Arabidopsis CHOLINE/ETHANOLAMINE KINASE 1 (CEK1) is a primary choline kinase localized at the endoplasmic reticulum (ER) and involved in ER stress tolerance.

Choline kinase catalyzes the initial reaction step of choline metabolism that produces phosphocholine, a prerequisite for the biosynthesis of a primary phospholipid phosphatidylcholine. However, the primary choline kinase and its role in plant growth remained elusive in seed plants. Here, we showed that Arabidopsis CHOLINE/ETHANOLAMINE KINASE 1 (CEK1) encodes functional CEK that prefers choline than ethanolamine as a substrate in vitro and affects contents of choline and phosphocholine but not phosphatidylcholine in vivo. CEK1 is localized at endoplasmic reticulum (ER); upon tunicamycin-induced ER stress, a null mutant of CEK1 showed hypersensitive phenotype in seedlings, albeit with no enhanced choline kinase activity. Our results demonstrate that CEK1 is a primary ER-localized choline kinase in vivo that is required for ER stress tolerance possibly through the modulation of choline metabolites.