ABSENCE OF HEAT-SHOCK PROTEIN-SYNTHESIS IN ISOLATED RICE (ORYZA-SATIVA L) MITOCHONDRIA

We investigated whether isolated mitochondria of rice (Oryza sativa L.) respond to elevated temperatures by synthesizing a set of unique polypeptides in organello. Mitochondrial samples were prepared by conventional procedures that included surface sterilization and sucrose-gradient purification. The heat-shock treatment adopted for isolated mitochondria was based on the most effective temperatures to induce in vitro synthesis of heat-shock proteins by cytosolic polysomes isolated from etiolated rice seedlings. Heat-induced unique polypeptides were detected in mitochondrial samples prepared from growth-chamber-grown seedlings. In contrast, no heat-shock protein was detected in mitochondrial samples isolated from aseptically grown rice seedlings. Three different rice varieties, including a cytoplasmic male-sterile line, Zhen Shen 97A, gave the same results. We found that the extent of bacterial contamination in mitochondrial samples could be readily determined by plating. Mitochondrial preparations of growth-chamber-grown rice seedlings were found to contain a significant number of bacteria, whereas very few or no bacteria were detected in mitochondrial samples isolated from aseptically grown seedlings. We conclude that isolated rice mitochondria do not respond to elevated temperatures by synthesizing a unique set of proteins and that bacteria are responsible for the synthesis of heat-shock proteins in contaminated rice mitochondrial samples.