Functional characterization of a plastidial cytochrome b5-fused Δ4-desaturase from Ostreococcus tauri in higher plants
Résumé
Marine microalgae are the primary producers of important lipids in oceanic ecosystems. In particular, they sustain the food web with omega-3 very-long chain polyunsaturated fatty acids (n3PUFAs), which play a protective role against various human metabolic disorders and are thus considered highly health beneficial. Ostreococcus tauri is a marine pico-eukaryote that contains high levels of several n3PUFAs, including docosahexaenoic acid (22:6n3; DHA), octadecapentaenoic acid (18:5n3), and hexadecatetraenoic acid (16:4n3), each with a distinct distribution. While docosahexaenoic and octadecapentaenoic acids are restricted to microsomal and plastidial lipids, respectively, hexadecatetraenoic acid is found in galactolipids as well as in bethaine and neutral lipids [1]. The genome of O. tauri contains 14 genes encoding fatty acid desaturases. In this study, we characterized the enzyme encoded by OT_ostta13g01550 (Ot13bDES) as a plastidial cytochrome b5-fused delta-4 desaturase involved in 16:4n3 biosynthesis. Transient heterologous expression of Ot13bDES in Nicotiana benthamiana led to the production of 16:4n3 and 16:3n6, but failed to produce 18:5n3 when Ot13bDES was coexpressed with plastidial Δ6-desaturases, suggesting Ot13bDES has a strict Δ4 regioselectivity. Lipidomic analyses of stable transgenic Arabidopsis lines further showed a nearly 100% conversion rate of 16:3n3 to 16:4n3 in the best-performing lines, demonstrating that Ot13bDES has a very high catalytic activity. Additionally, 16:4n3 was predominantly localized to monogalactosyldiacylglycerol. This study provides the first functional characterization of a plastidial cytochrome b5-fused delta-4 desaturase through heterologous expression in higher plants.
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