Acclimation of photosystem II to high temperature in a suspension culture of soybean (Glycine max) cells requires proteins that are associated with the thylakoid membrane.

In a study of the responses of photosystem II (PSII) to high temperature in suspension-cultured cells of soybean (Glycine max L. Merr.), we found that high temperatures inactivated PSII via two distinct pathways. Inactivation of PSII by moderately high temperatures, such as 41 degrees C, was reversed upon transfer of cells to 25 degrees C. The recovery of PSII required light, but not the synthesis of proteins de novo. By contrast, temperatures higher than 45 degrees C inactivated PSII irreversibly. An increase in the growth temperature from 25 to 35 degrees C resulted in an upward shift of 3 degrees C in the profile of the heat-induced inactivation of PSII, which indicated that the thermal stability of PSII had been enhanced. This acclimative response was reflected by the properties of isolated thylakoid membranes: PSII in thylakoid membranes from cells that had been grown at 35 degrees C exhibited greater thermal stability than that from cells grown at 25 degrees C. Disruption of the vesicular structure of thylakoid membranes with 0.05% Triton X-100 decreased the thermal stability of PSII to a similar level in both types of thylakoid membrane. Proteins released by Triton X-100 from thylakoid membranes from cells grown at 35 degrees C were able to increase the thermal stability of Triton-treated thylakoid membranes. These observations suggest that proteins that are associated with thylakoid membranes might be involved in the enhancement of the thermal stability of PSII.