Abstract
Main conclusionThe compatible solute sucrose reduces the efficiency of the enzymatic de-epoxidation of violaxanthin, probably by a direct effect on the protein parts of violaxanthin de-epoxidase which protrude from the lipid phase of the thylakoid membrane.The present study investigates the influence of the compatible solute sucrose on the violaxanthin cycle of higher plants in intact thylakoids and in in vitro enzyme assays with the isolated enzyme violaxanthin de-epoxidase at temperatures of 30 and 10 °C, respectively. In addition, the influence of sucrose on the lipid organization of thylakoid membranes and the MGDG phase in the in vitro assays is determined. The results show that sucrose leads to a pronounced inhibition of violaxanthin de-epoxidation both in intact thylakoid membranes and the enzyme assays. In general, the inhibition is similar at 30 and 10 °C. With respect to the lipid organization only minor changes can be seen in thylakoid membranes at 30 °C in the presence of sucrose. However, sucrose seems to stabilize the thylakoid membranes at lower temperatures and at 10 °C a comparable membrane organization to that at 30 °C can be observed, whereas control thylakoids show a significantly different membrane organization at the lower temperature. The MGDG phase in the in vitro assays is not substantially affected by the presence of sucrose or by changes of the temperature. We conclude that the presence of sucrose and the increased viscosity of the reaction buffers stabilize the protein part of the enzyme violaxanthin de-epoxidase, thereby decreasing the dynamic interactions between the catalytic site and the substrate violaxanthin. This indicates that sucrose interacts with those parts of the enzyme which are accessible at the membrane surface of the lipid phase of the thylakoid membrane or the MGDG phase of the in vitro enzyme assays.
Highlights
The violaxanthin (V) cycle represents an important photoprotection mechanism of higher plants and algae (Goss and Lepetit 2015)
The results of the present study show that sucrose leads to changes of the organization of the lipid phase of thylakoid membranes and the order of membrane lipids
In the present experiments the incubation of Laurdan with the thylakoid membranes was always carried out at a temperature of 30 °C to allow the complete incorporation of Laurdan into the membrane
Summary
The violaxanthin (V) cycle represents an important photoprotection mechanism of higher plants and algae (Goss and Lepetit 2015). It plays an important role in the structural re-arrangement of the light-harvesting complex of photosystem II (LHCII), which leads to the dissipation of excess. Planta (2021) 254:52 excitation energy as heat during high light illumination. The V cycle consists of a forward reaction which is typically driven by high light illumination. It comprises the stepwise de-epoxidation of the xanthophyll V, which contains two epoxy groups, to the mono-epoxide antheraxanthin (A) and to the epoxy-free zeaxanthin (Z). During low light or darkness the cycle is reverted and the two epoxy groups are re-introduced by the enzyme Z epoxidase (ZEP)
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