Eukaryotic Δ6-desaturases are microsomal enzymes that balance the synthesis of ω-3 and ω-6 C18-polyunsaturated-fatty-acids (PUFA) accordingly to their specificity. In several microalgae, including O. tauri, plastidic C18-PUFA are specifically regulated by environmental cues suggesting an autonomous control of Δ6-desaturation of plastidic PUFA. Sequence retrieval from O. tauri desaturases, highlighted two putative Δ6/Δ8-desaturases sequences clustering apart from other characterized Δ6-desaturases with other microalgal homologs. Their overexpression in heterologous hosts, including N. benthamiana and Synechocystis, unveiled their Δ6-desaturation activity and plastid localization. O. tauri lines overexpressing these Δ6-desaturases no longer adjusted their plastidic C18-PUFA amount under phosphate starvation but didn't show any obvious physiological alterations. Detailed lipid analyses from the various overexpressing hosts, unravelled that the substrate features involved in the Δ6-desaturase specificity importantly involved the lipid head-group and likely the non-substrate acyl-chain, in addition to the preference for the ω-class of the substrate acyl-chain. The most active desaturase displayed a broad range substrate specificity for plastidic lipids and a preference for ω-3 substrates, while the other was selective for ω-6 substrates, phosphatidylglycerol and 16:4-galactolipid species specific to the native host. The distribution of plastidial Δ6-desaturase products in eukaryotic hosts suggested the occurrence of C18-PUFA export from the plastid.

Plastidic Δ6 Fatty-Acid Desaturases With Distinctive Substrate Specificity Regulate The Pool Of C18-PUFAs In The Ancestral Picoalga Ostreococcus tauri