Regulation of zeaxanthin epoxidase activity

• Applicant: Professor Dr. Peter Jahns
• Subject Area: Plant Physiology
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 554388639

The xanthophyll zeaxanthin (Zx) serves central photoprotective functions in plants. Zx acts as antioxidant in the thylakoid membrane and essentially contributes to different processes of nonphotochemical quenching of excess excitation energy in photosystem II (PSII). Proper regulation of the Zx amount in the thylakoid membrane is required to optimize photosynthetic efficiency (under low and moderate light) and photoprotection (under high light). Zx is formed from violaxanthin (Vx) in the xanthophyll cycle by the Vx deepoxidase and is reconverted to Vx by the Zx epoxidase (ZEP). In preliminary work, we showed that light regulation of ZEP activity is essential for the short-term acclimation to high light stress. ZEP was found to be inactivated and finally degraded in response to high light stress along with photoinhibition of PSII and degradation of the D1 protein, indicating an essential role of Zx during the PSII repair cycle. We further provided evidence that high-light induced inactivation of ZEP is related to hydrogen peroxide formation. Moreover, we applied site-directed mutagenesis to replace six conserved cysteine residues by serine and seven conserved methionine residues by leucine. These conserved cysteine and methionine residues represent the most likely candidates for being oxidized by hydrogen peroxide in response to high-light stress. We established a simple in vitro assay of ZEP activity, which allowed the identification of several mutations that lead to the reduction of ZEP activity or even to complete inactivation of ZEP. For one cysteine residue we hypothesize an essential role in the formation of ZEP homodimers, for two methionine residues we hypothesize an essential role for the binding of a metal ion. In the planned project, the impact of the mutations on ZEP activity and in particular on ZEP regulation will be characterized in more detail under in vitro and in vivo conditions. For that, we will generate Arabidopsis mutant lines that stably express the different ZEP mutant variants. This approach will allow the detailed understanding of the importance of the different mutated cysteine and methionine residues for the regulation of ZEP activity under physiological conditions. Further analyses under in vitro conditions will clarify the role of specific cysteine and methionine residues for the formation of homodimers and for the binding of metal cofactors. This approach should allow the understanding of the functioning and regulation of ZEP in response to high light stress at the molecular level. The results are expected to provide important information on the molecular mechanisms of photoprotection in plants which might be useful for the improvement of photosynthetic performance and hence of biomass production of crop plants.

• DFG Programme: Research Grants