Lignin production marked a milestone in vascular plant evolution, and the emergence of syringyl (S)-lignin is lineage-specific. S-lignin biosynthesis in angiosperms, mediated by ferulate 5-hydroxylase (F5H, CYP84A1), has been considered a recent evolutionary event. F5H uniquely requires the cytochrome b₅ protein CB5D as an obligatory redox partner for catalysis.

 

However, it remains unclear how CB5D functionality originated and whether it co-evolved with F5H. We reveal here the ancient evolution of CB5D-type function supporting F5H-catalyzed S-lignin biosynthesis. CB5D emerged in charophyte algae, the closest relatives of land plants, and is conserved and proliferated in embryophytes, especially in angiosperms, suggesting functional diversification of the CB5 family before terrestrialization. A sequence motif containing acidic amino residues in helix 5 of the CB5 heme-binding domain contributes to the retention of CB5D function in land plants but not in algae.

 

Notably, CB5s in the S-lignin-producing lycophyte Selaginella lack these residues, resulting in no CB5D-type function. An independently evolved S-lignin biosynthetic F5H (CYP788A1) in Selaginella relies on NADPH-dependent cytochrome P450 reductase as sole redox partner, distinct from angiosperms. These results suggest that angiosperm F5Hs co-opted the ancient CB5D, forming a modern cytochrome P450 monooxygenase system for aromatic ring meta-hydroxylation, enabling the re-emergence of S-lignin biosynthesis in angiosperms.