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Abstract Polyunsaturated N-acylethanolamines (NAEs) can be hydrolyzed by fatty acid amide hydrolase (FAAH) or oxidized by lipoxygenase (LOX). In Arabidopsis (Arabidopsis thaliana), the 9-LOX product of linoleoylethanolamide, namely, 9-hydroxy linoleoylethanolamide (9-NAE-HOD), is reported to negatively regulate seedling development during secondary dormancy. In upland cotton (Gossypium hirsutum L.), six putative FAAH genes (from two diverged groups) and six potential 9-LOX genes are present; however, their involvement in 9-NAE-HOD metabolism and its regulation of seedling development remain unexplored. Here, we report that in cotton plants, two specific FAAH isoforms (GhFAAH Ib and GhFAAH IIb) are needed for hydrolysis of certain endogenous NAEs. Virus-induced gene silencing (VIGS) of either or both FAAHs led to reduced seedling growth and this coincided with reduced amidohydrolase activities and elevated quantities of endogenous 9-NAE-HOD. Transcripts of GhLOX21 were consistently elevated in FAAH-silenced tissues, and co-silencing of GhLOX21 and GhFAAH (Ib and/or IIb) led to reversal of seedling growth to normal levels (comparable with no silencing). This was concomitant with reductions in the levels of 9-NAE-HOD, but not of 13-NAE-HOD. Pharmacological experiments corroborated the genetic and biochemical evidence, demonstrating that direct application of 9-NAE-HOD, but not 13-NAE-HOD or their corresponding free fatty acid oxylipins, inhibited the growth of cotton seedlings. Additionally, VIGS of GhLOX21 in cotton lines overexpressing AtFAAH exhibited enhanced growth and no detectable 9-NAE-HOD. Altogether, we conclude that the growth of cotton seedlings involves fine-tuning of 9-NAE-HOD levels via FAAH-mediated hydrolysis and LOX-mediated production, expanding the mechanistic understanding of plant growth modulation by NAE oxylipins to a perennial crop species. 

Abstract Fatty acid amide hydrolase (FAAH) is a conserved amidase that is known to modulate the levels of endogenousN‐acylethanolamines (NAEs) in both plants and animals. The activity of FAAH is enhancedin vitroby synthetic phenoxyacylethanolamides resulting in greater hydrolysis of NAEs. Previously, 3‐n‐pentadecylphenolethanolamide (PDP‐EA) was shown to exert positive effects on the development of Arabidopsis seedlings by enhancing Arabidopsis FAAH (AtFAAH) activity. However, there is little information regarding FAAH activity and the impact of PDP‐EA in the development of seedlings of other plant species. Here, we examined the effects of PDP‐EA on growth of upland cotton (Gossypium hirsutumL. cv Coker 312) seedlings including two lines of transgenic seedlings overexpressingAtFAAH. Independent transgenic events showed accelerated true‐leaf emergence compared with non‐transgenic controls. Exogenous applications of PDP‐EA led to increases in overall seedling growth in AtFAAH transgenic lines. These enhanced‐growth phenotypes coincided with elevated FAAH activities toward NAEs and NAE oxylipins. Conversely, the endogenous contents of NAEs and NAE‐oxylipin species, especially linoleoylethanolamide and 9‐hydroxy linoleoylethanolamide, were lower in PDP‐EA treated seedlings than in controls. Further, transcripts for endogenous cottonFAAHgenes were increased following PDP‐EA exposure. Collectively, our data corroborate that the enhancement of FAAH enzyme activity by PDP‐EA stimulates NAE‐hydrolysis and that this results in enhanced growth in seedlings of a perennial crop species, extending the role of NAE metabolism in seedling development beyond the model annual plant species,Arabidopsis thaliana. 

Fatty acid amide hydrolase (FAAH) is a conserved hydrolase in eukaryotes with promiscuous activity toward a range of acylamide substrates. The native substrate repertoire for FAAH has just begun to be explored in plant systems outside the modelArabidopsis thaliana. Here, we usedex vivolipidomics to identify potential endogenous substrates forMedicago truncatulaFAAH1 (MtFAAH1). We incubated recombinant MtFAAH1 with lipid mixtures extracted fromM. truncatulaand resolved their profiles via gas chromatography–mass spectrometry (GC–MS). Data revealed that besidesN‐acylethanolamines (NAEs),sn‐1orsn‐2isomers of monoacylglycerols (MAGs) were substrates for MtFAAH1. Combined within vitroand computational approaches, our data support both amidase and esterase activities for MtFAAH1. MAG‐mediated hydrolysis via MtFAAH1 may be linked to biological roles that are yet to be discovered. 

Abstract Fatty acid amide hydrolase (FAAH) is a widely conserved amidase in eukaryotes, perhaps best known for inactivating N -acylethanolamine lipid mediators. However, FAAH enzymes hydrolyze a wide range of acylamide substrates. Analysis of FAAHs from multiple angiosperm species revealed two conserved phylogenetic groups that differed in key conserved residues in the substrate binding pocket. While the foundation group of plant FAAHs, designated FAAH1, has been studied at the structural and functional level in Arabidopsis thaliana , nothing is known about FAAH2 members. Here, we combined computational and biochemical approaches to compare the structural and enzymatic properties of two FAAH isoforms in the legume Medicago truncatula designated MtFAAH1 and MtFAAH2a. Differences in structural and physicochemical properties of the substrate binding pockets, predicted from homology modeling, molecular docking, and molecular dynamic simulation experiments, suggested that these two FAAH isoforms would exhibit differences in their amidohydrolase activity profiles. Indeed, kinetic studies of purified, recombinant MtFAAHs indicated a reciprocal preference for acylamide substrates with MtFAAH1 more efficiently utilizing long-chain acylamides, and MtFAAH2a more efficiently hydrolyzing short-chain and aromatic acylamides. This first report of the enzymatic behavior of two phylogenetically distinct plant FAAHs will provide a foundation for further investigations regarding FAAH isoforms in legumes and other plant species. 

N-acylethanolamines (NAEs) are a group of lipid signaling molecules derived from the phospholipid precursor N-acylphosphatidylethanolamine (NAPE). NAEs can be processed by a wide range of metabolic processes including hydrolysis by fatty acid amide hydrolase (FAAH), peroxidation by lipoxygenases (LOX), and conjugation by glycosyl- and malonyl-transferases. The diversity of NAE metabolites points to participation in multiple downstream pathways for regulation and function. NAEs with acyl chains of 18C are typically the most predominant types in vascular plants. Whereas in nonvascular plants and some algae, the arachidonic acid-containing NAE, anandamide (a functional “endocannabinoid” in animal systems), was recently reported. A signaling role for anandamide and other NAEs is well established in vertebrates, while NAEs and their oxylipin metabolites are recently becoming appreciated for lipid mediator roles in vascular plants. Here, the NAE metabolism and function in plants are overviewed, with particular emphasis on processes described in vascular plants where most attention has been focused.