Light-dependent conversion of carotenoids in the parasitic angiosperm Cuscuta reflexa L.

The parasitic angiosperm Cuscuta reflexa contains unusually high amounts of the carotenoids lutein-5,6-epoxide and 9-cis-violaxanthin. In this study the light-dependent conversions of these carotenoids in entire plant tissue and purified LHCIIb was compared with that of the xanthophyll cycle carotenoid violaxanthin when plants are exposed to high irradiance followed by low irradiance. In entire tissue under high irradiance, similar conversion kinetics and stoichiometry with de-epoxidation products suggest that both lutein-5,6-epoxide and all-trans-violaxanthin are equally suitable substrates for de-epoxidase. This is not the case under low irradiance as, although epoxidation of zeaxanthin and antheraxanthin rapidly restores the violaxanthin pool, the recovery of the lutein-5,6-epoxide pool is comparatively slow and has no stoichiometric relationship with its de-epoxidation product, lutein. Light-dependent changes in the concentration of 9-cis-violaxanthin mimic violaxanthin. However, the inability to detect de-epoxidation products or to de-epoxidize 9-cis-violaxanthin in vitro suggests that it is not subject to de-epoxidation and, instead, its concentration changes may reflect the equilibrium between isomers of violaxanthin. Light exposure did not affect the composition of carotenoids bound to purified LHCIIb, indicating that these bound carotenoids are not subject to de-epoxidation and do not contribute to the isomer pool equilibrium. The biosynthetic origins of lutein-5,6-epoxide and the potential role of these carotenoid cycles in photoprotection are discussed.