Fig. 1.

Pathway showing all possible routes to xanthophyll synthesis in Arabidopsis. Enzymatic reactions are indicated by numbers: , ε-cyclization; , β-cyclization; , β-ring hydroxylation of β,ε- and β,ψ-carotenoids; , ε-ring hydroxylation;, , β-ring hydroxylation of β,β-carotenoids. Reactions blocked by mutation of the indicated loci are shown: lut1 (ε-ring hydroxylase), lut2 (ε-cyclase), lut5, b1, and b2 (three β-ring hydroxylases). Solid gray arrows indicate a reaction sequence that is supported by mutant phenotypes and/or enzyme activity assays in E. coli, whereas dashed gray arrows are not. Solid black arrows, compounds, and mutant loci indicate major biosynthetic routes.