N-Acylethanolamines are fatty acid derivatives that are amide-linked to an ethanolamine moiety. They have been shown to have potent biological activities in the plant and animal kingdoms, but much of what is known about them pertains to their regulation of animal physiology and behavior.
|Fig. 1. Molecular structure of three NAEs. *Specific N-acylethanolamine types are identified by numerical designation of their acyl chain with number of carbons: number of double bonds.
N-Acylethanolamines, or NAEs,* are fatty acid derivatives that are amide-linked to an ethanolamine moiety (Fig. 1). They differ in their acyl chain length and number of double bonds, and they are present at trace concentrations in organisms throughout the eukaryotic domain. These lipids have been shown to have potent biological activities in the plant and animal kingdoms, but much of what is known about them pertains to their regulation of animal physiology and behavior.
Several NAE types act in the endocannabinoid signaling system of vertebrates by serving as endogenous ligands to the cannabinoid receptors (CB1 and CB2). Additional studies have shown that the complement of NAEs present in animal tissues, including those that are inactive as CB receptors, act either as entourage lipids or directly on targets other than CB receptors, such as vanilloid receptor ion channels and peroxisome proliferator activated receptor transcription factors (1). Regardless of NAE type, the bioactivity in mammalian systems appears to be terminated mostly through hydrolysis via fatty acid amide hydrolase, or FAAH (2).
The prevalence of NAEs in plant systems, particularly in seeds, has been recognized for many years. More recently it has become apparent that these NAEs are metabolized by a pathway analogous to that found in animal species. Furthermore, certain NAE types are known to have potent biological activities in plant cells at micromolar concentrations, prompting speculation that an NAE lipid mediator pathway may influence growth processes and stress responses in plants (3).
Several years ago, a functional homologue of rat FAAH was identified in Arabidopsis and other plant species (4). Biochemical and molecular characterization of this enzyme from plants confirmed that it hydrolyzes NAEs, which supports the hypothesis that NAE lipid mediators and their metabolism by FAAH facilitate plant growth regulation (5), interaction with phytohormone signaling (6) and responses of plants to pathogens (7).
While NAEs may not necessarily act in plants as ligands for G-protein coupled receptors like some do in animal systems, the evolutionary conservation of the occurrence of these lipid mediators and their metabolic machinery is striking (4). Furthermore, it is the arachidonic acid-containing NAE (anandamide), or NAE20:4 (Fig. 1), that functions as the endogenous NAE ligand for the CB receptors in neuronal and peripheral signaling, whereas the CB receptor inactive NAEs in animals seem to act through other means and overlap with the most abundant NAEs in eukaryotes in general (1). In other words, NAE metabolism itself may be more central (ancient) in eukaryotic biology, and the evolution of the endocannabinoid signaling system in vertebrates may have capitalized on this pathway and paralleled the development of arachidonic acid-based signaling and the expansion of sensory perception.
Studies of the NAE regulatory pathway in plants have begun to reveal how this lipid-based signaling pathway modulates plant growth and responses to the environment. Biochemical and genetic approaches have demonstrated that NAE metabolism interacts, at least partly, with abscisic acid signaling in plants (6). Overall, the experimental evidence suggests that the efficient depletion of both NAE and ABA is important for normal seedling establishment and that these two compounds can interact through the ABA signaling pathway to arrest normal seedling growth via modulation of ABI3 transcript levels (a key regulator of embryo-to-seedling transition).
|Fig 2. Hypothetical model for the interaction of NAE metabolism with ABA signaling to regulate growth during Arabidopsis seedling establishment. The red arrows indicate negative regulation of growth, and the green arrows indicate conditions that lead to enhanced growth. The large blue arrow indicates changes in concentration of NAEs.
Seedlings overexpressing AtFAAH exhibited enhanced growth under optimal conditions; however, they were exceptionally sensitive to biotic and abiotic stresses and the phytohormones known to be involved in these stresses (ABA and salicylic acid, or SA; 6,7), placing NAE metabolism and FAAH at a balance point between plant growth and responses to stress (4). Unexpectedly, active-site-directed mutations in AtFAAH that abolished catalytic activity in vitro toward all amide- and ester-linked fatty acids retained ABA hypersensitivity and compromised immunity but lost the capacity for enhanced growth (8). Hence, NAE hydrolysis by FAAH was important for enhancing seedling growth but not for influencing responses to ABA (or to SA and pathogens), demonstrating that the FAAH protein has bifurcating action, with discrete functions that are dependent and independent of its catalytic activity (4).
In a proposed model (Fig. 2), FAAH itself acts to regulate seedling growth by pathways that depend on fluctuating NAE levels as well as pathways that are independent of NAE hydrolysis. This represents a significant departure from mammalian paradigms for endocannabinoid signaling in neurotransmission, in which the hydrolysis of anandamide modulates G-protein signaling via plasma membrane receptors. On the other hand, plant systems likely have evolved alternative strategies from animals for using NAE metabolism and FAAH to regulate various processes, and the NAE regulatory pathway may be far more central to the overall control of plant physiology than previously appreciated.
1. De Petrocellis, L., and Di Marzo, V. (2009) An introduction to the endocannabinoid system: from the early to the latest concepts. Best Pract. Res. Clin. Endocrinol. Metab. 23, 1 – 15.
2. McKinney, M. K., and Cravatt, B. F. (2005) Structure and function of fatty acid amide hydrolase. Annu. Rev. Biochem. 74, 411 – 32.
3. Kilaru, A., Blancaflor, E. B., Venables, B. J., Tripathy, S., Mysore, K. S., and Chapman, K. D. (2007) The N-acylethanolamine-mediated regulatory pathway in plants. Chem.Biodivers. 4, 1933 – 1955.
4. Kim, S.-C., Chapman, K. D., and Blancaflor, E. B. (2010). Fatty acid amide lipid mediators in plants. Plant Sci. 178, 411 – 419.
5. Wang, Y.-S., Shrestha, R., Kilaru, A., Wiant, W., Venables, B. J., Chapman, K. D., and Blancaflor, E. B. (2006) Manipulation of Arabidopsis fatty acid amide hydrolase expression modifies plant growth and sensitivity to N-acylethanolamines. Proc. Natl. Acad. Sci., USA 103, 12197 – 12202.
6. Teaster, N. D., Motes, C. M., Tang, Y., Wiant, W. C., Cotter, M. Q., Wang,Y.-S., Kilaru, A., Venables, B. J., Hasenstein, K. H., Gonzalez, G., Blancaflor, E. B., and Chapman, K. D. (2007) N-Acylethanolamine metabolism interacts with abscisic acid signaling in Arabidopsis thaliana seedlings. Plant Cell 19, 2454 – 2469.
7. Kang, L., Wang, Y.-S., Uppalapati, S. R., Wang, K., Tang, Y., Vadapalli, V., Venables, B. J., Chapman, K. D., Blancaflor, E. B., and Mysore, K. S. (2008) Overexpression of a fatty acid amide hydrolase compromises innate immunity in Arabidopsis. Plant J. 56, 336 – 349.
8. Kim, S.-C., Kang, L., Nagaraj, S., Blancaflor, E. B., Mysore, K. S., and Chapman, K. D. (2009) Mutations in Arabidopsis fatty acid amide hydrolase reveal that catalytic activity influences growth but not sensitivity to abscisic acid or pathogens. J. Biol. Chem. 284, 34065 – 34074.
Kent D. Chapman (firstname.lastname@example.org) is a regents professor of biochemistry at the University of North Texas, and Elison B. Blancaflor (email@example.com) is an associate professor at the Samuel Roberts Noble Foundation.