The amount of zeaxanthin epoxidase but not the amount of violaxanthin de-epoxidase is a critical determinant of zeaxanthin accumulation in Arabidopsis thaliana and Nicotiana tabacum.

The generation of VPZ lines, which simultaneously overexpress violaxanthin de-epoxidase (VDE), PsbS and zeaxanthin epoxidase (ZEP), has been successfully used to accelerate the kinetics of the induction and relaxation of non-photochemical quenching (NPQ). Here, we studied the impact of the over-expression of VDE and ZEP on the conversion of the xanthophyll cycle pigments in VPZ lines of Arabidopsis thaliana and Nicotiana tabacum. The protein amount of both VDE and ZEP was determined to be increased to about 3-5fold levels of wild-type (WT) plants for both species. Compared to WT plants, the conversion of violaxanthin (Vx) to zeaxanthin (Zx), and hence VDE activity, was only marginally accelerated in VPZ lines, whereas the conversion of Zx to Vx, and thus ZEP activity, was strongly increased in VPZ lines. This indicates, that the amount of ZEP but not the amount of VDE is a critical determinant of the equilibrium of the de-epoxidation state of the xanthophyll cycle pigments under saturating light conditions. Comparing the two steps of epoxidation, particularly the second step (antheraxanthin (Ax) to Vx) was found to be accelerated in VPZ lines, implying that the intermediate Ax is released into the membrane during epoxidation by ZEP.