Ceramidases 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 At ACER, 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 micro RNA-mediated silenced line amiR- ACER-1 have high ceramide levels and decreased long chain bases. At ACER localizes predominantly to the endoplasmic reticulum, and partially to the Golgi complex. Furthermore, we found that acer-1 mutants and At ACER RNAi lines showed increased sensitivity to salt stress, and lines overexpressing At ACER showed increased tolerance to salt stress. Reduction of At ACER also increased plant susceptibility to Pseudomonas syringae. Our data highlight the key biological functions of ceramidases in biotic and abiotic stresses in plants. [ABSTRACT FROM AUTHOR]