N-Acylphosphotidylethanolamine (NAPE) is an unusual phospholipid with a third acyl moiety linked to the ethanolamine head group of phosphatidylethanolamine (PE). NAPE is present in membranes of plants, animals, and some microorganisms, where it is normally present at only a few percent of the total phospholipid fraction. The activation of N-acylphosphatidylethanolamine (NAPE) metabolism in plants appears to be associated mostly with cellular stresses. In response to pathogen elicitors, NAPE is hydrolyzed by phospholipase-D (PLD), and corresponding medium-chain, saturated N-acylethanolamines (NAEs) are released by plant cells where they act as lipid mediators to modulate ion flux and activate defense gene expression. In desiccated seeds of higher plants, long-chain, saturated and unsaturated NAEs are prevalent, but are rapidly metabolized during the first few hours of imbibition, a period of substantial osmotic stress. NAPE synthesis is increased in seeds during this same period of rapid rehydration. A membrane-bound enzyme designated NAPE synthase has been purified from imbibed cottonseeds and its unusual biochemical properties suggest that it may scavenge free fatty acids in vivo. This feature of NAPE metabolism may be unique to higher plants a may be a mechanism for the rapid recycling of fatty acids back into membrane-associated NAPE (below). Altogether, increasing evidence indicates that NAPE metabolism in plants shares functional similarities with NAPE metabolism in animal systems, including signal transduction and cellular protection. In particular, the emerging role of released NAEs as lipid mediators in plant defense signaling represents an intriguing parallel to “endocannabinoid signaling” in several mammalian cell types. In support of this concept, an ortholog of the mammalian fatty acid amide hydrolase was identified in Arabidopsis and other plant species indicating conservation of some of the machinery for NAE metabolism between plants and animals.
