Biosynthesis of ω–hydroxy fatty acid polyesters in Nicotiana tabacum stigmas

Abstract

The aerial surfaces of all land plants are covered by hydrophobic lipid barriers that contain a core structure of polyesters derived from hydroxy fatty acids and glycerol. One type of surface lipid polyester is found in the exudates of wet stigmas of Solanaceae species, such as Nicotiana tabacum. These polyesters consist of ω-hydroxy fatty acids and glycerol, are extracellular, and have the experimental advantage of being soluble in organic solvents. However, very little is known about the biosynthesis mechanisms of stigma lipid polyesters. To gain insight into the biosynthesis of stigma lipid polyesters, I have characterized their structure and composition through stigma development and monitored polyester assembly through [14C]-acetate and [14C]-glycerol radiolabeling. Tobacco stigma lipids accumulate estolides consisting of ester-bonded ω-hydroxy fatty acids end-capped by normal fatty acids in the form of triacylglycerol, TAG(n), diacylglycerol, DAG(n), and free estolides, FFA(n), where n denotes the estolide bond count. These are composed primarily of oleic acid, linoleic acid, 18-hydroxyoleic acid, and 18-hydroxylinoleic acid and the estolides were identified to form on any position of the glycerol backbone. Chemical characterization of stigma lipid estolides suggest that polyester assembly occurs within the glandular zone of stigmatic tissue and then exported to the stigma surface; however, no distinctions have been made whether this process is intracellular or extracellular. Phosphatidylcholine (PC) is involved in stigma glycerolipid synthesis but not in acyl hydroxylation or estolide formation. Radiolabeled substrate accumulation suggests non-estolide glycerolipids to be synthesized through a combined de novo and PC-derived DAG(0)/TAG(0) pathways. A precursor-product relationship was observed in TAG(n) species and suggests that TAG(n) remodeling occurs, such that estolide polymerization occurs in a stepwise manner. These oxygenated fatty acids and estolides have physiochemical properties suitable for potential industrial uses, such as biolubricants. Identifying the mechanisms involved in the production of stigma lipid polyesters will help develop alternative sources of bio-based polymers by bioengineering plant surfaces or microorganisms. The sustainable and environmentally friendly production of these polyesters by engineered organisms is expected to result in substantial economic and environmental benefits.