceramidases function as key regulators of sphingolipid homeostasis, but little is known about their roles in plants. Here we characterize the Arabidopsis ceramidase AtACER, a homolog of human alkaline ceramidases. The acer-1 T-DNA insertion mutant has pleiotropic phenotypes, including reduction of leaf size, dwarfing and an irregular wax layer, compared with wild-type plants. Quantitative sphingolipid profiling showed that acer-1 mutants and the artificial microRNA-mediated silenced line amiR-ACER-1 have high ceramide levels and decreased long chain bases. AtACER localizes predominantly to the endoplasmic reticulum, and partially to the Golgi complex. Furthermore, we found that acer-1 mutants and AtACER RNAi lines showed increased sensitivity to salt stress, and lines overexpressing AtACER showed increased tolerance to salt stress. Reduction of AtA-CER also increased plant susceptibility to Pseudomonas syringae. Our data highlight the key biological functions of ceramidases in biotic and abiotic stresses in plants.

Serine palmitoyltransferase (SPT), a pyridoxyl-59-phosphate-dependent enzyme, catalyzes the first and rate-limiting step in sphingolipid biosynthesis. In humans and yeast, orosomucoid proteins (ORMs) negatively regulate SPT and thus play an important role in maintaining sphingolipid levels. Despite the importance of sphingoid intermediates as bioactive molecules, the regulation of sphingolipid biosynthesis through SPT is not well understood in plants. Here, we identified and characterized the Arabidopsis thaliana ORMs, ORM1 and ORM2. Loss of function of both ORM1 and ORM2 (orm1 amiR-ORM2) stimulated de novo sphingolipid biosynthesis, leading to strong sphingolipid accumulation, especially of long-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 H 2 O 2 production at the cell wall and in mitochondria, active vesicular trafficking, and formation of cell wall appositions. Strikingly, the orm1 amiR-ORM2 plants showed increased expression of genes related to endoplasmic reticulum stress and defenses and also had enhanced resistance to oxidative stress and pathogen infection. Taken together, our findings indicate that ORMs interact with SPT to regulate sphingolipid homeostasis and play a pivotal role in environmental stress tolerance in plants.

Ceramidases hydrolyze ceramide into sphingosine and fatty acids and, although ceramidases function as key regulators of sphingolipid homeostasis in mammals, their roles in plants remain largely unknown. Here, we characterized the Arabidopsis thaliana ceramidase AtNCER1, a homolog of human neutral ceramidase. AtNCER1 localizes predominantly on the endoplasmic reticulum. The ncer1 T-DNA insertion mutants had no visible phenotype, but accumulated hydroxyceramides, and showed increased sensitivity to oxidative stress induced by methyl viologen. Plants over-expressing AtNCER1 showed increased tolerance to oxidative stress. These data indicate that the Arabidopsis neutral ceramidase affects sphingolipid homeostasis and oxidative stress responses.

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