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BIOENERGETICS AND PHOTOSYNTHESIS

Heat Stress Induces an Aggregation of the Light-Harvesting Complex of Photosystem II in Spinach Plants¹

Key Laboratory of Photosynthesis and Environmental Molecular Physiology, Institute of Botany (Y.T., X.W., Q.L., Z.Y., C.L.) and State Key Laboratory of Plant Genomics, Institute of Genetics and Developmental Biology (Z.C.), Chinese Academy of Sciences, Beijing 100101, People's Republic of China

Whole spinach (Spinacia oleracea) plants were subjected to heat stress (25°C–50°C) in the dark for 30 min. At temperatures higher than 35°C, CO₂ assimilation rate decreased significantly. The maximal efficiency of photosystem II (PSII) photochemistry remained unchanged until 45°C and decreased only slightly at 50°C. Nonphotochemical quenching increased significantly either in the absence or presence of dithiothreitol. There was an appearance of the characteristic band at around 698 nm in 77 K fluorescence emission spectra of leaves. Native green gel of thylakoid membranes isolated immediately from heat-stressed leaves showed that many pigment-protein complexes remained aggregated in the stacking gel. The analyses of sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting demonstrated that the aggregates were composed of the main light-harvesting complex of PSII (LHCIIb). To characterize the aggregates, isolated PSII core complexes were incubated at 25°C to 50°C in the dark for 10 min. At temperatures over 35°C, many pigment-protein complexes remained aggregated in the stacking gel of native green gel, and immunoblotting analyses showed that the aggregates were composed of LHCIIb. In addition, isolated LHCII was also incubated at 25°C to 50°C in the dark for 10 min. LHCII remained aggregated in the stacking gel of native green gel at temperatures over 35°C. Massive aggregation of LHCII was clearly observed by using microscope images, which was accompanied by a significant increase in fluorescence quenching. There was a linear relationship between the formation of LHCII aggregates and nonphotochemical quenching in vivo. The results in this study suggest that LHCII aggregates may represent a protective mechanism to dissipate excess excitation energy in heat-stressed plants.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Congming Lu (lucm{at}ibcas.ac.cn).

www.plantphysiol.org/cgi/doi/10.1104/pp.106.090712

^* Corresponding author; e-mail lucm{at}ibcas.ac.cn; fax 86–10–62595516.

Received October 2, 2006; accepted November 26, 2006; published December 1, 2006.

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