Elucidation of the beta-carotene hydroxylation pathway in Arabidopsis thaliana reveals a fundamental role of epoxixanthophylls in photoprotection.

The first dedicated step in plant xanthophyll biosynthesis is carotenoid hydroxylation. In Arabidopsis thaliana, this reaction is performed by both heme (LUT1 and LUT5) and non-heme (CHY1 and CHY2) hydroxylases. No mutant completely abolishing α- or β-carotene hydroxylation has been described to date. We constructed double and triple mutant combinations in CHY1, CHY2, LUT1, LUT5 and LUT2 (lycopene epsilon-cyclase). In chy1chy2lut2, 80% of leaf carotenoids is represented by β-carotene. In chy1chy2lut5, β-carotene hydroxylation is completely abolished, while hydroxylation of the β-ring of α-carotene is still observed. The data are consistent with a role of LUT5 in β-ring hydroxylation, and with the existence of an additional hydroxylase, acting on the β-ring of α-, but not β-carotene