Expression of the chloroplast-localized small heat shock protein by oxidative stress in rice

A rice (Oryza sativa L. cv. Nakdong) cDNA clone, Oshsp26, encoding the chloroplast-localized small heat shock protein (smHSP) was isolated. Southern blot analysis of genomic DNA and the result of screening of a cDNA library indicated that the Oshsp26 gene is encoded by a single gene in the rice genome. The Oshsp26 gene was expressed following heat stress: the transcript level was highest when rice leaves were treated at high temperatures for 2h at 42 degrees C, and the transcripts became detectable after 20min and reached a maximum level after 2h. It was also found that the Oshsp26 gene was expressed following oxidative stress even in the absence of heat stress. Treatment of rice plants with methyl viologen (MV) in the light and treatment with hydrogen peroxide (H(2)O(2)), either in the light or in the dark, both caused a significant accumulation of the transcripts and the protein. Since MV treatment in the light leads to the generation of H(2)O(2) inside the chloroplast, it is likely that H(2)O(2) by itself acts to induce the expression of the Oshsp26 gene. These results suggest that the chloroplast smHSP plays an important role in protecting the chloroplast against damage caused by oxidative stress as well as by heat stress.