Dimorphecolic Acid Is Synthesized by the Coordinate Activities of Two Divergent Δ¹²-Oleic Acid Desaturases

Dimorphecolic acid (9-OH-18:2Δ^{10trans,12trans}) is the major fatty acid of seeds of Dimorphotheca species. This fatty acid contains structural features that are not typically in plant fatty acids, including a C-9 hydroxyl group, Δ¹⁰,Δ¹²-conjugated double bonds, and trans-Δ12 unsaturation. Expressed sequence tag analysis was conducted to determine the biosynthetic origin of dimorphecolic acid. cDNAs for two divergent forms of Δ12-oleic acid desaturase, designated DsFAD2-1 and DsFAD2-2, were identified among expressed sequence tags generated from developing Dimorphotheca sinuata seeds. Expression of DsFAD2-1 in Saccharomyces cerevisiae and soybean somatic embryos resulted in the accumulation of the trans-Δ¹² isomer of linoleic acid (18: 2Δ ^9cis,12trans) rather than the more typical cis-Δ¹² isomer. When co-expressed with DsFAD2-1 in soybean embryos or yeast, DsFAD2-2 converted 18:2Δ^9cis,12trans into dimorphecolic acid. When DsFAD2-2 was expressed alone in soybean embryos or together with a typical cis-Δ¹²-oleic acid desaturase in yeast, trace amounts of the cis-Δ¹² isomer of dimorphecolic acid (9-OH-18:2Δ ^9cis,12cis) were formed from DsFAD2-2 activity with cis-Δ ¹²-linoleic acid. These results indicate that DsFAD2-2 catalyzes the conversion of the Δ⁹ double bond of linoleic acid into a C-9 hydroxyl group and Δ^10trans double bond and displays a substrate preference for the trans-Δ¹², rather than the cis-Δ¹², isomer of linoleic acid. Overall these data are consistent with a biosynthetic pathway of dimorphecolic acid involving the concerted activities of DsFAD2-1 and DsFAD2-2. The evolution of two divergent Δ¹²-oleic acid desaturases for the biosynthesis of an unusual fatty acid is unprecedented in plants.