Unsaturation of very-long-chain ceramides regulates hypoxia response by modulating ethylene signaling in Arabidopsis

Lipid remodeling is crucial for hypoxic tolerance in animals, whilst little is known about the hypoxia-induced lipid dynamics in plant cells. Here we performed a mass spectrometry-based analysis to survey the lipid profiles of Arabidopsis rosettes under various hypoxic conditions. We observed that hypoxia caused a significant increase in total amounts of phosphatidylserine, phosphatidic acid and oxylipins, but a decrease in phosphatidylcholine (PC) and phosphatidylethanolamine (PE). Particularly, significant gains in the polyunsaturated species of PC, PE and phosphatidylinositol, and losses in their saturated and mono-unsaturated species were evident during hypoxia. Moreover, hypoxia led to a remarkable elevation of ceramides and hydroxyceramides. Depletion of ceramide synthases LOH1, LOH2, and LOH3 enhanced plant sensitivity to dark submergence (DS), but displayed more resistance to submergence under light than wild type. Consistently, levels of unsaturated ceramide species (22:1, 24:1, and 26:1) predominantly declined in the loh1, loh2, and loh3 mutants under DS. Evidence that C24:1-ceramide interacted with recombinant CTR1 protein in vitro, enhanced ER-to-nucleus translocation of EIN2-GFP and stabilization of EIN3-GFP in vivo, suggests a role of ceramides in modulating ethylene signaling. The DS-sensitive phenotypes of loh mutants were rescued by a ctr1-1 mutation. Thus, our findings demonstrate that unsaturation of very-long-chain ceramides is a protective strategy for hypoxic tolerance in Arabidopsis.

6