The fern CYPome: Fern-specific cytochrome P450 family involved in convergent evolution of chemical defense.

Plant natural products encompass an enormous chemical diversity bearing witness to great molecular innovation that occured throughout land plant evolution. Cytochrome P450 monooxygenases (CYPs) catalyze a wide variety of monooxygenation reactions essential to the metabolic repertoire of plants natural products. Ferns constitute the second largest group of vascular plants and hold a significant phylogenetic position in land evolution, lying sister to seed plants. To date, CYP diversity has not been described for this taxon and pathway discovery in ferns in general is scarce, despite possessing a rich diversity of natural products. We analysed over 8000 available fern CYPs, classifing and characterizing the landscape of this super-enzyme group. Fern CYPs are dominated by fern-specific families ([~]60%), with the largest family - CYP981 - constituting approximately 15% of all predicted fern CYPs in the dataset. The abundancy and dynamics of the CYP981 family suggest a position equivalent to the CYP71 family present in seed plants, with potential roles in natural product biosynthesis. Ferns are the evolutionary oldest group to biosynthesize cyanogenic glycosides; amino acid-derived defense compounds. We show that CYP981F5 from the highly cyanogenic fern Phlebodium aureum catalyzes the conversion of phenylacetonitrile to mandelonitrile, an intermediate step in cyanogenic glycoside biosynthesis. The fern CYPome provides an important platform to further understand evolution of metabolite biosynthesis throughout the plant kingdom, and in ferns specifically.