SYNTAXIN OF PLANTS 81 regulates fatty acid desaturation by mediating Acyl Carrier Protein Desaturase 5 during seed development in Arabidopsis thaliana.

Lipids are essential for building cells and are used as important seed reserves. Fatty acids (FAs) are the key structural units of lipids, forming their hydrophobic tails in triglycerides and phospholipids. FA synthesis starts in plastids and is completed in the endoplasmic reticulum (ER). SYNTAXIN OF PLANTS 81 (SYP81), a Qa-soluble N-ETHYLMALEIMIDE SENSITIVE FACTOR attachment protein receptor (Qa-SNARE), regulates vesicle trafficking between the ER and Golgi apparatus, yet its role in FA synthesis is unknown. Here, we examined the expression of SYP81 during Arabidopsis thaliana seed development and found that the syp81 mutation reduced Acyl-Acyl Desaturase 5 (AAD5) accumulation in plastids of Arabidopsis embryonic cells and thus significantly decreased unsaturated FA production. Pull-down experiments identified possible interactors with SYP81, including various Stearoyl-Acyl carrier protein Desaturases (SADs), notably AAD5, and Translocons at Outer envelope membranes of Chloroplast (TOCs), especially TOC33. To validate these interactions, CoIP, yeast-two-hybridization assays, and bimolecular-fluorescence-complementation experiments were performed. The results of these experiments supported the interaction between SYP81 and AAD5, as well as SYP81 and TOC33. Based on these findings, a model was proposed, suggesting that pre-AAD5, recruited by SYP81, translocates from the ER to the plastids through the TIC-TOC complex mediations. Within the plastids, pre-AAD5 then matures into its catalytically active form, enabling subsequent FA desaturation.