Progress in understanding the origin and functions of carotenoid hydroxylases in plants

The dihydroxy xanthophylls lutein (β,ε-carotene-3,3′-diol) and zeaxanthin (β,β-carotene-3,3′-diol) are well known for their critical roles in photosystem structure and adaptation of plants to high light stress. As a group, carotenoid hydroxylases catalyze the formation of lutein and zeaxanthin from their corresponding cyclic carotene precursors. Carotenoid β-ring hydroxylases (β-hydroxylases) have been isolated and characterized from various organisms and have invariably been shown to be non-heme di-iron monooxygenases. The presence of an ε-ring specific hydroxylase (ε-hydroxylase) was genetically demonstrated by isolation of mutants at the LUT1 locus in Arabidopsis. This review focuses on progress in understanding the molecular and biochemical nature of the ε-hydroxylase and the in vivo overlapping functions of the various carotenoid β- and ε-hydroxylases in Arabidopsis.