Molecular mechanisms of chilling injury and freezing injury

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Experimental results obtained through this Research Project are summarized as follows: 1) Molecular mechanisms of chilling injury. Following two factors were determined to be primary important for chilling injury in plant cells. First, a) a dysfunction of chloroplasts due to a low temperature-induced phase separation of disaturated molecular species of phosphatidylglycerol in the lipid bilayers. The correlation between the content of disaturated phosphatidylglycerol and the chilling sensitivity of plants was established both in hevaceous and woody plant species. The phase separation of disaturated phosphatoidylglycerol in intact membrane systems under low temperatures was determined by an analysis of msec delayed fluorescence.The key enzyme which controls the synthesis of the disaturated molecular species of phosphatidylglycerol was purified from chilling sensitive squash cotyledons and the CDNA was successfully cloned. Second, b) a disturbance of an iconic environment of cytoplasms du … More e to a low temperature-induced dysfunction of tonoplast proton transports. In mung bean cells, the tonoplast H^+-ATPase is impaired by a relatively short time of chilling in vivo, giving rise to an acidification of the cytoplasms. Two types of the proton transporting enzymes, i.e., H^+-ATPase and H^+-PPase, were purified from mung bean seedlings and characterized. The low temperature-induced inactivation of the H^+-ATPase is found to be related to a specific release of 57, 68 and 32 kDa subunits. 2) Mechanisms of freezing injury. Based on electronmicroscopic studies, it was confirmed that a freeze-induced ultrastructural changes in plasma membranes is primary important factor for cell injury. Upon a lethal freezing of cells, an aggegation of intramebranous particles and a formation of particle-free area, and a formation of Hexagonal THY phase structure were observed in plasma membranes specifically related to chill injury. However, these ultrastructural changes seemed to be dependent on plant species and the cell types, suggesting a diversity in the mechanisms of freezing injury. In sold acclimation of plants, a molecular rearrangement of plasma membranes, which includes qualitative changes in membrane proteins and enrichment of unsaturated phospholipids, is considered to play an important role in stabilization of plasma membranes against freezing stress.These results may provide a basis for molecularbiological approaches to the mechanisms of chilling injury and freezing injury in plants in future. Less