January 2011

N-Acylethanolamine metabolism in plants – a regulatory pathway diverged from endocannabinoid signaling in mammals?

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.

References

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 (chapman@unt.edu) is a regents professor of biochemistry at the University of North Texas, and Elison B. Blancaflor (eblancaflor@noble.org) is an associate professor at the Samuel Roberts Noble Foundation.

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