Regulation of xanthophyll conversion and the function of zeaxanthin in land plants

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The xanthophyll zeaxanthin (Zx) serves central photoprotective functions in land plants as regulator of the dissipation of excess light energy as heat (non-photochemical quenching, NPQ) in photosystem II (PSII) and as antioxidant in the thylakoid membrane. Zx is formed in the thylakoid membrane from violaxanthin (Vx) under high light (HL) in the de-epoxidation reactions (catalyzed by the lumen-localized Vx de-epoxidase) of the xanthophyll cycle and is reconverted back to Vx under low light or in darkness in the epoxidation reactions, catalyzed by the stroma-localized Zx epoxidase (ZEP). ZEP activity determines the period of the maintenance of a high Zx amount, and thus of Zx-dependent NPQ, in the chloroplast after HL stress. We have shown that ZEP activity is gradually down-regulated in response to increasing HL stress and photoinhibition of PSII, and that ZEP protein is degraded concomitant with the D1 protein upon PSII photoinhibition. The HL-induced down-regulation of ZEP activity ensures that high levels of Zx are retained in response to prolonged HL periods to allow for efficient reactivation (or retention) of NPQ and thus photoprotection after (or during) transient low light periods under fluctuating light conditions. Therefore, Zx is supposed to be a key component in the “memory” of the chloroplast with respect to preceding photo-oxidative stress. Detailed knowledge of the functioning and (light) regulation of ZEP activity is thus central for the understanding of the acclimation of photoprotective NPQ mechanisms in plants to HL or to fluctuating light. In preliminary work, we provided evidence that the HL-induced inactivation of ZEP may be caused by hydrogen peroxide, a reactive oxygen species (ROS) that is frequently formed in chloroplasts under HL stress. Analyses of the in vivo and in vitro activity of ZEP further showed that ZEP activity likely requires specific protein interactions in the thylakoid membrane and yet unknown cofactors. To characterize the properties and regulation of ZEP in detail at the molecular level, we will address three key issues in the planned project: (i) the exact localization and interaction partners of ZEP in the thylakoid membrane, (ii) the molecular basis of HL-induced ZEP inactivation, and (iii) the detailed biochemical properties of ZEP with focus on essential cofactors. This approach should allow the understanding of the functioning of ZEP in photoprotection at the molecular level.