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

The dihydroxy xanthophylls lutein (beta,epsilon-carotene-3,3'-diol) and zeaxanthin (beta,beta-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 beta-ring hydroxylases (beta-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 epsilon-ring specific hydroxylase (epsilon-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 epsilon-hydroxylase and the in vivo overlapping functions of the various carotenoid beta- and epsilon-hydroxylases in Arabidopsis.